LZMA

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The Lempel-Ziv-Markov chain-Algorithm (LZMA) is an algorithm for data compression in development since 1998^[1] and used in the 7z format of the 7-Zip archiver. It uses a dictionary compression scheme somewhat similar to LZ77 and features a high compression ratio (generally higher than bzip2) and a variable compression-dictionary size (up to 4 GB).^[2]

Overview

The LZMA uses an improved LZ77 compression algorithm, backed by a range encoder.

Streams for data, repeated-sequence size and repeated-sequence location seem to be compressed separately.

7-Zip reference implementation

The reference implementation of LZMA is included as part of the 7z and 7-Zip suite of tools. Source code is distributed under the terms of the GNU LGPL license with a special exception for linked binaries. The special exception allows redistribution of binaries linked to unmodified LZMA to be free of any LGPL requirements (e.g., they do not need to allow reverse engineering or binary modifications.)

The reference open source LZMA compression library is written in C++ and has the following properties:

• Compression speed: approximately 1 MB per second on a 2 GHz CPU
• Decompression speed: between 10 and 20 MB per second on a 2 GHz CPU
• Support for multi-threading.

The 7-Zip implementation uses several variants of hash chains, binary trees and Patricia tries as the basis for its dictionary search algorithm.

Decompression-only code for LZMA generally compiles to around 5kB and the amount of RAM required during decompression is principally determined by the size of the sliding window used during compression. Small code size and relatively low memory overhead, particularly with smaller dictionary lengths, make the LZMA decompression algorithm well-suited to embedded applications.

Algorithm

The documentation in the SDK is very limited, providing no design information.

For that, rely on third-party discussion, e.g., its user forum.^[3] Digesting that (or some other source), Paul Sladen summarizes the algorithm:^[4]

LZMA is effectively deflate (zlib, gzip, zip) with a larger dictionary size, 32MB instead of 32kB. LZMA stands for Lempel-Ziv-Markov chain-Algorithm, after string back-references have been located, values are reduced using a Markov chain range-encoder (aka arithmetic coding) instead of huffman.

Users

Software that uses or supports LZMA:

• cramfs and SquashFS, with applied patches
• Inno Setup
• Nullsoft Scriptable Install System
• Peazip, GUI frontend to command-line 7z and POSIX 7z binaries
• UPX, from version 2.92 (beta) and above, features optional LZMA compression
• The RPM Package Manager, from version 5 onwards
• dpkg, from version 1.13.35.
• GNU Tar, from version 1.20 (using the --lzma switch).

Notes

External links

• 7-Zip Official homepage.
• p7zip 7-Zip homepage for Linux.
• Data compression, Compressors & Archivers
• LZMA Utils, command line compression program similar to gzip and bzip2
• PylibLZMA, a python interface using the liblzma API.

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