From e6918187568dbd01842d8d1d2c808ce16a894239 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 21 Apr 2024 13:54:28 +0200 Subject: Adding upstream version 18.2.2. Signed-off-by: Daniel Baumann --- src/zstd/doc/README.md | 25 + src/zstd/doc/educational_decoder/.gitignore | 2 + src/zstd/doc/educational_decoder/Makefile | 62 + src/zstd/doc/educational_decoder/README.md | 36 + src/zstd/doc/educational_decoder/harness.c | 119 + src/zstd/doc/educational_decoder/zstd_decompress.c | 2320 ++++++++++++++++++++ src/zstd/doc/educational_decoder/zstd_decompress.h | 61 + src/zstd/doc/images/CSpeed2.png | Bin 0 -> 73335 bytes src/zstd/doc/images/DCspeed5.png | Bin 0 -> 69278 bytes src/zstd/doc/images/DSpeed3.png | Bin 0 -> 27123 bytes src/zstd/doc/images/cdict_v136.png | Bin 0 -> 33330 bytes src/zstd/doc/images/dict-cr.png | Bin 0 -> 90412 bytes src/zstd/doc/images/dict-cs.png | Bin 0 -> 91518 bytes src/zstd/doc/images/dict-ds.png | Bin 0 -> 98316 bytes src/zstd/doc/images/zstd_cdict_v1_3_5.png | Bin 0 -> 93969 bytes src/zstd/doc/images/zstd_logo86.png | Bin 0 -> 5963 bytes src/zstd/doc/zstd_compression_format.md | 1694 ++++++++++++++ src/zstd/doc/zstd_manual.html | 1680 ++++++++++++++ 18 files changed, 5999 insertions(+) create mode 100644 src/zstd/doc/README.md create mode 100644 src/zstd/doc/educational_decoder/.gitignore create mode 100644 src/zstd/doc/educational_decoder/Makefile create mode 100644 src/zstd/doc/educational_decoder/README.md create mode 100644 src/zstd/doc/educational_decoder/harness.c create mode 100644 src/zstd/doc/educational_decoder/zstd_decompress.c create mode 100644 src/zstd/doc/educational_decoder/zstd_decompress.h create mode 100644 src/zstd/doc/images/CSpeed2.png create mode 100644 src/zstd/doc/images/DCspeed5.png create mode 100644 src/zstd/doc/images/DSpeed3.png create mode 100644 src/zstd/doc/images/cdict_v136.png create mode 100644 src/zstd/doc/images/dict-cr.png create mode 100644 src/zstd/doc/images/dict-cs.png create mode 100644 src/zstd/doc/images/dict-ds.png create mode 100644 src/zstd/doc/images/zstd_cdict_v1_3_5.png create mode 100644 src/zstd/doc/images/zstd_logo86.png create mode 100644 src/zstd/doc/zstd_compression_format.md create mode 100644 src/zstd/doc/zstd_manual.html (limited to 'src/zstd/doc') diff --git a/src/zstd/doc/README.md b/src/zstd/doc/README.md new file mode 100644 index 000000000..bb7a3e490 --- /dev/null +++ b/src/zstd/doc/README.md @@ -0,0 +1,25 @@ +Zstandard Documentation +======================= + +This directory contains material defining the Zstandard format, +as well as detailed instructions to use `zstd` library. + +__`zstd_manual.html`__ : Documentation of `zstd.h` API, in html format. +Click on this link: [http://zstd.net/zstd_manual.html](http://zstd.net/zstd_manual.html) +to display documentation of latest release in readable format within a browser. + +__`zstd_compression_format.md`__ : This document defines the Zstandard compression format. +Compliant decoders must adhere to this document, +and compliant encoders must generate data that follows it. + +Should you look for resources to develop your own port of Zstandard algorithm, +you may find the following resources useful : + +__`educational_decoder`__ : This directory contains an implementation of a Zstandard decoder, +compliant with the Zstandard compression format. +It can be used, for example, to better understand the format, +or as the basis for a separate implementation of Zstandard decoder. + +[__`decode_corpus`__](https://github.com/facebook/zstd/tree/dev/tests#decodecorpus---tool-to-generate-zstandard-frames-for-decoder-testing) : +This tool, stored in `/tests` directory, is able to generate random valid frames, +which is useful if you wish to test your decoder and verify it fully supports the specification. diff --git a/src/zstd/doc/educational_decoder/.gitignore b/src/zstd/doc/educational_decoder/.gitignore new file mode 100644 index 000000000..b801306fa --- /dev/null +++ b/src/zstd/doc/educational_decoder/.gitignore @@ -0,0 +1,2 @@ +# Build artifacts +harness diff --git a/src/zstd/doc/educational_decoder/Makefile b/src/zstd/doc/educational_decoder/Makefile new file mode 100644 index 000000000..316c6eadc --- /dev/null +++ b/src/zstd/doc/educational_decoder/Makefile @@ -0,0 +1,62 @@ +# ################################################################ +# Copyright (c) 2016-2020, Yann Collet, Facebook, Inc. +# All rights reserved. +# +# This source code is licensed under both the BSD-style license (found in the +# LICENSE file in the root directory of this source tree) and the GPLv2 (found +# in the COPYING file in the root directory of this source tree). +# You may select, at your option, one of the above-listed licenses. +# ################################################################ + +ZSTD ?= zstd # note: requires zstd installation on local system + +UNAME?= $(shell uname) +ifeq ($(UNAME), SunOS) +DIFF ?= gdiff +else +DIFF ?= diff +endif + +HARNESS_FILES=*.c + +MULTITHREAD_LDFLAGS = -pthread +DEBUGFLAGS= -g -DZSTD_DEBUG=1 +CPPFLAGS += -I$(ZSTDDIR) -I$(ZSTDDIR)/common -I$(ZSTDDIR)/compress \ + -I$(ZSTDDIR)/dictBuilder -I$(ZSTDDIR)/deprecated -I$(PRGDIR) +CFLAGS ?= -O2 +CFLAGS += -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow \ + -Wstrict-aliasing=1 -Wswitch-enum \ + -Wredundant-decls -Wstrict-prototypes -Wundef \ + -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings \ + -std=c99 +CFLAGS += $(DEBUGFLAGS) +CFLAGS += $(MOREFLAGS) +FLAGS = $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) $(MULTITHREAD_LDFLAGS) + +harness: $(HARNESS_FILES) + $(CC) $(FLAGS) $^ -o $@ + +clean: + @$(RM) harness *.o + @$(RM) -rf harness.dSYM # MacOS specific + +test: harness + # + # Testing single-file decompression with educational decoder + # + @$(ZSTD) -f README.md -o tmp.zst + @./harness tmp.zst tmp + @$(DIFF) -s tmp README.md + @$(RM) tmp* + # + # Testing dictionary decompression with education decoder + # + # note : files are presented multiple for training, to reach minimum threshold + @$(ZSTD) --train harness.c zstd_decompress.c zstd_decompress.h README.md \ + harness.c zstd_decompress.c zstd_decompress.h README.md \ + harness.c zstd_decompress.c zstd_decompress.h README.md \ + -o dictionary + @$(ZSTD) -f README.md -D dictionary -o tmp.zst + @./harness tmp.zst tmp dictionary + @$(DIFF) -s tmp README.md + @$(RM) tmp* dictionary diff --git a/src/zstd/doc/educational_decoder/README.md b/src/zstd/doc/educational_decoder/README.md new file mode 100644 index 000000000..c89451ca0 --- /dev/null +++ b/src/zstd/doc/educational_decoder/README.md @@ -0,0 +1,36 @@ +Educational Decoder +=================== + +`zstd_decompress.c` is a self-contained implementation in C99 of a decoder, +according to the [Zstandard format specification]. +While it does not implement as many features as the reference decoder, +such as the streaming API or content checksums, it is written to be easy to +follow and understand, to help understand how the Zstandard format works. +It's laid out to match the [format specification], +so it can be used to understand how complex segments could be implemented. +It also contains implementations of Huffman and FSE table decoding. + +[Zstandard format specification]: https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md +[format specification]: https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md + +While the library's primary objective is code clarity, +it also happens to compile into a small object file. +The object file can be made even smaller by removing error messages, +using the macro directive `ZDEC_NO_MESSAGE` at compilation time. +This can be reduced even further by foregoing dictionary support, +by defining `ZDEC_NO_DICTIONARY`. + +`harness.c` provides a simple test harness around the decoder: + + harness [dictionary] + +As an additional resource to be used with this decoder, +see the `decodecorpus` tool in the [tests] directory. +It generates valid Zstandard frames that can be used to verify +a Zstandard decoder implementation. +Note that to use the tool to verify this decoder implementation, +the --content-size flag should be set, +as this decoder does not handle streaming decoding, +and so it must know the decompressed size in advance. + +[tests]: https://github.com/facebook/zstd/blob/dev/tests/ diff --git a/src/zstd/doc/educational_decoder/harness.c b/src/zstd/doc/educational_decoder/harness.c new file mode 100644 index 000000000..1403a6ed6 --- /dev/null +++ b/src/zstd/doc/educational_decoder/harness.c @@ -0,0 +1,119 @@ +/* + * Copyright (c) 2017-2020, Facebook, Inc. + * All rights reserved. + * + * This source code is licensed under both the BSD-style license (found in the + * LICENSE file in the root directory of this source tree) and the GPLv2 (found + * in the COPYING file in the root directory of this source tree). + * You may select, at your option, one of the above-listed licenses. + */ + +#include +#include + +#include "zstd_decompress.h" + +typedef unsigned char u8; + +// If the data doesn't have decompressed size with it, fallback on assuming the +// compression ratio is at most 16 +#define MAX_COMPRESSION_RATIO (16) + +// Protect against allocating too much memory for output +#define MAX_OUTPUT_SIZE ((size_t)1024 * 1024 * 1024) + +// Error message then exit +#define ERR_OUT(...) { fprintf(stderr, __VA_ARGS__); exit(1); } + + +typedef struct { + u8* address; + size_t size; +} buffer_s; + +static void freeBuffer(buffer_s b) { free(b.address); } + +static buffer_s read_file(const char *path) +{ + FILE* const f = fopen(path, "rb"); + if (!f) ERR_OUT("failed to open file %s \n", path); + + fseek(f, 0L, SEEK_END); + size_t const size = (size_t)ftell(f); + rewind(f); + + void* const ptr = malloc(size); + if (!ptr) ERR_OUT("failed to allocate memory to hold %s \n", path); + + size_t const read = fread(ptr, 1, size, f); + if (read != size) ERR_OUT("error while reading file %s \n", path); + + fclose(f); + buffer_s const b = { ptr, size }; + return b; +} + +static void write_file(const char* path, const u8* ptr, size_t size) +{ + FILE* const f = fopen(path, "wb"); + if (!f) ERR_OUT("failed to open file %s \n", path); + + size_t written = 0; + while (written < size) { + written += fwrite(ptr+written, 1, size, f); + if (ferror(f)) ERR_OUT("error while writing file %s\n", path); + } + + fclose(f); +} + +int main(int argc, char **argv) +{ + if (argc < 3) + ERR_OUT("usage: %s [dictionary] \n", argv[0]); + + buffer_s const input = read_file(argv[1]); + + buffer_s dict = { NULL, 0 }; + if (argc >= 4) { + dict = read_file(argv[3]); + } + + size_t out_capacity = ZSTD_get_decompressed_size(input.address, input.size); + if (out_capacity == (size_t)-1) { + out_capacity = MAX_COMPRESSION_RATIO * input.size; + fprintf(stderr, "WARNING: Compressed data does not contain " + "decompressed size, going to assume the compression " + "ratio is at most %d (decompressed size of at most " + "%u) \n", + MAX_COMPRESSION_RATIO, (unsigned)out_capacity); + } + if (out_capacity > MAX_OUTPUT_SIZE) + ERR_OUT("Required output size too large for this implementation \n"); + + u8* const output = malloc(out_capacity); + if (!output) ERR_OUT("failed to allocate memory \n"); + + dictionary_t* const parsed_dict = create_dictionary(); + if (dict.size) { +#if defined (ZDEC_NO_DICTIONARY) + printf("dict.size = %zu \n", dict.size); + ERR_OUT("no dictionary support \n"); +#else + parse_dictionary(parsed_dict, dict.address, dict.size); +#endif + } + size_t const decompressed_size = + ZSTD_decompress_with_dict(output, out_capacity, + input.address, input.size, + parsed_dict); + + free_dictionary(parsed_dict); + + write_file(argv[2], output, decompressed_size); + + freeBuffer(input); + freeBuffer(dict); + free(output); + return 0; +} diff --git a/src/zstd/doc/educational_decoder/zstd_decompress.c b/src/zstd/doc/educational_decoder/zstd_decompress.c new file mode 100644 index 000000000..605918b39 --- /dev/null +++ b/src/zstd/doc/educational_decoder/zstd_decompress.c @@ -0,0 +1,2320 @@ +/* + * Copyright (c) 2017-2020, Facebook, Inc. + * All rights reserved. + * + * This source code is licensed under both the BSD-style license (found in the + * LICENSE file in the root directory of this source tree) and the GPLv2 (found + * in the COPYING file in the root directory of this source tree). + * You may select, at your option, one of the above-listed licenses. + */ + +/// Zstandard educational decoder implementation +/// See https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md + +#include // uint8_t, etc. +#include // malloc, free, exit +#include // fprintf +#include // memset, memcpy +#include "zstd_decompress.h" + + +/******* IMPORTANT CONSTANTS *********************************************/ + +// Zstandard frame +// "Magic_Number +// 4 Bytes, little-endian format. Value : 0xFD2FB528" +#define ZSTD_MAGIC_NUMBER 0xFD2FB528U + +// The size of `Block_Content` is limited by `Block_Maximum_Size`, +#define ZSTD_BLOCK_SIZE_MAX ((size_t)128 * 1024) + +// literal blocks can't be larger than their block +#define MAX_LITERALS_SIZE ZSTD_BLOCK_SIZE_MAX + + +/******* UTILITY MACROS AND TYPES *********************************************/ +#define MAX(a, b) ((a) > (b) ? (a) : (b)) +#define MIN(a, b) ((a) < (b) ? (a) : (b)) + +#if defined(ZDEC_NO_MESSAGE) +#define MESSAGE(...) +#else +#define MESSAGE(...) fprintf(stderr, "" __VA_ARGS__) +#endif + +/// This decoder calls exit(1) when it encounters an error, however a production +/// library should propagate error codes +#define ERROR(s) \ + do { \ + MESSAGE("Error: %s\n", s); \ + exit(1); \ + } while (0) +#define INP_SIZE() \ + ERROR("Input buffer smaller than it should be or input is " \ + "corrupted") +#define OUT_SIZE() ERROR("Output buffer too small for output") +#define CORRUPTION() ERROR("Corruption detected while decompressing") +#define BAD_ALLOC() ERROR("Memory allocation error") +#define IMPOSSIBLE() ERROR("An impossibility has occurred") + +typedef uint8_t u8; +typedef uint16_t u16; +typedef uint32_t u32; +typedef uint64_t u64; + +typedef int8_t i8; +typedef int16_t i16; +typedef int32_t i32; +typedef int64_t i64; +/******* END UTILITY MACROS AND TYPES *****************************************/ + +/******* IMPLEMENTATION PRIMITIVE PROTOTYPES **********************************/ +/// The implementations for these functions can be found at the bottom of this +/// file. They implement low-level functionality needed for the higher level +/// decompression functions. + +/*** IO STREAM OPERATIONS *************/ + +/// ostream_t/istream_t are used to wrap the pointers/length data passed into +/// ZSTD_decompress, so that all IO operations are safely bounds checked +/// They are written/read forward, and reads are treated as little-endian +/// They should be used opaquely to ensure safety +typedef struct { + u8 *ptr; + size_t len; +} ostream_t; + +typedef struct { + const u8 *ptr; + size_t len; + + // Input often reads a few bits at a time, so maintain an internal offset + int bit_offset; +} istream_t; + +/// The following two functions are the only ones that allow the istream to be +/// non-byte aligned + +/// Reads `num` bits from a bitstream, and updates the internal offset +static inline u64 IO_read_bits(istream_t *const in, const int num_bits); +/// Backs-up the stream by `num` bits so they can be read again +static inline void IO_rewind_bits(istream_t *const in, const int num_bits); +/// If the remaining bits in a byte will be unused, advance to the end of the +/// byte +static inline void IO_align_stream(istream_t *const in); + +/// Write the given byte into the output stream +static inline void IO_write_byte(ostream_t *const out, u8 symb); + +/// Returns the number of bytes left to be read in this stream. The stream must +/// be byte aligned. +static inline size_t IO_istream_len(const istream_t *const in); + +/// Advances the stream by `len` bytes, and returns a pointer to the chunk that +/// was skipped. The stream must be byte aligned. +static inline const u8 *IO_get_read_ptr(istream_t *const in, size_t len); +/// Advances the stream by `len` bytes, and returns a pointer to the chunk that +/// was skipped so it can be written to. +static inline u8 *IO_get_write_ptr(ostream_t *const out, size_t len); + +/// Advance the inner state by `len` bytes. The stream must be byte aligned. +static inline void IO_advance_input(istream_t *const in, size_t len); + +/// Returns an `ostream_t` constructed from the given pointer and length. +static inline ostream_t IO_make_ostream(u8 *out, size_t len); +/// Returns an `istream_t` constructed from the given pointer and length. +static inline istream_t IO_make_istream(const u8 *in, size_t len); + +/// Returns an `istream_t` with the same base as `in`, and length `len`. +/// Then, advance `in` to account for the consumed bytes. +/// `in` must be byte aligned. +static inline istream_t IO_make_sub_istream(istream_t *const in, size_t len); +/*** END IO STREAM OPERATIONS *********/ + +/*** BITSTREAM OPERATIONS *************/ +/// Read `num` bits (up to 64) from `src + offset`, where `offset` is in bits, +/// and return them interpreted as a little-endian unsigned integer. +static inline u64 read_bits_LE(const u8 *src, const int num_bits, + const size_t offset); + +/// Read bits from the end of a HUF or FSE bitstream. `offset` is in bits, so +/// it updates `offset` to `offset - bits`, and then reads `bits` bits from +/// `src + offset`. If the offset becomes negative, the extra bits at the +/// bottom are filled in with `0` bits instead of reading from before `src`. +static inline u64 STREAM_read_bits(const u8 *src, const int bits, + i64 *const offset); +/*** END BITSTREAM OPERATIONS *********/ + +/*** BIT COUNTING OPERATIONS **********/ +/// Returns the index of the highest set bit in `num`, or `-1` if `num == 0` +static inline int highest_set_bit(const u64 num); +/*** END BIT COUNTING OPERATIONS ******/ + +/*** HUFFMAN PRIMITIVES ***************/ +// Table decode method uses exponential memory, so we need to limit depth +#define HUF_MAX_BITS (16) + +// Limit the maximum number of symbols to 256 so we can store a symbol in a byte +#define HUF_MAX_SYMBS (256) + +/// Structure containing all tables necessary for efficient Huffman decoding +typedef struct { + u8 *symbols; + u8 *num_bits; + int max_bits; +} HUF_dtable; + +/// Decode a single symbol and read in enough bits to refresh the state +static inline u8 HUF_decode_symbol(const HUF_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset); +/// Read in a full state's worth of bits to initialize it +static inline void HUF_init_state(const HUF_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset); + +/// Decompresses a single Huffman stream, returns the number of bytes decoded. +/// `src_len` must be the exact length of the Huffman-coded block. +static size_t HUF_decompress_1stream(const HUF_dtable *const dtable, + ostream_t *const out, istream_t *const in); +/// Same as previous but decodes 4 streams, formatted as in the Zstandard +/// specification. +/// `src_len` must be the exact length of the Huffman-coded block. +static size_t HUF_decompress_4stream(const HUF_dtable *const dtable, + ostream_t *const out, istream_t *const in); + +/// Initialize a Huffman decoding table using the table of bit counts provided +static void HUF_init_dtable(HUF_dtable *const table, const u8 *const bits, + const int num_symbs); +/// Initialize a Huffman decoding table using the table of weights provided +/// Weights follow the definition provided in the Zstandard specification +static void HUF_init_dtable_usingweights(HUF_dtable *const table, + const u8 *const weights, + const int num_symbs); + +/// Free the malloc'ed parts of a decoding table +static void HUF_free_dtable(HUF_dtable *const dtable); +/*** END HUFFMAN PRIMITIVES ***********/ + +/*** FSE PRIMITIVES *******************/ +/// For more description of FSE see +/// https://github.com/Cyan4973/FiniteStateEntropy/ + +// FSE table decoding uses exponential memory, so limit the maximum accuracy +#define FSE_MAX_ACCURACY_LOG (15) +// Limit the maximum number of symbols so they can be stored in a single byte +#define FSE_MAX_SYMBS (256) + +/// The tables needed to decode FSE encoded streams +typedef struct { + u8 *symbols; + u8 *num_bits; + u16 *new_state_base; + int accuracy_log; +} FSE_dtable; + +/// Return the symbol for the current state +static inline u8 FSE_peek_symbol(const FSE_dtable *const dtable, + const u16 state); +/// Read the number of bits necessary to update state, update, and shift offset +/// back to reflect the bits read +static inline void FSE_update_state(const FSE_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset); + +/// Combine peek and update: decode a symbol and update the state +static inline u8 FSE_decode_symbol(const FSE_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset); + +/// Read bits from the stream to initialize the state and shift offset back +static inline void FSE_init_state(const FSE_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset); + +/// Decompress two interleaved bitstreams (e.g. compressed Huffman weights) +/// using an FSE decoding table. `src_len` must be the exact length of the +/// block. +static size_t FSE_decompress_interleaved2(const FSE_dtable *const dtable, + ostream_t *const out, + istream_t *const in); + +/// Initialize a decoding table using normalized frequencies. +static void FSE_init_dtable(FSE_dtable *const dtable, + const i16 *const norm_freqs, const int num_symbs, + const int accuracy_log); + +/// Decode an FSE header as defined in the Zstandard format specification and +/// use the decoded frequencies to initialize a decoding table. +static void FSE_decode_header(FSE_dtable *const dtable, istream_t *const in, + const int max_accuracy_log); + +/// Initialize an FSE table that will always return the same symbol and consume +/// 0 bits per symbol, to be used for RLE mode in sequence commands +static void FSE_init_dtable_rle(FSE_dtable *const dtable, const u8 symb); + +/// Free the malloc'ed parts of a decoding table +static void FSE_free_dtable(FSE_dtable *const dtable); +/*** END FSE PRIMITIVES ***************/ + +/******* END IMPLEMENTATION PRIMITIVE PROTOTYPES ******************************/ + +/******* ZSTD HELPER STRUCTS AND PROTOTYPES ***********************************/ + +/// A small structure that can be reused in various places that need to access +/// frame header information +typedef struct { + // The size of window that we need to be able to contiguously store for + // references + size_t window_size; + // The total output size of this compressed frame + size_t frame_content_size; + + // The dictionary id if this frame uses one + u32 dictionary_id; + + // Whether or not the content of this frame has a checksum + int content_checksum_flag; + // Whether or not the output for this frame is in a single segment + int single_segment_flag; +} frame_header_t; + +/// The context needed to decode blocks in a frame +typedef struct { + frame_header_t header; + + // The total amount of data available for backreferences, to determine if an + // offset too large to be correct + size_t current_total_output; + + const u8 *dict_content; + size_t dict_content_len; + + // Entropy encoding tables so they can be repeated by future blocks instead + // of retransmitting + HUF_dtable literals_dtable; + FSE_dtable ll_dtable; + FSE_dtable ml_dtable; + FSE_dtable of_dtable; + + // The last 3 offsets for the special "repeat offsets". + u64 previous_offsets[3]; +} frame_context_t; + +/// The decoded contents of a dictionary so that it doesn't have to be repeated +/// for each frame that uses it +struct dictionary_s { + // Entropy tables + HUF_dtable literals_dtable; + FSE_dtable ll_dtable; + FSE_dtable ml_dtable; + FSE_dtable of_dtable; + + // Raw content for backreferences + u8 *content; + size_t content_size; + + // Offset history to prepopulate the frame's history + u64 previous_offsets[3]; + + u32 dictionary_id; +}; + +/// A tuple containing the parts necessary to decode and execute a ZSTD sequence +/// command +typedef struct { + u32 literal_length; + u32 match_length; + u32 offset; +} sequence_command_t; + +/// The decoder works top-down, starting at the high level like Zstd frames, and +/// working down to lower more technical levels such as blocks, literals, and +/// sequences. The high-level functions roughly follow the outline of the +/// format specification: +/// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md + +/// Before the implementation of each high-level function declared here, the +/// prototypes for their helper functions are defined and explained + +/// Decode a single Zstd frame, or error if the input is not a valid frame. +/// Accepts a dict argument, which may be NULL indicating no dictionary. +/// See +/// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#frame-concatenation +static void decode_frame(ostream_t *const out, istream_t *const in, + const dictionary_t *const dict); + +// Decode data in a compressed block +static void decompress_block(frame_context_t *const ctx, ostream_t *const out, + istream_t *const in); + +// Decode the literals section of a block +static size_t decode_literals(frame_context_t *const ctx, istream_t *const in, + u8 **const literals); + +// Decode the sequences part of a block +static size_t decode_sequences(frame_context_t *const ctx, istream_t *const in, + sequence_command_t **const sequences); + +// Execute the decoded sequences on the literals block +static void execute_sequences(frame_context_t *const ctx, ostream_t *const out, + const u8 *const literals, + const size_t literals_len, + const sequence_command_t *const sequences, + const size_t num_sequences); + +// Copies literals and returns the total literal length that was copied +static u32 copy_literals(const size_t seq, istream_t *litstream, + ostream_t *const out); + +// Given an offset code from a sequence command (either an actual offset value +// or an index for previous offset), computes the correct offset and updates +// the offset history +static size_t compute_offset(sequence_command_t seq, u64 *const offset_hist); + +// Given an offset, match length, and total output, as well as the frame +// context for the dictionary, determines if the dictionary is used and +// executes the copy operation +static void execute_match_copy(frame_context_t *const ctx, size_t offset, + size_t match_length, size_t total_output, + ostream_t *const out); + +/******* END ZSTD HELPER STRUCTS AND PROTOTYPES *******************************/ + +size_t ZSTD_decompress(void *const dst, const size_t dst_len, + const void *const src, const size_t src_len) { + dictionary_t* const uninit_dict = create_dictionary(); + size_t const decomp_size = ZSTD_decompress_with_dict(dst, dst_len, src, + src_len, uninit_dict); + free_dictionary(uninit_dict); + return decomp_size; +} + +size_t ZSTD_decompress_with_dict(void *const dst, const size_t dst_len, + const void *const src, const size_t src_len, + dictionary_t* parsed_dict) { + + istream_t in = IO_make_istream(src, src_len); + ostream_t out = IO_make_ostream(dst, dst_len); + + // "A content compressed by Zstandard is transformed into a Zstandard frame. + // Multiple frames can be appended into a single file or stream. A frame is + // totally independent, has a defined beginning and end, and a set of + // parameters which tells the decoder how to decompress it." + + /* this decoder assumes decompression of a single frame */ + decode_frame(&out, &in, parsed_dict); + + return (size_t)(out.ptr - (u8 *)dst); +} + +/******* FRAME DECODING ******************************************************/ + +static void decode_data_frame(ostream_t *const out, istream_t *const in, + const dictionary_t *const dict); +static void init_frame_context(frame_context_t *const context, + istream_t *const in, + const dictionary_t *const dict); +static void free_frame_context(frame_context_t *const context); +static void parse_frame_header(frame_header_t *const header, + istream_t *const in); +static void frame_context_apply_dict(frame_context_t *const ctx, + const dictionary_t *const dict); + +static void decompress_data(frame_context_t *const ctx, ostream_t *const out, + istream_t *const in); + +static void decode_frame(ostream_t *const out, istream_t *const in, + const dictionary_t *const dict) { + const u32 magic_number = (u32)IO_read_bits(in, 32); + if (magic_number == ZSTD_MAGIC_NUMBER) { + // ZSTD frame + decode_data_frame(out, in, dict); + + return; + } + + // not a real frame or a skippable frame + ERROR("Tried to decode non-ZSTD frame"); +} + +/// Decode a frame that contains compressed data. Not all frames do as there +/// are skippable frames. +/// See +/// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#general-structure-of-zstandard-frame-format +static void decode_data_frame(ostream_t *const out, istream_t *const in, + const dictionary_t *const dict) { + frame_context_t ctx; + + // Initialize the context that needs to be carried from block to block + init_frame_context(&ctx, in, dict); + + if (ctx.header.frame_content_size != 0 && + ctx.header.frame_content_size > out->len) { + OUT_SIZE(); + } + + decompress_data(&ctx, out, in); + + free_frame_context(&ctx); +} + +/// Takes the information provided in the header and dictionary, and initializes +/// the context for this frame +static void init_frame_context(frame_context_t *const context, + istream_t *const in, + const dictionary_t *const dict) { + // Most fields in context are correct when initialized to 0 + memset(context, 0, sizeof(frame_context_t)); + + // Parse data from the frame header + parse_frame_header(&context->header, in); + + // Set up the offset history for the repeat offset commands + context->previous_offsets[0] = 1; + context->previous_offsets[1] = 4; + context->previous_offsets[2] = 8; + + // Apply details from the dict if it exists + frame_context_apply_dict(context, dict); +} + +static void free_frame_context(frame_context_t *const context) { + HUF_free_dtable(&context->literals_dtable); + + FSE_free_dtable(&context->ll_dtable); + FSE_free_dtable(&context->ml_dtable); + FSE_free_dtable(&context->of_dtable); + + memset(context, 0, sizeof(frame_context_t)); +} + +static void parse_frame_header(frame_header_t *const header, + istream_t *const in) { + // "The first header's byte is called the Frame_Header_Descriptor. It tells + // which other fields are present. Decoding this byte is enough to tell the + // size of Frame_Header. + // + // Bit number Field name + // 7-6 Frame_Content_Size_flag + // 5 Single_Segment_flag + // 4 Unused_bit + // 3 Reserved_bit + // 2 Content_Checksum_flag + // 1-0 Dictionary_ID_flag" + const u8 descriptor = (u8)IO_read_bits(in, 8); + + // decode frame header descriptor into flags + const u8 frame_content_size_flag = descriptor >> 6; + const u8 single_segment_flag = (descriptor >> 5) & 1; + const u8 reserved_bit = (descriptor >> 3) & 1; + const u8 content_checksum_flag = (descriptor >> 2) & 1; + const u8 dictionary_id_flag = descriptor & 3; + + if (reserved_bit != 0) { + CORRUPTION(); + } + + header->single_segment_flag = single_segment_flag; + header->content_checksum_flag = content_checksum_flag; + + // decode window size + if (!single_segment_flag) { + // "Provides guarantees on maximum back-reference distance that will be + // used within compressed data. This information is important for + // decoders to allocate enough memory. + // + // Bit numbers 7-3 2-0 + // Field name Exponent Mantissa" + u8 window_descriptor = (u8)IO_read_bits(in, 8); + u8 exponent = window_descriptor >> 3; + u8 mantissa = window_descriptor & 7; + + // Use the algorithm from the specification to compute window size + // https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#window_descriptor + size_t window_base = (size_t)1 << (10 + exponent); + size_t window_add = (window_base / 8) * mantissa; + header->window_size = window_base + window_add; + } + + // decode dictionary id if it exists + if (dictionary_id_flag) { + // "This is a variable size field, which contains the ID of the + // dictionary required to properly decode the frame. Note that this + // field is optional. When it's not present, it's up to the caller to + // make sure it uses the correct dictionary. Format is little-endian." + const int bytes_array[] = {0, 1, 2, 4}; + const int bytes = bytes_array[dictionary_id_flag]; + + header->dictionary_id = (u32)IO_read_bits(in, bytes * 8); + } else { + header->dictionary_id = 0; + } + + // decode frame content size if it exists + if (single_segment_flag || frame_content_size_flag) { + // "This is the original (uncompressed) size. This information is + // optional. The Field_Size is provided according to value of + // Frame_Content_Size_flag. The Field_Size can be equal to 0 (not + // present), 1, 2, 4 or 8 bytes. Format is little-endian." + // + // if frame_content_size_flag == 0 but single_segment_flag is set, we + // still have a 1 byte field + const int bytes_array[] = {1, 2, 4, 8}; + const int bytes = bytes_array[frame_content_size_flag]; + + header->frame_content_size = IO_read_bits(in, bytes * 8); + if (bytes == 2) { + // "When Field_Size is 2, the offset of 256 is added." + header->frame_content_size += 256; + } + } else { + header->frame_content_size = 0; + } + + if (single_segment_flag) { + // "The Window_Descriptor byte is optional. It is absent when + // Single_Segment_flag is set. In this case, the maximum back-reference + // distance is the content size itself, which can be any value from 1 to + // 2^64-1 bytes (16 EB)." + header->window_size = header->frame_content_size; + } +} + +/// Decompress the data from a frame block by block +static void decompress_data(frame_context_t *const ctx, ostream_t *const out, + istream_t *const in) { + // "A frame encapsulates one or multiple blocks. Each block can be + // compressed or not, and has a guaranteed maximum content size, which + // depends on frame parameters. Unlike frames, each block depends on + // previous blocks for proper decoding. However, each block can be + // decompressed without waiting for its successor, allowing streaming + // operations." + int last_block = 0; + do { + // "Last_Block + // + // The lowest bit signals if this block is the last one. Frame ends + // right after this block. + // + // Block_Type and Block_Size + // + // The next 2 bits represent the Block_Type, while the remaining 21 bits + // represent the Block_Size. Format is little-endian." + last_block = (int)IO_read_bits(in, 1); + const int block_type = (int)IO_read_bits(in, 2); + const size_t block_len = IO_read_bits(in, 21); + + switch (block_type) { + case 0: { + // "Raw_Block - this is an uncompressed block. Block_Size is the + // number of bytes to read and copy." + const u8 *const read_ptr = IO_get_read_ptr(in, block_len); + u8 *const write_ptr = IO_get_write_ptr(out, block_len); + + // Copy the raw data into the output + memcpy(write_ptr, read_ptr, block_len); + + ctx->current_total_output += block_len; + break; + } + case 1: { + // "RLE_Block - this is a single byte, repeated N times. In which + // case, Block_Size is the size to regenerate, while the + // "compressed" block is just 1 byte (the byte to repeat)." + const u8 *const read_ptr = IO_get_read_ptr(in, 1); + u8 *const write_ptr = IO_get_write_ptr(out, block_len); + + // Copy `block_len` copies of `read_ptr[0]` to the output + memset(write_ptr, read_ptr[0], block_len); + + ctx->current_total_output += block_len; + break; + } + case 2: { + // "Compressed_Block - this is a Zstandard compressed block, + // detailed in another section of this specification. Block_Size is + // the compressed size. + + // Create a sub-stream for the block + istream_t block_stream = IO_make_sub_istream(in, block_len); + decompress_block(ctx, out, &block_stream); + break; + } + case 3: + // "Reserved - this is not a block. This value cannot be used with + // current version of this specification." + CORRUPTION(); + break; + default: + IMPOSSIBLE(); + } + } while (!last_block); + + if (ctx->header.content_checksum_flag) { + // This program does not support checking the checksum, so skip over it + // if it's present + IO_advance_input(in, 4); + } +} +/******* END FRAME DECODING ***************************************************/ + +/******* BLOCK DECOMPRESSION **************************************************/ +static void decompress_block(frame_context_t *const ctx, ostream_t *const out, + istream_t *const in) { + // "A compressed block consists of 2 sections : + // + // Literals_Section + // Sequences_Section" + + + // Part 1: decode the literals block + u8 *literals = NULL; + const size_t literals_size = decode_literals(ctx, in, &literals); + + // Part 2: decode the sequences block + sequence_command_t *sequences = NULL; + const size_t num_sequences = + decode_sequences(ctx, in, &sequences); + + // Part 3: combine literals and sequence commands to generate output + execute_sequences(ctx, out, literals, literals_size, sequences, + num_sequences); + free(literals); + free(sequences); +} +/******* END BLOCK DECOMPRESSION **********************************************/ + +/******* LITERALS DECODING ****************************************************/ +static size_t decode_literals_simple(istream_t *const in, u8 **const literals, + const int block_type, + const int size_format); +static size_t decode_literals_compressed(frame_context_t *const ctx, + istream_t *const in, + u8 **const literals, + const int block_type, + const int size_format); +static void decode_huf_table(HUF_dtable *const dtable, istream_t *const in); +static void fse_decode_hufweights(ostream_t *weights, istream_t *const in, + int *const num_symbs); + +static size_t decode_literals(frame_context_t *const ctx, istream_t *const in, + u8 **const literals) { + // "Literals can be stored uncompressed or compressed using Huffman prefix + // codes. When compressed, an optional tree description can be present, + // followed by 1 or 4 streams." + // + // "Literals_Section_Header + // + // Header is in charge of describing how literals are packed. It's a + // byte-aligned variable-size bitfield, ranging from 1 to 5 bytes, using + // little-endian convention." + // + // "Literals_Block_Type + // + // This field uses 2 lowest bits of first byte, describing 4 different block + // types" + // + // size_format takes between 1 and 2 bits + int block_type = (int)IO_read_bits(in, 2); + int size_format = (int)IO_read_bits(in, 2); + + if (block_type <= 1) { + // Raw or RLE literals block + return decode_literals_simple(in, literals, block_type, + size_format); + } else { + // Huffman compressed literals + return decode_literals_compressed(ctx, in, literals, block_type, + size_format); + } +} + +/// Decodes literals blocks in raw or RLE form +static size_t decode_literals_simple(istream_t *const in, u8 **const literals, + const int block_type, + const int size_format) { + size_t size; + switch (size_format) { + // These cases are in the form ?0 + // In this case, the ? bit is actually part of the size field + case 0: + case 2: + // "Size_Format uses 1 bit. Regenerated_Size uses 5 bits (0-31)." + IO_rewind_bits(in, 1); + size = IO_read_bits(in, 5); + break; + case 1: + // "Size_Format uses 2 bits. Regenerated_Size uses 12 bits (0-4095)." + size = IO_read_bits(in, 12); + break; + case 3: + // "Size_Format uses 2 bits. Regenerated_Size uses 20 bits (0-1048575)." + size = IO_read_bits(in, 20); + break; + default: + // Size format is in range 0-3 + IMPOSSIBLE(); + } + + if (size > MAX_LITERALS_SIZE) { + CORRUPTION(); + } + + *literals = malloc(size); + if (!*literals) { + BAD_ALLOC(); + } + + switch (block_type) { + case 0: { + // "Raw_Literals_Block - Literals are stored uncompressed." + const u8 *const read_ptr = IO_get_read_ptr(in, size); + memcpy(*literals, read_ptr, size); + break; + } + case 1: { + // "RLE_Literals_Block - Literals consist of a single byte value repeated N times." + const u8 *const read_ptr = IO_get_read_ptr(in, 1); + memset(*literals, read_ptr[0], size); + break; + } + default: + IMPOSSIBLE(); + } + + return size; +} + +/// Decodes Huffman compressed literals +static size_t decode_literals_compressed(frame_context_t *const ctx, + istream_t *const in, + u8 **const literals, + const int block_type, + const int size_format) { + size_t regenerated_size, compressed_size; + // Only size_format=0 has 1 stream, so default to 4 + int num_streams = 4; + switch (size_format) { + case 0: + // "A single stream. Both Compressed_Size and Regenerated_Size use 10 + // bits (0-1023)." + num_streams = 1; + // Fall through as it has the same size format + /* fallthrough */ + case 1: + // "4 streams. Both Compressed_Size and Regenerated_Size use 10 bits + // (0-1023)." + regenerated_size = IO_read_bits(in, 10); + compressed_size = IO_read_bits(in, 10); + break; + case 2: + // "4 streams. Both Compressed_Size and Regenerated_Size use 14 bits + // (0-16383)." + regenerated_size = IO_read_bits(in, 14); + compressed_size = IO_read_bits(in, 14); + break; + case 3: + // "4 streams. Both Compressed_Size and Regenerated_Size use 18 bits + // (0-262143)." + regenerated_size = IO_read_bits(in, 18); + compressed_size = IO_read_bits(in, 18); + break; + default: + // Impossible + IMPOSSIBLE(); + } + if (regenerated_size > MAX_LITERALS_SIZE) { + CORRUPTION(); + } + + *literals = malloc(regenerated_size); + if (!*literals) { + BAD_ALLOC(); + } + + ostream_t lit_stream = IO_make_ostream(*literals, regenerated_size); + istream_t huf_stream = IO_make_sub_istream(in, compressed_size); + + if (block_type == 2) { + // Decode the provided Huffman table + // "This section is only present when Literals_Block_Type type is + // Compressed_Literals_Block (2)." + + HUF_free_dtable(&ctx->literals_dtable); + decode_huf_table(&ctx->literals_dtable, &huf_stream); + } else { + // If the previous Huffman table is being repeated, ensure it exists + if (!ctx->literals_dtable.symbols) { + CORRUPTION(); + } + } + + size_t symbols_decoded; + if (num_streams == 1) { + symbols_decoded = HUF_decompress_1stream(&ctx->literals_dtable, &lit_stream, &huf_stream); + } else { + symbols_decoded = HUF_decompress_4stream(&ctx->literals_dtable, &lit_stream, &huf_stream); + } + + if (symbols_decoded != regenerated_size) { + CORRUPTION(); + } + + return regenerated_size; +} + +// Decode the Huffman table description +static void decode_huf_table(HUF_dtable *const dtable, istream_t *const in) { + // "All literal values from zero (included) to last present one (excluded) + // are represented by Weight with values from 0 to Max_Number_of_Bits." + + // "This is a single byte value (0-255), which describes how to decode the list of weights." + const u8 header = IO_read_bits(in, 8); + + u8 weights[HUF_MAX_SYMBS]; + memset(weights, 0, sizeof(weights)); + + int num_symbs; + + if (header >= 128) { + // "This is a direct representation, where each Weight is written + // directly as a 4 bits field (0-15). The full representation occupies + // ((Number_of_Symbols+1)/2) bytes, meaning it uses a last full byte + // even if Number_of_Symbols is odd. Number_of_Symbols = headerByte - + // 127" + num_symbs = header - 127; + const size_t bytes = (num_symbs + 1) / 2; + + const u8 *const weight_src = IO_get_read_ptr(in, bytes); + + for (int i = 0; i < num_symbs; i++) { + // "They are encoded forward, 2 + // weights to a byte with the first weight taking the top four bits + // and the second taking the bottom four (e.g. the following + // operations could be used to read the weights: Weight[0] = + // (Byte[0] >> 4), Weight[1] = (Byte[0] & 0xf), etc.)." + if (i % 2 == 0) { + weights[i] = weight_src[i / 2] >> 4; + } else { + weights[i] = weight_src[i / 2] & 0xf; + } + } + } else { + // The weights are FSE encoded, decode them before we can construct the + // table + istream_t fse_stream = IO_make_sub_istream(in, header); + ostream_t weight_stream = IO_make_ostream(weights, HUF_MAX_SYMBS); + fse_decode_hufweights(&weight_stream, &fse_stream, &num_symbs); + } + + // Construct the table using the decoded weights + HUF_init_dtable_usingweights(dtable, weights, num_symbs); +} + +static void fse_decode_hufweights(ostream_t *weights, istream_t *const in, + int *const num_symbs) { + const int MAX_ACCURACY_LOG = 7; + + FSE_dtable dtable; + + // "An FSE bitstream starts by a header, describing probabilities + // distribution. It will create a Decoding Table. For a list of Huffman + // weights, maximum accuracy is 7 bits." + FSE_decode_header(&dtable, in, MAX_ACCURACY_LOG); + + // Decode the weights + *num_symbs = FSE_decompress_interleaved2(&dtable, weights, in); + + FSE_free_dtable(&dtable); +} +/******* END LITERALS DECODING ************************************************/ + +/******* SEQUENCE DECODING ****************************************************/ +/// The combination of FSE states needed to decode sequences +typedef struct { + FSE_dtable ll_table; + FSE_dtable of_table; + FSE_dtable ml_table; + + u16 ll_state; + u16 of_state; + u16 ml_state; +} sequence_states_t; + +/// Different modes to signal to decode_seq_tables what to do +typedef enum { + seq_literal_length = 0, + seq_offset = 1, + seq_match_length = 2, +} seq_part_t; + +typedef enum { + seq_predefined = 0, + seq_rle = 1, + seq_fse = 2, + seq_repeat = 3, +} seq_mode_t; + +/// The predefined FSE distribution tables for `seq_predefined` mode +static const i16 SEQ_LITERAL_LENGTH_DEFAULT_DIST[36] = { + 4, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 2, 2, + 2, 2, 2, 2, 2, 2, 2, 3, 2, 1, 1, 1, 1, 1, -1, -1, -1, -1}; +static const i16 SEQ_OFFSET_DEFAULT_DIST[29] = { + 1, 1, 1, 1, 1, 1, 2, 2, 2, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, 1, -1, -1, -1, -1, -1}; +static const i16 SEQ_MATCH_LENGTH_DEFAULT_DIST[53] = { + 1, 4, 3, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, -1, -1, -1, -1, -1, -1, -1}; + +/// The sequence decoding baseline and number of additional bits to read/add +/// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#the-codes-for-literals-lengths-match-lengths-and-offsets +static const u32 SEQ_LITERAL_LENGTH_BASELINES[36] = { + 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, + 12, 13, 14, 15, 16, 18, 20, 22, 24, 28, 32, 40, + 48, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536}; +static const u8 SEQ_LITERAL_LENGTH_EXTRA_BITS[36] = { + 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, + 1, 1, 2, 2, 3, 3, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}; + +static const u32 SEQ_MATCH_LENGTH_BASELINES[53] = { + 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, + 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, + 31, 32, 33, 34, 35, 37, 39, 41, 43, 47, 51, 59, 67, 83, + 99, 131, 259, 515, 1027, 2051, 4099, 8195, 16387, 32771, 65539}; +static const u8 SEQ_MATCH_LENGTH_EXTRA_BITS[53] = { + 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, + 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, + 2, 2, 3, 3, 4, 4, 5, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}; + +/// Offset decoding is simpler so we just need a maximum code value +static const u8 SEQ_MAX_CODES[3] = {35, (u8)-1, 52}; + +static void decompress_sequences(frame_context_t *const ctx, + istream_t *const in, + sequence_command_t *const sequences, + const size_t num_sequences); +static sequence_command_t decode_sequence(sequence_states_t *const state, + const u8 *const src, + i64 *const offset); +static void decode_seq_table(FSE_dtable *const table, istream_t *const in, + const seq_part_t type, const seq_mode_t mode); + +static size_t decode_sequences(frame_context_t *const ctx, istream_t *in, + sequence_command_t **const sequences) { + // "A compressed block is a succession of sequences . A sequence is a + // literal copy command, followed by a match copy command. A literal copy + // command specifies a length. It is the number of bytes to be copied (or + // extracted) from the literal section. A match copy command specifies an + // offset and a length. The offset gives the position to copy from, which + // can be within a previous block." + + size_t num_sequences; + + // "Number_of_Sequences + // + // This is a variable size field using between 1 and 3 bytes. Let's call its + // first byte byte0." + u8 header = IO_read_bits(in, 8); + if (header == 0) { + // "There are no sequences. The sequence section stops there. + // Regenerated content is defined entirely by literals section." + *sequences = NULL; + return 0; + } else if (header < 128) { + // "Number_of_Sequences = byte0 . Uses 1 byte." + num_sequences = header; + } else if (header < 255) { + // "Number_of_Sequences = ((byte0-128) << 8) + byte1 . Uses 2 bytes." + num_sequences = ((header - 128) << 8) + IO_read_bits(in, 8); + } else { + // "Number_of_Sequences = byte1 + (byte2<<8) + 0x7F00 . Uses 3 bytes." + num_sequences = IO_read_bits(in, 16) + 0x7F00; + } + + *sequences = malloc(num_sequences * sizeof(sequence_command_t)); + if (!*sequences) { + BAD_ALLOC(); + } + + decompress_sequences(ctx, in, *sequences, num_sequences); + return num_sequences; +} + +/// Decompress the FSE encoded sequence commands +static void decompress_sequences(frame_context_t *const ctx, istream_t *in, + sequence_command_t *const sequences, + const size_t num_sequences) { + // "The Sequences_Section regroup all symbols required to decode commands. + // There are 3 symbol types : literals lengths, offsets and match lengths. + // They are encoded together, interleaved, in a single bitstream." + + // "Symbol compression modes + // + // This is a single byte, defining the compression mode of each symbol + // type." + // + // Bit number : Field name + // 7-6 : Literals_Lengths_Mode + // 5-4 : Offsets_Mode + // 3-2 : Match_Lengths_Mode + // 1-0 : Reserved + u8 compression_modes = IO_read_bits(in, 8); + + if ((compression_modes & 3) != 0) { + // Reserved bits set + CORRUPTION(); + } + + // "Following the header, up to 3 distribution tables can be described. When + // present, they are in this order : + // + // Literals lengths + // Offsets + // Match Lengths" + // Update the tables we have stored in the context + decode_seq_table(&ctx->ll_dtable, in, seq_literal_length, + (compression_modes >> 6) & 3); + + decode_seq_table(&ctx->of_dtable, in, seq_offset, + (compression_modes >> 4) & 3); + + decode_seq_table(&ctx->ml_dtable, in, seq_match_length, + (compression_modes >> 2) & 3); + + + sequence_states_t states; + + // Initialize the decoding tables + { + states.ll_table = ctx->ll_dtable; + states.of_table = ctx->of_dtable; + states.ml_table = ctx->ml_dtable; + } + + const size_t len = IO_istream_len(in); + const u8 *const src = IO_get_read_ptr(in, len); + + // "After writing the last bit containing information, the compressor writes + // a single 1-bit and then fills the byte with 0-7 0 bits of padding." + const int padding = 8 - highest_set_bit(src[len - 1]); + // The offset starts at the end because FSE streams are read backwards + i64 bit_offset = (i64)(len * 8 - (size_t)padding); + + // "The bitstream starts with initial state values, each using the required + // number of bits in their respective accuracy, decoded previously from + // their normalized distribution. + // + // It starts by Literals_Length_State, followed by Offset_State, and finally + // Match_Length_State." + FSE_init_state(&states.ll_table, &states.ll_state, src, &bit_offset); + FSE_init_state(&states.of_table, &states.of_state, src, &bit_offset); + FSE_init_state(&states.ml_table, &states.ml_state, src, &bit_offset); + + for (size_t i = 0; i < num_sequences; i++) { + // Decode sequences one by one + sequences[i] = decode_sequence(&states, src, &bit_offset); + } + + if (bit_offset != 0) { + CORRUPTION(); + } +} + +// Decode a single sequence and update the state +static sequence_command_t decode_sequence(sequence_states_t *const states, + const u8 *const src, + i64 *const offset) { + // "Each symbol is a code in its own context, which specifies Baseline and + // Number_of_Bits to add. Codes are FSE compressed, and interleaved with raw + // additional bits in the same bitstream." + + // Decode symbols, but don't update states + const u8 of_code = FSE_peek_symbol(&states->of_table, states->of_state); + const u8 ll_code = FSE_peek_symbol(&states->ll_table, states->ll_state); + const u8 ml_code = FSE_peek_symbol(&states->ml_table, states->ml_state); + + // Offset doesn't need a max value as it's not decoded using a table + if (ll_code > SEQ_MAX_CODES[seq_literal_length] || + ml_code > SEQ_MAX_CODES[seq_match_length]) { + CORRUPTION(); + } + + // Read the interleaved bits + sequence_command_t seq; + // "Decoding starts by reading the Number_of_Bits required to decode Offset. + // It then does the same for Match_Length, and then for Literals_Length." + seq.offset = ((u32)1 << of_code) + STREAM_read_bits(src, of_code, offset); + + seq.match_length = + SEQ_MATCH_LENGTH_BASELINES[ml_code] + + STREAM_read_bits(src, SEQ_MATCH_LENGTH_EXTRA_BITS[ml_code], offset); + + seq.literal_length = + SEQ_LITERAL_LENGTH_BASELINES[ll_code] + + STREAM_read_bits(src, SEQ_LITERAL_LENGTH_EXTRA_BITS[ll_code], offset); + + // "If it is not the last sequence in the block, the next operation is to + // update states. Using the rules pre-calculated in the decoding tables, + // Literals_Length_State is updated, followed by Match_Length_State, and + // then Offset_State." + // If the stream is complete don't read bits to update state + if (*offset != 0) { + FSE_update_state(&states->ll_table, &states->ll_state, src, offset); + FSE_update_state(&states->ml_table, &states->ml_state, src, offset); + FSE_update_state(&states->of_table, &states->of_state, src, offset); + } + + return seq; +} + +/// Given a sequence part and table mode, decode the FSE distribution +/// Errors if the mode is `seq_repeat` without a pre-existing table in `table` +static void decode_seq_table(FSE_dtable *const table, istream_t *const in, + const seq_part_t type, const seq_mode_t mode) { + // Constant arrays indexed by seq_part_t + const i16 *const default_distributions[] = {SEQ_LITERAL_LENGTH_DEFAULT_DIST, + SEQ_OFFSET_DEFAULT_DIST, + SEQ_MATCH_LENGTH_DEFAULT_DIST}; + const size_t default_distribution_lengths[] = {36, 29, 53}; + const size_t default_distribution_accuracies[] = {6, 5, 6}; + + const size_t max_accuracies[] = {9, 8, 9}; + + if (mode != seq_repeat) { + // Free old one before overwriting + FSE_free_dtable(table); + } + + switch (mode) { + case seq_predefined: { + // "Predefined_Mode : uses a predefined distribution table." + const i16 *distribution = default_distributions[type]; + const size_t symbs = default_distribution_lengths[type]; + const size_t accuracy_log = default_distribution_accuracies[type]; + + FSE_init_dtable(table, distribution, symbs, accuracy_log); + break; + } + case seq_rle: { + // "RLE_Mode : it's a single code, repeated Number_of_Sequences times." + const u8 symb = IO_get_read_ptr(in, 1)[0]; + FSE_init_dtable_rle(table, symb); + break; + } + case seq_fse: { + // "FSE_Compressed_Mode : standard FSE compression. A distribution table + // will be present " + FSE_decode_header(table, in, max_accuracies[type]); + break; + } + case seq_repeat: + // "Repeat_Mode : re-use distribution table from previous compressed + // block." + // Nothing to do here, table will be unchanged + if (!table->symbols) { + // This mode is invalid if we don't already have a table + CORRUPTION(); + } + break; + default: + // Impossible, as mode is from 0-3 + IMPOSSIBLE(); + break; + } + +} +/******* END SEQUENCE DECODING ************************************************/ + +/******* SEQUENCE EXECUTION ***************************************************/ +static void execute_sequences(frame_context_t *const ctx, ostream_t *const out, + const u8 *const literals, + const size_t literals_len, + const sequence_command_t *const sequences, + const size_t num_sequences) { + istream_t litstream = IO_make_istream(literals, literals_len); + + u64 *const offset_hist = ctx->previous_offsets; + size_t total_output = ctx->current_total_output; + + for (size_t i = 0; i < num_sequences; i++) { + const sequence_command_t seq = sequences[i]; + { + const u32 literals_size = copy_literals(seq.literal_length, &litstream, out); + total_output += literals_size; + } + + size_t const offset = compute_offset(seq, offset_hist); + + size_t const match_length = seq.match_length; + + execute_match_copy(ctx, offset, match_length, total_output, out); + + total_output += match_length; + } + + // Copy any leftover literals + { + size_t len = IO_istream_len(&litstream); + copy_literals(len, &litstream, out); + total_output += len; + } + + ctx->current_total_output = total_output; +} + +static u32 copy_literals(const size_t literal_length, istream_t *litstream, + ostream_t *const out) { + // If the sequence asks for more literals than are left, the + // sequence must be corrupted + if (literal_length > IO_istream_len(litstream)) { + CORRUPTION(); + } + + u8 *const write_ptr = IO_get_write_ptr(out, literal_length); + const u8 *const read_ptr = + IO_get_read_ptr(litstream, literal_length); + // Copy literals to output + memcpy(write_ptr, read_ptr, literal_length); + + return literal_length; +} + +static size_t compute_offset(sequence_command_t seq, u64 *const offset_hist) { + size_t offset; + // Offsets are special, we need to handle the repeat offsets + if (seq.offset <= 3) { + // "The first 3 values define a repeated offset and we will call + // them Repeated_Offset1, Repeated_Offset2, and Repeated_Offset3. + // They are sorted in recency order, with Repeated_Offset1 meaning + // 'most recent one'". + + // Use 0 indexing for the array + u32 idx = seq.offset - 1; + if (seq.literal_length == 0) { + // "There is an exception though, when current sequence's + // literals length is 0. In this case, repeated offsets are + // shifted by one, so Repeated_Offset1 becomes Repeated_Offset2, + // Repeated_Offset2 becomes Repeated_Offset3, and + // Repeated_Offset3 becomes Repeated_Offset1 - 1_byte." + idx++; + } + + if (idx == 0) { + offset = offset_hist[0]; + } else { + // If idx == 3 then literal length was 0 and the offset was 3, + // as per the exception listed above + offset = idx < 3 ? offset_hist[idx] : offset_hist[0] - 1; + + // If idx == 1 we don't need to modify offset_hist[2], since + // we're using the second-most recent code + if (idx > 1) { + offset_hist[2] = offset_hist[1]; + } + offset_hist[1] = offset_hist[0]; + offset_hist[0] = offset; + } + } else { + // When it's not a repeat offset: + // "if (Offset_Value > 3) offset = Offset_Value - 3;" + offset = seq.offset - 3; + + // Shift back history + offset_hist[2] = offset_hist[1]; + offset_hist[1] = offset_hist[0]; + offset_hist[0] = offset; + } + return offset; +} + +static void execute_match_copy(frame_context_t *const ctx, size_t offset, + size_t match_length, size_t total_output, + ostream_t *const out) { + u8 *write_ptr = IO_get_write_ptr(out, match_length); + if (total_output <= ctx->header.window_size) { + // In this case offset might go back into the dictionary + if (offset > total_output + ctx->dict_content_len) { + // The offset goes beyond even the dictionary + CORRUPTION(); + } + + if (offset > total_output) { + // "The rest of the dictionary is its content. The content act + // as a "past" in front of data to compress or decompress, so it + // can be referenced in sequence commands." + const size_t dict_copy = + MIN(offset - total_output, match_length); + const size_t dict_offset = + ctx->dict_content_len - (offset - total_output); + + memcpy(write_ptr, ctx->dict_content + dict_offset, dict_copy); + write_ptr += dict_copy; + match_length -= dict_copy; + } + } else if (offset > ctx->header.window_size) { + CORRUPTION(); + } + + // We must copy byte by byte because the match length might be larger + // than the offset + // ex: if the output so far was "abc", a command with offset=3 and + // match_length=6 would produce "abcabcabc" as the new output + for (size_t j = 0; j < match_length; j++) { + *write_ptr = *(write_ptr - offset); + write_ptr++; + } +} +/******* END SEQUENCE EXECUTION ***********************************************/ + +/******* OUTPUT SIZE COUNTING *************************************************/ +/// Get the decompressed size of an input stream so memory can be allocated in +/// advance. +/// This implementation assumes `src` points to a single ZSTD-compressed frame +size_t ZSTD_get_decompressed_size(const void *src, const size_t src_len) { + istream_t in = IO_make_istream(src, src_len); + + // get decompressed size from ZSTD frame header + { + const u32 magic_number = (u32)IO_read_bits(&in, 32); + + if (magic_number == ZSTD_MAGIC_NUMBER) { + // ZSTD frame + frame_header_t header; + parse_frame_header(&header, &in); + + if (header.frame_content_size == 0 && !header.single_segment_flag) { + // Content size not provided, we can't tell + return (size_t)-1; + } + + return header.frame_content_size; + } else { + // not a real frame or skippable frame + ERROR("ZSTD frame magic number did not match"); + } + } +} +/******* END OUTPUT SIZE COUNTING *********************************************/ + +/******* DICTIONARY PARSING ***************************************************/ +dictionary_t* create_dictionary() { + dictionary_t* const dict = calloc(1, sizeof(dictionary_t)); + if (!dict) { + BAD_ALLOC(); + } + return dict; +} + +/// Free an allocated dictionary +void free_dictionary(dictionary_t *const dict) { + HUF_free_dtable(&dict->literals_dtable); + FSE_free_dtable(&dict->ll_dtable); + FSE_free_dtable(&dict->of_dtable); + FSE_free_dtable(&dict->ml_dtable); + + free(dict->content); + + memset(dict, 0, sizeof(dictionary_t)); + + free(dict); +} + + +#if !defined(ZDEC_NO_DICTIONARY) +#define DICT_SIZE_ERROR() ERROR("Dictionary size cannot be less than 8 bytes") +#define NULL_SRC() ERROR("Tried to create dictionary with pointer to null src"); + +static void init_dictionary_content(dictionary_t *const dict, + istream_t *const in); + +void parse_dictionary(dictionary_t *const dict, const void *src, + size_t src_len) { + const u8 *byte_src = (const u8 *)src; + memset(dict, 0, sizeof(dictionary_t)); + if (src == NULL) { /* cannot initialize dictionary with null src */ + NULL_SRC(); + } + if (src_len < 8) { + DICT_SIZE_ERROR(); + } + + istream_t in = IO_make_istream(byte_src, src_len); + + const u32 magic_number = IO_read_bits(&in, 32); + if (magic_number != 0xEC30A437) { + // raw content dict + IO_rewind_bits(&in, 32); + init_dictionary_content(dict, &in); + return; + } + + dict->dictionary_id = IO_read_bits(&in, 32); + + // "Entropy_Tables : following the same format as the tables in compressed + // blocks. They are stored in following order : Huffman tables for literals, + // FSE table for offsets, FSE table for match lengths, and FSE table for + // literals lengths. It's finally followed by 3 offset values, populating + // recent offsets (instead of using {1,4,8}), stored in order, 4-bytes + // little-endian each, for a total of 12 bytes. Each recent offset must have + // a value < dictionary size." + decode_huf_table(&dict->literals_dtable, &in); + decode_seq_table(&dict->of_dtable, &in, seq_offset, seq_fse); + decode_seq_table(&dict->ml_dtable, &in, seq_match_length, seq_fse); + decode_seq_table(&dict->ll_dtable, &in, seq_literal_length, seq_fse); + + // Read in the previous offset history + dict->previous_offsets[0] = IO_read_bits(&in, 32); + dict->previous_offsets[1] = IO_read_bits(&in, 32); + dict->previous_offsets[2] = IO_read_bits(&in, 32); + + // Ensure the provided offsets aren't too large + // "Each recent offset must have a value < dictionary size." + for (int i = 0; i < 3; i++) { + if (dict->previous_offsets[i] > src_len) { + ERROR("Dictionary corrupted"); + } + } + + // "Content : The rest of the dictionary is its content. The content act as + // a "past" in front of data to compress or decompress, so it can be + // referenced in sequence commands." + init_dictionary_content(dict, &in); +} + +static void init_dictionary_content(dictionary_t *const dict, + istream_t *const in) { + // Copy in the content + dict->content_size = IO_istream_len(in); + dict->content = malloc(dict->content_size); + if (!dict->content) { + BAD_ALLOC(); + } + + const u8 *const content = IO_get_read_ptr(in, dict->content_size); + + memcpy(dict->content, content, dict->content_size); +} + +static void HUF_copy_dtable(HUF_dtable *const dst, + const HUF_dtable *const src) { + if (src->max_bits == 0) { + memset(dst, 0, sizeof(HUF_dtable)); + return; + } + + const size_t size = (size_t)1 << src->max_bits; + dst->max_bits = src->max_bits; + + dst->symbols = malloc(size); + dst->num_bits = malloc(size); + if (!dst->symbols || !dst->num_bits) { + BAD_ALLOC(); + } + + memcpy(dst->symbols, src->symbols, size); + memcpy(dst->num_bits, src->num_bits, size); +} + +static void FSE_copy_dtable(FSE_dtable *const dst, const FSE_dtable *const src) { + if (src->accuracy_log == 0) { + memset(dst, 0, sizeof(FSE_dtable)); + return; + } + + size_t size = (size_t)1 << src->accuracy_log; + dst->accuracy_log = src->accuracy_log; + + dst->symbols = malloc(size); + dst->num_bits = malloc(size); + dst->new_state_base = malloc(size * sizeof(u16)); + if (!dst->symbols || !dst->num_bits || !dst->new_state_base) { + BAD_ALLOC(); + } + + memcpy(dst->symbols, src->symbols, size); + memcpy(dst->num_bits, src->num_bits, size); + memcpy(dst->new_state_base, src->new_state_base, size * sizeof(u16)); +} + +/// A dictionary acts as initializing values for the frame context before +/// decompression, so we implement it by applying it's predetermined +/// tables and content to the context before beginning decompression +static void frame_context_apply_dict(frame_context_t *const ctx, + const dictionary_t *const dict) { + // If the content pointer is NULL then it must be an empty dict + if (!dict || !dict->content) + return; + + // If the requested dictionary_id is non-zero, the correct dictionary must + // be present + if (ctx->header.dictionary_id != 0 && + ctx->header.dictionary_id != dict->dictionary_id) { + ERROR("Wrong dictionary provided"); + } + + // Copy the dict content to the context for references during sequence + // execution + ctx->dict_content = dict->content; + ctx->dict_content_len = dict->content_size; + + // If it's a formatted dict copy the precomputed tables in so they can + // be used in the table repeat modes + if (dict->dictionary_id != 0) { + // Deep copy the entropy tables so they can be freed independently of + // the dictionary struct + HUF_copy_dtable(&ctx->literals_dtable, &dict->literals_dtable); + FSE_copy_dtable(&ctx->ll_dtable, &dict->ll_dtable); + FSE_copy_dtable(&ctx->of_dtable, &dict->of_dtable); + FSE_copy_dtable(&ctx->ml_dtable, &dict->ml_dtable); + + // Copy the repeated offsets + memcpy(ctx->previous_offsets, dict->previous_offsets, + sizeof(ctx->previous_offsets)); + } +} + +#else // ZDEC_NO_DICTIONARY is defined + +static void frame_context_apply_dict(frame_context_t *const ctx, + const dictionary_t *const dict) { + (void)ctx; + if (dict && dict->content) ERROR("dictionary not supported"); +} + +#endif +/******* END DICTIONARY PARSING ***********************************************/ + +/******* IO STREAM OPERATIONS *************************************************/ + +/// Reads `num` bits from a bitstream, and updates the internal offset +static inline u64 IO_read_bits(istream_t *const in, const int num_bits) { + if (num_bits > 64 || num_bits <= 0) { + ERROR("Attempt to read an invalid number of bits"); + } + + const size_t bytes = (num_bits + in->bit_offset + 7) / 8; + const size_t full_bytes = (num_bits + in->bit_offset) / 8; + if (bytes > in->len) { + INP_SIZE(); + } + + const u64 result = read_bits_LE(in->ptr, num_bits, in->bit_offset); + + in->bit_offset = (num_bits + in->bit_offset) % 8; + in->ptr += full_bytes; + in->len -= full_bytes; + + return result; +} + +/// If a non-zero number of bits have been read from the current byte, advance +/// the offset to the next byte +static inline void IO_rewind_bits(istream_t *const in, int num_bits) { + if (num_bits < 0) { + ERROR("Attempting to rewind stream by a negative number of bits"); + } + + // move the offset back by `num_bits` bits + const int new_offset = in->bit_offset - num_bits; + // determine the number of whole bytes we have to rewind, rounding up to an + // integer number (e.g. if `new_offset == -5`, `bytes == 1`) + const i64 bytes = -(new_offset - 7) / 8; + + in->ptr -= bytes; + in->len += bytes; + // make sure the resulting `bit_offset` is positive, as mod in C does not + // convert numbers from negative to positive (e.g. -22 % 8 == -6) + in->bit_offset = ((new_offset % 8) + 8) % 8; +} + +/// If the remaining bits in a byte will be unused, advance to the end of the +/// byte +static inline void IO_align_stream(istream_t *const in) { + if (in->bit_offset != 0) { + if (in->len == 0) { + INP_SIZE(); + } + in->ptr++; + in->len--; + in->bit_offset = 0; + } +} + +/// Write the given byte into the output stream +static inline void IO_write_byte(ostream_t *const out, u8 symb) { + if (out->len == 0) { + OUT_SIZE(); + } + + out->ptr[0] = symb; + out->ptr++; + out->len--; +} + +/// Returns the number of bytes left to be read in this stream. The stream must +/// be byte aligned. +static inline size_t IO_istream_len(const istream_t *const in) { + return in->len; +} + +/// Returns a pointer where `len` bytes can be read, and advances the internal +/// state. The stream must be byte aligned. +static inline const u8 *IO_get_read_ptr(istream_t *const in, size_t len) { + if (len > in->len) { + INP_SIZE(); + } + if (in->bit_offset != 0) { + ERROR("Attempting to operate on a non-byte aligned stream"); + } + const u8 *const ptr = in->ptr; + in->ptr += len; + in->len -= len; + + return ptr; +} +/// Returns a pointer to write `len` bytes to, and advances the internal state +static inline u8 *IO_get_write_ptr(ostream_t *const out, size_t len) { + if (len > out->len) { + OUT_SIZE(); + } + u8 *const ptr = out->ptr; + out->ptr += len; + out->len -= len; + + return ptr; +} + +/// Advance the inner state by `len` bytes +static inline void IO_advance_input(istream_t *const in, size_t len) { + if (len > in->len) { + INP_SIZE(); + } + if (in->bit_offset != 0) { + ERROR("Attempting to operate on a non-byte aligned stream"); + } + + in->ptr += len; + in->len -= len; +} + +/// Returns an `ostream_t` constructed from the given pointer and length +static inline ostream_t IO_make_ostream(u8 *out, size_t len) { + return (ostream_t) { out, len }; +} + +/// Returns an `istream_t` constructed from the given pointer and length +static inline istream_t IO_make_istream(const u8 *in, size_t len) { + return (istream_t) { in, len, 0 }; +} + +/// Returns an `istream_t` with the same base as `in`, and length `len` +/// Then, advance `in` to account for the consumed bytes +/// `in` must be byte aligned +static inline istream_t IO_make_sub_istream(istream_t *const in, size_t len) { + // Consume `len` bytes of the parent stream + const u8 *const ptr = IO_get_read_ptr(in, len); + + // Make a substream using the pointer to those `len` bytes + return IO_make_istream(ptr, len); +} +/******* END IO STREAM OPERATIONS *********************************************/ + +/******* BITSTREAM OPERATIONS *************************************************/ +/// Read `num` bits (up to 64) from `src + offset`, where `offset` is in bits +static inline u64 read_bits_LE(const u8 *src, const int num_bits, + const size_t offset) { + if (num_bits > 64) { + ERROR("Attempt to read an invalid number of bits"); + } + + // Skip over bytes that aren't in range + src += offset / 8; + size_t bit_offset = offset % 8; + u64 res = 0; + + int shift = 0; + int left = num_bits; + while (left > 0) { + u64 mask = left >= 8 ? 0xff : (((u64)1 << left) - 1); + // Read the next byte, shift it to account for the offset, and then mask + // out the top part if we don't need all the bits + res += (((u64)*src++ >> bit_offset) & mask) << shift; + shift += 8 - bit_offset; + left -= 8 - bit_offset; + bit_offset = 0; + } + + return res; +} + +/// Read bits from the end of a HUF or FSE bitstream. `offset` is in bits, so +/// it updates `offset` to `offset - bits`, and then reads `bits` bits from +/// `src + offset`. If the offset becomes negative, the extra bits at the +/// bottom are filled in with `0` bits instead of reading from before `src`. +static inline u64 STREAM_read_bits(const u8 *const src, const int bits, + i64 *const offset) { + *offset = *offset - bits; + size_t actual_off = *offset; + size_t actual_bits = bits; + // Don't actually read bits from before the start of src, so if `*offset < + // 0` fix actual_off and actual_bits to reflect the quantity to read + if (*offset < 0) { + actual_bits += *offset; + actual_off = 0; + } + u64 res = read_bits_LE(src, actual_bits, actual_off); + + if (*offset < 0) { + // Fill in the bottom "overflowed" bits with 0's + res = -*offset >= 64 ? 0 : (res << -*offset); + } + return res; +} +/******* END BITSTREAM OPERATIONS *********************************************/ + +/******* BIT COUNTING OPERATIONS **********************************************/ +/// Returns `x`, where `2^x` is the largest power of 2 less than or equal to +/// `num`, or `-1` if `num == 0`. +static inline int highest_set_bit(const u64 num) { + for (int i = 63; i >= 0; i--) { + if (((u64)1 << i) <= num) { + return i; + } + } + return -1; +} +/******* END BIT COUNTING OPERATIONS ******************************************/ + +/******* HUFFMAN PRIMITIVES ***************************************************/ +static inline u8 HUF_decode_symbol(const HUF_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset) { + // Look up the symbol and number of bits to read + const u8 symb = dtable->symbols[*state]; + const u8 bits = dtable->num_bits[*state]; + const u16 rest = STREAM_read_bits(src, bits, offset); + // Shift `bits` bits out of the state, keeping the low order bits that + // weren't necessary to determine this symbol. Then add in the new bits + // read from the stream. + *state = ((*state << bits) + rest) & (((u16)1 << dtable->max_bits) - 1); + + return symb; +} + +static inline void HUF_init_state(const HUF_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset) { + // Read in a full `dtable->max_bits` bits to initialize the state + const u8 bits = dtable->max_bits; + *state = STREAM_read_bits(src, bits, offset); +} + +static size_t HUF_decompress_1stream(const HUF_dtable *const dtable, + ostream_t *const out, + istream_t *const in) { + const size_t len = IO_istream_len(in); + if (len == 0) { + INP_SIZE(); + } + const u8 *const src = IO_get_read_ptr(in, len); + + // "Each bitstream must be read backward, that is starting from the end down + // to the beginning. Therefore it's necessary to know the size of each + // bitstream. + // + // It's also necessary to know exactly which bit is the latest. This is + // detected by a final bit flag : the highest bit of latest byte is a + // final-bit-flag. Consequently, a last byte of 0 is not possible. And the + // final-bit-flag itself is not part of the useful bitstream. Hence, the + // last byte contains between 0 and 7 useful bits." + const int padding = 8 - highest_set_bit(src[len - 1]); + + // Offset starts at the end because HUF streams are read backwards + i64 bit_offset = len * 8 - padding; + u16 state; + + HUF_init_state(dtable, &state, src, &bit_offset); + + size_t symbols_written = 0; + while (bit_offset > -dtable->max_bits) { + // Iterate over the stream, decoding one symbol at a time + IO_write_byte(out, HUF_decode_symbol(dtable, &state, src, &bit_offset)); + symbols_written++; + } + // "The process continues up to reading the required number of symbols per + // stream. If a bitstream is not entirely and exactly consumed, hence + // reaching exactly its beginning position with all bits consumed, the + // decoding process is considered faulty." + + // When all symbols have been decoded, the final state value shouldn't have + // any data from the stream, so it should have "read" dtable->max_bits from + // before the start of `src` + // Therefore `offset`, the edge to start reading new bits at, should be + // dtable->max_bits before the start of the stream + if (bit_offset != -dtable->max_bits) { + CORRUPTION(); + } + + return symbols_written; +} + +static size_t HUF_decompress_4stream(const HUF_dtable *const dtable, + ostream_t *const out, istream_t *const in) { + // "Compressed size is provided explicitly : in the 4-streams variant, + // bitstreams are preceded by 3 unsigned little-endian 16-bits values. Each + // value represents the compressed size of one stream, in order. The last + // stream size is deducted from total compressed size and from previously + // decoded stream sizes" + const size_t csize1 = IO_read_bits(in, 16); + const size_t csize2 = IO_read_bits(in, 16); + const size_t csize3 = IO_read_bits(in, 16); + + istream_t in1 = IO_make_sub_istream(in, csize1); + istream_t in2 = IO_make_sub_istream(in, csize2); + istream_t in3 = IO_make_sub_istream(in, csize3); + istream_t in4 = IO_make_sub_istream(in, IO_istream_len(in)); + + size_t total_output = 0; + // Decode each stream independently for simplicity + // If we wanted to we could decode all 4 at the same time for speed, + // utilizing more execution units + total_output += HUF_decompress_1stream(dtable, out, &in1); + total_output += HUF_decompress_1stream(dtable, out, &in2); + total_output += HUF_decompress_1stream(dtable, out, &in3); + total_output += HUF_decompress_1stream(dtable, out, &in4); + + return total_output; +} + +/// Initializes a Huffman table using canonical Huffman codes +/// For more explanation on canonical Huffman codes see +/// http://www.cs.uofs.edu/~mccloske/courses/cmps340/huff_canonical_dec2015.html +/// Codes within a level are allocated in symbol order (i.e. smaller symbols get +/// earlier codes) +static void HUF_init_dtable(HUF_dtable *const table, const u8 *const bits, + const int num_symbs) { + memset(table, 0, sizeof(HUF_dtable)); + if (num_symbs > HUF_MAX_SYMBS) { + ERROR("Too many symbols for Huffman"); + } + + u8 max_bits = 0; + u16 rank_count[HUF_MAX_BITS + 1]; + memset(rank_count, 0, sizeof(rank_count)); + + // Count the number of symbols for each number of bits, and determine the + // depth of the tree + for (int i = 0; i < num_symbs; i++) { + if (bits[i] > HUF_MAX_BITS) { + ERROR("Huffman table depth too large"); + } + max_bits = MAX(max_bits, bits[i]); + rank_count[bits[i]]++; + } + + const size_t table_size = 1 << max_bits; + table->max_bits = max_bits; + table->symbols = malloc(table_size); + table->num_bits = malloc(table_size); + + if (!table->symbols || !table->num_bits) { + free(table->symbols); + free(table->num_bits); + BAD_ALLOC(); + } + + // "Symbols are sorted by Weight. Within same Weight, symbols keep natural + // order. Symbols with a Weight of zero are removed. Then, starting from + // lowest weight, prefix codes are distributed in order." + + u32 rank_idx[HUF_MAX_BITS + 1]; + // Initialize the starting codes for each rank (number of bits) + rank_idx[max_bits] = 0; + for (int i = max_bits; i >= 1; i--) { + rank_idx[i - 1] = rank_idx[i] + rank_count[i] * (1 << (max_bits - i)); + // The entire range takes the same number of bits so we can memset it + memset(&table->num_bits[rank_idx[i]], i, rank_idx[i - 1] - rank_idx[i]); + } + + if (rank_idx[0] != table_size) { + CORRUPTION(); + } + + // Allocate codes and fill in the table + for (int i = 0; i < num_symbs; i++) { + if (bits[i] != 0) { + // Allocate a code for this symbol and set its range in the table + const u16 code = rank_idx[bits[i]]; + // Since the code doesn't care about the bottom `max_bits - bits[i]` + // bits of state, it gets a range that spans all possible values of + // the lower bits + const u16 len = 1 << (max_bits - bits[i]); + memset(&table->symbols[code], i, len); + rank_idx[bits[i]] += len; + } + } +} + +static void HUF_init_dtable_usingweights(HUF_dtable *const table, + const u8 *const weights, + const int num_symbs) { + // +1 because the last weight is not transmitted in the header + if (num_symbs + 1 > HUF_MAX_SYMBS) { + ERROR("Too many symbols for Huffman"); + } + + u8 bits[HUF_MAX_SYMBS]; + + u64 weight_sum = 0; + for (int i = 0; i < num_symbs; i++) { + // Weights are in the same range as bit count + if (weights[i] > HUF_MAX_BITS) { + CORRUPTION(); + } + weight_sum += weights[i] > 0 ? (u64)1 << (weights[i] - 1) : 0; + } + + // Find the first power of 2 larger than the sum + const int max_bits = highest_set_bit(weight_sum) + 1; + const u64 left_over = ((u64)1 << max_bits) - weight_sum; + // If the left over isn't a power of 2, the weights are invalid + if (left_over & (left_over - 1)) { + CORRUPTION(); + } + + // left_over is used to find the last weight as it's not transmitted + // by inverting 2^(weight - 1) we can determine the value of last_weight + const int last_weight = highest_set_bit(left_over) + 1; + + for (int i = 0; i < num_symbs; i++) { + // "Number_of_Bits = Number_of_Bits ? Max_Number_of_Bits + 1 - Weight : 0" + bits[i] = weights[i] > 0 ? (max_bits + 1 - weights[i]) : 0; + } + bits[num_symbs] = + max_bits + 1 - last_weight; // Last weight is always non-zero + + HUF_init_dtable(table, bits, num_symbs + 1); +} + +static void HUF_free_dtable(HUF_dtable *const dtable) { + free(dtable->symbols); + free(dtable->num_bits); + memset(dtable, 0, sizeof(HUF_dtable)); +} +/******* END HUFFMAN PRIMITIVES ***********************************************/ + +/******* FSE PRIMITIVES *******************************************************/ +/// For more description of FSE see +/// https://github.com/Cyan4973/FiniteStateEntropy/ + +/// Allow a symbol to be decoded without updating state +static inline u8 FSE_peek_symbol(const FSE_dtable *const dtable, + const u16 state) { + return dtable->symbols[state]; +} + +/// Consumes bits from the input and uses the current state to determine the +/// next state +static inline void FSE_update_state(const FSE_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset) { + const u8 bits = dtable->num_bits[*state]; + const u16 rest = STREAM_read_bits(src, bits, offset); + *state = dtable->new_state_base[*state] + rest; +} + +/// Decodes a single FSE symbol and updates the offset +static inline u8 FSE_decode_symbol(const FSE_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset) { + const u8 symb = FSE_peek_symbol(dtable, *state); + FSE_update_state(dtable, state, src, offset); + return symb; +} + +static inline void FSE_init_state(const FSE_dtable *const dtable, + u16 *const state, const u8 *const src, + i64 *const offset) { + // Read in a full `accuracy_log` bits to initialize the state + const u8 bits = dtable->accuracy_log; + *state = STREAM_read_bits(src, bits, offset); +} + +static size_t FSE_decompress_interleaved2(const FSE_dtable *const dtable, + ostream_t *const out, + istream_t *const in) { + const size_t len = IO_istream_len(in); + if (len == 0) { + INP_SIZE(); + } + const u8 *const src = IO_get_read_ptr(in, len); + + // "Each bitstream must be read backward, that is starting from the end down + // to the beginning. Therefore it's necessary to know the size of each + // bitstream. + // + // It's also necessary to know exactly which bit is the latest. This is + // detected by a final bit flag : the highest bit of latest byte is a + // final-bit-flag. Consequently, a last byte of 0 is not possible. And the + // final-bit-flag itself is not part of the useful bitstream. Hence, the + // last byte contains between 0 and 7 useful bits." + const int padding = 8 - highest_set_bit(src[len - 1]); + i64 offset = len * 8 - padding; + + u16 state1, state2; + // "The first state (State1) encodes the even indexed symbols, and the + // second (State2) encodes the odd indexes. State1 is initialized first, and + // then State2, and they take turns decoding a single symbol and updating + // their state." + FSE_init_state(dtable, &state1, src, &offset); + FSE_init_state(dtable, &state2, src, &offset); + + // Decode until we overflow the stream + // Since we decode in reverse order, overflowing the stream is offset going + // negative + size_t symbols_written = 0; + while (1) { + // "The number of symbols to decode is determined by tracking bitStream + // overflow condition: If updating state after decoding a symbol would + // require more bits than remain in the stream, it is assumed the extra + // bits are 0. Then, the symbols for each of the final states are + // decoded and the process is complete." + IO_write_byte(out, FSE_decode_symbol(dtable, &state1, src, &offset)); + symbols_written++; + if (offset < 0) { + // There's still a symbol to decode in state2 + IO_write_byte(out, FSE_peek_symbol(dtable, state2)); + symbols_written++; + break; + } + + IO_write_byte(out, FSE_decode_symbol(dtable, &state2, src, &offset)); + symbols_written++; + if (offset < 0) { + // There's still a symbol to decode in state1 + IO_write_byte(out, FSE_peek_symbol(dtable, state1)); + symbols_written++; + break; + } + } + + return symbols_written; +} + +static void FSE_init_dtable(FSE_dtable *const dtable, + const i16 *const norm_freqs, const int num_symbs, + const int accuracy_log) { + if (accuracy_log > FSE_MAX_ACCURACY_LOG) { + ERROR("FSE accuracy too large"); + } + if (num_symbs > FSE_MAX_SYMBS) { + ERROR("Too many symbols for FSE"); + } + + dtable->accuracy_log = accuracy_log; + + const size_t size = (size_t)1 << accuracy_log; + dtable->symbols = malloc(size * sizeof(u8)); + dtable->num_bits = malloc(size * sizeof(u8)); + dtable->new_state_base = malloc(size * sizeof(u16)); + + if (!dtable->symbols || !dtable->num_bits || !dtable->new_state_base) { + BAD_ALLOC(); + } + + // Used to determine how many bits need to be read for each state, + // and where the destination range should start + // Needs to be u16 because max value is 2 * max number of symbols, + // which can be larger than a byte can store + u16 state_desc[FSE_MAX_SYMBS]; + + // "Symbols are scanned in their natural order for "less than 1" + // probabilities. Symbols with this probability are being attributed a + // single cell, starting from the end of the table. These symbols define a + // full state reset, reading Accuracy_Log bits." + int high_threshold = size; + for (int s = 0; s < num_symbs; s++) { + // Scan for low probability symbols to put at the top + if (norm_freqs[s] == -1) { + dtable->symbols[--high_threshold] = s; + state_desc[s] = 1; + } + } + + // "All remaining symbols are sorted in their natural order. Starting from + // symbol 0 and table position 0, each symbol gets attributed as many cells + // as its probability. Cell allocation is spreaded, not linear." + // Place the rest in the table + const u16 step = (size >> 1) + (size >> 3) + 3; + const u16 mask = size - 1; + u16 pos = 0; + for (int s = 0; s < num_symbs; s++) { + if (norm_freqs[s] <= 0) { + continue; + } + + state_desc[s] = norm_freqs[s]; + + for (int i = 0; i < norm_freqs[s]; i++) { + // Give `norm_freqs[s]` states to symbol s + dtable->symbols[pos] = s; + // "A position is skipped if already occupied, typically by a "less + // than 1" probability symbol." + do { + pos = (pos + step) & mask; + } while (pos >= + high_threshold); + // Note: no other collision checking is necessary as `step` is + // coprime to `size`, so the cycle will visit each position exactly + // once + } + } + if (pos != 0) { + CORRUPTION(); + } + + // Now we can fill baseline and num bits + for (size_t i = 0; i < size; i++) { + u8 symbol = dtable->symbols[i]; + u16 next_state_desc = state_desc[symbol]++; + // Fills in the table appropriately, next_state_desc increases by symbol + // over time, decreasing number of bits + dtable->num_bits[i] = (u8)(accuracy_log - highest_set_bit(next_state_desc)); + // Baseline increases until the bit threshold is passed, at which point + // it resets to 0 + dtable->new_state_base[i] = + ((u16)next_state_desc << dtable->num_bits[i]) - size; + } +} + +/// Decode an FSE header as defined in the Zstandard format specification and +/// use the decoded frequencies to initialize a decoding table. +static void FSE_decode_header(FSE_dtable *const dtable, istream_t *const in, + const int max_accuracy_log) { + // "An FSE distribution table describes the probabilities of all symbols + // from 0 to the last present one (included) on a normalized scale of 1 << + // Accuracy_Log . + // + // It's a bitstream which is read forward, in little-endian fashion. It's + // not necessary to know its exact size, since it will be discovered and + // reported by the decoding process. + if (max_accuracy_log > FSE_MAX_ACCURACY_LOG) { + ERROR("FSE accuracy too large"); + } + + // The bitstream starts by reporting on which scale it operates. + // Accuracy_Log = low4bits + 5. Note that maximum Accuracy_Log for literal + // and match lengths is 9, and for offsets is 8. Higher values are + // considered errors." + const int accuracy_log = 5 + IO_read_bits(in, 4); + if (accuracy_log > max_accuracy_log) { + ERROR("FSE accuracy too large"); + } + + // "Then follows each symbol value, from 0 to last present one. The number + // of bits used by each field is variable. It depends on : + // + // Remaining probabilities + 1 : example : Presuming an Accuracy_Log of 8, + // and presuming 100 probabilities points have already been distributed, the + // decoder may read any value from 0 to 255 - 100 + 1 == 156 (inclusive). + // Therefore, it must read log2sup(156) == 8 bits. + // + // Value decoded : small values use 1 less bit : example : Presuming values + // from 0 to 156 (inclusive) are possible, 255-156 = 99 values are remaining + // in an 8-bits field. They are used this way : first 99 values (hence from + // 0 to 98) use only 7 bits, values from 99 to 156 use 8 bits. " + + i32 remaining = 1 << accuracy_log; + i16 frequencies[FSE_MAX_SYMBS]; + + int symb = 0; + while (remaining > 0 && symb < FSE_MAX_SYMBS) { + // Log of the number of possible values we could read + int bits = highest_set_bit(remaining + 1) + 1; + + u16 val = IO_read_bits(in, bits); + + // Try to mask out the lower bits to see if it qualifies for the "small + // value" threshold + const u16 lower_mask = ((u16)1 << (bits - 1)) - 1; + const u16 threshold = ((u16)1 << bits) - 1 - (remaining + 1); + + if ((val & lower_mask) < threshold) { + IO_rewind_bits(in, 1); + val = val & lower_mask; + } else if (val > lower_mask) { + val = val - threshold; + } + + // "Probability is obtained from Value decoded by following formula : + // Proba = value - 1" + const i16 proba = (i16)val - 1; + + // "It means value 0 becomes negative probability -1. -1 is a special + // probability, which means "less than 1". Its effect on distribution + // table is described in next paragraph. For the purpose of calculating + // cumulated distribution, it counts as one." + remaining -= proba < 0 ? -proba : proba; + + frequencies[symb] = proba; + symb++; + + // "When a symbol has a probability of zero, it is followed by a 2-bits + // repeat flag. This repeat flag tells how many probabilities of zeroes + // follow the current one. It provides a number ranging from 0 to 3. If + // it is a 3, another 2-bits repeat flag follows, and so on." + if (proba == 0) { + // Read the next two bits to see how many more 0s + int repeat = IO_read_bits(in, 2); + + while (1) { + for (int i = 0; i < repeat && symb < FSE_MAX_SYMBS; i++) { + frequencies[symb++] = 0; + } + if (repeat == 3) { + repeat = IO_read_bits(in, 2); + } else { + break; + } + } + } + } + IO_align_stream(in); + + // "When last symbol reaches cumulated total of 1 << Accuracy_Log, decoding + // is complete. If the last symbol makes cumulated total go above 1 << + // Accuracy_Log, distribution is considered corrupted." + if (remaining != 0 || symb >= FSE_MAX_SYMBS) { + CORRUPTION(); + } + + // Initialize the decoding table using the determined weights + FSE_init_dtable(dtable, frequencies, symb, accuracy_log); +} + +static void FSE_init_dtable_rle(FSE_dtable *const dtable, const u8 symb) { + dtable->symbols = malloc(sizeof(u8)); + dtable->num_bits = malloc(sizeof(u8)); + dtable->new_state_base = malloc(sizeof(u16)); + + if (!dtable->symbols || !dtable->num_bits || !dtable->new_state_base) { + BAD_ALLOC(); + } + + // This setup will always have a state of 0, always return symbol `symb`, + // and never consume any bits + dtable->symbols[0] = symb; + dtable->num_bits[0] = 0; + dtable->new_state_base[0] = 0; + dtable->accuracy_log = 0; +} + +static void FSE_free_dtable(FSE_dtable *const dtable) { + free(dtable->symbols); + free(dtable->num_bits); + free(dtable->new_state_base); + memset(dtable, 0, sizeof(FSE_dtable)); +} +/******* END FSE PRIMITIVES ***************************************************/ diff --git a/src/zstd/doc/educational_decoder/zstd_decompress.h b/src/zstd/doc/educational_decoder/zstd_decompress.h new file mode 100644 index 000000000..2b44eee95 --- /dev/null +++ b/src/zstd/doc/educational_decoder/zstd_decompress.h @@ -0,0 +1,61 @@ +/* + * Copyright (c) 2016-2020, Facebook, Inc. + * All rights reserved. + * + * This source code is licensed under both the BSD-style license (found in the + * LICENSE file in the root directory of this source tree) and the GPLv2 (found + * in the COPYING file in the root directory of this source tree). + * You may select, at your option, one of the above-listed licenses. + */ + +#include /* size_t */ + +/******* EXPOSED TYPES ********************************************************/ +/* +* Contains the parsed contents of a dictionary +* This includes Huffman and FSE tables used for decoding and data on offsets +*/ +typedef struct dictionary_s dictionary_t; +/******* END EXPOSED TYPES ****************************************************/ + +/******* DECOMPRESSION FUNCTIONS **********************************************/ +/// Zstandard decompression functions. +/// `dst` must point to a space at least as large as the reconstructed output. +size_t ZSTD_decompress(void *const dst, const size_t dst_len, + const void *const src, const size_t src_len); + +/// If `dict != NULL` and `dict_len >= 8`, does the same thing as +/// `ZSTD_decompress` but uses the provided dict +size_t ZSTD_decompress_with_dict(void *const dst, const size_t dst_len, + const void *const src, const size_t src_len, + dictionary_t* parsed_dict); + +/// Get the decompressed size of an input stream so memory can be allocated in +/// advance +/// Returns -1 if the size can't be determined +/// Assumes decompression of a single frame +size_t ZSTD_get_decompressed_size(const void *const src, const size_t src_len); +/******* END DECOMPRESSION FUNCTIONS ******************************************/ + +/******* DICTIONARY MANAGEMENT ***********************************************/ +/* + * Return a valid dictionary_t pointer for use with dictionary initialization + * or decompression + */ +dictionary_t* create_dictionary(void); + +/* + * Parse a provided dictionary blob for use in decompression + * `src` -- must point to memory space representing the dictionary + * `src_len` -- must provide the dictionary size + * `dict` -- will contain the parsed contents of the dictionary and + * can be used for decompression + */ +void parse_dictionary(dictionary_t *const dict, const void *src, + size_t src_len); + +/* + * Free internal Huffman tables, FSE tables, and dictionary content + */ +void free_dictionary(dictionary_t *const dict); +/******* END DICTIONARY MANAGEMENT *******************************************/ diff --git a/src/zstd/doc/images/CSpeed2.png b/src/zstd/doc/images/CSpeed2.png new file mode 100644 index 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000000000..a0d8d2505 Binary files /dev/null and b/src/zstd/doc/images/dict-cs.png differ diff --git a/src/zstd/doc/images/dict-ds.png b/src/zstd/doc/images/dict-ds.png new file mode 100644 index 000000000..5b9c360c9 Binary files /dev/null and b/src/zstd/doc/images/dict-ds.png differ diff --git a/src/zstd/doc/images/zstd_cdict_v1_3_5.png b/src/zstd/doc/images/zstd_cdict_v1_3_5.png new file mode 100644 index 000000000..cce67c83a Binary files /dev/null and b/src/zstd/doc/images/zstd_cdict_v1_3_5.png differ diff --git a/src/zstd/doc/images/zstd_logo86.png b/src/zstd/doc/images/zstd_logo86.png new file mode 100644 index 000000000..216f22806 Binary files /dev/null and b/src/zstd/doc/images/zstd_logo86.png differ diff --git a/src/zstd/doc/zstd_compression_format.md b/src/zstd/doc/zstd_compression_format.md new file mode 100644 index 000000000..fc61726fc --- /dev/null +++ b/src/zstd/doc/zstd_compression_format.md @@ -0,0 +1,1694 @@ +Zstandard Compression Format +============================ + +### Notices + +Copyright (c) 2016-present Yann Collet, Facebook, Inc. + +Permission is granted to copy and distribute this document +for any purpose and without charge, +including translations into other languages +and incorporation into compilations, +provided that the copyright notice and this notice are preserved, +and that any substantive changes or deletions from the original +are clearly marked. +Distribution of this document is unlimited. + +### Version + +0.3.5 (13/11/19) + + +Introduction +------------ + +The purpose of this document is to define a lossless compressed data format, +that is independent of CPU type, operating system, +file system and character set, suitable for +file compression, pipe and streaming compression, +using the [Zstandard algorithm](http://www.zstandard.org). +The text of the specification assumes a basic background in programming +at the level of bits and other primitive data representations. + +The data can be produced or consumed, +even for an arbitrarily long sequentially presented input data stream, +using only an a priori bounded amount of intermediate storage, +and hence can be used in data communications. +The format uses the Zstandard compression method, +and optional [xxHash-64 checksum method](http://www.xxhash.org), +for detection of data corruption. + +The data format defined by this specification +does not attempt to allow random access to compressed data. + +Unless otherwise indicated below, +a compliant compressor must produce data sets +that conform to the specifications presented here. +It doesn’t need to support all options though. + +A compliant decompressor must be able to decompress +at least one working set of parameters +that conforms to the specifications presented here. +It may also ignore informative fields, such as checksum. +Whenever it does not support a parameter defined in the compressed stream, +it must produce a non-ambiguous error code and associated error message +explaining which parameter is unsupported. + +This specification is intended for use by implementers of software +to compress data into Zstandard format and/or decompress data from Zstandard format. +The Zstandard format is supported by an open source reference implementation, +written in portable C, and available at : https://github.com/facebook/zstd . + + +### Overall conventions +In this document: +- square brackets i.e. `[` and `]` are used to indicate optional fields or parameters. +- the naming convention for identifiers is `Mixed_Case_With_Underscores` + +### Definitions +Content compressed by Zstandard is transformed into a Zstandard __frame__. +Multiple frames can be appended into a single file or stream. +A frame is completely independent, has a defined beginning and end, +and a set of parameters which tells the decoder how to decompress it. + +A frame encapsulates one or multiple __blocks__. +Each block contains arbitrary content, which is described by its header, +and has a guaranteed maximum content size, which depends on frame parameters. +Unlike frames, each block depends on previous blocks for proper decoding. +However, each block can be decompressed without waiting for its successor, +allowing streaming operations. + +Overview +--------- +- [Frames](#frames) + - [Zstandard frames](#zstandard-frames) + - [Blocks](#blocks) + - [Literals Section](#literals-section) + - [Sequences Section](#sequences-section) + - [Sequence Execution](#sequence-execution) + - [Skippable frames](#skippable-frames) +- [Entropy Encoding](#entropy-encoding) + - [FSE](#fse) + - [Huffman Coding](#huffman-coding) +- [Dictionary Format](#dictionary-format) + +Frames +------ +Zstandard compressed data is made of one or more __frames__. +Each frame is independent and can be decompressed independently of other frames. +The decompressed content of multiple concatenated frames is the concatenation of +each frame decompressed content. + +There are two frame formats defined by Zstandard: + Zstandard frames and Skippable frames. +Zstandard frames contain compressed data, while +skippable frames contain custom user metadata. + +## Zstandard frames +The structure of a single Zstandard frame is following: + +| `Magic_Number` | `Frame_Header` |`Data_Block`| [More data blocks] | [`Content_Checksum`] | +|:--------------:|:--------------:|:----------:| ------------------ |:--------------------:| +| 4 bytes | 2-14 bytes | n bytes | | 0-4 bytes | + +__`Magic_Number`__ + +4 Bytes, __little-endian__ format. +Value : 0xFD2FB528 +Note: This value was selected to be less probable to find at the beginning of some random file. +It avoids trivial patterns (0x00, 0xFF, repeated bytes, increasing bytes, etc.), +contains byte values outside of ASCII range, +and doesn't map into UTF8 space. +It reduces the chances that a text file represent this value by accident. + +__`Frame_Header`__ + +2 to 14 Bytes, detailed in [`Frame_Header`](#frame_header). + +__`Data_Block`__ + +Detailed in [`Blocks`](#blocks). +That’s where compressed data is stored. + +__`Content_Checksum`__ + +An optional 32-bit checksum, only present if `Content_Checksum_flag` is set. +The content checksum is the result +of [xxh64() hash function](http://www.xxhash.org) +digesting the original (decoded) data as input, and a seed of zero. +The low 4 bytes of the checksum are stored in __little-endian__ format. + +### `Frame_Header` + +The `Frame_Header` has a variable size, with a minimum of 2 bytes, +and up to 14 bytes depending on optional parameters. +The structure of `Frame_Header` is following: + +| `Frame_Header_Descriptor` | [`Window_Descriptor`] | [`Dictionary_ID`] | [`Frame_Content_Size`] | +| ------------------------- | --------------------- | ----------------- | ---------------------- | +| 1 byte | 0-1 byte | 0-4 bytes | 0-8 bytes | + +#### `Frame_Header_Descriptor` + +The first header's byte is called the `Frame_Header_Descriptor`. +It describes which other fields are present. +Decoding this byte is enough to tell the size of `Frame_Header`. + +| Bit number | Field name | +| ---------- | ---------- | +| 7-6 | `Frame_Content_Size_flag` | +| 5 | `Single_Segment_flag` | +| 4 | `Unused_bit` | +| 3 | `Reserved_bit` | +| 2 | `Content_Checksum_flag` | +| 1-0 | `Dictionary_ID_flag` | + +In this table, bit 7 is the highest bit, while bit 0 is the lowest one. + +__`Frame_Content_Size_flag`__ + +This is a 2-bits flag (`= Frame_Header_Descriptor >> 6`), +specifying if `Frame_Content_Size` (the decompressed data size) +is provided within the header. +`Flag_Value` provides `FCS_Field_Size`, +which is the number of bytes used by `Frame_Content_Size` +according to the following table: + +| `Flag_Value` | 0 | 1 | 2 | 3 | +| -------------- | ------ | --- | --- | --- | +|`FCS_Field_Size`| 0 or 1 | 2 | 4 | 8 | + +When `Flag_Value` is `0`, `FCS_Field_Size` depends on `Single_Segment_flag` : +if `Single_Segment_flag` is set, `FCS_Field_Size` is 1. +Otherwise, `FCS_Field_Size` is 0 : `Frame_Content_Size` is not provided. + +__`Single_Segment_flag`__ + +If this flag is set, +data must be regenerated within a single continuous memory segment. + +In this case, `Window_Descriptor` byte is skipped, +but `Frame_Content_Size` is necessarily present. +As a consequence, the decoder must allocate a memory segment +of size equal or larger than `Frame_Content_Size`. + +In order to preserve the decoder from unreasonable memory requirements, +a decoder is allowed to reject a compressed frame +which requests a memory size beyond decoder's authorized range. + +For broader compatibility, decoders are recommended to support +memory sizes of at least 8 MB. +This is only a recommendation, +each decoder is free to support higher or lower limits, +depending on local limitations. + +__`Unused_bit`__ + +A decoder compliant with this specification version shall not interpret this bit. +It might be used in any future version, +to signal a property which is transparent to properly decode the frame. +An encoder compliant with this specification version must set this bit to zero. + +__`Reserved_bit`__ + +This bit is reserved for some future feature. +Its value _must be zero_. +A decoder compliant with this specification version must ensure it is not set. +This bit may be used in a future revision, +to signal a feature that must be interpreted to decode the frame correctly. + +__`Content_Checksum_flag`__ + +If this flag is set, a 32-bits `Content_Checksum` will be present at frame's end. +See `Content_Checksum` paragraph. + +__`Dictionary_ID_flag`__ + +This is a 2-bits flag (`= FHD & 3`), +telling if a dictionary ID is provided within the header. +It also specifies the size of this field as `DID_Field_Size`. + +|`Flag_Value` | 0 | 1 | 2 | 3 | +| -------------- | --- | --- | --- | --- | +|`DID_Field_Size`| 0 | 1 | 2 | 4 | + +#### `Window_Descriptor` + +Provides guarantees on minimum memory buffer required to decompress a frame. +This information is important for decoders to allocate enough memory. + +The `Window_Descriptor` byte is optional. +When `Single_Segment_flag` is set, `Window_Descriptor` is not present. +In this case, `Window_Size` is `Frame_Content_Size`, +which can be any value from 0 to 2^64-1 bytes (16 ExaBytes). + +| Bit numbers | 7-3 | 2-0 | +| ----------- | ---------- | ---------- | +| Field name | `Exponent` | `Mantissa` | + +The minimum memory buffer size is called `Window_Size`. +It is described by the following formulas : +``` +windowLog = 10 + Exponent; +windowBase = 1 << windowLog; +windowAdd = (windowBase / 8) * Mantissa; +Window_Size = windowBase + windowAdd; +``` +The minimum `Window_Size` is 1 KB. +The maximum `Window_Size` is `(1<<41) + 7*(1<<38)` bytes, which is 3.75 TB. + +In general, larger `Window_Size` tend to improve compression ratio, +but at the cost of memory usage. + +To properly decode compressed data, +a decoder will need to allocate a buffer of at least `Window_Size` bytes. + +In order to preserve decoder from unreasonable memory requirements, +a decoder is allowed to reject a compressed frame +which requests a memory size beyond decoder's authorized range. + +For improved interoperability, +it's recommended for decoders to support `Window_Size` of up to 8 MB, +and it's recommended for encoders to not generate frame requiring `Window_Size` larger than 8 MB. +It's merely a recommendation though, +decoders are free to support larger or lower limits, +depending on local limitations. + +#### `Dictionary_ID` + +This is a variable size field, which contains +the ID of the dictionary required to properly decode the frame. +`Dictionary_ID` field is optional. When it's not present, +it's up to the decoder to know which dictionary to use. + +`Dictionary_ID` field size is provided by `DID_Field_Size`. +`DID_Field_Size` is directly derived from value of `Dictionary_ID_flag`. +1 byte can represent an ID 0-255. +2 bytes can represent an ID 0-65535. +4 bytes can represent an ID 0-4294967295. +Format is __little-endian__. + +It's allowed to represent a small ID (for example `13`) +with a large 4-bytes dictionary ID, even if it is less efficient. + +_Reserved ranges :_ +Within private environments, any `Dictionary_ID` can be used. + +However, for frames and dictionaries distributed in public space, +`Dictionary_ID` must be attributed carefully. +Rules for public environment are not yet decided, +but the following ranges are reserved for some future registrar : +- low range : `<= 32767` +- high range : `>= (1 << 31)` + +Outside of these ranges, any value of `Dictionary_ID` +which is both `>= 32768` and `< (1<<31)` can be used freely, +even in public environment. + + + +#### `Frame_Content_Size` + +This is the original (uncompressed) size. This information is optional. +`Frame_Content_Size` uses a variable number of bytes, provided by `FCS_Field_Size`. +`FCS_Field_Size` is provided by the value of `Frame_Content_Size_flag`. +`FCS_Field_Size` can be equal to 0 (not present), 1, 2, 4 or 8 bytes. + +| `FCS_Field_Size` | Range | +| ---------------- | ---------- | +| 0 | unknown | +| 1 | 0 - 255 | +| 2 | 256 - 65791| +| 4 | 0 - 2^32-1 | +| 8 | 0 - 2^64-1 | + +`Frame_Content_Size` format is __little-endian__. +When `FCS_Field_Size` is 1, 4 or 8 bytes, the value is read directly. +When `FCS_Field_Size` is 2, _the offset of 256 is added_. +It's allowed to represent a small size (for example `18`) using any compatible variant. + + +Blocks +------- + +After `Magic_Number` and `Frame_Header`, there are some number of blocks. +Each frame must have at least one block, +but there is no upper limit on the number of blocks per frame. + +The structure of a block is as follows: + +| `Block_Header` | `Block_Content` | +|:--------------:|:---------------:| +| 3 bytes | n bytes | + +__`Block_Header`__ + +`Block_Header` uses 3 bytes, written using __little-endian__ convention. +It contains 3 fields : + +| `Last_Block` | `Block_Type` | `Block_Size` | +|:------------:|:------------:|:------------:| +| bit 0 | bits 1-2 | bits 3-23 | + +__`Last_Block`__ + +The lowest bit signals if this block is the last one. +The frame will end after this last block. +It may be followed by an optional `Content_Checksum` +(see [Zstandard Frames](#zstandard-frames)). + +__`Block_Type`__ + +The next 2 bits represent the `Block_Type`. +`Block_Type` influences the meaning of `Block_Size`. +There are 4 block types : + +| Value | 0 | 1 | 2 | 3 | +| ------------ | ----------- | ----------- | ------------------ | --------- | +| `Block_Type` | `Raw_Block` | `RLE_Block` | `Compressed_Block` | `Reserved`| + +- `Raw_Block` - this is an uncompressed block. + `Block_Content` contains `Block_Size` bytes. + +- `RLE_Block` - this is a single byte, repeated `Block_Size` times. + `Block_Content` consists of a single byte. + On the decompression side, this byte must be repeated `Block_Size` times. + +- `Compressed_Block` - this is a [Zstandard compressed block](#compressed-blocks), + explained later on. + `Block_Size` is the length of `Block_Content`, the compressed data. + The decompressed size is not known, + but its maximum possible value is guaranteed (see below) + +- `Reserved` - this is not a block. + This value cannot be used with current version of this specification. + If such a value is present, it is considered corrupted data. + +__`Block_Size`__ + +The upper 21 bits of `Block_Header` represent the `Block_Size`. + +When `Block_Type` is `Compressed_Block` or `Raw_Block`, +`Block_Size` is the size of `Block_Content` (hence excluding `Block_Header`). + +When `Block_Type` is `RLE_Block`, since `Block_Content`’s size is always 1, +`Block_Size` represents the number of times this byte must be repeated. + +`Block_Size` is limited by `Block_Maximum_Size` (see below). + +__`Block_Content`__ and __`Block_Maximum_Size`__ + +The size of `Block_Content` is limited by `Block_Maximum_Size`, +which is the smallest of: +- `Window_Size` +- 128 KB + +`Block_Maximum_Size` is constant for a given frame. +This maximum is applicable to both the decompressed size +and the compressed size of any block in the frame. + +The reasoning for this limit is that a decoder can read this information +at the beginning of a frame and use it to allocate buffers. +The guarantees on the size of blocks ensure that +the buffers will be large enough for any following block of the valid frame. + + +Compressed Blocks +----------------- +To decompress a compressed block, the compressed size must be provided +from `Block_Size` field within `Block_Header`. + +A compressed block consists of 2 sections : +- [Literals Section](#literals-section) +- [Sequences Section](#sequences-section) + +The results of the two sections are then combined to produce the decompressed +data in [Sequence Execution](#sequence-execution) + +#### Prerequisites +To decode a compressed block, the following elements are necessary : +- Previous decoded data, up to a distance of `Window_Size`, + or beginning of the Frame, whichever is smaller. +- List of "recent offsets" from previous `Compressed_Block`. +- The previous Huffman tree, required by `Treeless_Literals_Block` type +- Previous FSE decoding tables, required by `Repeat_Mode` + for each symbol type (literals lengths, match lengths, offsets) + +Note that decoding tables aren't always from the previous `Compressed_Block`. + +- Every decoding table can come from a dictionary. +- The Huffman tree comes from the previous `Compressed_Literals_Block`. + +Literals Section +---------------- +All literals are regrouped in the first part of the block. +They can be decoded first, and then copied during [Sequence Execution], +or they can be decoded on the flow during [Sequence Execution]. + +Literals can be stored uncompressed or compressed using Huffman prefix codes. +When compressed, an optional tree description can be present, +followed by 1 or 4 streams. + +| `Literals_Section_Header` | [`Huffman_Tree_Description`] | [jumpTable] | Stream1 | [Stream2] | [Stream3] | [Stream4] | +| ------------------------- | ---------------------------- | ----------- | ------- | --------- | --------- | --------- | + + +### `Literals_Section_Header` + +Header is in charge of describing how literals are packed. +It's a byte-aligned variable-size bitfield, ranging from 1 to 5 bytes, +using __little-endian__ convention. + +| `Literals_Block_Type` | `Size_Format` | `Regenerated_Size` | [`Compressed_Size`] | +| --------------------- | ------------- | ------------------ | ------------------- | +| 2 bits | 1 - 2 bits | 5 - 20 bits | 0 - 18 bits | + +In this representation, bits on the left are the lowest bits. + +__`Literals_Block_Type`__ + +This field uses 2 lowest bits of first byte, describing 4 different block types : + +| `Literals_Block_Type` | Value | +| --------------------------- | ----- | +| `Raw_Literals_Block` | 0 | +| `RLE_Literals_Block` | 1 | +| `Compressed_Literals_Block` | 2 | +| `Treeless_Literals_Block` | 3 | + +- `Raw_Literals_Block` - Literals are stored uncompressed. +- `RLE_Literals_Block` - Literals consist of a single byte value + repeated `Regenerated_Size` times. +- `Compressed_Literals_Block` - This is a standard Huffman-compressed block, + starting with a Huffman tree description. + See details below. +- `Treeless_Literals_Block` - This is a Huffman-compressed block, + using Huffman tree _from previous Huffman-compressed literals block_. + `Huffman_Tree_Description` will be skipped. + Note: If this mode is triggered without any previous Huffman-table in the frame + (or [dictionary](#dictionary-format)), this should be treated as data corruption. + +__`Size_Format`__ + +`Size_Format` is divided into 2 families : + +- For `Raw_Literals_Block` and `RLE_Literals_Block`, + it's only necessary to decode `Regenerated_Size`. + There is no `Compressed_Size` field. +- For `Compressed_Block` and `Treeless_Literals_Block`, + it's required to decode both `Compressed_Size` + and `Regenerated_Size` (the decompressed size). + It's also necessary to decode the number of streams (1 or 4). + +For values spanning several bytes, convention is __little-endian__. + +__`Size_Format` for `Raw_Literals_Block` and `RLE_Literals_Block`__ : + +`Size_Format` uses 1 _or_ 2 bits. +Its value is : `Size_Format = (Literals_Section_Header[0]>>2) & 3` + +- `Size_Format` == 00 or 10 : `Size_Format` uses 1 bit. + `Regenerated_Size` uses 5 bits (0-31). + `Literals_Section_Header` uses 1 byte. + `Regenerated_Size = Literals_Section_Header[0]>>3` +- `Size_Format` == 01 : `Size_Format` uses 2 bits. + `Regenerated_Size` uses 12 bits (0-4095). + `Literals_Section_Header` uses 2 bytes. + `Regenerated_Size = (Literals_Section_Header[0]>>4) + (Literals_Section_Header[1]<<4)` +- `Size_Format` == 11 : `Size_Format` uses 2 bits. + `Regenerated_Size` uses 20 bits (0-1048575). + `Literals_Section_Header` uses 3 bytes. + `Regenerated_Size = (Literals_Section_Header[0]>>4) + (Literals_Section_Header[1]<<4) + (Literals_Section_Header[2]<<12)` + +Only Stream1 is present for these cases. +Note : it's allowed to represent a short value (for example `13`) +using a long format, even if it's less efficient. + +__`Size_Format` for `Compressed_Literals_Block` and `Treeless_Literals_Block`__ : + +`Size_Format` always uses 2 bits. + +- `Size_Format` == 00 : _A single stream_. + Both `Regenerated_Size` and `Compressed_Size` use 10 bits (0-1023). + `Literals_Section_Header` uses 3 bytes. +- `Size_Format` == 01 : 4 streams. + Both `Regenerated_Size` and `Compressed_Size` use 10 bits (0-1023). + `Literals_Section_Header` uses 3 bytes. +- `Size_Format` == 10 : 4 streams. + Both `Regenerated_Size` and `Compressed_Size` use 14 bits (0-16383). + `Literals_Section_Header` uses 4 bytes. +- `Size_Format` == 11 : 4 streams. + Both `Regenerated_Size` and `Compressed_Size` use 18 bits (0-262143). + `Literals_Section_Header` uses 5 bytes. + +Both `Compressed_Size` and `Regenerated_Size` fields follow __little-endian__ convention. +Note: `Compressed_Size` __includes__ the size of the Huffman Tree description +_when_ it is present. + +#### Raw Literals Block +The data in Stream1 is `Regenerated_Size` bytes long, +it contains the raw literals data to be used during [Sequence Execution]. + +#### RLE Literals Block +Stream1 consists of a single byte which should be repeated `Regenerated_Size` times +to generate the decoded literals. + +#### Compressed Literals Block and Treeless Literals Block +Both of these modes contain Huffman encoded data. + +For `Treeless_Literals_Block`, +the Huffman table comes from previously compressed literals block, +or from a dictionary. + + +### `Huffman_Tree_Description` +This section is only present when `Literals_Block_Type` type is `Compressed_Literals_Block` (`2`). +The format of the Huffman tree description can be found at [Huffman Tree description](#huffman-tree-description). +The size of `Huffman_Tree_Description` is determined during decoding process, +it must be used to determine where streams begin. +`Total_Streams_Size = Compressed_Size - Huffman_Tree_Description_Size`. + + +### Jump Table +The Jump Table is only present when there are 4 Huffman-coded streams. + +Reminder : Huffman compressed data consists of either 1 or 4 Huffman-coded streams. + +If only one stream is present, it is a single bitstream occupying the entire +remaining portion of the literals block, encoded as described within +[Huffman-Coded Streams](#huffman-coded-streams). + +If there are four streams, `Literals_Section_Header` only provided +enough information to know the decompressed and compressed sizes +of all four streams _combined_. +The decompressed size of _each_ stream is equal to `(Regenerated_Size+3)/4`, +except for the last stream which may be up to 3 bytes smaller, +to reach a total decompressed size as specified in `Regenerated_Size`. + +The compressed size of each stream is provided explicitly in the Jump Table. +Jump Table is 6 bytes long, and consist of three 2-byte __little-endian__ fields, +describing the compressed sizes of the first three streams. +`Stream4_Size` is computed from total `Total_Streams_Size` minus sizes of other streams. + +`Stream4_Size = Total_Streams_Size - 6 - Stream1_Size - Stream2_Size - Stream3_Size`. + +Note: if `Stream1_Size + Stream2_Size + Stream3_Size > Total_Streams_Size`, +data is considered corrupted. + +Each of these 4 bitstreams is then decoded independently as a Huffman-Coded stream, +as described at [Huffman-Coded Streams](#huffman-coded-streams) + + +Sequences Section +----------------- +A compressed block is a succession of _sequences_ . +A sequence is a literal copy command, followed by a match copy command. +A literal copy command specifies a length. +It is the number of bytes to be copied (or extracted) from the Literals Section. +A match copy command specifies an offset and a length. + +When all _sequences_ are decoded, +if there are literals left in the _literals section_, +these bytes are added at the end of the block. + +This is described in more detail in [Sequence Execution](#sequence-execution). + +The `Sequences_Section` regroup all symbols required to decode commands. +There are 3 symbol types : literals lengths, offsets and match lengths. +They are encoded together, interleaved, in a single _bitstream_. + +The `Sequences_Section` starts by a header, +followed by optional probability tables for each symbol type, +followed by the bitstream. + +| `Sequences_Section_Header` | [`Literals_Length_Table`] | [`Offset_Table`] | [`Match_Length_Table`] | bitStream | +| -------------------------- | ------------------------- | ---------------- | ---------------------- | --------- | + +To decode the `Sequences_Section`, it's required to know its size. +Its size is deduced from the size of `Literals_Section`: +`Sequences_Section_Size = Block_Size - Literals_Section_Size`. + + +#### `Sequences_Section_Header` + +Consists of 2 items: +- `Number_of_Sequences` +- Symbol compression modes + +__`Number_of_Sequences`__ + +This is a variable size field using between 1 and 3 bytes. +Let's call its first byte `byte0`. +- `if (byte0 == 0)` : there are no sequences. + The sequence section stops there. + Decompressed content is defined entirely as Literals Section content. + The FSE tables used in `Repeat_Mode` aren't updated. +- `if (byte0 < 128)` : `Number_of_Sequences = byte0` . Uses 1 byte. +- `if (byte0 < 255)` : `Number_of_Sequences = ((byte0-128) << 8) + byte1` . Uses 2 bytes. +- `if (byte0 == 255)`: `Number_of_Sequences = byte1 + (byte2<<8) + 0x7F00` . Uses 3 bytes. + +__Symbol compression modes__ + +This is a single byte, defining the compression mode of each symbol type. + +|Bit number| 7-6 | 5-4 | 3-2 | 1-0 | +| -------- | ----------------------- | -------------- | -------------------- | ---------- | +|Field name| `Literals_Lengths_Mode` | `Offsets_Mode` | `Match_Lengths_Mode` | `Reserved` | + +The last field, `Reserved`, must be all-zeroes. + +`Literals_Lengths_Mode`, `Offsets_Mode` and `Match_Lengths_Mode` define the `Compression_Mode` of +literals lengths, offsets, and match lengths symbols respectively. + +They follow the same enumeration : + +| Value | 0 | 1 | 2 | 3 | +| ------------------ | ----------------- | ---------- | --------------------- | ------------- | +| `Compression_Mode` | `Predefined_Mode` | `RLE_Mode` | `FSE_Compressed_Mode` | `Repeat_Mode` | + +- `Predefined_Mode` : A predefined FSE distribution table is used, defined in + [default distributions](#default-distributions). + No distribution table will be present. +- `RLE_Mode` : The table description consists of a single byte, which contains the symbol's value. + This symbol will be used for all sequences. +- `FSE_Compressed_Mode` : standard FSE compression. + A distribution table will be present. + The format of this distribution table is described in [FSE Table Description](#fse-table-description). + Note that the maximum allowed accuracy log for literals length and match length tables is 9, + and the maximum accuracy log for the offsets table is 8. + `FSE_Compressed_Mode` must not be used when only one symbol is present, + `RLE_Mode` should be used instead (although any other mode will work). +- `Repeat_Mode` : The table used in the previous `Compressed_Block` with `Number_of_Sequences > 0` will be used again, + or if this is the first block, table in the dictionary will be used. + Note that this includes `RLE_mode`, so if `Repeat_Mode` follows `RLE_Mode`, the same symbol will be repeated. + It also includes `Predefined_Mode`, in which case `Repeat_Mode` will have same outcome as `Predefined_Mode`. + No distribution table will be present. + If this mode is used without any previous sequence table in the frame + (nor [dictionary](#dictionary-format)) to repeat, this should be treated as corruption. + +#### The codes for literals lengths, match lengths, and offsets. + +Each symbol is a _code_ in its own context, +which specifies `Baseline` and `Number_of_Bits` to add. +_Codes_ are FSE compressed, +and interleaved with raw additional bits in the same bitstream. + +##### Literals length codes + +Literals length codes are values ranging from `0` to `35` included. +They define lengths from 0 to 131071 bytes. +The literals length is equal to the decoded `Baseline` plus +the result of reading `Number_of_Bits` bits from the bitstream, +as a __little-endian__ value. + +| `Literals_Length_Code` | 0-15 | +| ---------------------- | ---------------------- | +| length | `Literals_Length_Code` | +| `Number_of_Bits` | 0 | + +| `Literals_Length_Code` | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | +| ---------------------- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | +| `Baseline` | 16 | 18 | 20 | 22 | 24 | 28 | 32 | 40 | +| `Number_of_Bits` | 1 | 1 | 1 | 1 | 2 | 2 | 3 | 3 | + +| `Literals_Length_Code` | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | +| ---------------------- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | +| `Baseline` | 48 | 64 | 128 | 256 | 512 | 1024 | 2048 | 4096 | +| `Number_of_Bits` | 4 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | + +| `Literals_Length_Code` | 32 | 33 | 34 | 35 | +| ---------------------- | ---- | ---- | ---- | ---- | +| `Baseline` | 8192 |16384 |32768 |65536 | +| `Number_of_Bits` | 13 | 14 | 15 | 16 | + + +##### Match length codes + +Match length codes are values ranging from `0` to `52` included. +They define lengths from 3 to 131074 bytes. +The match length is equal to the decoded `Baseline` plus +the result of reading `Number_of_Bits` bits from the bitstream, +as a __little-endian__ value. + +| `Match_Length_Code` | 0-31 | +| ------------------- | ----------------------- | +| value | `Match_Length_Code` + 3 | +| `Number_of_Bits` | 0 | + +| `Match_Length_Code` | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | +| ------------------- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | +| `Baseline` | 35 | 37 | 39 | 41 | 43 | 47 | 51 | 59 | +| `Number_of_Bits` | 1 | 1 | 1 | 1 | 2 | 2 | 3 | 3 | + +| `Match_Length_Code` | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | +| ------------------- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | +| `Baseline` | 67 | 83 | 99 | 131 | 259 | 515 | 1027 | 2051 | +| `Number_of_Bits` | 4 | 4 | 5 | 7 | 8 | 9 | 10 | 11 | + +| `Match_Length_Code` | 48 | 49 | 50 | 51 | 52 | +| ------------------- | ---- | ---- | ---- | ---- | ---- | +| `Baseline` | 4099 | 8195 |16387 |32771 |65539 | +| `Number_of_Bits` | 12 | 13 | 14 | 15 | 16 | + +##### Offset codes + +Offset codes are values ranging from `0` to `N`. + +A decoder is free to limit its maximum `N` supported. +Recommendation is to support at least up to `22`. +For information, at the time of this writing. +the reference decoder supports a maximum `N` value of `31`. + +An offset code is also the number of additional bits to read in __little-endian__ fashion, +and can be translated into an `Offset_Value` using the following formulas : + +``` +Offset_Value = (1 << offsetCode) + readNBits(offsetCode); +if (Offset_Value > 3) offset = Offset_Value - 3; +``` +It means that maximum `Offset_Value` is `(2^(N+1))-1` +supporting back-reference distances up to `(2^(N+1))-4`, +but is limited by [maximum back-reference distance](#window_descriptor). + +`Offset_Value` from 1 to 3 are special : they define "repeat codes". +This is described in more detail in [Repeat Offsets](#repeat-offsets). + +#### Decoding Sequences +FSE bitstreams are read in reverse direction than written. In zstd, +the compressor writes bits forward into a block and the decompressor +must read the bitstream _backwards_. + +To find the start of the bitstream it is therefore necessary to +know the offset of the last byte of the block which can be found +by counting `Block_Size` bytes after the block header. + +After writing the last bit containing information, the compressor +writes a single `1`-bit and then fills the byte with 0-7 `0` bits of +padding. The last byte of the compressed bitstream cannot be `0` for +that reason. + +When decompressing, the last byte containing the padding is the first +byte to read. The decompressor needs to skip 0-7 initial `0`-bits and +the first `1`-bit it occurs. Afterwards, the useful part of the bitstream +begins. + +FSE decoding requires a 'state' to be carried from symbol to symbol. +For more explanation on FSE decoding, see the [FSE section](#fse). + +For sequence decoding, a separate state keeps track of each +literal lengths, offsets, and match lengths symbols. +Some FSE primitives are also used. +For more details on the operation of these primitives, see the [FSE section](#fse). + +##### Starting states +The bitstream starts with initial FSE state values, +each using the required number of bits in their respective _accuracy_, +decoded previously from their normalized distribution. + +It starts by `Literals_Length_State`, +followed by `Offset_State`, +and finally `Match_Length_State`. + +Reminder : always keep in mind that all values are read _backward_, +so the 'start' of the bitstream is at the highest position in memory, +immediately before the last `1`-bit for padding. + +After decoding the starting states, a single sequence is decoded +`Number_Of_Sequences` times. +These sequences are decoded in order from first to last. +Since the compressor writes the bitstream in the forward direction, +this means the compressor must encode the sequences starting with the last +one and ending with the first. + +##### Decoding a sequence +For each of the symbol types, the FSE state can be used to determine the appropriate code. +The code then defines the `Baseline` and `Number_of_Bits` to read for each type. +See the [description of the codes] for how to determine these values. + +[description of the codes]: #the-codes-for-literals-lengths-match-lengths-and-offsets + +Decoding starts by reading the `Number_of_Bits` required to decode `Offset`. +It then does the same for `Match_Length`, and then for `Literals_Length`. +This sequence is then used for [sequence execution](#sequence-execution). + +If it is not the last sequence in the block, +the next operation is to update states. +Using the rules pre-calculated in the decoding tables, +`Literals_Length_State` is updated, +followed by `Match_Length_State`, +and then `Offset_State`. +See the [FSE section](#fse) for details on how to update states from the bitstream. + +This operation will be repeated `Number_of_Sequences` times. +At the end, the bitstream shall be entirely consumed, +otherwise the bitstream is considered corrupted. + +#### Default Distributions +If `Predefined_Mode` is selected for a symbol type, +its FSE decoding table is generated from a predefined distribution table defined here. +For details on how to convert this distribution into a decoding table, see the [FSE section]. + +[FSE section]: #from-normalized-distribution-to-decoding-tables + +##### Literals Length +The decoding table uses an accuracy log of 6 bits (64 states). +``` +short literalsLength_defaultDistribution[36] = + { 4, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, + 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 2, 1, 1, 1, 1, 1, + -1,-1,-1,-1 }; +``` + +##### Match Length +The decoding table uses an accuracy log of 6 bits (64 states). +``` +short matchLengths_defaultDistribution[53] = + { 1, 4, 3, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,-1,-1, + -1,-1,-1,-1,-1 }; +``` + +##### Offset Codes +The decoding table uses an accuracy log of 5 bits (32 states), +and supports a maximum `N` value of 28, allowing offset values up to 536,870,908 . + +If any sequence in the compressed block requires a larger offset than this, +it's not possible to use the default distribution to represent it. +``` +short offsetCodes_defaultDistribution[29] = + { 1, 1, 1, 1, 1, 1, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1,-1,-1,-1,-1,-1 }; +``` + + +Sequence Execution +------------------ +Once literals and sequences have been decoded, +they are combined to produce the decoded content of a block. + +Each sequence consists of a tuple of (`literals_length`, `offset_value`, `match_length`), +decoded as described in the [Sequences Section](#sequences-section). +To execute a sequence, first copy `literals_length` bytes +from the decoded literals to the output. + +Then `match_length` bytes are copied from previous decoded data. +The offset to copy from is determined by `offset_value`: +if `offset_value > 3`, then the offset is `offset_value - 3`. +If `offset_value` is from 1-3, the offset is a special repeat offset value. +See the [repeat offset](#repeat-offsets) section for how the offset is determined +in this case. + +The offset is defined as from the current position, so an offset of 6 +and a match length of 3 means that 3 bytes should be copied from 6 bytes back. +Note that all offsets leading to previously decoded data +must be smaller than `Window_Size` defined in `Frame_Header_Descriptor`. + +#### Repeat offsets +As seen in [Sequence Execution](#sequence-execution), +the first 3 values define a repeated offset and we will call them +`Repeated_Offset1`, `Repeated_Offset2`, and `Repeated_Offset3`. +They are sorted in recency order, with `Repeated_Offset1` meaning "most recent one". + +If `offset_value == 1`, then the offset used is `Repeated_Offset1`, etc. + +There is an exception though, when current sequence's `literals_length = 0`. +In this case, repeated offsets are shifted by one, +so an `offset_value` of 1 means `Repeated_Offset2`, +an `offset_value` of 2 means `Repeated_Offset3`, +and an `offset_value` of 3 means `Repeated_Offset1 - 1_byte`. + +For the first block, the starting offset history is populated with following values : +`Repeated_Offset1`=1, `Repeated_Offset2`=4, `Repeated_Offset3`=8, +unless a dictionary is used, in which case they come from the dictionary. + +Then each block gets its starting offset history from the ending values of the most recent `Compressed_Block`. +Note that blocks which are not `Compressed_Block` are skipped, they do not contribute to offset history. + +[Offset Codes]: #offset-codes + +###### Offset updates rules + +The newest offset takes the lead in offset history, +shifting others back by one rank, +up to the previous rank of the new offset _if it was present in history_. + +__Examples__ : + +In the common case, when new offset is not part of history : +`Repeated_Offset3` = `Repeated_Offset2` +`Repeated_Offset2` = `Repeated_Offset1` +`Repeated_Offset1` = `NewOffset` + +When the new offset _is_ part of history, there may be specific adjustments. + +When `NewOffset` == `Repeated_Offset1`, offset history remains actually unmodified. + +When `NewOffset` == `Repeated_Offset2`, +`Repeated_Offset1` and `Repeated_Offset2` ranks are swapped. +`Repeated_Offset3` is unmodified. + +When `NewOffset` == `Repeated_Offset3`, +there is actually no difference with the common case : +all offsets are shifted by one rank, +`NewOffset` (== `Repeated_Offset3`) becomes the new `Repeated_Offset1`. + +Also worth mentioning, the specific corner case when `offset_value` == 3, +and the literal length of the current sequence is zero. +In which case , `NewOffset` = `Repeated_Offset1` - 1_byte. +Here also, from an offset history update perspective, it's just a common case : +`Repeated_Offset3` = `Repeated_Offset2` +`Repeated_Offset2` = `Repeated_Offset1` +`Repeated_Offset1` = `NewOffset` ( == `Repeated_Offset1` - 1_byte ) + + + +Skippable Frames +---------------- + +| `Magic_Number` | `Frame_Size` | `User_Data` | +|:--------------:|:------------:|:-----------:| +| 4 bytes | 4 bytes | n bytes | + +Skippable frames allow the insertion of user-defined metadata +into a flow of concatenated frames. + +Skippable frames defined in this specification are compatible with [LZ4] ones. + +[LZ4]:http://www.lz4.org + +From a compliant decoder perspective, skippable frames need just be skipped, +and their content ignored, resuming decoding after the skippable frame. + +It can be noted that a skippable frame +can be used to watermark a stream of concatenated frames +embedding any kind of tracking information (even just an UUID). +Users wary of such possibility should scan the stream of concatenated frames +in an attempt to detect such frame for analysis or removal. + +__`Magic_Number`__ + +4 Bytes, __little-endian__ format. +Value : 0x184D2A5?, which means any value from 0x184D2A50 to 0x184D2A5F. +All 16 values are valid to identify a skippable frame. +This specification doesn't detail any specific tagging for skippable frames. + +__`Frame_Size`__ + +This is the size, in bytes, of the following `User_Data` +(without including the magic number nor the size field itself). +This field is represented using 4 Bytes, __little-endian__ format, unsigned 32-bits. +This means `User_Data` can’t be bigger than (2^32-1) bytes. + +__`User_Data`__ + +The `User_Data` can be anything. Data will just be skipped by the decoder. + + + +Entropy Encoding +---------------- +Two types of entropy encoding are used by the Zstandard format: +FSE, and Huffman coding. +Huffman is used to compress literals, +while FSE is used for all other symbols +(`Literals_Length_Code`, `Match_Length_Code`, offset codes) +and to compress Huffman headers. + + +FSE +--- +FSE, short for Finite State Entropy, is an entropy codec based on [ANS]. +FSE encoding/decoding involves a state that is carried over between symbols, +so decoding must be done in the opposite direction as encoding. +Therefore, all FSE bitstreams are read from end to beginning. +Note that the order of the bits in the stream is not reversed, +we just read the elements in the reverse order they are written. + +For additional details on FSE, see [Finite State Entropy]. + +[Finite State Entropy]:https://github.com/Cyan4973/FiniteStateEntropy/ + +FSE decoding involves a decoding table which has a power of 2 size, and contain three elements: +`Symbol`, `Num_Bits`, and `Baseline`. +The `log2` of the table size is its `Accuracy_Log`. +An FSE state value represents an index in this table. + +To obtain the initial state value, consume `Accuracy_Log` bits from the stream as a __little-endian__ value. +The next symbol in the stream is the `Symbol` indicated in the table for that state. +To obtain the next state value, +the decoder should consume `Num_Bits` bits from the stream as a __little-endian__ value and add it to `Baseline`. + +[ANS]: https://en.wikipedia.org/wiki/Asymmetric_Numeral_Systems + +### FSE Table Description +To decode FSE streams, it is necessary to construct the decoding table. +The Zstandard format encodes FSE table descriptions as follows: + +An FSE distribution table describes the probabilities of all symbols +from `0` to the last present one (included) +on a normalized scale of `1 << Accuracy_Log` . +Note that there must be two or more symbols with nonzero probability. + +It's a bitstream which is read forward, in __little-endian__ fashion. +It's not necessary to know bitstream exact size, +it will be discovered and reported by the decoding process. + +The bitstream starts by reporting on which scale it operates. +Let's `low4Bits` designate the lowest 4 bits of the first byte : +`Accuracy_Log = low4bits + 5`. + +Then follows each symbol value, from `0` to last present one. +The number of bits used by each field is variable. +It depends on : + +- Remaining probabilities + 1 : + __example__ : + Presuming an `Accuracy_Log` of 8, + and presuming 100 probabilities points have already been distributed, + the decoder may read any value from `0` to `256 - 100 + 1 == 157` (inclusive). + Therefore, it must read `log2sup(157) == 8` bits. + +- Value decoded : small values use 1 less bit : + __example__ : + Presuming values from 0 to 157 (inclusive) are possible, + 255-157 = 98 values are remaining in an 8-bits field. + They are used this way : + first 98 values (hence from 0 to 97) use only 7 bits, + values from 98 to 157 use 8 bits. + This is achieved through this scheme : + + | Value read | Value decoded | Number of bits used | + | ---------- | ------------- | ------------------- | + | 0 - 97 | 0 - 97 | 7 | + | 98 - 127 | 98 - 127 | 8 | + | 128 - 225 | 0 - 97 | 7 | + | 226 - 255 | 128 - 157 | 8 | + +Symbols probabilities are read one by one, in order. + +Probability is obtained from Value decoded by following formula : +`Proba = value - 1` + +It means value `0` becomes negative probability `-1`. +`-1` is a special probability, which means "less than 1". +Its effect on distribution table is described in the [next section]. +For the purpose of calculating total allocated probability points, it counts as one. + +[next section]:#from-normalized-distribution-to-decoding-tables + +When a symbol has a __probability__ of `zero`, +it is followed by a 2-bits repeat flag. +This repeat flag tells how many probabilities of zeroes follow the current one. +It provides a number ranging from 0 to 3. +If it is a 3, another 2-bits repeat flag follows, and so on. + +When last symbol reaches cumulated total of `1 << Accuracy_Log`, +decoding is complete. +If the last symbol makes cumulated total go above `1 << Accuracy_Log`, +distribution is considered corrupted. + +Then the decoder can tell how many bytes were used in this process, +and how many symbols are present. +The bitstream consumes a round number of bytes. +Any remaining bit within the last byte is just unused. + +#### From normalized distribution to decoding tables + +The distribution of normalized probabilities is enough +to create a unique decoding table. + +It follows the following build rule : + +The table has a size of `Table_Size = 1 << Accuracy_Log`. +Each cell describes the symbol decoded, +and instructions to get the next state (`Number_of_Bits` and `Baseline`). + +Symbols are scanned in their natural order for "less than 1" probabilities. +Symbols with this probability are being attributed a single cell, +starting from the end of the table and retreating. +These symbols define a full state reset, reading `Accuracy_Log` bits. + +Then, all remaining symbols, sorted in natural order, are allocated cells. +Starting from symbol `0` (if it exists), and table position `0`, +each symbol gets allocated as many cells as its probability. +Cell allocation is spreaded, not linear : +each successor position follows this rule : + +``` +position += (tableSize>>1) + (tableSize>>3) + 3; +position &= tableSize-1; +``` + +A position is skipped if already occupied by a "less than 1" probability symbol. +`position` does not reset between symbols, it simply iterates through +each position in the table, switching to the next symbol when enough +states have been allocated to the current one. + +The process guarantees that the table is entirely filled. +Each cell corresponds to a state value, which contains the symbol being decoded. + +To add the `Number_of_Bits` and `Baseline` required to retrieve next state, +it's first necessary to sort all occurrences of each symbol in state order. +Lower states will need 1 more bit than higher ones. +The process is repeated for each symbol. + +__Example__ : +Presuming a symbol has a probability of 5, +it receives 5 cells, corresponding to 5 state values. +These state values are then sorted in natural order. + +Next power of 2 after 5 is 8. +Space of probabilities must be divided into 8 equal parts. +Presuming the `Accuracy_Log` is 7, it defines a space of 128 states. +Divided by 8, each share is 16 large. + +In order to reach 8 shares, 8-5=3 lowest states will count "double", +doubling their shares (32 in width), hence requiring one more bit. + +Baseline is assigned starting from the higher states using fewer bits, +increasing at each state, then resuming at the first state, +each state takes its allocated width from Baseline. + +| state value | 1 | 39 | 77 | 84 | 122 | +| state order | 0 | 1 | 2 | 3 | 4 | +| ---------------- | ----- | ----- | ------ | ---- | ------ | +| width | 32 | 32 | 32 | 16 | 16 | +| `Number_of_Bits` | 5 | 5 | 5 | 4 | 4 | +| range number | 2 | 4 | 6 | 0 | 1 | +| `Baseline` | 32 | 64 | 96 | 0 | 16 | +| range | 32-63 | 64-95 | 96-127 | 0-15 | 16-31 | + +During decoding, the next state value is determined from current state value, +by reading the required `Number_of_Bits`, and adding the specified `Baseline`. + +See [Appendix A] for the results of this process applied to the default distributions. + +[Appendix A]: #appendix-a---decoding-tables-for-predefined-codes + + +Huffman Coding +-------------- +Zstandard Huffman-coded streams are read backwards, +similar to the FSE bitstreams. +Therefore, to find the start of the bitstream, it is therefore to +know the offset of the last byte of the Huffman-coded stream. + +After writing the last bit containing information, the compressor +writes a single `1`-bit and then fills the byte with 0-7 `0` bits of +padding. The last byte of the compressed bitstream cannot be `0` for +that reason. + +When decompressing, the last byte containing the padding is the first +byte to read. The decompressor needs to skip 0-7 initial `0`-bits and +the first `1`-bit it occurs. Afterwards, the useful part of the bitstream +begins. + +The bitstream contains Huffman-coded symbols in __little-endian__ order, +with the codes defined by the method below. + +### Huffman Tree Description + +Prefix coding represents symbols from an a priori known alphabet +by bit sequences (codewords), one codeword for each symbol, +in a manner such that different symbols may be represented +by bit sequences of different lengths, +but a parser can always parse an encoded string +unambiguously symbol-by-symbol. + +Given an alphabet with known symbol frequencies, +the Huffman algorithm allows the construction of an optimal prefix code +using the fewest bits of any possible prefix codes for that alphabet. + +Prefix code must not exceed a maximum code length. +More bits improve accuracy but cost more header size, +and require more memory or more complex decoding operations. +This specification limits maximum code length to 11 bits. + +#### Representation + +All literal values from zero (included) to last present one (excluded) +are represented by `Weight` with values from `0` to `Max_Number_of_Bits`. +Transformation from `Weight` to `Number_of_Bits` follows this formula : +``` +Number_of_Bits = Weight ? (Max_Number_of_Bits + 1 - Weight) : 0 +``` +The last symbol's `Weight` is deduced from previously decoded ones, +by completing to the nearest power of 2. +This power of 2 gives `Max_Number_of_Bits`, the depth of the current tree. +`Max_Number_of_Bits` must be <= 11, +otherwise the representation is considered corrupted. + +__Example__ : +Let's presume the following Huffman tree must be described : + +| literal value | 0 | 1 | 2 | 3 | 4 | 5 | +| ---------------- | --- | --- | --- | --- | --- | --- | +| `Number_of_Bits` | 1 | 2 | 3 | 0 | 4 | 4 | + +The tree depth is 4, since its longest elements uses 4 bits +(longest elements are the one with smallest frequency). +Value `5` will not be listed, as it can be determined from values for 0-4, +nor will values above `5` as they are all 0. +Values from `0` to `4` will be listed using `Weight` instead of `Number_of_Bits`. +Weight formula is : +``` +Weight = Number_of_Bits ? (Max_Number_of_Bits + 1 - Number_of_Bits) : 0 +``` +It gives the following series of weights : + +| literal value | 0 | 1 | 2 | 3 | 4 | +| ------------- | --- | --- | --- | --- | --- | +| `Weight` | 4 | 3 | 2 | 0 | 1 | + +The decoder will do the inverse operation : +having collected weights of literal symbols from `0` to `4`, +it knows the last literal, `5`, is present with a non-zero `Weight`. +The `Weight` of `5` can be determined by advancing to the next power of 2. +The sum of `2^(Weight-1)` (excluding 0's) is : +`8 + 4 + 2 + 0 + 1 = 15`. +Nearest larger power of 2 value is 16. +Therefore, `Max_Number_of_Bits = 4` and `Weight[5] = 16-15 = 1`. + +#### Huffman Tree header + +This is a single byte value (0-255), +which describes how the series of weights is encoded. + +- if `headerByte` < 128 : + the series of weights is compressed using FSE (see below). + The length of the FSE-compressed series is equal to `headerByte` (0-127). + +- if `headerByte` >= 128 : + + the series of weights uses a direct representation, + where each `Weight` is encoded directly as a 4 bits field (0-15). + + They are encoded forward, 2 weights to a byte, + first weight taking the top four bits and second one taking the bottom four. + * e.g. the following operations could be used to read the weights: + `Weight[0] = (Byte[0] >> 4), Weight[1] = (Byte[0] & 0xf)`, etc. + + The full representation occupies `Ceiling(Number_of_Weights/2)` bytes, + meaning it uses only full bytes even if `Number_of_Weights` is odd. + + `Number_of_Weights = headerByte - 127`. + * Note that maximum `Number_of_Weights` is 255-127 = 128, + therefore, only up to 128 `Weight` can be encoded using direct representation. + * Since the last non-zero `Weight` is _not_ encoded, + this scheme is compatible with alphabet sizes of up to 129 symbols, + hence including literal symbol 128. + * If any literal symbol > 128 has a non-zero `Weight`, + direct representation is not possible. + In such case, it's necessary to use FSE compression. + + +#### Finite State Entropy (FSE) compression of Huffman weights + +In this case, the series of Huffman weights is compressed using FSE compression. +It's a single bitstream with 2 interleaved states, +sharing a single distribution table. + +To decode an FSE bitstream, it is necessary to know its compressed size. +Compressed size is provided by `headerByte`. +It's also necessary to know its _maximum possible_ decompressed size, +which is `255`, since literal values span from `0` to `255`, +and last symbol's `Weight` is not represented. + +An FSE bitstream starts by a header, describing probabilities distribution. +It will create a Decoding Table. +For a list of Huffman weights, the maximum accuracy log is 6 bits. +For more description see the [FSE header description](#fse-table-description) + +The Huffman header compression uses 2 states, +which share the same FSE distribution table. +The first state (`State1`) encodes the even indexed symbols, +and the second (`State2`) encodes the odd indexed symbols. +`State1` is initialized first, and then `State2`, and they take turns +decoding a single symbol and updating their state. +For more details on these FSE operations, see the [FSE section](#fse). + +The number of symbols to decode is determined +by tracking bitStream overflow condition: +If updating state after decoding a symbol would require more bits than +remain in the stream, it is assumed that extra bits are 0. Then, +symbols for each of the final states are decoded and the process is complete. + +#### Conversion from weights to Huffman prefix codes + +All present symbols shall now have a `Weight` value. +It is possible to transform weights into `Number_of_Bits`, using this formula: +``` +Number_of_Bits = (Weight>0) ? Max_Number_of_Bits + 1 - Weight : 0 +``` +Symbols are sorted by `Weight`. +Within same `Weight`, symbols keep natural sequential order. +Symbols with a `Weight` of zero are removed. +Then, starting from lowest `Weight`, prefix codes are distributed in sequential order. + +__Example__ : +Let's presume the following list of weights has been decoded : + +| Literal | 0 | 1 | 2 | 3 | 4 | 5 | +| -------- | --- | --- | --- | --- | --- | --- | +| `Weight` | 4 | 3 | 2 | 0 | 1 | 1 | + +Sorted by weight and then natural sequential order, +it gives the following distribution : + +| Literal | 3 | 4 | 5 | 2 | 1 | 0 | +| ---------------- | --- | --- | --- | --- | --- | ---- | +| `Weight` | 0 | 1 | 1 | 2 | 3 | 4 | +| `Number_of_Bits` | 0 | 4 | 4 | 3 | 2 | 1 | +| prefix codes | N/A | 0000| 0001| 001 | 01 | 1 | + +### Huffman-coded Streams + +Given a Huffman decoding table, +it's possible to decode a Huffman-coded stream. + +Each bitstream must be read _backward_, +that is starting from the end down to the beginning. +Therefore it's necessary to know the size of each bitstream. + +It's also necessary to know exactly which _bit_ is the last one. +This is detected by a final bit flag : +the highest bit of latest byte is a final-bit-flag. +Consequently, a last byte of `0` is not possible. +And the final-bit-flag itself is not part of the useful bitstream. +Hence, the last byte contains between 0 and 7 useful bits. + +Starting from the end, +it's possible to read the bitstream in a __little-endian__ fashion, +keeping track of already used bits. Since the bitstream is encoded in reverse +order, starting from the end read symbols in forward order. + +For example, if the literal sequence "0145" was encoded using above prefix code, +it would be encoded (in reverse order) as: + +|Symbol | 5 | 4 | 1 | 0 | Padding | +|--------|------|------|----|---|---------| +|Encoding|`0000`|`0001`|`01`|`1`| `00001` | + +Resulting in following 2-bytes bitstream : +``` +00010000 00001101 +``` + +Here is an alternative representation with the symbol codes separated by underscore: +``` +0001_0000 00001_1_01 +``` + +Reading highest `Max_Number_of_Bits` bits, +it's possible to compare extracted value to decoding table, +determining the symbol to decode and number of bits to discard. + +The process continues up to reading the required number of symbols per stream. +If a bitstream is not entirely and exactly consumed, +hence reaching exactly its beginning position with _all_ bits consumed, +the decoding process is considered faulty. + + +Dictionary Format +----------------- + +Zstandard is compatible with "raw content" dictionaries, +free of any format restriction, except that they must be at least 8 bytes. +These dictionaries function as if they were just the `Content` part +of a formatted dictionary. + +But dictionaries created by `zstd --train` follow a format, described here. + +__Pre-requisites__ : a dictionary has a size, + defined either by a buffer limit, or a file size. + +| `Magic_Number` | `Dictionary_ID` | `Entropy_Tables` | `Content` | +| -------------- | --------------- | ---------------- | --------- | + +__`Magic_Number`__ : 4 bytes ID, value 0xEC30A437, __little-endian__ format + +__`Dictionary_ID`__ : 4 bytes, stored in __little-endian__ format. + `Dictionary_ID` can be any value, except 0 (which means no `Dictionary_ID`). + It's used by decoders to check if they use the correct dictionary. + +_Reserved ranges :_ + If the frame is going to be distributed in a private environment, + any `Dictionary_ID` can be used. + However, for public distribution of compressed frames, + the following ranges are reserved and shall not be used : + + - low range : <= 32767 + - high range : >= (2^31) + +__`Entropy_Tables`__ : follow the same format as tables in [compressed blocks]. + See the relevant [FSE](#fse-table-description) + and [Huffman](#huffman-tree-description) sections for how to decode these tables. + They are stored in following order : + Huffman tables for literals, FSE table for offsets, + FSE table for match lengths, and FSE table for literals lengths. + These tables populate the Repeat Stats literals mode and + Repeat distribution mode for sequence decoding. + It's finally followed by 3 offset values, populating recent offsets (instead of using `{1,4,8}`), + stored in order, 4-bytes __little-endian__ each, for a total of 12 bytes. + Each recent offset must have a value < dictionary size. + +__`Content`__ : The rest of the dictionary is its content. + The content act as a "past" in front of data to compress or decompress, + so it can be referenced in sequence commands. + As long as the amount of data decoded from this frame is less than or + equal to `Window_Size`, sequence commands may specify offsets longer + than the total length of decoded output so far to reference back to the + dictionary, even parts of the dictionary with offsets larger than `Window_Size`. + After the total output has surpassed `Window_Size` however, + this is no longer allowed and the dictionary is no longer accessible. + +[compressed blocks]: #the-format-of-compressed_block + +If a dictionary is provided by an external source, +it should be loaded with great care, its content considered untrusted. + + + +Appendix A - Decoding tables for predefined codes +------------------------------------------------- + +This appendix contains FSE decoding tables +for the predefined literal length, match length, and offset codes. +The tables have been constructed using the algorithm as given above in chapter +"from normalized distribution to decoding tables". +The tables here can be used as examples +to crosscheck that an implementation build its decoding tables correctly. + +#### Literal Length Code: + +| State | Symbol | Number_Of_Bits | Base | +| ----- | ------ | -------------- | ---- | +| 0 | 0 | 4 | 0 | +| 1 | 0 | 4 | 16 | +| 2 | 1 | 5 | 32 | +| 3 | 3 | 5 | 0 | +| 4 | 4 | 5 | 0 | +| 5 | 6 | 5 | 0 | +| 6 | 7 | 5 | 0 | +| 7 | 9 | 5 | 0 | +| 8 | 10 | 5 | 0 | +| 9 | 12 | 5 | 0 | +| 10 | 14 | 6 | 0 | +| 11 | 16 | 5 | 0 | +| 12 | 18 | 5 | 0 | +| 13 | 19 | 5 | 0 | +| 14 | 21 | 5 | 0 | +| 15 | 22 | 5 | 0 | +| 16 | 24 | 5 | 0 | +| 17 | 25 | 5 | 32 | +| 18 | 26 | 5 | 0 | +| 19 | 27 | 6 | 0 | +| 20 | 29 | 6 | 0 | +| 21 | 31 | 6 | 0 | +| 22 | 0 | 4 | 32 | +| 23 | 1 | 4 | 0 | +| 24 | 2 | 5 | 0 | +| 25 | 4 | 5 | 32 | +| 26 | 5 | 5 | 0 | +| 27 | 7 | 5 | 32 | +| 28 | 8 | 5 | 0 | +| 29 | 10 | 5 | 32 | +| 30 | 11 | 5 | 0 | +| 31 | 13 | 6 | 0 | +| 32 | 16 | 5 | 32 | +| 33 | 17 | 5 | 0 | +| 34 | 19 | 5 | 32 | +| 35 | 20 | 5 | 0 | +| 36 | 22 | 5 | 32 | +| 37 | 23 | 5 | 0 | +| 38 | 25 | 4 | 0 | +| 39 | 25 | 4 | 16 | +| 40 | 26 | 5 | 32 | +| 41 | 28 | 6 | 0 | +| 42 | 30 | 6 | 0 | +| 43 | 0 | 4 | 48 | +| 44 | 1 | 4 | 16 | +| 45 | 2 | 5 | 32 | +| 46 | 3 | 5 | 32 | +| 47 | 5 | 5 | 32 | +| 48 | 6 | 5 | 32 | +| 49 | 8 | 5 | 32 | +| 50 | 9 | 5 | 32 | +| 51 | 11 | 5 | 32 | +| 52 | 12 | 5 | 32 | +| 53 | 15 | 6 | 0 | +| 54 | 17 | 5 | 32 | +| 55 | 18 | 5 | 32 | +| 56 | 20 | 5 | 32 | +| 57 | 21 | 5 | 32 | +| 58 | 23 | 5 | 32 | +| 59 | 24 | 5 | 32 | +| 60 | 35 | 6 | 0 | +| 61 | 34 | 6 | 0 | +| 62 | 33 | 6 | 0 | +| 63 | 32 | 6 | 0 | + +#### Match Length Code: + +| State | Symbol | Number_Of_Bits | Base | +| ----- | ------ | -------------- | ---- | +| 0 | 0 | 6 | 0 | +| 1 | 1 | 4 | 0 | +| 2 | 2 | 5 | 32 | +| 3 | 3 | 5 | 0 | +| 4 | 5 | 5 | 0 | +| 5 | 6 | 5 | 0 | +| 6 | 8 | 5 | 0 | +| 7 | 10 | 6 | 0 | +| 8 | 13 | 6 | 0 | +| 9 | 16 | 6 | 0 | +| 10 | 19 | 6 | 0 | +| 11 | 22 | 6 | 0 | +| 12 | 25 | 6 | 0 | +| 13 | 28 | 6 | 0 | +| 14 | 31 | 6 | 0 | +| 15 | 33 | 6 | 0 | +| 16 | 35 | 6 | 0 | +| 17 | 37 | 6 | 0 | +| 18 | 39 | 6 | 0 | +| 19 | 41 | 6 | 0 | +| 20 | 43 | 6 | 0 | +| 21 | 45 | 6 | 0 | +| 22 | 1 | 4 | 16 | +| 23 | 2 | 4 | 0 | +| 24 | 3 | 5 | 32 | +| 25 | 4 | 5 | 0 | +| 26 | 6 | 5 | 32 | +| 27 | 7 | 5 | 0 | +| 28 | 9 | 6 | 0 | +| 29 | 12 | 6 | 0 | +| 30 | 15 | 6 | 0 | +| 31 | 18 | 6 | 0 | +| 32 | 21 | 6 | 0 | +| 33 | 24 | 6 | 0 | +| 34 | 27 | 6 | 0 | +| 35 | 30 | 6 | 0 | +| 36 | 32 | 6 | 0 | +| 37 | 34 | 6 | 0 | +| 38 | 36 | 6 | 0 | +| 39 | 38 | 6 | 0 | +| 40 | 40 | 6 | 0 | +| 41 | 42 | 6 | 0 | +| 42 | 44 | 6 | 0 | +| 43 | 1 | 4 | 32 | +| 44 | 1 | 4 | 48 | +| 45 | 2 | 4 | 16 | +| 46 | 4 | 5 | 32 | +| 47 | 5 | 5 | 32 | +| 48 | 7 | 5 | 32 | +| 49 | 8 | 5 | 32 | +| 50 | 11 | 6 | 0 | +| 51 | 14 | 6 | 0 | +| 52 | 17 | 6 | 0 | +| 53 | 20 | 6 | 0 | +| 54 | 23 | 6 | 0 | +| 55 | 26 | 6 | 0 | +| 56 | 29 | 6 | 0 | +| 57 | 52 | 6 | 0 | +| 58 | 51 | 6 | 0 | +| 59 | 50 | 6 | 0 | +| 60 | 49 | 6 | 0 | +| 61 | 48 | 6 | 0 | +| 62 | 47 | 6 | 0 | +| 63 | 46 | 6 | 0 | + +#### Offset Code: + +| State | Symbol | Number_Of_Bits | Base | +| ----- | ------ | -------------- | ---- | +| 0 | 0 | 5 | 0 | +| 1 | 6 | 4 | 0 | +| 2 | 9 | 5 | 0 | +| 3 | 15 | 5 | 0 | +| 4 | 21 | 5 | 0 | +| 5 | 3 | 5 | 0 | +| 6 | 7 | 4 | 0 | +| 7 | 12 | 5 | 0 | +| 8 | 18 | 5 | 0 | +| 9 | 23 | 5 | 0 | +| 10 | 5 | 5 | 0 | +| 11 | 8 | 4 | 0 | +| 12 | 14 | 5 | 0 | +| 13 | 20 | 5 | 0 | +| 14 | 2 | 5 | 0 | +| 15 | 7 | 4 | 16 | +| 16 | 11 | 5 | 0 | +| 17 | 17 | 5 | 0 | +| 18 | 22 | 5 | 0 | +| 19 | 4 | 5 | 0 | +| 20 | 8 | 4 | 16 | +| 21 | 13 | 5 | 0 | +| 22 | 19 | 5 | 0 | +| 23 | 1 | 5 | 0 | +| 24 | 6 | 4 | 16 | +| 25 | 10 | 5 | 0 | +| 26 | 16 | 5 | 0 | +| 27 | 28 | 5 | 0 | +| 28 | 27 | 5 | 0 | +| 29 | 26 | 5 | 0 | +| 30 | 25 | 5 | 0 | +| 31 | 24 | 5 | 0 | + + + +Appendix B - Resources for implementers +------------------------------------------------- + +An open source reference implementation is available on : +https://github.com/facebook/zstd + +The project contains a frame generator, called [decodeCorpus], +which can be used by any 3rd-party implementation +to verify that a tested decoder is compliant with the specification. + +[decodeCorpus]: https://github.com/facebook/zstd/tree/v1.3.4/tests#decodecorpus---tool-to-generate-zstandard-frames-for-decoder-testing + +`decodeCorpus` generates random valid frames. +A compliant decoder should be able to decode them all, +or at least provide a meaningful error code explaining for which reason it cannot +(memory limit restrictions for example). + + +Version changes +--------------- +- 0.3.5 : clarifications for Block_Maximum_Size +- 0.3.4 : clarifications for FSE decoding table +- 0.3.3 : clarifications for field Block_Size +- 0.3.2 : remove additional block size restriction on compressed blocks +- 0.3.1 : minor clarification regarding offset history update rules +- 0.3.0 : minor edits to match RFC8478 +- 0.2.9 : clarifications for huffman weights direct representation, by Ulrich Kunitz +- 0.2.8 : clarifications for IETF RFC discuss +- 0.2.7 : clarifications from IETF RFC review, by Vijay Gurbani and Nick Terrell +- 0.2.6 : fixed an error in huffman example, by Ulrich Kunitz +- 0.2.5 : minor typos and clarifications +- 0.2.4 : section restructuring, by Sean Purcell +- 0.2.3 : clarified several details, by Sean Purcell +- 0.2.2 : added predefined codes, by Johannes Rudolph +- 0.2.1 : clarify field names, by Przemyslaw Skibinski +- 0.2.0 : numerous format adjustments for zstd v0.8+ +- 0.1.2 : limit Huffman tree depth to 11 bits +- 0.1.1 : reserved dictID ranges +- 0.1.0 : initial release diff --git a/src/zstd/doc/zstd_manual.html b/src/zstd/doc/zstd_manual.html new file mode 100644 index 000000000..fe58f78cb --- /dev/null +++ b/src/zstd/doc/zstd_manual.html @@ -0,0 +1,1680 @@ + + + +zstd 1.4.5 Manual + + +

zstd 1.4.5 Manual

+
+

Contents

+
    +
  1. Introduction
    2. +
  2. Version
    3. +
  3. Simple API
    4. +
  4. Explicit context
    5. +
  5. Advanced compression API
    6. +
  6. Advanced decompression API
    7. +
  7. Streaming
    8. +
  8. Streaming compression - HowTo
    9. +
  9. Streaming decompression - HowTo
    10. +
  10. Simple dictionary API
    11. +
  11. Bulk processing dictionary API
    12. +
  12. Dictionary helper functions
    13. +
  13. Advanced dictionary and prefix API
    14. +
  14. experimental API (static linking only)
    15. +
  15. Frame size functions
    16. +
  16. Memory management
    17. +
  17. Advanced compression functions
    18. +
  18. Advanced decompression functions
    19. +
  19. Advanced streaming functions
    20. +
  20. ! ZSTD_initCStream_usingDict() :
    21. +
  21. ! ZSTD_initCStream_advanced() :
    22. +
  22. ! ZSTD_initCStream_usingCDict() :
    23. +
  23. ! ZSTD_initCStream_usingCDict_advanced() :
    24. +
  24. This function is deprecated, and is equivalent to:
    25. +
  25. This function is deprecated, and is equivalent to:
    26. +
  26. Buffer-less and synchronous inner streaming functions
    27. +
  27. Buffer-less streaming compression (synchronous mode)
    28. +
  28. Buffer-less streaming decompression (synchronous mode)
    29. +
  29. Block level API
    30. +
+
+

Introduction

+  zstd, short for Zstandard, is a fast lossless compression algorithm, targeting
+  real-time compression scenarios at zlib-level and better compression ratios.
+  The zstd compression library provides in-memory compression and decompression
+  functions.
+
+  The library supports regular compression levels from 1 up to ZSTD_maxCLevel(),
+  which is currently 22. Levels >= 20, labeled `--ultra`, should be used with
+  caution, as they require more memory. The library also offers negative
+  compression levels, which extend the range of speed vs. ratio preferences.
+  The lower the level, the faster the speed (at the cost of compression).
+
+  Compression can be done in:
+    - a single step (described as Simple API)
+    - a single step, reusing a context (described as Explicit context)
+    - unbounded multiple steps (described as Streaming compression)
+
+  The compression ratio achievable on small data can be highly improved using
+  a dictionary. Dictionary compression can be performed in:
+    - a single step (described as Simple dictionary API)
+    - a single step, reusing a dictionary (described as Bulk-processing
+      dictionary API)
+
+  Advanced experimental functions can be accessed using
+  `#define ZSTD_STATIC_LINKING_ONLY` before including zstd.h.
+
+  Advanced experimental APIs should never be used with a dynamically-linked
+  library. They are not "stable"; their definitions or signatures may change in
+  the future. Only static linking is allowed.
+
+ +

Version


+
+
unsigned ZSTD_versionNumber(void);   /**< to check runtime library version */
+


+
Simple API

+
+
size_t ZSTD_compress( void* dst, size_t dstCapacity,
+                const void* src, size_t srcSize,
+                      int compressionLevel);
+
  Compresses `src` content as a single zstd compressed frame into already allocated `dst`.
+  Hint : compression runs faster if `dstCapacity` >=  `ZSTD_compressBound(srcSize)`.
+  @return : compressed size written into `dst` (<= `dstCapacity),
+            or an error code if it fails (which can be tested using ZSTD_isError()). 
+


+
+
size_t ZSTD_decompress( void* dst, size_t dstCapacity,
+                  const void* src, size_t compressedSize);
+
  `compressedSize` : must be the _exact_ size of some number of compressed and/or skippable frames.
+  `dstCapacity` is an upper bound of originalSize to regenerate.
+  If user cannot imply a maximum upper bound, it's better to use streaming mode to decompress data.
+  @return : the number of bytes decompressed into `dst` (<= `dstCapacity`),
+            or an errorCode if it fails (which can be tested using ZSTD_isError()). 
+


+
+
#define ZSTD_CONTENTSIZE_UNKNOWN (0ULL - 1)
+#define ZSTD_CONTENTSIZE_ERROR   (0ULL - 2)
+unsigned long long ZSTD_getFrameContentSize(const void *src, size_t srcSize);
+
  `src` should point to the start of a ZSTD encoded frame.
+  `srcSize` must be at least as large as the frame header.
+            hint : any size >= `ZSTD_frameHeaderSize_max` is large enough.
+  @return : - decompressed size of `src` frame content, if known
+            - ZSTD_CONTENTSIZE_UNKNOWN if the size cannot be determined
+            - ZSTD_CONTENTSIZE_ERROR if an error occurred (e.g. invalid magic number, srcSize too small)
+   note 1 : a 0 return value means the frame is valid but "empty".
+   note 2 : decompressed size is an optional field, it may not be present, typically in streaming mode.
+            When `return==ZSTD_CONTENTSIZE_UNKNOWN`, data to decompress could be any size.
+            In which case, it's necessary to use streaming mode to decompress data.
+            Optionally, application can rely on some implicit limit,
+            as ZSTD_decompress() only needs an upper bound of decompressed size.
+            (For example, data could be necessarily cut into blocks <= 16 KB).
+   note 3 : decompressed size is always present when compression is completed using single-pass functions,
+            such as ZSTD_compress(), ZSTD_compressCCtx() ZSTD_compress_usingDict() or ZSTD_compress_usingCDict().
+   note 4 : decompressed size can be very large (64-bits value),
+            potentially larger than what local system can handle as a single memory segment.
+            In which case, it's necessary to use streaming mode to decompress data.
+   note 5 : If source is untrusted, decompressed size could be wrong or intentionally modified.
+            Always ensure return value fits within application's authorized limits.
+            Each application can set its own limits.
+   note 6 : This function replaces ZSTD_getDecompressedSize() 
+


+
+
unsigned long long ZSTD_getDecompressedSize(const void* src, size_t srcSize);
+
  NOTE: This function is now obsolete, in favor of ZSTD_getFrameContentSize().
+  Both functions work the same way, but ZSTD_getDecompressedSize() blends
+  "empty", "unknown" and "error" results to the same return value (0),
+  while ZSTD_getFrameContentSize() gives them separate return values.
+ @return : decompressed size of `src` frame content _if known and not empty_, 0 otherwise. 
+


+
+
size_t ZSTD_findFrameCompressedSize(const void* src, size_t srcSize);
+
 `src` should point to the start of a ZSTD frame or skippable frame.
+ `srcSize` must be >= first frame size
+ @return : the compressed size of the first frame starting at `src`,
+           suitable to pass as `srcSize` to `ZSTD_decompress` or similar,
+        or an error code if input is invalid 
+


+
+
Helper functions
#define ZSTD_COMPRESSBOUND(srcSize)   ((srcSize) + ((srcSize)>>8) + (((srcSize) < (128<<10)) ? (((128<<10) - (srcSize)) >> 11) /* margin, from 64 to 0 */ : 0))  /* this formula ensures that bound(A) + bound(B) <= bound(A+B) as long as A and B >= 128 KB */
+size_t      ZSTD_compressBound(size_t srcSize); /*!< maximum compressed size in worst case single-pass scenario */
+unsigned    ZSTD_isError(size_t code);          /*!< tells if a `size_t` function result is an error code */
+const char* ZSTD_getErrorName(size_t code);     /*!< provides readable string from an error code */
+int         ZSTD_minCLevel(void);               /*!< minimum negative compression level allowed */
+int         ZSTD_maxCLevel(void);               /*!< maximum compression level available */
+


+
Explicit context

+
+
Compression context
  When compressing many times,
+  it is recommended to allocate a context just once,
+  and re-use it for each successive compression operation.
+  This will make workload friendlier for system's memory.
+  Note : re-using context is just a speed / resource optimization.
+         It doesn't change the compression ratio, which remains identical.
+  Note 2 : In multi-threaded environments,
+         use one different context per thread for parallel execution.
+ 
+
typedef struct ZSTD_CCtx_s ZSTD_CCtx;
+ZSTD_CCtx* ZSTD_createCCtx(void);
+size_t     ZSTD_freeCCtx(ZSTD_CCtx* cctx);
+


+
size_t ZSTD_compressCCtx(ZSTD_CCtx* cctx,
+                         void* dst, size_t dstCapacity,
+                   const void* src, size_t srcSize,
+                         int compressionLevel);
+
  Same as ZSTD_compress(), using an explicit ZSTD_CCtx.
+  Important : in order to behave similarly to `ZSTD_compress()`,
+  this function compresses at requested compression level,
+  __ignoring any other parameter__ .
+  If any advanced parameter was set using the advanced API,
+  they will all be reset. Only `compressionLevel` remains.
+ 
+


+
+
Decompression context
  When decompressing many times,
+  it is recommended to allocate a context only once,
+  and re-use it for each successive compression operation.
+  This will make workload friendlier for system's memory.
+  Use one context per thread for parallel execution. 
+
typedef struct ZSTD_DCtx_s ZSTD_DCtx;
+ZSTD_DCtx* ZSTD_createDCtx(void);
+size_t     ZSTD_freeDCtx(ZSTD_DCtx* dctx);
+


+
size_t ZSTD_decompressDCtx(ZSTD_DCtx* dctx,
+                           void* dst, size_t dstCapacity,
+                     const void* src, size_t srcSize);
+
  Same as ZSTD_decompress(),
+  requires an allocated ZSTD_DCtx.
+  Compatible with sticky parameters.
+ 
+


+
+
Advanced compression API

+
+
typedef enum { ZSTD_fast=1,
+               ZSTD_dfast=2,
+               ZSTD_greedy=3,
+               ZSTD_lazy=4,
+               ZSTD_lazy2=5,
+               ZSTD_btlazy2=6,
+               ZSTD_btopt=7,
+               ZSTD_btultra=8,
+               ZSTD_btultra2=9
+               /* note : new strategies _might_ be added in the future.
+                         Only the order (from fast to strong) is guaranteed */
+} ZSTD_strategy;
+


+
typedef enum {
+
+    /* compression parameters
+     * Note: When compressing with a ZSTD_CDict these parameters are superseded
+     * by the parameters used to construct the ZSTD_CDict.
+     * See ZSTD_CCtx_refCDict() for more info (superseded-by-cdict). */
+    ZSTD_c_compressionLevel=100, /* Set compression parameters according to pre-defined cLevel table.
+                              * Note that exact compression parameters are dynamically determined,
+                              * depending on both compression level and srcSize (when known).
+                              * Default level is ZSTD_CLEVEL_DEFAULT==3.
+                              * Special: value 0 means default, which is controlled by ZSTD_CLEVEL_DEFAULT.
+                              * Note 1 : it's possible to pass a negative compression level.
+                              * Note 2 : setting a level does not automatically set all other compression parameters
+                              *   to default. Setting this will however eventually dynamically impact the compression
+                              *   parameters which have not been manually set. The manually set
+                              *   ones will 'stick'. */
+    /* Advanced compression parameters :
+     * It's possible to pin down compression parameters to some specific values.
+     * In which case, these values are no longer dynamically selected by the compressor */
+    ZSTD_c_windowLog=101,    /* Maximum allowed back-reference distance, expressed as power of 2.
+                              * This will set a memory budget for streaming decompression,
+                              * with larger values requiring more memory
+                              * and typically compressing more.
+                              * Must be clamped between ZSTD_WINDOWLOG_MIN and ZSTD_WINDOWLOG_MAX.
+                              * Special: value 0 means "use default windowLog".
+                              * Note: Using a windowLog greater than ZSTD_WINDOWLOG_LIMIT_DEFAULT
+                              *       requires explicitly allowing such size at streaming decompression stage. */
+    ZSTD_c_hashLog=102,      /* Size of the initial probe table, as a power of 2.
+                              * Resulting memory usage is (1 << (hashLog+2)).
+                              * Must be clamped between ZSTD_HASHLOG_MIN and ZSTD_HASHLOG_MAX.
+                              * Larger tables improve compression ratio of strategies <= dFast,
+                              * and improve speed of strategies > dFast.
+                              * Special: value 0 means "use default hashLog". */
+    ZSTD_c_chainLog=103,     /* Size of the multi-probe search table, as a power of 2.
+                              * Resulting memory usage is (1 << (chainLog+2)).
+                              * Must be clamped between ZSTD_CHAINLOG_MIN and ZSTD_CHAINLOG_MAX.
+                              * Larger tables result in better and slower compression.
+                              * This parameter is useless for "fast" strategy.
+                              * It's still useful when using "dfast" strategy,
+                              * in which case it defines a secondary probe table.
+                              * Special: value 0 means "use default chainLog". */
+    ZSTD_c_searchLog=104,    /* Number of search attempts, as a power of 2.
+                              * More attempts result in better and slower compression.
+                              * This parameter is useless for "fast" and "dFast" strategies.
+                              * Special: value 0 means "use default searchLog". */
+    ZSTD_c_minMatch=105,     /* Minimum size of searched matches.
+                              * Note that Zstandard can still find matches of smaller size,
+                              * it just tweaks its search algorithm to look for this size and larger.
+                              * Larger values increase compression and decompression speed, but decrease ratio.
+                              * Must be clamped between ZSTD_MINMATCH_MIN and ZSTD_MINMATCH_MAX.
+                              * Note that currently, for all strategies < btopt, effective minimum is 4.
+                              *                    , for all strategies > fast, effective maximum is 6.
+                              * Special: value 0 means "use default minMatchLength". */
+    ZSTD_c_targetLength=106, /* Impact of this field depends on strategy.
+                              * For strategies btopt, btultra & btultra2:
+                              *     Length of Match considered "good enough" to stop search.
+                              *     Larger values make compression stronger, and slower.
+                              * For strategy fast:
+                              *     Distance between match sampling.
+                              *     Larger values make compression faster, and weaker.
+                              * Special: value 0 means "use default targetLength". */
+    ZSTD_c_strategy=107,     /* See ZSTD_strategy enum definition.
+                              * The higher the value of selected strategy, the more complex it is,
+                              * resulting in stronger and slower compression.
+                              * Special: value 0 means "use default strategy". */
+
+    /* LDM mode parameters */
+    ZSTD_c_enableLongDistanceMatching=160, /* Enable long distance matching.
+                                     * This parameter is designed to improve compression ratio
+                                     * for large inputs, by finding large matches at long distance.
+                                     * It increases memory usage and window size.
+                                     * Note: enabling this parameter increases default ZSTD_c_windowLog to 128 MB
+                                     * except when expressly set to a different value. */
+    ZSTD_c_ldmHashLog=161,   /* Size of the table for long distance matching, as a power of 2.
+                              * Larger values increase memory usage and compression ratio,
+                              * but decrease compression speed.
+                              * Must be clamped between ZSTD_HASHLOG_MIN and ZSTD_HASHLOG_MAX
+                              * default: windowlog - 7.
+                              * Special: value 0 means "automatically determine hashlog". */
+    ZSTD_c_ldmMinMatch=162,  /* Minimum match size for long distance matcher.
+                              * Larger/too small values usually decrease compression ratio.
+                              * Must be clamped between ZSTD_LDM_MINMATCH_MIN and ZSTD_LDM_MINMATCH_MAX.
+                              * Special: value 0 means "use default value" (default: 64). */
+    ZSTD_c_ldmBucketSizeLog=163, /* Log size of each bucket in the LDM hash table for collision resolution.
+                              * Larger values improve collision resolution but decrease compression speed.
+                              * The maximum value is ZSTD_LDM_BUCKETSIZELOG_MAX.
+                              * Special: value 0 means "use default value" (default: 3). */
+    ZSTD_c_ldmHashRateLog=164, /* Frequency of inserting/looking up entries into the LDM hash table.
+                              * Must be clamped between 0 and (ZSTD_WINDOWLOG_MAX - ZSTD_HASHLOG_MIN).
+                              * Default is MAX(0, (windowLog - ldmHashLog)), optimizing hash table usage.
+                              * Larger values improve compression speed.
+                              * Deviating far from default value will likely result in a compression ratio decrease.
+                              * Special: value 0 means "automatically determine hashRateLog". */
+
+    /* frame parameters */
+    ZSTD_c_contentSizeFlag=200, /* Content size will be written into frame header _whenever known_ (default:1)
+                              * Content size must be known at the beginning of compression.
+                              * This is automatically the case when using ZSTD_compress2(),
+                              * For streaming scenarios, content size must be provided with ZSTD_CCtx_setPledgedSrcSize() */
+    ZSTD_c_checksumFlag=201, /* A 32-bits checksum of content is written at end of frame (default:0) */
+    ZSTD_c_dictIDFlag=202,   /* When applicable, dictionary's ID is written into frame header (default:1) */
+
+    /* multi-threading parameters */
+    /* These parameters are only useful if multi-threading is enabled (compiled with build macro ZSTD_MULTITHREAD).
+     * They return an error otherwise. */
+    ZSTD_c_nbWorkers=400,    /* Select how many threads will be spawned to compress in parallel.
+                              * When nbWorkers >= 1, triggers asynchronous mode when used with ZSTD_compressStream*() :
+                              * ZSTD_compressStream*() consumes input and flush output if possible, but immediately gives back control to caller,
+                              * while compression work is performed in parallel, within worker threads.
+                              * (note : a strong exception to this rule is when first invocation of ZSTD_compressStream2() sets ZSTD_e_end :
+                              *  in which case, ZSTD_compressStream2() delegates to ZSTD_compress2(), which is always a blocking call).
+                              * More workers improve speed, but also increase memory usage.
+                              * Default value is `0`, aka "single-threaded mode" : no worker is spawned, compression is performed inside Caller's thread, all invocations are blocking */
+    ZSTD_c_jobSize=401,      /* Size of a compression job. This value is enforced only when nbWorkers >= 1.
+                              * Each compression job is completed in parallel, so this value can indirectly impact the nb of active threads.
+                              * 0 means default, which is dynamically determined based on compression parameters.
+                              * Job size must be a minimum of overlap size, or 1 MB, whichever is largest.
+                              * The minimum size is automatically and transparently enforced. */
+    ZSTD_c_overlapLog=402,   /* Control the overlap size, as a fraction of window size.
+                              * The overlap size is an amount of data reloaded from previous job at the beginning of a new job.
+                              * It helps preserve compression ratio, while each job is compressed in parallel.
+                              * This value is enforced only when nbWorkers >= 1.
+                              * Larger values increase compression ratio, but decrease speed.
+                              * Possible values range from 0 to 9 :
+                              * - 0 means "default" : value will be determined by the library, depending on strategy
+                              * - 1 means "no overlap"
+                              * - 9 means "full overlap", using a full window size.
+                              * Each intermediate rank increases/decreases load size by a factor 2 :
+                              * 9: full window;  8: w/2;  7: w/4;  6: w/8;  5:w/16;  4: w/32;  3:w/64;  2:w/128;  1:no overlap;  0:default
+                              * default value varies between 6 and 9, depending on strategy */
+
+    /* note : additional experimental parameters are also available
+     * within the experimental section of the API.
+     * At the time of this writing, they include :
+     * ZSTD_c_rsyncable
+     * ZSTD_c_format
+     * ZSTD_c_forceMaxWindow
+     * ZSTD_c_forceAttachDict
+     * ZSTD_c_literalCompressionMode
+     * ZSTD_c_targetCBlockSize
+     * ZSTD_c_srcSizeHint
+     * Because they are not stable, it's necessary to define ZSTD_STATIC_LINKING_ONLY to access them.
+     * note : never ever use experimentalParam? names directly;
+     *        also, the enums values themselves are unstable and can still change.
+     */
+     ZSTD_c_experimentalParam1=500,
+     ZSTD_c_experimentalParam2=10,
+     ZSTD_c_experimentalParam3=1000,
+     ZSTD_c_experimentalParam4=1001,
+     ZSTD_c_experimentalParam5=1002,
+     ZSTD_c_experimentalParam6=1003,
+     ZSTD_c_experimentalParam7=1004
+} ZSTD_cParameter;
+


+
typedef struct {
+    size_t error;
+    int lowerBound;
+    int upperBound;
+} ZSTD_bounds;
+


+
ZSTD_bounds ZSTD_cParam_getBounds(ZSTD_cParameter cParam);
+
  All parameters must belong to an interval with lower and upper bounds,
+  otherwise they will either trigger an error or be automatically clamped.
+ @return : a structure, ZSTD_bounds, which contains
+         - an error status field, which must be tested using ZSTD_isError()
+         - lower and upper bounds, both inclusive
+ 
+


+
+
size_t ZSTD_CCtx_setParameter(ZSTD_CCtx* cctx, ZSTD_cParameter param, int value);
+
  Set one compression parameter, selected by enum ZSTD_cParameter.
+  All parameters have valid bounds. Bounds can be queried using ZSTD_cParam_getBounds().
+  Providing a value beyond bound will either clamp it, or trigger an error (depending on parameter).
+  Setting a parameter is generally only possible during frame initialization (before starting compression).
+  Exception : when using multi-threading mode (nbWorkers >= 1),
+              the following parameters can be updated _during_ compression (within same frame):
+              => compressionLevel, hashLog, chainLog, searchLog, minMatch, targetLength and strategy.
+              new parameters will be active for next job only (after a flush()).
+ @return : an error code (which can be tested using ZSTD_isError()).
+ 
+


+
+
size_t ZSTD_CCtx_setPledgedSrcSize(ZSTD_CCtx* cctx, unsigned long long pledgedSrcSize);
+
  Total input data size to be compressed as a single frame.
+  Value will be written in frame header, unless if explicitly forbidden using ZSTD_c_contentSizeFlag.
+  This value will also be controlled at end of frame, and trigger an error if not respected.
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+  Note 1 : pledgedSrcSize==0 actually means zero, aka an empty frame.
+           In order to mean "unknown content size", pass constant ZSTD_CONTENTSIZE_UNKNOWN.
+           ZSTD_CONTENTSIZE_UNKNOWN is default value for any new frame.
+  Note 2 : pledgedSrcSize is only valid once, for the next frame.
+           It's discarded at the end of the frame, and replaced by ZSTD_CONTENTSIZE_UNKNOWN.
+  Note 3 : Whenever all input data is provided and consumed in a single round,
+           for example with ZSTD_compress2(),
+           or invoking immediately ZSTD_compressStream2(,,,ZSTD_e_end),
+           this value is automatically overridden by srcSize instead.
+ 
+


+
+
typedef enum {
+    ZSTD_reset_session_only = 1,
+    ZSTD_reset_parameters = 2,
+    ZSTD_reset_session_and_parameters = 3
+} ZSTD_ResetDirective;
+


+
size_t ZSTD_CCtx_reset(ZSTD_CCtx* cctx, ZSTD_ResetDirective reset);
+
  There are 2 different things that can be reset, independently or jointly :
+  - The session : will stop compressing current frame, and make CCtx ready to start a new one.
+                  Useful after an error, or to interrupt any ongoing compression.
+                  Any internal data not yet flushed is cancelled.
+                  Compression parameters and dictionary remain unchanged.
+                  They will be used to compress next frame.
+                  Resetting session never fails.
+  - The parameters : changes all parameters back to "default".
+                  This removes any reference to any dictionary too.
+                  Parameters can only be changed between 2 sessions (i.e. no compression is currently ongoing)
+                  otherwise the reset fails, and function returns an error value (which can be tested using ZSTD_isError())
+  - Both : similar to resetting the session, followed by resetting parameters.
+ 
+


+
+
size_t ZSTD_compress2( ZSTD_CCtx* cctx,
+                       void* dst, size_t dstCapacity,
+                 const void* src, size_t srcSize);
+
  Behave the same as ZSTD_compressCCtx(), but compression parameters are set using the advanced API.
+  ZSTD_compress2() always starts a new frame.
+  Should cctx hold data from a previously unfinished frame, everything about it is forgotten.
+  - Compression parameters are pushed into CCtx before starting compression, using ZSTD_CCtx_set*()
+  - The function is always blocking, returns when compression is completed.
+  Hint : compression runs faster if `dstCapacity` >=  `ZSTD_compressBound(srcSize)`.
+ @return : compressed size written into `dst` (<= `dstCapacity),
+           or an error code if it fails (which can be tested using ZSTD_isError()).
+ 
+


+
+
Advanced decompression API

+
+
typedef enum {
+
+    ZSTD_d_windowLogMax=100, /* Select a size limit (in power of 2) beyond which
+                              * the streaming API will refuse to allocate memory buffer
+                              * in order to protect the host from unreasonable memory requirements.
+                              * This parameter is only useful in streaming mode, since no internal buffer is allocated in single-pass mode.
+                              * By default, a decompression context accepts window sizes <= (1 << ZSTD_WINDOWLOG_LIMIT_DEFAULT).
+                              * Special: value 0 means "use default maximum windowLog". */
+
+    /* note : additional experimental parameters are also available
+     * within the experimental section of the API.
+     * At the time of this writing, they include :
+     * ZSTD_d_format
+     * ZSTD_d_stableOutBuffer
+     * Because they are not stable, it's necessary to define ZSTD_STATIC_LINKING_ONLY to access them.
+     * note : never ever use experimentalParam? names directly
+     */
+     ZSTD_d_experimentalParam1=1000,
+     ZSTD_d_experimentalParam2=1001
+
+} ZSTD_dParameter;
+


+
ZSTD_bounds ZSTD_dParam_getBounds(ZSTD_dParameter dParam);
+
  All parameters must belong to an interval with lower and upper bounds,
+  otherwise they will either trigger an error or be automatically clamped.
+ @return : a structure, ZSTD_bounds, which contains
+         - an error status field, which must be tested using ZSTD_isError()
+         - both lower and upper bounds, inclusive
+ 
+


+
+
size_t ZSTD_DCtx_setParameter(ZSTD_DCtx* dctx, ZSTD_dParameter param, int value);
+
  Set one compression parameter, selected by enum ZSTD_dParameter.
+  All parameters have valid bounds. Bounds can be queried using ZSTD_dParam_getBounds().
+  Providing a value beyond bound will either clamp it, or trigger an error (depending on parameter).
+  Setting a parameter is only possible during frame initialization (before starting decompression).
+ @return : 0, or an error code (which can be tested using ZSTD_isError()).
+ 
+


+
+
size_t ZSTD_DCtx_reset(ZSTD_DCtx* dctx, ZSTD_ResetDirective reset);
+
  Return a DCtx to clean state.
+  Session and parameters can be reset jointly or separately.
+  Parameters can only be reset when no active frame is being decompressed.
+ @return : 0, or an error code, which can be tested with ZSTD_isError()
+ 
+


+
+
Streaming

+
+
typedef struct ZSTD_inBuffer_s {
+  const void* src;    /**< start of input buffer */
+  size_t size;        /**< size of input buffer */
+  size_t pos;         /**< position where reading stopped. Will be updated. Necessarily 0 <= pos <= size */
+} ZSTD_inBuffer;
+


+
typedef struct ZSTD_outBuffer_s {
+  void*  dst;         /**< start of output buffer */
+  size_t size;        /**< size of output buffer */
+  size_t pos;         /**< position where writing stopped. Will be updated. Necessarily 0 <= pos <= size */
+} ZSTD_outBuffer;
+


+
Streaming compression - HowTo
+  A ZSTD_CStream object is required to track streaming operation.
+  Use ZSTD_createCStream() and ZSTD_freeCStream() to create/release resources.
+  ZSTD_CStream objects can be reused multiple times on consecutive compression operations.
+  It is recommended to re-use ZSTD_CStream since it will play nicer with system's memory, by re-using already allocated memory.
+
+  For parallel execution, use one separate ZSTD_CStream per thread.
+
+  note : since v1.3.0, ZSTD_CStream and ZSTD_CCtx are the same thing.
+
+  Parameters are sticky : when starting a new compression on the same context,
+  it will re-use the same sticky parameters as previous compression session.
+  When in doubt, it's recommended to fully initialize the context before usage.
+  Use ZSTD_CCtx_reset() to reset the context and ZSTD_CCtx_setParameter(),
+  ZSTD_CCtx_setPledgedSrcSize(), or ZSTD_CCtx_loadDictionary() and friends to
+  set more specific parameters, the pledged source size, or load a dictionary.
+
+  Use ZSTD_compressStream2() with ZSTD_e_continue as many times as necessary to
+  consume input stream. The function will automatically update both `pos`
+  fields within `input` and `output`.
+  Note that the function may not consume the entire input, for example, because
+  the output buffer is already full, in which case `input.pos < input.size`.
+  The caller must check if input has been entirely consumed.
+  If not, the caller must make some room to receive more compressed data,
+  and then present again remaining input data.
+  note: ZSTD_e_continue is guaranteed to make some forward progress when called,
+        but doesn't guarantee maximal forward progress. This is especially relevant
+        when compressing with multiple threads. The call won't block if it can
+        consume some input, but if it can't it will wait for some, but not all,
+        output to be flushed.
+ @return : provides a minimum amount of data remaining to be flushed from internal buffers
+           or an error code, which can be tested using ZSTD_isError().
+
+  At any moment, it's possible to flush whatever data might remain stuck within internal buffer,
+  using ZSTD_compressStream2() with ZSTD_e_flush. `output->pos` will be updated.
+  Note that, if `output->size` is too small, a single invocation with ZSTD_e_flush might not be enough (return code > 0).
+  In which case, make some room to receive more compressed data, and call again ZSTD_compressStream2() with ZSTD_e_flush.
+  You must continue calling ZSTD_compressStream2() with ZSTD_e_flush until it returns 0, at which point you can change the
+  operation.
+  note: ZSTD_e_flush will flush as much output as possible, meaning when compressing with multiple threads, it will
+        block until the flush is complete or the output buffer is full.
+  @return : 0 if internal buffers are entirely flushed,
+            >0 if some data still present within internal buffer (the value is minimal estimation of remaining size),
+            or an error code, which can be tested using ZSTD_isError().
+
+  Calling ZSTD_compressStream2() with ZSTD_e_end instructs to finish a frame.
+  It will perform a flush and write frame epilogue.
+  The epilogue is required for decoders to consider a frame completed.
+  flush operation is the same, and follows same rules as calling ZSTD_compressStream2() with ZSTD_e_flush.
+  You must continue calling ZSTD_compressStream2() with ZSTD_e_end until it returns 0, at which point you are free to
+  start a new frame.
+  note: ZSTD_e_end will flush as much output as possible, meaning when compressing with multiple threads, it will
+        block until the flush is complete or the output buffer is full.
+  @return : 0 if frame fully completed and fully flushed,
+            >0 if some data still present within internal buffer (the value is minimal estimation of remaining size),
+            or an error code, which can be tested using ZSTD_isError().
+
+ 
+

+
+
typedef ZSTD_CCtx ZSTD_CStream;  /**< CCtx and CStream are now effectively same object (>= v1.3.0) */
+


+
ZSTD_CStream management functions
ZSTD_CStream* ZSTD_createCStream(void);
+size_t ZSTD_freeCStream(ZSTD_CStream* zcs);
+


+
Streaming compression functions
typedef enum {
+    ZSTD_e_continue=0, /* collect more data, encoder decides when to output compressed result, for optimal compression ratio */
+    ZSTD_e_flush=1,    /* flush any data provided so far,
+                        * it creates (at least) one new block, that can be decoded immediately on reception;
+                        * frame will continue: any future data can still reference previously compressed data, improving compression.
+                        * note : multithreaded compression will block to flush as much output as possible. */
+    ZSTD_e_end=2       /* flush any remaining data _and_ close current frame.
+                        * note that frame is only closed after compressed data is fully flushed (return value == 0).
+                        * After that point, any additional data starts a new frame.
+                        * note : each frame is independent (does not reference any content from previous frame).
+                        : note : multithreaded compression will block to flush as much output as possible. */
+} ZSTD_EndDirective;
+


+
size_t ZSTD_compressStream2( ZSTD_CCtx* cctx,
+                             ZSTD_outBuffer* output,
+                             ZSTD_inBuffer* input,
+                             ZSTD_EndDirective endOp);
+
  Behaves about the same as ZSTD_compressStream, with additional control on end directive.
+  - Compression parameters are pushed into CCtx before starting compression, using ZSTD_CCtx_set*()
+  - Compression parameters cannot be changed once compression is started (save a list of exceptions in multi-threading mode)
+  - output->pos must be <= dstCapacity, input->pos must be <= srcSize
+  - output->pos and input->pos will be updated. They are guaranteed to remain below their respective limit.
+  - When nbWorkers==0 (default), function is blocking : it completes its job before returning to caller.
+  - When nbWorkers>=1, function is non-blocking : it just acquires a copy of input, and distributes jobs to internal worker threads, flush whatever is available,
+                                                  and then immediately returns, just indicating that there is some data remaining to be flushed.
+                                                  The function nonetheless guarantees forward progress : it will return only after it reads or write at least 1+ byte.
+  - Exception : if the first call requests a ZSTD_e_end directive and provides enough dstCapacity, the function delegates to ZSTD_compress2() which is always blocking.
+  - @return provides a minimum amount of data remaining to be flushed from internal buffers
+            or an error code, which can be tested using ZSTD_isError().
+            if @return != 0, flush is not fully completed, there is still some data left within internal buffers.
+            This is useful for ZSTD_e_flush, since in this case more flushes are necessary to empty all buffers.
+            For ZSTD_e_end, @return == 0 when internal buffers are fully flushed and frame is completed.
+  - after a ZSTD_e_end directive, if internal buffer is not fully flushed (@return != 0),
+            only ZSTD_e_end or ZSTD_e_flush operations are allowed.
+            Before starting a new compression job, or changing compression parameters,
+            it is required to fully flush internal buffers.
+ 
+


+
+
size_t ZSTD_CStreamInSize(void);    /**< recommended size for input buffer */
+


+
size_t ZSTD_CStreamOutSize(void);   /**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block. */
+


+
size_t ZSTD_initCStream(ZSTD_CStream* zcs, int compressionLevel);
+/*!
+ * Alternative for ZSTD_compressStream2(zcs, output, input, ZSTD_e_continue).
+ * NOTE: The return value is different. ZSTD_compressStream() returns a hint for
+ * the next read size (if non-zero and not an error). ZSTD_compressStream2()
+ * returns the minimum nb of bytes left to flush (if non-zero and not an error).
+ */
+size_t ZSTD_compressStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output, ZSTD_inBuffer* input);
+/*! Equivalent to ZSTD_compressStream2(zcs, output, &emptyInput, ZSTD_e_flush). */
+size_t ZSTD_flushStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output);
+/*! Equivalent to ZSTD_compressStream2(zcs, output, &emptyInput, ZSTD_e_end). */
+size_t ZSTD_endStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output);
+

+     ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
+     ZSTD_CCtx_refCDict(zcs, NULL); // clear the dictionary (if any)
+     ZSTD_CCtx_setParameter(zcs, ZSTD_c_compressionLevel, compressionLevel);
+ 
+


+
+
Streaming decompression - HowTo
+  A ZSTD_DStream object is required to track streaming operations.
+  Use ZSTD_createDStream() and ZSTD_freeDStream() to create/release resources.
+  ZSTD_DStream objects can be re-used multiple times.
+
+  Use ZSTD_initDStream() to start a new decompression operation.
+ @return : recommended first input size
+  Alternatively, use advanced API to set specific properties.
+
+  Use ZSTD_decompressStream() repetitively to consume your input.
+  The function will update both `pos` fields.
+  If `input.pos < input.size`, some input has not been consumed.
+  It's up to the caller to present again remaining data.
+  The function tries to flush all data decoded immediately, respecting output buffer size.
+  If `output.pos < output.size`, decoder has flushed everything it could.
+  But if `output.pos == output.size`, there might be some data left within internal buffers.,
+  In which case, call ZSTD_decompressStream() again to flush whatever remains in the buffer.
+  Note : with no additional input provided, amount of data flushed is necessarily <= ZSTD_BLOCKSIZE_MAX.
+ @return : 0 when a frame is completely decoded and fully flushed,
+        or an error code, which can be tested using ZSTD_isError(),
+        or any other value > 0, which means there is still some decoding or flushing to do to complete current frame :
+                                the return value is a suggested next input size (just a hint for better latency)
+                                that will never request more than the remaining frame size.
+ 
+

+
+
typedef ZSTD_DCtx ZSTD_DStream;  /**< DCtx and DStream are now effectively same object (>= v1.3.0) */
+


+
ZSTD_DStream management functions
ZSTD_DStream* ZSTD_createDStream(void);
+size_t ZSTD_freeDStream(ZSTD_DStream* zds);
+


+
Streaming decompression functions


+
size_t ZSTD_DStreamInSize(void);    /*!< recommended size for input buffer */
+


+
size_t ZSTD_DStreamOutSize(void);   /*!< recommended size for output buffer. Guarantee to successfully flush at least one complete block in all circumstances. */
+


+
Simple dictionary API

+
+
size_t ZSTD_compress_usingDict(ZSTD_CCtx* ctx,
+                               void* dst, size_t dstCapacity,
+                         const void* src, size_t srcSize,
+                         const void* dict,size_t dictSize,
+                               int compressionLevel);
+
  Compression at an explicit compression level using a Dictionary.
+  A dictionary can be any arbitrary data segment (also called a prefix),
+  or a buffer with specified information (see dictBuilder/zdict.h).
+  Note : This function loads the dictionary, resulting in significant startup delay.
+         It's intended for a dictionary used only once.
+  Note 2 : When `dict == NULL || dictSize < 8` no dictionary is used. 
+


+
+
size_t ZSTD_decompress_usingDict(ZSTD_DCtx* dctx,
+                                 void* dst, size_t dstCapacity,
+                           const void* src, size_t srcSize,
+                           const void* dict,size_t dictSize);
+
  Decompression using a known Dictionary.
+  Dictionary must be identical to the one used during compression.
+  Note : This function loads the dictionary, resulting in significant startup delay.
+         It's intended for a dictionary used only once.
+  Note : When `dict == NULL || dictSize < 8` no dictionary is used. 
+


+
+
Bulk processing dictionary API

+
+
ZSTD_CDict* ZSTD_createCDict(const void* dictBuffer, size_t dictSize,
+                             int compressionLevel);
+
  When compressing multiple messages or blocks using the same dictionary,
+  it's recommended to digest the dictionary only once, since it's a costly operation.
+  ZSTD_createCDict() will create a state from digesting a dictionary.
+  The resulting state can be used for future compression operations with very limited startup cost.
+  ZSTD_CDict can be created once and shared by multiple threads concurrently, since its usage is read-only.
+ @dictBuffer can be released after ZSTD_CDict creation, because its content is copied within CDict.
+  Note 1 : Consider experimental function `ZSTD_createCDict_byReference()` if you prefer to not duplicate @dictBuffer content.
+  Note 2 : A ZSTD_CDict can be created from an empty @dictBuffer,
+      in which case the only thing that it transports is the @compressionLevel.
+      This can be useful in a pipeline featuring ZSTD_compress_usingCDict() exclusively,
+      expecting a ZSTD_CDict parameter with any data, including those without a known dictionary. 
+


+
+
size_t      ZSTD_freeCDict(ZSTD_CDict* CDict);
+
  Function frees memory allocated by ZSTD_createCDict(). 
+


+
+
size_t ZSTD_compress_usingCDict(ZSTD_CCtx* cctx,
+                                void* dst, size_t dstCapacity,
+                          const void* src, size_t srcSize,
+                          const ZSTD_CDict* cdict);
+
  Compression using a digested Dictionary.
+  Recommended when same dictionary is used multiple times.
+  Note : compression level is _decided at dictionary creation time_,
+     and frame parameters are hardcoded (dictID=yes, contentSize=yes, checksum=no) 
+


+
+
ZSTD_DDict* ZSTD_createDDict(const void* dictBuffer, size_t dictSize);
+
  Create a digested dictionary, ready to start decompression operation without startup delay.
+  dictBuffer can be released after DDict creation, as its content is copied inside DDict. 
+


+
+
size_t      ZSTD_freeDDict(ZSTD_DDict* ddict);
+
  Function frees memory allocated with ZSTD_createDDict() 
+


+
+
size_t ZSTD_decompress_usingDDict(ZSTD_DCtx* dctx,
+                                  void* dst, size_t dstCapacity,
+                            const void* src, size_t srcSize,
+                            const ZSTD_DDict* ddict);
+
  Decompression using a digested Dictionary.
+  Recommended when same dictionary is used multiple times. 
+


+
+
Dictionary helper functions

+
+
unsigned ZSTD_getDictID_fromDict(const void* dict, size_t dictSize);
+
  Provides the dictID stored within dictionary.
+  if @return == 0, the dictionary is not conformant with Zstandard specification.
+  It can still be loaded, but as a content-only dictionary. 
+


+
+
unsigned ZSTD_getDictID_fromDDict(const ZSTD_DDict* ddict);
+
  Provides the dictID of the dictionary loaded into `ddict`.
+  If @return == 0, the dictionary is not conformant to Zstandard specification, or empty.
+  Non-conformant dictionaries can still be loaded, but as content-only dictionaries. 
+


+
+
unsigned ZSTD_getDictID_fromFrame(const void* src, size_t srcSize);
+
  Provides the dictID required to decompressed the frame stored within `src`.
+  If @return == 0, the dictID could not be decoded.
+  This could for one of the following reasons :
+  - The frame does not require a dictionary to be decoded (most common case).
+  - The frame was built with dictID intentionally removed. Whatever dictionary is necessary is a hidden information.
+    Note : this use case also happens when using a non-conformant dictionary.
+  - `srcSize` is too small, and as a result, the frame header could not be decoded (only possible if `srcSize < ZSTD_FRAMEHEADERSIZE_MAX`).
+  - This is not a Zstandard frame.
+  When identifying the exact failure cause, it's possible to use ZSTD_getFrameHeader(), which will provide a more precise error code. 
+


+
+
Advanced dictionary and prefix API
+ This API allows dictionaries to be used with ZSTD_compress2(),
+ ZSTD_compressStream2(), and ZSTD_decompress(). Dictionaries are sticky, and
+ only reset with the context is reset with ZSTD_reset_parameters or
+ ZSTD_reset_session_and_parameters. Prefixes are single-use.
+

+
+
size_t ZSTD_CCtx_loadDictionary(ZSTD_CCtx* cctx, const void* dict, size_t dictSize);
+
  Create an internal CDict from `dict` buffer.
+  Decompression will have to use same dictionary.
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+  Special: Loading a NULL (or 0-size) dictionary invalidates previous dictionary,
+           meaning "return to no-dictionary mode".
+  Note 1 : Dictionary is sticky, it will be used for all future compressed frames.
+           To return to "no-dictionary" situation, load a NULL dictionary (or reset parameters).
+  Note 2 : Loading a dictionary involves building tables.
+           It's also a CPU consuming operation, with non-negligible impact on latency.
+           Tables are dependent on compression parameters, and for this reason,
+           compression parameters can no longer be changed after loading a dictionary.
+  Note 3 :`dict` content will be copied internally.
+           Use experimental ZSTD_CCtx_loadDictionary_byReference() to reference content instead.
+           In such a case, dictionary buffer must outlive its users.
+  Note 4 : Use ZSTD_CCtx_loadDictionary_advanced()
+           to precisely select how dictionary content must be interpreted. 
+


+
+
size_t ZSTD_CCtx_refCDict(ZSTD_CCtx* cctx, const ZSTD_CDict* cdict);
+
  Reference a prepared dictionary, to be used for all next compressed frames.
+  Note that compression parameters are enforced from within CDict,
+  and supersede any compression parameter previously set within CCtx.
+  The parameters ignored are labled as "superseded-by-cdict" in the ZSTD_cParameter enum docs.
+  The ignored parameters will be used again if the CCtx is returned to no-dictionary mode.
+  The dictionary will remain valid for future compressed frames using same CCtx.
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+  Special : Referencing a NULL CDict means "return to no-dictionary mode".
+  Note 1 : Currently, only one dictionary can be managed.
+           Referencing a new dictionary effectively "discards" any previous one.
+  Note 2 : CDict is just referenced, its lifetime must outlive its usage within CCtx. 
+


+
+
size_t ZSTD_CCtx_refPrefix(ZSTD_CCtx* cctx,
+                     const void* prefix, size_t prefixSize);
+
  Reference a prefix (single-usage dictionary) for next compressed frame.
+  A prefix is **only used once**. Tables are discarded at end of frame (ZSTD_e_end).
+  Decompression will need same prefix to properly regenerate data.
+  Compressing with a prefix is similar in outcome as performing a diff and compressing it,
+  but performs much faster, especially during decompression (compression speed is tunable with compression level).
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+  Special: Adding any prefix (including NULL) invalidates any previous prefix or dictionary
+  Note 1 : Prefix buffer is referenced. It **must** outlive compression.
+           Its content must remain unmodified during compression.
+  Note 2 : If the intention is to diff some large src data blob with some prior version of itself,
+           ensure that the window size is large enough to contain the entire source.
+           See ZSTD_c_windowLog.
+  Note 3 : Referencing a prefix involves building tables, which are dependent on compression parameters.
+           It's a CPU consuming operation, with non-negligible impact on latency.
+           If there is a need to use the same prefix multiple times, consider loadDictionary instead.
+  Note 4 : By default, the prefix is interpreted as raw content (ZSTD_dct_rawContent).
+           Use experimental ZSTD_CCtx_refPrefix_advanced() to alter dictionary interpretation. 
+


+
+
size_t ZSTD_DCtx_loadDictionary(ZSTD_DCtx* dctx, const void* dict, size_t dictSize);
+
  Create an internal DDict from dict buffer,
+  to be used to decompress next frames.
+  The dictionary remains valid for all future frames, until explicitly invalidated.
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+  Special : Adding a NULL (or 0-size) dictionary invalidates any previous dictionary,
+            meaning "return to no-dictionary mode".
+  Note 1 : Loading a dictionary involves building tables,
+           which has a non-negligible impact on CPU usage and latency.
+           It's recommended to "load once, use many times", to amortize the cost
+  Note 2 :`dict` content will be copied internally, so `dict` can be released after loading.
+           Use ZSTD_DCtx_loadDictionary_byReference() to reference dictionary content instead.
+  Note 3 : Use ZSTD_DCtx_loadDictionary_advanced() to take control of
+           how dictionary content is loaded and interpreted.
+ 
+


+
+
size_t ZSTD_DCtx_refDDict(ZSTD_DCtx* dctx, const ZSTD_DDict* ddict);
+
  Reference a prepared dictionary, to be used to decompress next frames.
+  The dictionary remains active for decompression of future frames using same DCtx.
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+  Note 1 : Currently, only one dictionary can be managed.
+           Referencing a new dictionary effectively "discards" any previous one.
+  Special: referencing a NULL DDict means "return to no-dictionary mode".
+  Note 2 : DDict is just referenced, its lifetime must outlive its usage from DCtx.
+ 
+


+
+
size_t ZSTD_DCtx_refPrefix(ZSTD_DCtx* dctx,
+                     const void* prefix, size_t prefixSize);
+
  Reference a prefix (single-usage dictionary) to decompress next frame.
+  This is the reverse operation of ZSTD_CCtx_refPrefix(),
+  and must use the same prefix as the one used during compression.
+  Prefix is **only used once**. Reference is discarded at end of frame.
+  End of frame is reached when ZSTD_decompressStream() returns 0.
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+  Note 1 : Adding any prefix (including NULL) invalidates any previously set prefix or dictionary
+  Note 2 : Prefix buffer is referenced. It **must** outlive decompression.
+           Prefix buffer must remain unmodified up to the end of frame,
+           reached when ZSTD_decompressStream() returns 0.
+  Note 3 : By default, the prefix is treated as raw content (ZSTD_dct_rawContent).
+           Use ZSTD_CCtx_refPrefix_advanced() to alter dictMode (Experimental section)
+  Note 4 : Referencing a raw content prefix has almost no cpu nor memory cost.
+           A full dictionary is more costly, as it requires building tables.
+ 
+


+
+
size_t ZSTD_sizeof_CCtx(const ZSTD_CCtx* cctx);
+size_t ZSTD_sizeof_DCtx(const ZSTD_DCtx* dctx);
+size_t ZSTD_sizeof_CStream(const ZSTD_CStream* zcs);
+size_t ZSTD_sizeof_DStream(const ZSTD_DStream* zds);
+size_t ZSTD_sizeof_CDict(const ZSTD_CDict* cdict);
+size_t ZSTD_sizeof_DDict(const ZSTD_DDict* ddict);
+
  These functions give the _current_ memory usage of selected object.
+  Note that object memory usage can evolve (increase or decrease) over time. 
+


+
+
experimental API (static linking only)
+ The following symbols and constants
+ are not planned to join "stable API" status in the near future.
+ They can still change in future versions.
+ Some of them are planned to remain in the static_only section indefinitely.
+ Some of them might be removed in the future (especially when redundant with existing stable functions)
+ 
+

+
+
typedef struct {
+    unsigned int matchPos; /* Match pos in dst */
+    /* If seqDef.offset > 3, then this is seqDef.offset - 3
+     * If seqDef.offset < 3, then this is the corresponding repeat offset
+     * But if seqDef.offset < 3 and litLength == 0, this is the
+     *   repeat offset before the corresponding repeat offset
+     * And if seqDef.offset == 3 and litLength == 0, this is the
+     *   most recent repeat offset - 1
+     */
+    unsigned int offset;
+    unsigned int litLength; /* Literal length */
+    unsigned int matchLength; /* Match length */
+    /* 0 when seq not rep and seqDef.offset otherwise
+     * when litLength == 0 this will be <= 4, otherwise <= 3 like normal
+     */
+    unsigned int rep;
+} ZSTD_Sequence;
+


+
typedef struct {
+    unsigned windowLog;       /**< largest match distance : larger == more compression, more memory needed during decompression */
+    unsigned chainLog;        /**< fully searched segment : larger == more compression, slower, more memory (useless for fast) */
+    unsigned hashLog;         /**< dispatch table : larger == faster, more memory */
+    unsigned searchLog;       /**< nb of searches : larger == more compression, slower */
+    unsigned minMatch;        /**< match length searched : larger == faster decompression, sometimes less compression */
+    unsigned targetLength;    /**< acceptable match size for optimal parser (only) : larger == more compression, slower */
+    ZSTD_strategy strategy;   /**< see ZSTD_strategy definition above */
+} ZSTD_compressionParameters;
+


+
typedef struct {
+    int contentSizeFlag; /**< 1: content size will be in frame header (when known) */
+    int checksumFlag;    /**< 1: generate a 32-bits checksum using XXH64 algorithm at end of frame, for error detection */
+    int noDictIDFlag;    /**< 1: no dictID will be saved into frame header (dictID is only useful for dictionary compression) */
+} ZSTD_frameParameters;
+


+
typedef struct {
+    ZSTD_compressionParameters cParams;
+    ZSTD_frameParameters fParams;
+} ZSTD_parameters;
+


+
typedef enum {
+    ZSTD_dct_auto = 0,       /* dictionary is "full" when starting with ZSTD_MAGIC_DICTIONARY, otherwise it is "rawContent" */
+    ZSTD_dct_rawContent = 1, /* ensures dictionary is always loaded as rawContent, even if it starts with ZSTD_MAGIC_DICTIONARY */
+    ZSTD_dct_fullDict = 2    /* refuses to load a dictionary if it does not respect Zstandard's specification, starting with ZSTD_MAGIC_DICTIONARY */
+} ZSTD_dictContentType_e;
+


+
typedef enum {
+    ZSTD_dlm_byCopy = 0,  /**< Copy dictionary content internally */
+    ZSTD_dlm_byRef = 1    /**< Reference dictionary content -- the dictionary buffer must outlive its users. */
+} ZSTD_dictLoadMethod_e;
+


+
typedef enum {
+    ZSTD_f_zstd1 = 0,           /* zstd frame format, specified in zstd_compression_format.md (default) */
+    ZSTD_f_zstd1_magicless = 1  /* Variant of zstd frame format, without initial 4-bytes magic number.
+                                 * Useful to save 4 bytes per generated frame.
+                                 * Decoder cannot recognise automatically this format, requiring this instruction. */
+} ZSTD_format_e;
+


+
typedef enum {
+    /* Note: this enum and the behavior it controls are effectively internal
+     * implementation details of the compressor. They are expected to continue
+     * to evolve and should be considered only in the context of extremely
+     * advanced performance tuning.
+     *
+     * Zstd currently supports the use of a CDict in three ways:
+     *
+     * - The contents of the CDict can be copied into the working context. This
+     *   means that the compression can search both the dictionary and input
+     *   while operating on a single set of internal tables. This makes
+     *   the compression faster per-byte of input. However, the initial copy of
+     *   the CDict's tables incurs a fixed cost at the beginning of the
+     *   compression. For small compressions (< 8 KB), that copy can dominate
+     *   the cost of the compression.
+     *
+     * - The CDict's tables can be used in-place. In this model, compression is
+     *   slower per input byte, because the compressor has to search two sets of
+     *   tables. However, this model incurs no start-up cost (as long as the
+     *   working context's tables can be reused). For small inputs, this can be
+     *   faster than copying the CDict's tables.
+     *
+     * - The CDict's tables are not used at all, and instead we use the working
+     *   context alone to reload the dictionary and use params based on the source
+     *   size. See ZSTD_compress_insertDictionary() and ZSTD_compress_usingDict().
+     *   This method is effective when the dictionary sizes are very small relative
+     *   to the input size, and the input size is fairly large to begin with.
+     *
+     * Zstd has a simple internal heuristic that selects which strategy to use
+     * at the beginning of a compression. However, if experimentation shows that
+     * Zstd is making poor choices, it is possible to override that choice with
+     * this enum.
+     */
+    ZSTD_dictDefaultAttach = 0, /* Use the default heuristic. */
+    ZSTD_dictForceAttach   = 1, /* Never copy the dictionary. */
+    ZSTD_dictForceCopy     = 2, /* Always copy the dictionary. */
+    ZSTD_dictForceLoad     = 3  /* Always reload the dictionary */
+} ZSTD_dictAttachPref_e;
+


+
typedef enum {
+  ZSTD_lcm_auto = 0,          /**< Automatically determine the compression mode based on the compression level.
+                               *   Negative compression levels will be uncompressed, and positive compression
+                               *   levels will be compressed. */
+  ZSTD_lcm_huffman = 1,       /**< Always attempt Huffman compression. Uncompressed literals will still be
+                               *   emitted if Huffman compression is not profitable. */
+  ZSTD_lcm_uncompressed = 2   /**< Always emit uncompressed literals. */
+} ZSTD_literalCompressionMode_e;
+


+
Frame size functions

+
+
unsigned long long ZSTD_findDecompressedSize(const void* src, size_t srcSize);
+
  `src` should point to the start of a series of ZSTD encoded and/or skippable frames
+  `srcSize` must be the _exact_ size of this series
+       (i.e. there should be a frame boundary at `src + srcSize`)
+  @return : - decompressed size of all data in all successive frames
+            - if the decompressed size cannot be determined: ZSTD_CONTENTSIZE_UNKNOWN
+            - if an error occurred: ZSTD_CONTENTSIZE_ERROR
+
+   note 1 : decompressed size is an optional field, that may not be present, especially in streaming mode.
+            When `return==ZSTD_CONTENTSIZE_UNKNOWN`, data to decompress could be any size.
+            In which case, it's necessary to use streaming mode to decompress data.
+   note 2 : decompressed size is always present when compression is done with ZSTD_compress()
+   note 3 : decompressed size can be very large (64-bits value),
+            potentially larger than what local system can handle as a single memory segment.
+            In which case, it's necessary to use streaming mode to decompress data.
+   note 4 : If source is untrusted, decompressed size could be wrong or intentionally modified.
+            Always ensure result fits within application's authorized limits.
+            Each application can set its own limits.
+   note 5 : ZSTD_findDecompressedSize handles multiple frames, and so it must traverse the input to
+            read each contained frame header.  This is fast as most of the data is skipped,
+            however it does mean that all frame data must be present and valid. 
+


+
+
unsigned long long ZSTD_decompressBound(const void* src, size_t srcSize);
+
  `src` should point to the start of a series of ZSTD encoded and/or skippable frames
+  `srcSize` must be the _exact_ size of this series
+       (i.e. there should be a frame boundary at `src + srcSize`)
+  @return : - upper-bound for the decompressed size of all data in all successive frames
+            - if an error occured: ZSTD_CONTENTSIZE_ERROR
+
+  note 1  : an error can occur if `src` contains an invalid or incorrectly formatted frame.
+  note 2  : the upper-bound is exact when the decompressed size field is available in every ZSTD encoded frame of `src`.
+            in this case, `ZSTD_findDecompressedSize` and `ZSTD_decompressBound` return the same value.
+  note 3  : when the decompressed size field isn't available, the upper-bound for that frame is calculated by:
+              upper-bound = # blocks * min(128 KB, Window_Size)
+ 
+


+
+
size_t ZSTD_frameHeaderSize(const void* src, size_t srcSize);
+
  srcSize must be >= ZSTD_FRAMEHEADERSIZE_PREFIX.
+ @return : size of the Frame Header,
+           or an error code (if srcSize is too small) 
+


+
+
size_t ZSTD_getSequences(ZSTD_CCtx* zc, ZSTD_Sequence* outSeqs,
+    size_t outSeqsSize, const void* src, size_t srcSize);
+
 Extract sequences from the sequence store
+ zc can be used to insert custom compression params.
+ This function invokes ZSTD_compress2
+ @return : number of sequences extracted
+ 
+


+
+
Memory management

+
+
size_t ZSTD_estimateCCtxSize(int compressionLevel);
+size_t ZSTD_estimateCCtxSize_usingCParams(ZSTD_compressionParameters cParams);
+size_t ZSTD_estimateCCtxSize_usingCCtxParams(const ZSTD_CCtx_params* params);
+size_t ZSTD_estimateDCtxSize(void);
+
  These functions make it possible to estimate memory usage
+  of a future {D,C}Ctx, before its creation.
+
+  ZSTD_estimateCCtxSize() will provide a memory budget large enough
+  for any compression level up to selected one.
+  Note : Unlike ZSTD_estimateCStreamSize*(), this estimate
+         does not include space for a window buffer.
+         Therefore, the estimation is only guaranteed for single-shot compressions, not streaming.
+  The estimate will assume the input may be arbitrarily large,
+  which is the worst case.
+
+  When srcSize can be bound by a known and rather "small" value,
+  this fact can be used to provide a tighter estimation
+  because the CCtx compression context will need less memory.
+  This tighter estimation can be provided by more advanced functions
+  ZSTD_estimateCCtxSize_usingCParams(), which can be used in tandem with ZSTD_getCParams(),
+  and ZSTD_estimateCCtxSize_usingCCtxParams(), which can be used in tandem with ZSTD_CCtxParams_setParameter().
+  Both can be used to estimate memory using custom compression parameters and arbitrary srcSize limits.
+
+  Note 2 : only single-threaded compression is supported.
+  ZSTD_estimateCCtxSize_usingCCtxParams() will return an error code if ZSTD_c_nbWorkers is >= 1.
+ 
+


+
+
size_t ZSTD_estimateCStreamSize(int compressionLevel);
+size_t ZSTD_estimateCStreamSize_usingCParams(ZSTD_compressionParameters cParams);
+size_t ZSTD_estimateCStreamSize_usingCCtxParams(const ZSTD_CCtx_params* params);
+size_t ZSTD_estimateDStreamSize(size_t windowSize);
+size_t ZSTD_estimateDStreamSize_fromFrame(const void* src, size_t srcSize);
+
  ZSTD_estimateCStreamSize() will provide a budget large enough for any compression level up to selected one.
+  It will also consider src size to be arbitrarily "large", which is worst case.
+  If srcSize is known to always be small, ZSTD_estimateCStreamSize_usingCParams() can provide a tighter estimation.
+  ZSTD_estimateCStreamSize_usingCParams() can be used in tandem with ZSTD_getCParams() to create cParams from compressionLevel.
+  ZSTD_estimateCStreamSize_usingCCtxParams() can be used in tandem with ZSTD_CCtxParams_setParameter(). Only single-threaded compression is supported. This function will return an error code if ZSTD_c_nbWorkers is >= 1.
+  Note : CStream size estimation is only correct for single-threaded compression.
+  ZSTD_DStream memory budget depends on window Size.
+  This information can be passed manually, using ZSTD_estimateDStreamSize,
+  or deducted from a valid frame Header, using ZSTD_estimateDStreamSize_fromFrame();
+  Note : if streaming is init with function ZSTD_init?Stream_usingDict(),
+         an internal ?Dict will be created, which additional size is not estimated here.
+         In this case, get total size by adding ZSTD_estimate?DictSize 
+


+
+
size_t ZSTD_estimateCDictSize(size_t dictSize, int compressionLevel);
+size_t ZSTD_estimateCDictSize_advanced(size_t dictSize, ZSTD_compressionParameters cParams, ZSTD_dictLoadMethod_e dictLoadMethod);
+size_t ZSTD_estimateDDictSize(size_t dictSize, ZSTD_dictLoadMethod_e dictLoadMethod);
+
  ZSTD_estimateCDictSize() will bet that src size is relatively "small", and content is copied, like ZSTD_createCDict().
+  ZSTD_estimateCDictSize_advanced() makes it possible to control compression parameters precisely, like ZSTD_createCDict_advanced().
+  Note : dictionaries created by reference (`ZSTD_dlm_byRef`) are logically smaller.
+ 
+


+
+
ZSTD_CCtx*    ZSTD_initStaticCCtx(void* workspace, size_t workspaceSize);
+ZSTD_CStream* ZSTD_initStaticCStream(void* workspace, size_t workspaceSize);    /**< same as ZSTD_initStaticCCtx() */
+
  Initialize an object using a pre-allocated fixed-size buffer.
+  workspace: The memory area to emplace the object into.
+             Provided pointer *must be 8-bytes aligned*.
+             Buffer must outlive object.
+  workspaceSize: Use ZSTD_estimate*Size() to determine
+                 how large workspace must be to support target scenario.
+ @return : pointer to object (same address as workspace, just different type),
+           or NULL if error (size too small, incorrect alignment, etc.)
+  Note : zstd will never resize nor malloc() when using a static buffer.
+         If the object requires more memory than available,
+         zstd will just error out (typically ZSTD_error_memory_allocation).
+  Note 2 : there is no corresponding "free" function.
+           Since workspace is allocated externally, it must be freed externally too.
+  Note 3 : cParams : use ZSTD_getCParams() to convert a compression level
+           into its associated cParams.
+  Limitation 1 : currently not compatible with internal dictionary creation, triggered by
+                 ZSTD_CCtx_loadDictionary(), ZSTD_initCStream_usingDict() or ZSTD_initDStream_usingDict().
+  Limitation 2 : static cctx currently not compatible with multi-threading.
+  Limitation 3 : static dctx is incompatible with legacy support.
+ 
+


+
+
ZSTD_DStream* ZSTD_initStaticDStream(void* workspace, size_t workspaceSize);    /**< same as ZSTD_initStaticDCtx() */
+


+
typedef void* (*ZSTD_allocFunction) (void* opaque, size_t size);
+typedef void  (*ZSTD_freeFunction) (void* opaque, void* address);
+typedef struct { ZSTD_allocFunction customAlloc; ZSTD_freeFunction customFree; void* opaque; } ZSTD_customMem;
+static ZSTD_customMem const ZSTD_defaultCMem = { NULL, NULL, NULL };  /**< this constant defers to stdlib's functions */
+
  These prototypes make it possible to pass your own allocation/free functions.
+  ZSTD_customMem is provided at creation time, using ZSTD_create*_advanced() variants listed below.
+  All allocation/free operations will be completed using these custom variants instead of regular  ones.
+ 
+


+
+
Advanced compression functions

+
+
ZSTD_CDict* ZSTD_createCDict_byReference(const void* dictBuffer, size_t dictSize, int compressionLevel);
+
  Create a digested dictionary for compression
+  Dictionary content is just referenced, not duplicated.
+  As a consequence, `dictBuffer` **must** outlive CDict,
+  and its content must remain unmodified throughout the lifetime of CDict.
+  note: equivalent to ZSTD_createCDict_advanced(), with dictLoadMethod==ZSTD_dlm_byRef 
+


+
+
ZSTD_compressionParameters ZSTD_getCParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
+
 @return ZSTD_compressionParameters structure for a selected compression level and estimated srcSize.
+ `estimatedSrcSize` value is optional, select 0 if not known 
+


+
+
ZSTD_parameters ZSTD_getParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
+
  same as ZSTD_getCParams(), but @return a full `ZSTD_parameters` object instead of sub-component `ZSTD_compressionParameters`.
+  All fields of `ZSTD_frameParameters` are set to default : contentSize=1, checksum=0, noDictID=0 
+


+
+
size_t ZSTD_checkCParams(ZSTD_compressionParameters params);
+
  Ensure param values remain within authorized range.
+ @return 0 on success, or an error code (can be checked with ZSTD_isError()) 
+


+
+
ZSTD_compressionParameters ZSTD_adjustCParams(ZSTD_compressionParameters cPar, unsigned long long srcSize, size_t dictSize);
+
  optimize params for a given `srcSize` and `dictSize`.
+ `srcSize` can be unknown, in which case use ZSTD_CONTENTSIZE_UNKNOWN.
+ `dictSize` must be `0` when there is no dictionary.
+  cPar can be invalid : all parameters will be clamped within valid range in the @return struct.
+  This function never fails (wide contract) 
+


+
+
size_t ZSTD_compress_advanced(ZSTD_CCtx* cctx,
+                              void* dst, size_t dstCapacity,
+                        const void* src, size_t srcSize,
+                        const void* dict,size_t dictSize,
+                              ZSTD_parameters params);
+
  Note : this function is now DEPRECATED.
+         It can be replaced by ZSTD_compress2(), in combination with ZSTD_CCtx_setParameter() and other parameter setters.
+  This prototype will be marked as deprecated and generate compilation warning on reaching v1.5.x 
+


+
+
size_t ZSTD_compress_usingCDict_advanced(ZSTD_CCtx* cctx,
+                                  void* dst, size_t dstCapacity,
+                            const void* src, size_t srcSize,
+                            const ZSTD_CDict* cdict,
+                                  ZSTD_frameParameters fParams);
+
  Note : this function is now REDUNDANT.
+         It can be replaced by ZSTD_compress2(), in combination with ZSTD_CCtx_loadDictionary() and other parameter setters.
+  This prototype will be marked as deprecated and generate compilation warning in some future version 
+


+
+
size_t ZSTD_CCtx_loadDictionary_byReference(ZSTD_CCtx* cctx, const void* dict, size_t dictSize);
+
  Same as ZSTD_CCtx_loadDictionary(), but dictionary content is referenced, instead of being copied into CCtx.
+  It saves some memory, but also requires that `dict` outlives its usage within `cctx` 
+


+
+
size_t ZSTD_CCtx_loadDictionary_advanced(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, ZSTD_dictLoadMethod_e dictLoadMethod, ZSTD_dictContentType_e dictContentType);
+
  Same as ZSTD_CCtx_loadDictionary(), but gives finer control over
+  how to load the dictionary (by copy ? by reference ?)
+  and how to interpret it (automatic ? force raw mode ? full mode only ?) 
+


+
+
size_t ZSTD_CCtx_refPrefix_advanced(ZSTD_CCtx* cctx, const void* prefix, size_t prefixSize, ZSTD_dictContentType_e dictContentType);
+
  Same as ZSTD_CCtx_refPrefix(), but gives finer control over
+  how to interpret prefix content (automatic ? force raw mode (default) ? full mode only ?) 
+


+
+
size_t ZSTD_CCtx_getParameter(ZSTD_CCtx* cctx, ZSTD_cParameter param, int* value);
+
  Get the requested compression parameter value, selected by enum ZSTD_cParameter,
+  and store it into int* value.
+ @return : 0, or an error code (which can be tested with ZSTD_isError()).
+ 
+


+
+
ZSTD_CCtx_params* ZSTD_createCCtxParams(void);
+size_t ZSTD_freeCCtxParams(ZSTD_CCtx_params* params);
+
  Quick howto :
+  - ZSTD_createCCtxParams() : Create a ZSTD_CCtx_params structure
+  - ZSTD_CCtxParams_setParameter() : Push parameters one by one into
+                                     an existing ZSTD_CCtx_params structure.
+                                     This is similar to
+                                     ZSTD_CCtx_setParameter().
+  - ZSTD_CCtx_setParametersUsingCCtxParams() : Apply parameters to
+                                    an existing CCtx.
+                                    These parameters will be applied to
+                                    all subsequent frames.
+  - ZSTD_compressStream2() : Do compression using the CCtx.
+  - ZSTD_freeCCtxParams() : Free the memory.
+
+  This can be used with ZSTD_estimateCCtxSize_advanced_usingCCtxParams()
+  for static allocation of CCtx for single-threaded compression.
+ 
+


+
+
size_t ZSTD_CCtxParams_reset(ZSTD_CCtx_params* params);
+
  Reset params to default values.
+ 
+


+
+
size_t ZSTD_CCtxParams_init(ZSTD_CCtx_params* cctxParams, int compressionLevel);
+
  Initializes the compression parameters of cctxParams according to
+  compression level. All other parameters are reset to their default values.
+ 
+


+
+
size_t ZSTD_CCtxParams_init_advanced(ZSTD_CCtx_params* cctxParams, ZSTD_parameters params);
+
  Initializes the compression and frame parameters of cctxParams according to
+  params. All other parameters are reset to their default values.
+ 
+


+
+
size_t ZSTD_CCtxParams_setParameter(ZSTD_CCtx_params* params, ZSTD_cParameter param, int value);
+
  Similar to ZSTD_CCtx_setParameter.
+  Set one compression parameter, selected by enum ZSTD_cParameter.
+  Parameters must be applied to a ZSTD_CCtx using ZSTD_CCtx_setParametersUsingCCtxParams().
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+ 
+


+
+
size_t ZSTD_CCtxParams_getParameter(ZSTD_CCtx_params* params, ZSTD_cParameter param, int* value);
+
 Similar to ZSTD_CCtx_getParameter.
+ Get the requested value of one compression parameter, selected by enum ZSTD_cParameter.
+ @result : 0, or an error code (which can be tested with ZSTD_isError()).
+ 
+


+
+
size_t ZSTD_CCtx_setParametersUsingCCtxParams(
+        ZSTD_CCtx* cctx, const ZSTD_CCtx_params* params);
+
  Apply a set of ZSTD_CCtx_params to the compression context.
+  This can be done even after compression is started,
+    if nbWorkers==0, this will have no impact until a new compression is started.
+    if nbWorkers>=1, new parameters will be picked up at next job,
+       with a few restrictions (windowLog, pledgedSrcSize, nbWorkers, jobSize, and overlapLog are not updated).
+ 
+


+
+
size_t ZSTD_compressStream2_simpleArgs (
+                ZSTD_CCtx* cctx,
+                void* dst, size_t dstCapacity, size_t* dstPos,
+          const void* src, size_t srcSize, size_t* srcPos,
+                ZSTD_EndDirective endOp);
+
  Same as ZSTD_compressStream2(),
+  but using only integral types as arguments.
+  This variant might be helpful for binders from dynamic languages
+  which have troubles handling structures containing memory pointers.
+ 
+


+
+
Advanced decompression functions

+
+
unsigned ZSTD_isFrame(const void* buffer, size_t size);
+
  Tells if the content of `buffer` starts with a valid Frame Identifier.
+  Note : Frame Identifier is 4 bytes. If `size < 4`, @return will always be 0.
+  Note 2 : Legacy Frame Identifiers are considered valid only if Legacy Support is enabled.
+  Note 3 : Skippable Frame Identifiers are considered valid. 
+


+
+
ZSTD_DDict* ZSTD_createDDict_byReference(const void* dictBuffer, size_t dictSize);
+
  Create a digested dictionary, ready to start decompression operation without startup delay.
+  Dictionary content is referenced, and therefore stays in dictBuffer.
+  It is important that dictBuffer outlives DDict,
+  it must remain read accessible throughout the lifetime of DDict 
+


+
+
size_t ZSTD_DCtx_loadDictionary_byReference(ZSTD_DCtx* dctx, const void* dict, size_t dictSize);
+
  Same as ZSTD_DCtx_loadDictionary(),
+  but references `dict` content instead of copying it into `dctx`.
+  This saves memory if `dict` remains around.,
+  However, it's imperative that `dict` remains accessible (and unmodified) while being used, so it must outlive decompression. 
+


+
+
size_t ZSTD_DCtx_loadDictionary_advanced(ZSTD_DCtx* dctx, const void* dict, size_t dictSize, ZSTD_dictLoadMethod_e dictLoadMethod, ZSTD_dictContentType_e dictContentType);
+
  Same as ZSTD_DCtx_loadDictionary(),
+  but gives direct control over
+  how to load the dictionary (by copy ? by reference ?)
+  and how to interpret it (automatic ? force raw mode ? full mode only ?). 
+


+
+
size_t ZSTD_DCtx_refPrefix_advanced(ZSTD_DCtx* dctx, const void* prefix, size_t prefixSize, ZSTD_dictContentType_e dictContentType);
+
  Same as ZSTD_DCtx_refPrefix(), but gives finer control over
+  how to interpret prefix content (automatic ? force raw mode (default) ? full mode only ?) 
+


+
+
size_t ZSTD_DCtx_setMaxWindowSize(ZSTD_DCtx* dctx, size_t maxWindowSize);
+
  Refuses allocating internal buffers for frames requiring a window size larger than provided limit.
+  This protects a decoder context from reserving too much memory for itself (potential attack scenario).
+  This parameter is only useful in streaming mode, since no internal buffer is allocated in single-pass mode.
+  By default, a decompression context accepts all window sizes <= (1 << ZSTD_WINDOWLOG_LIMIT_DEFAULT)
+ @return : 0, or an error code (which can be tested using ZSTD_isError()).
+ 
+


+
+
size_t ZSTD_DCtx_setFormat(ZSTD_DCtx* dctx, ZSTD_format_e format);
+
  Instruct the decoder context about what kind of data to decode next.
+  This instruction is mandatory to decode data without a fully-formed header,
+  such ZSTD_f_zstd1_magicless for example.
+ @return : 0, or an error code (which can be tested using ZSTD_isError()). 
+


+
+
size_t ZSTD_decompressStream_simpleArgs (
+                ZSTD_DCtx* dctx,
+                void* dst, size_t dstCapacity, size_t* dstPos,
+          const void* src, size_t srcSize, size_t* srcPos);
+
  Same as ZSTD_decompressStream(),
+  but using only integral types as arguments.
+  This can be helpful for binders from dynamic languages
+  which have troubles handling structures containing memory pointers.
+ 
+


+
+
Advanced streaming functions
  Warning : most of these functions are now redundant with the Advanced API.
+  Once Advanced API reaches "stable" status,
+  redundant functions will be deprecated, and then at some point removed.
+

+
+
Advanced Streaming compression functions
/**! ZSTD_initCStream_srcSize() :
+ * This function is deprecated, and equivalent to:
+ *     ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
+ *     ZSTD_CCtx_refCDict(zcs, NULL); // clear the dictionary (if any)
+ *     ZSTD_CCtx_setParameter(zcs, ZSTD_c_compressionLevel, compressionLevel);
+ *     ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize);
+ *
+ * pledgedSrcSize must be correct. If it is not known at init time, use
+ * ZSTD_CONTENTSIZE_UNKNOWN. Note that, for compatibility with older programs,
+ * "0" also disables frame content size field. It may be enabled in the future.
+ * Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ */
+size_t
+ZSTD_initCStream_srcSize(ZSTD_CStream* zcs,
+                         int compressionLevel,
+                         unsigned long long pledgedSrcSize);
+


+
! ZSTD_initCStream_usingDict() :
 This function is deprecated, and is equivalent to:
+     ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
+     ZSTD_CCtx_setParameter(zcs, ZSTD_c_compressionLevel, compressionLevel);
+     ZSTD_CCtx_loadDictionary(zcs, dict, dictSize);
+
+ Creates of an internal CDict (incompatible with static CCtx), except if
+ dict == NULL or dictSize < 8, in which case no dict is used.
+ Note: dict is loaded with ZSTD_dct_auto (treated as a full zstd dictionary if
+ it begins with ZSTD_MAGIC_DICTIONARY, else as raw content) and ZSTD_dlm_byCopy.
+ Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ 
+

+
+
! ZSTD_initCStream_advanced() :
 This function is deprecated, and is approximately equivalent to:
+     ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
+     // Pseudocode: Set each zstd parameter and leave the rest as-is.
+     for ((param, value) : params) {
+         ZSTD_CCtx_setParameter(zcs, param, value);
+     }
+     ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize);
+     ZSTD_CCtx_loadDictionary(zcs, dict, dictSize);
+
+ dict is loaded with ZSTD_dct_auto and ZSTD_dlm_byCopy.
+ pledgedSrcSize must be correct.
+ If srcSize is not known at init time, use value ZSTD_CONTENTSIZE_UNKNOWN.
+ Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ 
+

+
+
! ZSTD_initCStream_usingCDict() :
 This function is deprecated, and equivalent to:
+     ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
+     ZSTD_CCtx_refCDict(zcs, cdict);
+
+ note : cdict will just be referenced, and must outlive compression session
+ Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ 
+

+
+
! ZSTD_initCStream_usingCDict_advanced() :
   This function is DEPRECATED, and is approximately equivalent to:
+     ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
+     // Pseudocode: Set each zstd frame parameter and leave the rest as-is.
+     for ((fParam, value) : fParams) {
+         ZSTD_CCtx_setParameter(zcs, fParam, value);
+     }
+     ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize);
+     ZSTD_CCtx_refCDict(zcs, cdict);
+
+ same as ZSTD_initCStream_usingCDict(), with control over frame parameters.
+ pledgedSrcSize must be correct. If srcSize is not known at init time, use
+ value ZSTD_CONTENTSIZE_UNKNOWN.
+ Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ 
+

+
+
size_t ZSTD_resetCStream(ZSTD_CStream* zcs, unsigned long long pledgedSrcSize);
+
 This function is deprecated, and is equivalent to:
+     ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only);
+     ZSTD_CCtx_setPledgedSrcSize(zcs, pledgedSrcSize);
+
+  start a new frame, using same parameters from previous frame.
+  This is typically useful to skip dictionary loading stage, since it will re-use it in-place.
+  Note that zcs must be init at least once before using ZSTD_resetCStream().
+  If pledgedSrcSize is not known at reset time, use macro ZSTD_CONTENTSIZE_UNKNOWN.
+  If pledgedSrcSize > 0, its value must be correct, as it will be written in header, and controlled at the end.
+  For the time being, pledgedSrcSize==0 is interpreted as "srcSize unknown" for compatibility with older programs,
+  but it will change to mean "empty" in future version, so use macro ZSTD_CONTENTSIZE_UNKNOWN instead.
+ @return : 0, or an error code (which can be tested using ZSTD_isError())
+  Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ 
+


+
+
typedef struct {
+    unsigned long long ingested;   /* nb input bytes read and buffered */
+    unsigned long long consumed;   /* nb input bytes actually compressed */
+    unsigned long long produced;   /* nb of compressed bytes generated and buffered */
+    unsigned long long flushed;    /* nb of compressed bytes flushed : not provided; can be tracked from caller side */
+    unsigned currentJobID;         /* MT only : latest started job nb */
+    unsigned nbActiveWorkers;      /* MT only : nb of workers actively compressing at probe time */
+} ZSTD_frameProgression;
+


+
size_t ZSTD_toFlushNow(ZSTD_CCtx* cctx);
+
  Tell how many bytes are ready to be flushed immediately.
+  Useful for multithreading scenarios (nbWorkers >= 1).
+  Probe the oldest active job, defined as oldest job not yet entirely flushed,
+  and check its output buffer.
+ @return : amount of data stored in oldest job and ready to be flushed immediately.
+  if @return == 0, it means either :
+  + there is no active job (could be checked with ZSTD_frameProgression()), or
+  + oldest job is still actively compressing data,
+    but everything it has produced has also been flushed so far,
+    therefore flush speed is limited by production speed of oldest job
+    irrespective of the speed of concurrent (and newer) jobs.
+ 
+


+
+
Advanced Streaming decompression functions
/**
+ * This function is deprecated, and is equivalent to:
+ *
+ *     ZSTD_DCtx_reset(zds, ZSTD_reset_session_only);
+ *     ZSTD_DCtx_loadDictionary(zds, dict, dictSize);
+ *
+ * note: no dictionary will be used if dict == NULL or dictSize < 8
+ * Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ */
+size_t ZSTD_initDStream_usingDict(ZSTD_DStream* zds, const void* dict, size_t dictSize);
+


+
This function is deprecated, and is equivalent to:
+     ZSTD_DCtx_reset(zds, ZSTD_reset_session_only);
+     ZSTD_DCtx_refDDict(zds, ddict);
+
+ note : ddict is referenced, it must outlive decompression session
+ Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ 
+

+
+
This function is deprecated, and is equivalent to:
+     ZSTD_DCtx_reset(zds, ZSTD_reset_session_only);
+
+ re-use decompression parameters from previous init; saves dictionary loading
+ Note : this prototype will be marked as deprecated and generate compilation warnings on reaching v1.5.x
+ 
+

+
+
Buffer-less and synchronous inner streaming functions
+  This is an advanced API, giving full control over buffer management, for users which need direct control over memory.
+  But it's also a complex one, with several restrictions, documented below.
+  Prefer normal streaming API for an easier experience.
+ 
+

+
+
Buffer-less streaming compression (synchronous mode)
+  A ZSTD_CCtx object is required to track streaming operations.
+  Use ZSTD_createCCtx() / ZSTD_freeCCtx() to manage resource.
+  ZSTD_CCtx object can be re-used multiple times within successive compression operations.
+
+  Start by initializing a context.
+  Use ZSTD_compressBegin(), or ZSTD_compressBegin_usingDict() for dictionary compression,
+  or ZSTD_compressBegin_advanced(), for finer parameter control.
+  It's also possible to duplicate a reference context which has already been initialized, using ZSTD_copyCCtx()
+
+  Then, consume your input using ZSTD_compressContinue().
+  There are some important considerations to keep in mind when using this advanced function :
+  - ZSTD_compressContinue() has no internal buffer. It uses externally provided buffers only.
+  - Interface is synchronous : input is consumed entirely and produces 1+ compressed blocks.
+  - Caller must ensure there is enough space in `dst` to store compressed data under worst case scenario.
+    Worst case evaluation is provided by ZSTD_compressBound().
+    ZSTD_compressContinue() doesn't guarantee recover after a failed compression.
+  - ZSTD_compressContinue() presumes prior input ***is still accessible and unmodified*** (up to maximum distance size, see WindowLog).
+    It remembers all previous contiguous blocks, plus one separated memory segment (which can itself consists of multiple contiguous blocks)
+  - ZSTD_compressContinue() detects that prior input has been overwritten when `src` buffer overlaps.
+    In which case, it will "discard" the relevant memory section from its history.
+
+  Finish a frame with ZSTD_compressEnd(), which will write the last block(s) and optional checksum.
+  It's possible to use srcSize==0, in which case, it will write a final empty block to end the frame.
+  Without last block mark, frames are considered unfinished (hence corrupted) by compliant decoders.
+
+  `ZSTD_CCtx` object can be re-used (ZSTD_compressBegin()) to compress again.
+

+
+
Buffer-less streaming compression functions
size_t ZSTD_compressBegin(ZSTD_CCtx* cctx, int compressionLevel);
+size_t ZSTD_compressBegin_usingDict(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, int compressionLevel);
+size_t ZSTD_compressBegin_advanced(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, ZSTD_parameters params, unsigned long long pledgedSrcSize); /**< pledgedSrcSize : If srcSize is not known at init time, use ZSTD_CONTENTSIZE_UNKNOWN */
+size_t ZSTD_compressBegin_usingCDict(ZSTD_CCtx* cctx, const ZSTD_CDict* cdict); /**< note: fails if cdict==NULL */
+size_t ZSTD_compressBegin_usingCDict_advanced(ZSTD_CCtx* const cctx, const ZSTD_CDict* const cdict, ZSTD_frameParameters const fParams, unsigned long long const pledgedSrcSize);   /* compression parameters are already set within cdict. pledgedSrcSize must be correct. If srcSize is not known, use macro ZSTD_CONTENTSIZE_UNKNOWN */
+size_t ZSTD_copyCCtx(ZSTD_CCtx* cctx, const ZSTD_CCtx* preparedCCtx, unsigned long long pledgedSrcSize); /**<  note: if pledgedSrcSize is not known, use ZSTD_CONTENTSIZE_UNKNOWN */
+


+
Buffer-less streaming decompression (synchronous mode)
+  A ZSTD_DCtx object is required to track streaming operations.
+  Use ZSTD_createDCtx() / ZSTD_freeDCtx() to manage it.
+  A ZSTD_DCtx object can be re-used multiple times.
+
+  First typical operation is to retrieve frame parameters, using ZSTD_getFrameHeader().
+  Frame header is extracted from the beginning of compressed frame, so providing only the frame's beginning is enough.
+  Data fragment must be large enough to ensure successful decoding.
+ `ZSTD_frameHeaderSize_max` bytes is guaranteed to always be large enough.
+  @result : 0 : successful decoding, the `ZSTD_frameHeader` structure is correctly filled.
+           >0 : `srcSize` is too small, please provide at least @result bytes on next attempt.
+           errorCode, which can be tested using ZSTD_isError().
+
+  It fills a ZSTD_frameHeader structure with important information to correctly decode the frame,
+  such as the dictionary ID, content size, or maximum back-reference distance (`windowSize`).
+  Note that these values could be wrong, either because of data corruption, or because a 3rd party deliberately spoofs false information.
+  As a consequence, check that values remain within valid application range.
+  For example, do not allocate memory blindly, check that `windowSize` is within expectation.
+  Each application can set its own limits, depending on local restrictions.
+  For extended interoperability, it is recommended to support `windowSize` of at least 8 MB.
+
+  ZSTD_decompressContinue() needs previous data blocks during decompression, up to `windowSize` bytes.
+  ZSTD_decompressContinue() is very sensitive to contiguity,
+  if 2 blocks don't follow each other, make sure that either the compressor breaks contiguity at the same place,
+  or that previous contiguous segment is large enough to properly handle maximum back-reference distance.
+  There are multiple ways to guarantee this condition.
+
+  The most memory efficient way is to use a round buffer of sufficient size.
+  Sufficient size is determined by invoking ZSTD_decodingBufferSize_min(),
+  which can @return an error code if required value is too large for current system (in 32-bits mode).
+  In a round buffer methodology, ZSTD_decompressContinue() decompresses each block next to previous one,
+  up to the moment there is not enough room left in the buffer to guarantee decoding another full block,
+  which maximum size is provided in `ZSTD_frameHeader` structure, field `blockSizeMax`.
+  At which point, decoding can resume from the beginning of the buffer.
+  Note that already decoded data stored in the buffer should be flushed before being overwritten.
+
+  There are alternatives possible, for example using two or more buffers of size `windowSize` each, though they consume more memory.
+
+  Finally, if you control the compression process, you can also ignore all buffer size rules,
+  as long as the encoder and decoder progress in "lock-step",
+  aka use exactly the same buffer sizes, break contiguity at the same place, etc.
+
+  Once buffers are setup, start decompression, with ZSTD_decompressBegin().
+  If decompression requires a dictionary, use ZSTD_decompressBegin_usingDict() or ZSTD_decompressBegin_usingDDict().
+
+  Then use ZSTD_nextSrcSizeToDecompress() and ZSTD_decompressContinue() alternatively.
+  ZSTD_nextSrcSizeToDecompress() tells how many bytes to provide as 'srcSize' to ZSTD_decompressContinue().
+  ZSTD_decompressContinue() requires this _exact_ amount of bytes, or it will fail.
+
+ @result of ZSTD_decompressContinue() is the number of bytes regenerated within 'dst' (necessarily <= dstCapacity).
+  It can be zero : it just means ZSTD_decompressContinue() has decoded some metadata item.
+  It can also be an error code, which can be tested with ZSTD_isError().
+
+  A frame is fully decoded when ZSTD_nextSrcSizeToDecompress() returns zero.
+  Context can then be reset to start a new decompression.
+
+  Note : it's possible to know if next input to present is a header or a block, using ZSTD_nextInputType().
+  This information is not required to properly decode a frame.
+
+  == Special case : skippable frames 
+
+  Skippable frames allow integration of user-defined data into a flow of concatenated frames.
+  Skippable frames will be ignored (skipped) by decompressor.
+  The format of skippable frames is as follows :
+  a) Skippable frame ID - 4 Bytes, Little endian format, any value from 0x184D2A50 to 0x184D2A5F
+  b) Frame Size - 4 Bytes, Little endian format, unsigned 32-bits
+  c) Frame Content - any content (User Data) of length equal to Frame Size
+  For skippable frames ZSTD_getFrameHeader() returns zfhPtr->frameType==ZSTD_skippableFrame.
+  For skippable frames ZSTD_decompressContinue() always returns 0 : it only skips the content.
+

+
+
Buffer-less streaming decompression functions
typedef enum { ZSTD_frame, ZSTD_skippableFrame } ZSTD_frameType_e;
+typedef struct {
+    unsigned long long frameContentSize; /* if == ZSTD_CONTENTSIZE_UNKNOWN, it means this field is not available. 0 means "empty" */
+    unsigned long long windowSize;       /* can be very large, up to <= frameContentSize */
+    unsigned blockSizeMax;
+    ZSTD_frameType_e frameType;          /* if == ZSTD_skippableFrame, frameContentSize is the size of skippable content */
+    unsigned headerSize;
+    unsigned dictID;
+    unsigned checksumFlag;
+} ZSTD_frameHeader;
+


+
size_t ZSTD_getFrameHeader(ZSTD_frameHeader* zfhPtr, const void* src, size_t srcSize);   /**< doesn't consume input */
+/*! ZSTD_getFrameHeader_advanced() :
+ *  same as ZSTD_getFrameHeader(),
+ *  with added capability to select a format (like ZSTD_f_zstd1_magicless) */
+size_t ZSTD_getFrameHeader_advanced(ZSTD_frameHeader* zfhPtr, const void* src, size_t srcSize, ZSTD_format_e format);
+size_t ZSTD_decodingBufferSize_min(unsigned long long windowSize, unsigned long long frameContentSize);  /**< when frame content size is not known, pass in frameContentSize == ZSTD_CONTENTSIZE_UNKNOWN */
+
  decode Frame Header, or requires larger `srcSize`.
+ @return : 0, `zfhPtr` is correctly filled,
+          >0, `srcSize` is too small, value is wanted `srcSize` amount,
+           or an error code, which can be tested using ZSTD_isError() 
+


+
+
typedef enum { ZSTDnit_frameHeader, ZSTDnit_blockHeader, ZSTDnit_block, ZSTDnit_lastBlock, ZSTDnit_checksum, ZSTDnit_skippableFrame } ZSTD_nextInputType_e;
+


+
Block level API

+
+
    Frame metadata cost is typically ~12 bytes, which can be non-negligible for very small blocks (< 100 bytes).
+    But users will have to take in charge needed metadata to regenerate data, such as compressed and content sizes.
+
+    A few rules to respect :
+    - Compressing and decompressing require a context structure
+      + Use ZSTD_createCCtx() and ZSTD_createDCtx()
+    - It is necessary to init context before starting
+      + compression : any ZSTD_compressBegin*() variant, including with dictionary
+      + decompression : any ZSTD_decompressBegin*() variant, including with dictionary
+      + copyCCtx() and copyDCtx() can be used too
+    - Block size is limited, it must be <= ZSTD_getBlockSize() <= ZSTD_BLOCKSIZE_MAX == 128 KB
+      + If input is larger than a block size, it's necessary to split input data into multiple blocks
+      + For inputs larger than a single block, consider using regular ZSTD_compress() instead.
+        Frame metadata is not that costly, and quickly becomes negligible as source size grows larger than a block.
+    - When a block is considered not compressible enough, ZSTD_compressBlock() result will be 0 (zero) !
+      ===> In which case, nothing is produced into `dst` !
+      + User __must__ test for such outcome and deal directly with uncompressed data
+      + A block cannot be declared incompressible if ZSTD_compressBlock() return value was != 0.
+        Doing so would mess up with statistics history, leading to potential data corruption.
+      + ZSTD_decompressBlock() _doesn't accept uncompressed data as input_ !!
+      + In case of multiple successive blocks, should some of them be uncompressed,
+        decoder must be informed of their existence in order to follow proper history.
+        Use ZSTD_insertBlock() for such a case.
+


+
+
Raw zstd block functions
size_t ZSTD_getBlockSize   (const ZSTD_CCtx* cctx);
+size_t ZSTD_compressBlock  (ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize);
+size_t ZSTD_decompressBlock(ZSTD_DCtx* dctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize);
+size_t ZSTD_insertBlock    (ZSTD_DCtx* dctx, const void* blockStart, size_t blockSize);  /**< insert uncompressed block into `dctx` history. Useful for multi-blocks decompression. */
+


+
+
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