lzw compression

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lzw compression - Free On-line Dictionary of Computing (26 May 2007) :

  Lempel-Ziv Welch compression
  LZW 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].