103 lines
5.1 KiB
Text
103 lines
5.1 KiB
Text
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.
|
|
|
|
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 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.
|
|
|
|
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 (LZ77/78) 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) 2009-2022 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.
|