LZX (algorithm)
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LZX is the name of an LZ77 family compression algorithm. It is also the name of a file archiver with the same name. Both were invented by Jonathan Forbes and Tomi Poutanen.
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Instances of use of the LZX algorithm
Amiga LZX
LZX was publicly released as an Amiga file archiver in 1995, while the authors were studying at the University of Waterloo in Canada. The software was shareware, which was common for compression software at the time. The registered version contained fixes and improvements not available in the evaluation version. In 1997, the authors gave away a free keyfile, which allowed anyone to use the registered version, as they had stopped work on the archiver and stopped accepting registrations.
Microsoft Cabinet files
In 1997, Jonathan went to work for Microsoft, and Microsoft's cabinet archiver was enhanced to include the LZX compression method. Improvements included a variable search window size; Amiga LZX was fixed to 64 kilobytes, Microsoft LZX could range on powers of two between 32 and 2048 kilobytes. A special pre-processor was added to detect Intel 80x86 "JMP" instructions, converting their operands from relative addressing to absolute addressing, thus jumps to the same location resulted in repeated strings that the compressor could match, improving compression of 80x86 binary code.
Microsoft HTMLHelp (CHM) files
When Microsoft introduced the replacement to their classic Help file format, they chose to compress all of the HTML data with the LZX algorithm. However, in order to improve random access speed, the compressor was altered to reset itself after every 32 kilobyte interval and re-align to a 16-bit boundary. Thus, the HTMLHelp software could immediately seek to the nearest 32 kilobyte interval and start decoding from there, rather than decoding from the beginning of the compressed datastream at all times.
Microsoft EBook Reader (LIT) files
Microsoft LIT files are simply an extension of the CHM file format, and thus also use LZX compression.
Decompressing LZX files
The unlzx program can unpack Amiga LZX archives. The cabextract program can unpack Microsoft cabinet files using the LZX method. There are a multitude of cross-platform tools for decompiling or viewing CHM files, as stated in the CHM article. LIT files can be unpacked using the Convert LIT software.
See also
External links
- The LZX page, including many versions of the Amiga LZX archiver (http://xavprods.free.fr/lzx/)
- unlzx source code (http://wuarchive.wustl.edu/pub/aminet/misc/unix/unlzx.c.gz)
- cabextract (includes source code) (http://www.kyz.uklinux.net/cabextract.php)
- Convert LIT (includes source code) (http://www.convertlit.com/)