The first
algorithm to use the
Lempel-Ziv {substitutional compression} schemes, proposed in 1977. LZ77 compression keeps track of the last n bytes of data seen, and when a phrase is encountered that has already been seen, it outputs a pair of values corresponding to the position of the phrase in the previously-seen buffer of data, and the length of the phrase. In effect the compressor moves a fixed-size "window" over the data (generally referred to as a "sliding window"), with the position p
art of the (position, length) pair referring to the position of the phrase within the window. The most commonly used
algorithms
are derived from the
LZSS scheme described by James Storer and Thomas Szymanski in 1982. In this the compressor maintains a window of size N bytes and a "lookahead buffer", the contents of which it tries to find a match for in the window: while (lookAheadBuffer not empty) { get a pointer (position, match) to the longest match in the window for the lookahead buffer if (length > MINIMUM_MATCH_LENGTH) { output a (position, length) pair shift the window length ch
aracters along } else { output the first ch
aracter in the lookahead buffer shift the window 1 ch
aracter along } } Decompression is simple and fast: whenever a (POSITION, LENGTH) pair is encountered, go to that POSITION in the window and copy LENGTH bytes to the output. Sliding-window-based schemes can be simplified by numbering the input text ch
aracters mod N, in effect creating a circul
ar buffer. The sliding window approach automatically creates the
LRU effect which must be done explicitly in
LZ78 schemes. V
ariants of this method apply additional compression to the output of the LZSS compressor, which include a simple v
ariable-length code (
LZB), dynamic
Huffman coding (
LZH), and
Shannon-Fano coding (
ZIP 1.x), all of which result in a certain degree of improvement over the basic scheme, especially when the data
are rather random and the LZSS compressor has little effect. An algorithm was developed which combines the ideas behind LZ77 and LZ78 to produce a hybrid called
LZFG. LZFG uses the stand
ard sliding window, but stores the data in a modified
trie data structure and produces as output the position of the text in the trie. Since LZFG only inserts complete *phrases* into the diction
ary, it should run faster than other LZ77-based compressors. All popul
ar archivers (
arj,
lha,
zip,
zoo)
are v
ariations on LZ77. [comp.compression
FAQ]. (1995-04-07)
In addition suitable contents:
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