path: root/doc/lzip.info

This is lzip.info, produced by makeinfo version 4.13+ from lzip.texi.

INFO-DIR-SECTION Data Compression
START-INFO-DIR-ENTRY
* Lzip: (lzip).                 LZMA lossless data compressor
END-INFO-DIR-ENTRY


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Lzip Manual
***********

This manual is for Lzip (version 1.16, 26 August 2014).

* Menu:

* Introduction::           Purpose and features of lzip
* Algorithm::              How lzip compresses the data
* Invoking lzip::          Command line interface
* File format::            Detailed format of the compressed file
* Stream format::          Format of the LZMA stream in lzip files
* Examples::               A small tutorial with examples
* Problems::               Reporting bugs
* Reference source code::  Source code illustrating stream format
* Concept index::          Index of concepts


   Copyright (C) 2008-2014 Antonio Diaz Diaz.

   This manual is free documentation: you have unlimited permission to
copy, distribute and modify it.


File: lzip.info,  Node: Introduction,  Next: Algorithm,  Prev: Top,  Up: Top

1 Introduction
**************

Lzip is a lossless data compressor with a user interface similar to the
one of gzip or bzip2. Lzip is about as fast as gzip, compresses most
files more than bzip2, and is better than both from a data recovery
perspective. Lzip is a clean implementation of the LZMA
(Lempel-Ziv-Markov chain-Algorithm) "algorithm".

   The lzip file format is designed for long-term data archiving, taking
into account both data integrity and decoder availability:

   * The lzip format provides very safe integrity checking and some data
     recovery means. The lziprecover program can repair bit-flip errors
     (one of the most common forms of data corruption) in lzip files,
     and provides data recovery capabilities, including error-checked
     merging of damaged copies of a file.

   * The lzip format is as simple as possible (but not simpler). The
     lzip manual provides the code of a simple decompressor along with
     a detailed explanation of how it works, so that with the only help
     of the lzip manual it would be possible for a digital
     archaeologist to extract the data from a lzip file long after
     quantum computers eventually render LZMA obsolete.

   * Additionally lzip is copylefted, which guarantees that it will
     remain free forever.

   A nice feature of the lzip format is that a corrupt byte is easier to
repair the nearer it is from the beginning of the file. Therefore, with
the help of lziprecover, losing an entire archive just because of a
corrupt byte near the beginning is a thing of the past.

   The member trailer stores the 32-bit CRC of the original data, the
size of the original data and the size of the member. These values,
together with the value remaining in the range decoder and the
end-of-stream marker, provide a 4 factor integrity checking which
guarantees that the decompressed version of the data is identical to
the original. This guards against corruption of the compressed data,
and against undetected bugs in lzip (hopefully very unlikely). The
chances of data corruption going undetected are microscopic. Be aware,
though, that the check occurs upon decompression, so it can only tell
you that something is wrong. It can't help you recover the original
uncompressed data.

   Lzip uses the same well-defined exit status values used by bzip2,
which makes it safer than compressors returning ambiguous warning
values (like gzip) when it is used as a back end for other programs
like tar or zutils.

   The amount of memory required for compression is about 1 or 2 times
the dictionary size limit (1 if input file size is less than dictionary
size limit, else 2) plus 9 times the dictionary size really used. The
option '-0' is special and only requires about 1.5 MiB at most. The
amount of memory required for decompression is about 46 kB larger than
the dictionary size really used.

   Lzip will automatically use the smallest possible dictionary size for
each file without exceeding the given limit. Keep in mind that the
decompression memory requirement is affected at compression time by the
choice of dictionary size limit.

   When compressing, lzip replaces every file given in the command line
with a compressed version of itself, with the name "original_name.lz".
When decompressing, lzip attempts to guess the name for the decompressed
file from that of the compressed file as follows:

filename.lz    becomes   filename
filename.tlz   becomes   filename.tar
anyothername   becomes   anyothername.out

   (De)compressing a file is much like copying or moving it; therefore
lzip preserves the access and modification dates, permissions, and, when
possible, ownership of the file just as "cp -p" does. (If the user ID or
the group ID can't be duplicated, the file permission bits S_ISUID and
S_ISGID are cleared).

   Lzip is able to read from some types of non regular files if the
'--stdout' option is specified.

   If no file names are specified, lzip compresses (or decompresses)
from standard input to standard output. In this case, lzip will decline
to write compressed output to a terminal, as this would be entirely
incomprehensible and therefore pointless.

   Lzip will correctly decompress a file which is the concatenation of
two or more compressed files. The result is the concatenation of the
corresponding uncompressed files. Integrity testing of concatenated
compressed files is also supported.

   Lzip can produce multi-member files and safely recover, with
lziprecover, the undamaged members in case of file damage. Lzip can also
split the compressed output in volumes of a given size, even when
reading from standard input. This allows the direct creation of
multivolume compressed tar archives.

   Lzip is able to compress and decompress streams of unlimited size by
automatically creating multi-member output. The members so created are
large, about 64 PiB each.


File: lzip.info,  Node: Algorithm,  Next: Invoking lzip,  Prev: Introduction,  Up: Top

2 Algorithm
***********

There is no such thing as a "LZMA algorithm"; it is more like a "LZMA
coding scheme". For example, the option '-0' of lzip uses the scheme in
almost the simplest way possible; issuing the longest match it can find,
or a literal byte if it can't find a match. Inversely, a much more
elaborated way of finding coding sequences of minimum price than the one
currently used by lzip could be developed, and the resulting sequence
could also be coded using the LZMA coding scheme.

   Lzip currently implements two variants of the LZMA algorithm; fast
(used by option -0) and normal (used by all other compression levels).

   The high compression of LZMA comes from combining two basic,
well-proven compression ideas: sliding dictionaries (LZ77/78) and
markov models (the thing used by every compression algorithm that uses
a range encoder or similar order-0 entropy coder as its last stage)
with segregation of contexts according to what the bits are used for.

   Lzip is a two stage compressor. The first stage is a Lempel-Ziv
coder, which reduces redundancy by translating chunks of data to their
corresponding distance-length pairs. The second stage is a range encoder
that uses a different probability model for each type of data;
distances, lengths, literal bytes, etc.

   Here is how it works, step by step:

   1) The member header is written to the output stream.

   2) The first byte is coded literally, because there are no previous
bytes to which the match finder can refer to.

   3) The main encoder advances to the next byte in the input data and
calls the match finder.

   4) The match finder fills an array with the minimum distances before
the current byte where a match of a given length can be found.

   5) Go back to step 3 until a sequence (formed of pairs, repeated
distances and literal bytes) of minimum price has been formed. Where the
price represents the number of output bits produced.

   6) The range encoder encodes the sequence produced by the main
encoder and sends the produced bytes to the output stream.

   7) Go back to step 3 until the input data are finished or until the
member or volume size limits are reached.

   8) The range encoder is flushed.

   9) The member trailer is written to the output stream.

   10) If there are more data to compress, go back to step 1.


The ideas embodied in lzip are due to (at least) the following people:
Abraham Lempel and Jacob Ziv (for the LZ algorithm), Andrey Markov (for
the definition of Markov chains), G.N.N. Martin (for the definition of
range encoding), Igor Pavlov (for putting all the above together in
LZMA), and Julian Seward (for bzip2's CLI).


File: lzip.info,  Node: Invoking lzip,  Next: File format,  Prev: Algorithm,  Up: Top

3 Invoking lzip
***************

The format for running lzip is:

     lzip [OPTIONS] [FILES]

   Lzip supports the following options:

'-h'
'--help'
     Print an informative help message describing the options and exit.

'-V'
'--version'
     Print the version number of lzip on the standard output and exit.

'-b BYTES'
'--member-size=BYTES'
     Set the member size limit to BYTES. A small member size may
     degrade compression ratio, so use it only when needed. Valid values
     range from 100 kB to 64 PiB. Defaults to 64 PiB.

'-c'
'--stdout'
     Compress or decompress to standard output. Needed when reading
     from a named pipe (fifo) or from a device. Use it to recover as
     much of the uncompressed data as possible when decompressing a
     corrupt file.

'-d'
'--decompress'
     Decompress.

'-f'
'--force'
     Force overwrite of output files.

'-F'
'--recompress'
     Force recompression of files whose name already has the '.lz' or
     '.tlz' suffix.

'-k'
'--keep'
     Keep (don't delete) input files during compression or
     decompression.

'-m BYTES'
'--match-length=BYTES'
     Set the match length limit in bytes. After a match this long is
     found, the search is finished. Valid values range from 5 to 273.
     Larger values usually give better compression ratios but longer
     compression times.

'-o FILE'
'--output=FILE'
     When reading from standard input and '--stdout' has not been
     specified, use 'FILE' as the virtual name of the uncompressed
     file. This produces a file named 'FILE' when decompressing, a file
     named 'FILE.lz' when compressing, and several files named
     'FILE00001.lz', 'FILE00002.lz', etc, when compressing and
     splitting the output in volumes.

'-q'
'--quiet'
     Quiet operation. Suppress all messages.

'-s BYTES'
'--dictionary-size=BYTES'
     Set the dictionary size limit in bytes. Valid values range from 4
     KiB to 512 MiB. Lzip will use the smallest possible dictionary
     size for each file without exceeding this limit. Note that
     dictionary sizes are quantized. If the specified size does not
     match one of the valid sizes, it will be rounded upwards by adding
     up to (BYTES / 16) to it.

     For maximum compression you should use a dictionary size limit as
     large as possible, but keep in mind that the decompression memory
     requirement is affected at compression time by the choice of
     dictionary size limit.

'-S BYTES'
'--volume-size=BYTES'
     Split the compressed output into several volume files with names
     'original_name00001.lz', 'original_name00002.lz', etc, and set the
     volume size limit to BYTES. Each volume is a complete, maybe
     multi-member, lzip file. A small volume size may degrade
     compression ratio, so use it only when needed. Valid values range
     from 100 kB to 4 EiB.

'-t'
'--test'
     Check integrity of the specified file(s), but don't decompress
     them.  This really performs a trial decompression and throws away
     the result.  Use it together with '-v' to see information about
     the file.

'-v'
'--verbose'
     Verbose mode.
     When compressing, show the compression ratio for each file
     processed. A second '-v' shows the progress of compression.
     When decompressing or testing, further -v's (up to 4) increase the
     verbosity level, showing status, compression ratio, dictionary
     size, trailer contents (CRC, data size, member size), and up to 6
     bytes of trailing garbage (if any).

'-0 .. -9'
     Set the compression parameters (dictionary size and match length
     limit) as shown in the table below. Note that '-9' can be much
     slower than '-0'. These options have no effect when decompressing.

     The bidimensional parameter space of LZMA can't be mapped to a
     linear scale optimal for all files. If your files are large, very
     repetitive, etc, you may need to use the '--match-length' and
     '--dictionary-size' options directly to achieve optimal
     performance. For example, '-9m64' usually compresses executables
     more (and faster) than '-9'.

     Level   Dictionary size   Match length limit
     -0      64 KiB            16 bytes
     -1      1 MiB             5 bytes
     -2      1.5 MiB           6 bytes
     -3      2 MiB             8 bytes
     -4      3 MiB             12 bytes
     -5      4 MiB             20 bytes
     -6      8 MiB             36 bytes
     -7      16 MiB            68 bytes
     -8      24 MiB            132 bytes
     -9      32 MiB            273 bytes

'--fast'
'--best'
     Aliases for GNU gzip compatibility.


   Numbers given as arguments to options may be followed by a multiplier
and an optional 'B' for "byte".

   Table of SI and binary prefixes (unit multipliers):

Prefix   Value                     |   Prefix   Value
k        kilobyte  (10^3 = 1000)   |   Ki       kibibyte (2^10 = 1024)
M        megabyte  (10^6)          |   Mi       mebibyte (2^20)
G        gigabyte  (10^9)          |   Gi       gibibyte (2^30)
T        terabyte  (10^12)         |   Ti       tebibyte (2^40)
P        petabyte  (10^15)         |   Pi       pebibyte (2^50)
E        exabyte   (10^18)         |   Ei       exbibyte (2^60)
Z        zettabyte (10^21)         |   Zi       zebibyte (2^70)
Y        yottabyte (10^24)         |   Yi       yobibyte (2^80)


   Exit status: 0 for a normal exit, 1 for environmental problems (file
not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or
invalid input file, 3 for an internal consistency error (eg, bug) which
caused lzip to panic.


File: lzip.info,  Node: File format,  Next: Stream format,  Prev: Invoking lzip,  Up: Top

4 File format
*************

Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away.
-- Antoine de Saint-Exupery


   In the diagram below, a box like this:
+---+
|   | <-- the vertical bars might be missing
+---+

   represents one byte; a box like this:
+==============+
|              |
+==============+

   represents a variable number of bytes.


   A lzip file consists of a series of "members" (compressed data sets).
The members simply appear one after another in the file, with no
additional information before, between, or after them.

   Each member has the following structure:
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID string | VN | DS | Lzma stream | CRC32 |   Data size   |  Member size  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All multibyte values are stored in little endian order.

'ID string'
     A four byte string, identifying the lzip format, with the value
     "LZIP" (0x4C, 0x5A, 0x49, 0x50).

'VN (version number, 1 byte)'
     Just in case something needs to be modified in the future. 1 for
     now.

'DS (coded dictionary size, 1 byte)'
     Lzip divides the distance between any two powers of 2 into 8
     equally spaced intervals, named "wedges". The dictionary size is
     calculated by taking a power of 2 (the base size) and substracting
     from it a number of wedges between 0 and 7. The size of a wedge is
     (base_size / 16).
     Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).
     Bits 7-5 contain the number of wedges (0 to 7) to substract from
     the base size to obtain the dictionary size.
     Example: 0xD3 = 2^19 - 6 * 2^15 = 512 KiB - 6 * 32 KiB = 320 KiB
     Valid values for dictionary size range from 4 KiB to 512 MiB.

'Lzma stream'
     The lzma stream, finished by an end of stream marker. Uses default
     values for encoder properties. See the chapter 'Stream format'
     (*note Stream format::) for a complete description.

'CRC32 (4 bytes)'
     CRC of the uncompressed original data.

'Data size (8 bytes)'
     Size of the uncompressed original data.

'Member size (8 bytes)'
     Total size of the member, including header and trailer. This field
     acts as a distributed index, allows the verification of stream
     integrity, and facilitates safe recovery of undamaged members from
     multi-member files.



File: lzip.info,  Node: Stream format,  Next: Examples,  Prev: File format,  Up: Top

5 Format of the LZMA stream in lzip files
*****************************************

The LZMA algorithm has three parameters, called "special LZMA
properties", to adjust it for some kinds of binary data. These
parameters are; 'literal_context_bits' (with a default value of 3),
'literal_pos_state_bits' (with a default value of 0), and
'pos_state_bits' (with a default value of 2). As a general purpose
compressor, lzip only uses the default values for these parameters.

   Lzip also finishes the LZMA stream with an "End Of Stream" marker
(the distance-length pair 0xFFFFFFFFU, 2), which in conjunction with the
"member size" field in the member trailer allows the verification of
stream integrity. The LZMA stream in lzip files always has these two
features (default properties and EOS marker) and is referred to in this
document as LZMA-302eos or LZMA-lzip.

   The second stage of LZMA is a range encoder that uses a different
probability model for each type of symbol; distances, lengths, literal
bytes, etc. Range encoding conceptually encodes all the symbols of the
message into one number. Unlike Huffman coding, which assigns to each
symbol a bit-pattern and concatenates all the bit-patterns together,
range encoding can compress one symbol to less than one bit. Therefore
the compressed data produced by a range encoder can't be split in pieces
that could be individually described.

   It seems that the only way of describing the LZMA-302eos stream is
describing the algorithm that decodes it. And given the many details
about the range decoder that need to be described accurately, the source
code of a real decoder seems the only appropriate reference to use.

   What follows is a description of the decoding algorithm for
LZMA-302eos streams using as reference the source code of "lzd", an
educational decompressor for lzip files which can be downloaded from
the lzip download directory. The source code of lzd is included in
appendix A.  *note Reference source code::


5.1 What is coded
=================

The LZMA stream includes literals, matches and repeated matches (matches
reusing a recently used distance). There are 7 different coding
sequences:

Bit sequence              Name       Description
--------------------------------------------------------------------------- 
0 + byte                  literal    literal byte
1 + 0 + len + dis         match      distance-length pair
1 + 1 + 0 + 0             shortrep   1 byte match at latest used distance
1 + 1 + 0 + 1 + len       rep0       len bytes match at latest used
                                     distance
1 + 1 + 1 + 0 + len       rep1       len bytes match at second latest
                                     used distance
1 + 1 + 1 + 1 + 0 + len   rep2       len bytes match at third latest used
                                     distance
1 + 1 + 1 + 1 + 1 + len   rep3       len bytes match at fourth latest
                                     used distance


   In the following tables, multi-bit sequences are coded in normal
order, from MSB to LSB, except where noted otherwise.

   Lengths (the 'len' in the table above) are coded as follows:

Bit sequence                         Description
-------------------------------------------------------------------------- 
0 + 3 bits                           lengths from 2 to 9
1 + 0 + 3 bits                       lengths from 10 to 17
1 + 1 + 8 bits                       lengths from 18 to 273


   The coding of distances is a little more complicated. LZMA divides
the interval between any two powers of 2 into 2 halves, named slots. As
possible distances range from 0 to (2^32 - 1), there are 64 slots (0 to
63). The slot number is context-coded in 6 bits. 'direct_bits' are the
remaining bits (from 0 to 30) needed to form a complete distance, and
are calculated as (slot >> 1) - 1. If a distance needs 6 or more
direct_bits, the last 4 bits are coded separately. The last piece
(direct_bits for distances 4 to 127 or the last 4 bits for distances >=
128) is context-coded in reverse order (from LSB to MSB). For distances
>= 128, the 'direct_bits - 4' part is coded with fixed 0.5 probability.

Bit sequence                         Description
-------------------------------------------------------------------------- 
slot                                 distances from 0 to 3
slot + direct_bits                   distances from 4 to 127
slot + (direct_bits - 4) + 4 bits    distances from 128 to 2^32 - 1


5.2 The coding contexts
=======================

These contexts ('Bit_model' in the source), are integers or arrays of
integers representing the probability of the corresponding bit being 0.

   The indices used in these arrays are:

'state'
     A state machine ('State' in the source) with 12 states (0 to 11),
     coding the latest 2 to 4 types of sequences processed. The initial
     state is 0.

'pos_state'
     Value of the 2 least significant bits of the current position in
     the decoded data.

'literal_state'
     Value of the 3 most significant bits of the latest byte decoded.

'len_state'
     Coded value of length (length - 2), with a maximum of 3. The
     resulting value is in the range 0 to 3.


   In the following table, '!literal' is any sequence except a literal
byte. 'rep' is any one of 'rep0', 'rep1', 'rep2' or 'rep3'. The types
of previous sequences corresponding to each state are:

State   Types of previous sequences
-------------------------------------------------------- 
0       literal, literal, literal
1       match, literal, literal
2       rep or (!literal, shortrep), literal, literal
3       literal, shortrep, literal, literal
4       match, literal
5       rep or (!literal, shortrep), literal
6       literal, shortrep, literal
7       literal, match
8       literal, rep
9       literal, shortrep
10      !literal, match
11      !literal, (rep or shortrep)


   The contexts for decoding the type of coding sequence are:

Name           Indices                       Used when
--------------------------------------------------------------------------- 
bm_match       state, pos_state              sequence start
bm_rep         state                         after sequence 1
bm_rep0        state                         after sequence 11
bm_rep1        state                         after sequence 111
bm_rep2        state                         after sequence 1111
bm_len         state, pos_state              after sequence 110


   The contexts for decoding distances are:

Name           Indices                       Used when
--------------------------------------------------------------------------- 
bm_dis_slot    len_state, bit tree           distance start
bm_dis         reverse bit tree              after slots 4 to 13
bm_align       reverse bit tree              for distances >= 128, after
                                             fixed probability bits


   There are two separate sets of contexts for lengths ('Len_model' in
the source). One for normal matches, the other for repeated matches. The
contexts in each Len_model are (see 'decode_len' in the source):

Name           Indices                       Used when
--------------------------------------------------------------------------- 
choice1        none                          length start
choice2        none                          after sequence 1
bm_low         pos_state, bit tree           after sequence 0
bm_mid         pos_state, bit tree           after sequence 10
bm_high        bit tree                      after sequence 11


   The context array 'bm_literal' is special. In principle it acts as a
normal bit tree context, the one selected by 'literal_state'. But if
the previous decoded byte was not a literal, two other bit tree
contexts are used depending on the value of each bit in 'match_byte'
(the byte at the latest used distance), until a bit is decoded that is
different from its corresponding bit in 'match_byte'. After the first
difference is found, the rest of the byte is decoded using the normal
bit tree context. (See 'decode_matched' in the source).


5.3 The range decoder
=====================

The LZMA stream is consumed one byte at a time by the range decoder.
(See 'normalize' in the source). Every byte consumed produces a
variable number of decoded bits, depending on how well these bits agree
with their context. (See 'decode_bit' in the source).

   The range decoder state consists of two unsigned 32-bit variables;
'range' (representing the most significant part of the range size not
yet decoded), and 'code' (representing the current point within
'range'). 'range' is initialized to (2^32 - 1), and 'code' is
initialized to 0.

   The range encoder produces a first 0 byte that must be ignored by the
range decoder. This is done by shifting 5 bytes in the initialization of
'code' instead of 4. (See the 'Range_decoder' constructor in the
source).


5.4 Decoding the LZMA stream
============================

After decoding the member header and obtaining the dictionary size, the
range decoder is initialized and then the LZMA decoder enters a loop
(See 'decode_member' in the source) where it invokes the range decoder
with the appropriate contexts to decode the different coding sequences
(matches, repeated matches, and literal bytes), until the "End Of
Stream" marker is decoded.


File: lzip.info,  Node: Examples,  Next: Problems,  Prev: Stream format,  Up: Top

6 A small tutorial with examples
********************************

WARNING! Even if lzip is bug-free, other causes may result in a corrupt
compressed file (bugs in the system libraries, memory errors, etc).
Therefore, if the data you are going to compress are important, give the
'--keep' option to lzip and do not remove the original file until you
verify the compressed file with a command like
'lzip -cd file.lz | cmp file -'.


Example 1: Replace a regular file with its compressed version 'file.lz'
and show the compression ratio.

     lzip -v file


Example 2: Like example 1 but the created 'file.lz' is multi-member
with a member size of 1 MiB. The compression ratio is not shown.

     lzip -b 1MiB file


Example 3: Restore a regular file from its compressed version
'file.lz'. If the operation is successful, 'file.lz' is removed.

     lzip -d file.lz


Example 4: Verify the integrity of the compressed file 'file.lz' and
show status.

     lzip -tv file.lz


Example 5: Compress a whole floppy in /dev/fd0 and send the output to
'file.lz'.

     lzip -c /dev/fd0 > file.lz


Example 6: Decompress 'file.lz' partially until 10 KiB of decompressed
data are produced.

     lzip -cd file.lz | dd bs=1024 count=10


Example 7: Decompress 'file.lz' partially from decompressed byte 10000
to decompressed byte 15000 (5000 bytes are produced).

     lzip -cd file.lz | dd bs=1000 skip=10 count=5


Example 8: Create a multivolume compressed tar archive with a volume
size of 1440 KiB.

     tar -c some_directory | lzip -S 1440KiB -o volume_name


Example 9: Extract a multivolume compressed tar archive.

     lzip -cd volume_name*.lz | tar -xf -


Example 10: Create a multivolume compressed backup of a large database
file with a volume size of 650 MB, where each volume is a multi-member
file with a member size of 32 MiB.

     lzip -b 32MiB -S 650MB big_db


File: lzip.info,  Node: Problems,  Next: Reference source code,  Prev: Examples,  Up: Top

7 Reporting bugs
****************

There are probably bugs in lzip. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
you don't, no one will ever know about them and they will remain unfixed
for all eternity, if not longer.

   If you find a bug in lzip, please send electronic mail to
<lzip-bug@nongnu.org>. Include the version number, which you can find
by running 'lzip --version'.


File: lzip.info,  Node: Reference source code,  Next: Concept index,  Prev: Problems,  Up: Top

Appendix A Reference source code
********************************

/*  Lzd - Educational decompressor for lzip files
    Copyright (C) 2013, 2014 Antonio Diaz Diaz.

    This program is free software: you have unlimited permission
    to copy, distribute and modify it.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
*/
/*
    Exit status: 0 for a normal exit, 1 for environmental problems
    (file not found, invalid flags, I/O errors, etc), 2 to indicate a
    corrupt or invalid input file.
*/

#include <algorithm>
#include <cerrno>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <stdint.h>
#include <unistd.h>


class State
  {
  int st;

public:
  enum { states = 12 };
  State() : st( 0 ) {}
  int operator()() const { return st; }
  bool is_char() const { return st < 7; }

  void set_char()
    {
    static const int next[states] = { 0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 4, 5 };
    st = next[st];
    }
  void set_match()     { st = ( st < 7 ) ? 7 : 10; }
  void set_rep()       { st = ( st < 7 ) ? 8 : 11; }
  void set_short_rep() { st = ( st < 7 ) ? 9 : 11; }
  };


enum {
  min_dictionary_size = 1 << 12,
  max_dictionary_size = 1 << 29,
  literal_context_bits = 3,
  pos_state_bits = 2,
  pos_states = 1 << pos_state_bits,
  pos_state_mask = pos_states - 1,

  len_states = 4,
  dis_slot_bits = 6,
  start_dis_model = 4,
  end_dis_model = 14,
  modeled_distances = 1 << (end_dis_model / 2),		// 128
  dis_align_bits = 4,
  dis_align_size = 1 << dis_align_bits,

  len_low_bits = 3,
  len_mid_bits = 3,
  len_high_bits = 8,
  len_low_symbols = 1 << len_low_bits,
  len_mid_symbols = 1 << len_mid_bits,
  len_high_symbols = 1 << len_high_bits,
  max_len_symbols = len_low_symbols + len_mid_symbols + len_high_symbols,

  min_match_len = 2,					// must be 2

  bit_model_move_bits = 5,
  bit_model_total_bits = 11,
  bit_model_total = 1 << bit_model_total_bits };

struct Bit_model
  {
  int probability;
  Bit_model() : probability( bit_model_total / 2 ) {}
  };

struct Len_model
  {
  Bit_model choice1;
  Bit_model choice2;
  Bit_model bm_low[pos_states][len_low_symbols];
  Bit_model bm_mid[pos_states][len_mid_symbols];
  Bit_model bm_high[len_high_symbols];
  };


class CRC32
  {
  uint32_t data[256];		// Table of CRCs of all 8-bit messages.

public:
  CRC32()
    {
    for( unsigned n = 0; n < 256; ++n )
      {
      unsigned c = n;
      for( int k = 0; k < 8; ++k )
        { if( c & 1 ) c = 0xEDB88320U ^ ( c >> 1 ); else c >>= 1; }
      data[n] = c;
      }
    }

  void update_buf( uint32_t & crc, const uint8_t * const buffer,
                   const int size ) const
    {
    for( int i = 0; i < size; ++i )
      crc = data[(crc^buffer[i])&0xFF] ^ ( crc >> 8 );
    }
  };

const CRC32 crc32;


typedef uint8_t File_header[6];	// 0-3 magic, 4 version, 5 coded_dict_size

typedef uint8_t File_trailer[20];
			//  0-3  CRC32 of the uncompressed data
			//  4-11 size of the uncompressed data
			// 12-19 member size including header and trailer

class Range_decoder
  {
  uint32_t code;
  uint32_t range;

public:
  Range_decoder() : code( 0 ), range( 0xFFFFFFFFU )
    {
    for( int i = 0; i < 5; ++i ) code = (code << 8) | get_byte();
    }

  uint8_t get_byte() { return std::getc( stdin ); }

  int decode( const int num_bits )
    {
    int symbol = 0;
    for( int i = 0; i < num_bits; ++i )
      {
      range >>= 1;
      symbol <<= 1;
      if( code >= range ) { code -= range; symbol |= 1; }
      if( range <= 0x00FFFFFFU )			// normalize
        { range <<= 8; code = (code << 8) | get_byte(); }
      }
    return symbol;
    }

  int decode_bit( Bit_model & bm )
    {
    int symbol;
    const uint32_t bound = ( range >> bit_model_total_bits ) * bm.probability;
    if( code < bound )
      {
      range = bound;
      bm.probability += (bit_model_total - bm.probability) >> bit_model_move_bits;
      symbol = 0;
      }
    else
      {
      range -= bound;
      code -= bound;
      bm.probability -= bm.probability >> bit_model_move_bits;
      symbol = 1;
      }
    if( range <= 0x00FFFFFFU )				// normalize
      { range <<= 8; code = (code << 8) | get_byte(); }
    return symbol;
    }

  int decode_tree( Bit_model bm[], const int num_bits )
    {
    int symbol = 1;
    for( int i = 0; i < num_bits; ++i )
      symbol = ( symbol << 1 ) | decode_bit( bm[symbol] );
    return symbol - (1 << num_bits);
    }

  int decode_tree_reversed( Bit_model bm[], const int num_bits )
    {
    int symbol = decode_tree( bm, num_bits );
    int reversed_symbol = 0;
    for( int i = 0; i < num_bits; ++i )
      {
      reversed_symbol = ( reversed_symbol << 1 ) | ( symbol & 1 );
      symbol >>= 1;
      }
    return reversed_symbol;
    }

  int decode_matched( Bit_model bm[], const int match_byte )
    {
    Bit_model * const bm1 = bm + 0x100;
    int symbol = 1;
    for( int i = 7; i >= 0; --i )
      {
      const int match_bit = ( match_byte >> i ) & 1;
      const int bit = decode_bit( bm1[(match_bit<<8)+symbol] );
      symbol = ( symbol << 1 ) | bit;
      if( match_bit != bit )
        {
        while( symbol < 0x100 )
          symbol = ( symbol << 1 ) | decode_bit( bm[symbol] );
        break;
        }
      }
    return symbol & 0xFF;
    }

  int decode_len( Len_model & lm, const int pos_state )
    {
    if( decode_bit( lm.choice1 ) == 0 )
      return decode_tree( lm.bm_low[pos_state], len_low_bits );
    if( decode_bit( lm.choice2 ) == 0 )
      return len_low_symbols +
             decode_tree( lm.bm_mid[pos_state], len_mid_bits );
    return len_low_symbols + len_mid_symbols +
           decode_tree( lm.bm_high, len_high_bits );
    }
  };


class LZ_decoder
  {
  unsigned long long partial_data_pos;
  Range_decoder rdec;
  const unsigned dictionary_size;
  uint8_t * const buffer;	// output buffer
  unsigned pos;			// current pos in buffer
  unsigned stream_pos;		// first byte not yet written to stdout
  uint32_t crc_;

  void flush_data();

  uint8_t get_byte( const unsigned distance ) const
    {
    unsigned i = pos - distance - 1;
    if( pos <= distance ) i += dictionary_size;
    return buffer[i];
    }

  void put_byte( const uint8_t b )
    {
    buffer[pos] = b;
    if( ++pos >= dictionary_size ) flush_data();
    }

public:
  LZ_decoder( const unsigned dict_size )
    :
    partial_data_pos( 0 ),
    dictionary_size( dict_size ),
    buffer( new uint8_t[dictionary_size] ),
    pos( 0 ),
    stream_pos( 0 ),
    crc_( 0xFFFFFFFFU )
    { buffer[dictionary_size-1] = 0; }		// prev_byte of first byte

  ~LZ_decoder() { delete[] buffer; }

  unsigned crc() const { return crc_ ^ 0xFFFFFFFFU; }
  unsigned long long data_position() const { return partial_data_pos + pos; }

  bool decode_member();
  };


void LZ_decoder::flush_data()
  {
  if( pos > stream_pos )
    {
    const unsigned size = pos - stream_pos;
    crc32.update_buf( crc_, buffer + stream_pos, size );
    errno = 0;
    if( std::fwrite( buffer + stream_pos, 1, size, stdout ) != size )
      { std::fprintf( stderr, "Write error: %s.\n", std::strerror( errno ) );
        std::exit( 1 ); }
    if( pos >= dictionary_size ) { partial_data_pos += pos; pos = 0; }
    stream_pos = pos;
    }
  }


bool LZ_decoder::decode_member()		// Returns false if error
  {
  Bit_model bm_literal[1<<literal_context_bits][0x300];
  Bit_model bm_match[State::states][pos_states];
  Bit_model bm_rep[State::states];
  Bit_model bm_rep0[State::states];
  Bit_model bm_rep1[State::states];
  Bit_model bm_rep2[State::states];
  Bit_model bm_len[State::states][pos_states];
  Bit_model bm_dis_slot[len_states][1<<dis_slot_bits];
  Bit_model bm_dis[modeled_distances-end_dis_model];
  Bit_model bm_align[dis_align_size];
  Len_model match_len_model;
  Len_model rep_len_model;
  unsigned rep0 = 0;			// rep[0-3] latest four distances
  unsigned rep1 = 0;			// used for efficient coding of
  unsigned rep2 = 0;			// repeated distances
  unsigned rep3 = 0;
  State state;

  while( !std::feof( stdin ) && !std::ferror( stdin ) )
    {
    const int pos_state = data_position() & pos_state_mask;
    if( rdec.decode_bit( bm_match[state()][pos_state] ) == 0 )	// 1st bit
      {
      const uint8_t prev_byte = get_byte( 0 );
      const int literal_state = prev_byte >> ( 8 - literal_context_bits );
      Bit_model * const bm = bm_literal[literal_state];
      if( state.is_char() )
        put_byte( rdec.decode_tree( bm, 8 ) );
      else
        put_byte( rdec.decode_matched( bm, get_byte( rep0 ) ) );
      state.set_char();
      }
    else
      {
      int len;
      if( rdec.decode_bit( bm_rep[state()] ) != 0 )		// 2nd bit
        {
        if( rdec.decode_bit( bm_rep0[state()] ) != 0 )		// 3rd bit
          {
          unsigned distance;
          if( rdec.decode_bit( bm_rep1[state()] ) == 0 )	// 4th bit
            distance = rep1;
          else
            {
            if( rdec.decode_bit( bm_rep2[state()] ) == 0 )	// 5th bit
              distance = rep2;
            else
              { distance = rep3; rep3 = rep2; }
            rep2 = rep1;
            }
          rep1 = rep0;
          rep0 = distance;
          }
        else
          {
          if( rdec.decode_bit( bm_len[state()][pos_state] ) == 0 ) // 4th bit
            { state.set_short_rep(); put_byte( get_byte( rep0 ) ); continue; }
          }
        state.set_rep();
        len = min_match_len + rdec.decode_len( rep_len_model, pos_state );
        }
      else
        {
        rep3 = rep2; rep2 = rep1; rep1 = rep0;
        len = min_match_len + rdec.decode_len( match_len_model, pos_state );
        const int len_state = std::min( len - min_match_len, len_states - 1 );
        const int dis_slot =
          rdec.decode_tree( bm_dis_slot[len_state], dis_slot_bits );
        if( dis_slot < start_dis_model ) rep0 = dis_slot;
        else
          {
          const int direct_bits = ( dis_slot >> 1 ) - 1;
          rep0 = ( 2 | ( dis_slot & 1 ) ) << direct_bits;
          if( dis_slot < end_dis_model )
            rep0 += rdec.decode_tree_reversed( bm_dis + rep0 - dis_slot - 1,
                                               direct_bits );
          else
            {
            rep0 += rdec.decode( direct_bits - dis_align_bits ) << dis_align_bits;
            rep0 += rdec.decode_tree_reversed( bm_align, dis_align_bits );
            if( rep0 == 0xFFFFFFFFU )		// Marker found
              {
              flush_data();
              return ( len == min_match_len );	// End Of Stream marker
              }
            }
          }
        state.set_match();
        if( rep0 >= dictionary_size || rep0 >= data_position() )
          { flush_data(); return false; }
        }
      for( int i = 0; i < len; ++i ) put_byte( get_byte( rep0 ) );
      }
    }
  flush_data();
  return false;
  }


int main( const int argc, const char * const argv[] )
  {
  if( argc > 1 )
    {
    std::printf( "Lzd %s - Educational decompressor for lzip files.\n",
                 PROGVERSION );
    std::printf( "Study the source to learn how a lzip decompressor works.\n"
                 "See the lzip manual for an explanation of the code.\n"
                 "It is not safe to use lzd for any real work.\n"
                 "\nUsage: %s < file.lz > file\n", argv[0] );
    std::printf( "Lzd decompresses from standard input to standard output.\n"
                 "\nCopyright (C) 2014 Antonio Diaz Diaz.\n"
                 "This is free software: you are free to change and redistribute it.\n"
                 "There is NO WARRANTY, to the extent permitted by law.\n"
                 "Report bugs to lzip-bug@nongnu.org\n"
                 "Lzd home page: http://www.nongnu.org/lzip/lzd.html\n" );
    return 0;
    }

  for( bool first_member = true; ; first_member = false )
    {
    File_header header;
    for( int i = 0; i < 6; ++i ) header[i] = std::getc( stdin );
    if( std::feof( stdin ) || std::memcmp( header, "LZIP", 4 ) != 0 )
      {
      if( first_member )
        { std::fprintf( stderr, "Bad magic number (file not in lzip format)\n" );
          return 2; }
      break;
      }
    if( header[4] != 1 )
      {
      std::fprintf( stderr, "Version %d member format not supported.\n",
                    header[4] );
      return 2;
      }
    unsigned dict_size = 1 << ( header[5] & 0x1F );
    dict_size -= ( dict_size / 16 ) * ( ( header[5] >> 5 ) & 7 );
    if( dict_size < min_dictionary_size || dict_size > max_dictionary_size )
      { std::fprintf( stderr, "Invalid dictionary size in member header\n" );
        return 2; }

    LZ_decoder decoder( dict_size );
    if( !decoder.decode_member() )
      { std::fprintf( stderr, "Data error\n" ); return 2; }

    File_trailer trailer;
    for( int i = 0; i < 20; ++i ) trailer[i] = std::getc( stdin );
    unsigned crc = 0;
    for( int i = 3; i >= 0; --i ) { crc <<= 8; crc += trailer[i]; }
    unsigned long long data_size = 0;
    for( int i = 11; i >= 4; --i ) { data_size <<= 8; data_size += trailer[i]; }
    if( crc != decoder.crc() || data_size != decoder.data_position() )
      { std::fprintf( stderr, "CRC error\n" ); return 2; }
    }

  if( std::fclose( stdout ) != 0 )
    { std::fprintf( stderr, "Can't close stdout: %s.\n", std::strerror( errno ) );
      return 1; }
  return 0;
  }


File: lzip.info,  Node: Concept index,  Prev: Reference source code,  Up: Top

Concept index
*************

[index]
* Menu:

* algorithm:                             Algorithm.             (line 6)
* bugs:                                  Problems.              (line 6)
* examples:                              Examples.              (line 6)
* file format:                           File format.           (line 6)
* format of the LZMA stream:             Stream format.         (line 6)
* getting help:                          Problems.              (line 6)
* introduction:                          Introduction.          (line 6)
* invoking:                              Invoking lzip.         (line 6)
* options:                               Invoking lzip.         (line 6)
* reference source code:                 Reference source code. (line 6)
* usage:                                 Invoking lzip.         (line 6)
* version:                               Invoking lzip.         (line 6)



Tag Table:
Node: Top208
Node: Introduction1022
Node: Algorithm5992
Node: Invoking lzip8750
Node: File format14426
Node: Stream format16974
Node: Examples26406
Node: Problems28363
Node: Reference source code28893
Node: Concept index42410

End Tag Table


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