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@@ -0,0 +1,108 @@ +Description + +Lzlib is a data compression library providing in-memory LZMA compression and +decompression functions, including integrity checking of the decompressed +data. The compressed data format used by the library is the lzip format. +Lzlib is written in C. + +The lzip file format is designed for data sharing and long-term archiving, +taking into account both data integrity and decoder availability: + + * The lzip format provides very safe integrity checking and some data + recovery means. The program lziprecover 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 source 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 the lzip reference implementation 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 functions and variables forming the interface of the compression library +are declared in the file 'lzlib.h'. Usage examples of the library are given +in the files 'bbexample.c', 'ffexample.c', and 'minilzip.c' from the source +distribution. + +As 'lzlib.h' can be used by C and C++ programs, it must not impose a choice +of system headers on the program by including one of them. Therefore it is +the responsibility of the program using lzlib to include before 'lzlib.h' +some header that declares the type 'uint8_t'. There are at least four such +headers in C and C++: 'stdint.h', 'cstdint', 'inttypes.h', and 'cinttypes'. + +All the library functions are thread safe. The library does not install any +signal handler. The decoder checks the consistency of the compressed data, +so the library should never crash even in case of corrupted input. + +Compression/decompression is done by repeatedly calling a couple of +read/write functions until all the data have been processed by the library. +This interface is safer and less error prone than the traditional zlib +interface. + +Compression/decompression is done when the read function is called. This +means the value returned by the position functions is not updated until a +read call, even if a lot of data are written. If you want the data to be +compressed in advance, just call the read function with a size equal to 0. + +If all the data to be compressed are written in advance, lzlib automatically +adjusts the header of the compressed data to use the largest dictionary size +that does not exceed neither the data size nor the limit given to +'LZ_compress_open'. This feature reduces the amount of memory needed for +decompression and allows minilzip to produce identical compressed output as +lzip. + +Lzlib correctly decompresses a data stream which is the concatenation of +two or more compressed data streams. The result is the concatenation of the +corresponding decompressed data streams. Integrity testing of concatenated +compressed data streams is also supported. + +Lzlib is able to compress and decompress streams of unlimited size by +automatically creating multimember output. The members so created are large, +about 2 PiB each. + +In spite of its name (Lempel-Ziv-Markov chain-Algorithm), LZMA is not a +concrete algorithm; it is more like "any algorithm using the 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 size than the one currently used by lzip +could be developed, and the resulting sequence could also be coded using the +LZMA coding scheme. + +Lzlib currently implements two variants of the LZMA algorithm: fast (used by +option '-0' of minilzip) 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) 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. + +The ideas embodied in lzlib are due to (at least) the following people: +Abraham Lempel and Jacob Ziv (for the LZ algorithm), Andrei 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). + +LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never have +been compressed. Decompressed is used to refer to data which have undergone +the process of decompression. + + +Copyright (C) 2009-2024 Antonio Diaz Diaz. + +This file is free documentation: you have unlimited permission to copy, +distribute, and modify it. + +The file Makefile.in is a data file used by configure to produce the Makefile. +It has the same copyright owner and permissions that configure itself. |