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 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 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 'main.c' and 'bbexample.c' from the
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 will not be 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 will
automatically adjust 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 will correctly decompress 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.
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.
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
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.
The ideas embodied in lzlib 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).
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) 2009-2019 Antonio Diaz Diaz.
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