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].