89 lines
4.2 KiB
Text
89 lines
4.2 KiB
Text
Description
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Lzlib is a data compression library providing in-memory LZMA compression
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and decompression functions, including integrity checking of the
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decompressed data. The compressed data format used by the library is the
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lzip format. Lzlib is written in C.
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The lzip file format is designed for long-term data archiving, taking
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into account both data integrity and decoder availability:
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* The lzip format provides very safe integrity checking and some data
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recovery means. The lziprecover program can repair bit-flip errors
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(one of the most common forms of data corruption) in lzip files,
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and provides data recovery capabilities, including error-checked
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merging of damaged copies of a file.
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* The lzip format is as simple as possible (but not simpler). The
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lzip manual provides the code of a simple decompressor along with a
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detailed explanation of how it works, so that with the only help of
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the lzip manual it would be possible for a digital archaeologist to
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extract the data from a lzip file long after quantum computers
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eventually render LZMA obsolete.
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* Additionally lzip is copylefted, which guarantees that it will
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remain free forever.
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A nice feature of the lzip format is that a corrupt byte is easier to
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repair the nearer it is from the beginning of the file. Therefore, with
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the help of lziprecover, losing an entire archive just because of a
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corrupt byte near the beginning is a thing of the past.
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The functions and variables forming the interface of the compression
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library are declared in the file 'lzlib.h'. Usage examples of the
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library are given in the files 'main.c' and 'bbexample.c' from the
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source distribution.
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Compression/decompression is done by repeatedly calling a couple of
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read/write functions until all the data has been processed by the
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library. This interface is safer and less error prone than the
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traditional zlib interface.
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Compression/decompression is done when the read function is called. This
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means the value returned by the position functions will not be updated
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until some data is read, even if you write a lot of data. If you want
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the data to be compressed in advance, just call the read function with a
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size equal to 0.
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Lzlib will correctly decompress a data stream which is the concatenation
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of two or more compressed data streams. The result is the concatenation
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of the corresponding decompressed data streams. Integrity testing of
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concatenated compressed data streams is also supported.
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All the library functions are thread safe. The library does not install
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any signal handler. The decoder checks the consistency of the compressed
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data, so the library should never crash even in case of corrupted input.
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There is no such thing as a "LZMA algorithm"; it is more like a "LZMA
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coding scheme". For example, the option '-0' of lzip uses the scheme in
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almost the simplest way possible; issuing the longest match it can find,
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or a literal byte if it can't find a match. Inversely, a much more
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elaborated way of finding coding sequences of minimum price than the one
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currently used by lzip could be developed, and the resulting sequence
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could also be coded using the LZMA coding scheme.
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Lzip currently implements two variants of the LZMA algorithm; fast (used
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by option -0) and normal (used by all other compression levels). Lzlib
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just implements the "normal" variant.
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The high compression of LZMA comes from combining two basic, well-proven
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compression ideas: sliding dictionaries (LZ77/78) and markov models (the
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thing used by every compression algorithm that uses a range encoder or
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similar order-0 entropy coder as its last stage) with segregation of
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contexts according to what the bits are used for.
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The ideas embodied in lzlib are due to (at least) the following people:
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Abraham Lempel and Jacob Ziv (for the LZ algorithm), Andrey Markov (for
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the definition of Markov chains), G.N.N. Martin (for the definition of
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range encoding), Igor Pavlov (for putting all the above together in
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LZMA), and Julian Seward (for bzip2's CLI).
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Copyright (C) 2009-2014 Antonio Diaz Diaz.
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This file is free documentation: you have unlimited permission to copy,
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distribute and modify it.
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The file Makefile.in is a data file used by configure to produce the
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Makefile. It has the same copyright owner and permissions that configure
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itself.
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