LZW compression ==>
Lempel-Ziv Welch compression
(LZW) The algorithm used by the Unix compress command to reduce the size of
files, e.g. for archival or transmission. LZW was designed by Terry Welch in
1984 for implementation in hardware for high-performance disk controllers. It is
a variant of LZ78, one of the two Lempel-Ziv compression schemes.
The LZW algorithm relies on reoccurrence of byte sequences (strings) in its
input. It maintains a table mapping input strings to their associated output
codes. The table initially contains mappings for all possible strings of length
one. Input is taken one byte at a time to find the longest initial string
present in the table. The code for that string is output and then the string is
extended with one more input byte, b. A new entry is added to the table mapping
the extended string to the next unused code (obtained by incrementing a
counter). The process repeats, starting from byte b. The number of bits in an
output code, and hence the maximum number of entries in the table is usually
fixed and once this limit is reached, no more entries are added.
LZW compression and decompression are licensed under Unisys Corporation's 1984
U.S. Patent 4,558,302 and equivalent foreign patents. This kind of patent isn't
legal in most coutries of the world (including the UK) except the USA. Patents
in the UK can't describe algorithms or mathematical methods.
[A Technique for High Performance Data Compression, Terry A. Welch, IEEE
Computer, 17(6), June 1984, pp. 8-19]
[J. Ziv and A. Lempel, "A Universal Algorithm for Sequential Data Compression,"
IEEE Transactions on Information Theory, Vol. IT-23, No. 3, May 1977, pp.
337-343].
Nearby terms:
Le-Lisp « lemma « Lempel-Ziv compression «
Lempel-Ziv Welch compression » Lenat, Doug »
lenient evaluation » LEO
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