Zip's deflation algorithm is a variation of LZ77 (Lempel-Ziv 1977, seereference below). It finds duplicated strings in the input data.  Thesecond occurrence of a string is replaced by a pointer to the previousstring, in the form of a pair (distance, length).  Distances arelimited to 32K bytes, and lengths are limited to 258 bytes. When astring does not occur anywhere in the previous 32K bytes, it isemitted as a sequence of literal bytes.  (In this description,'string' must be taken as an arbitrary sequence of bytes, and is notrestricted to printable characters.)Literals or match lengths are compressed with one Huffman tree, andmatch distances are compressed with another tree. The trees are storedin a compact form at the start of each block. The blocks can have anysize (except that the compressed data for one block must fit inavailable memory). A block is terminated when zip determines that itwould be useful to start another block with fresh trees. (This issomewhat similar to compress.)Duplicated strings are found using a hash table. All input strings oflength 3 are inserted in the hash table. A hash index is computed forthe next 3 bytes. If the hash chain for this index is not empty, allstrings in the chain are compared with the current input string, andthe longest match is selected.The hash chains are searched starting with the most recent strings, tofavor small distances and thus take advantage of the Huffman encoding.The hash chains are singly linked. There are no deletions from thehash chains, the algorithm simply discards matches that are too old.To avoid a worst-case situation, very long hash chains are arbitrarilytruncated at a certain length, determined by a runtime option (zip -1to -9). So zip does not always find the longest possible match butgenerally finds a match which is long enough.zip also defers the selection of matches with a lazy evaluationmechanism. After a match of length N has been found, zip searches for alonger match at the next input byte. If a longer match is found, theprevious match is truncated to a length of one (thus producing a singleliteral byte) and the longer match is emitted afterwards.  Otherwise,the original match is kept, and the next match search is attempted onlyN steps later.The lazy match evaluation is also subject to a runtime parameter. Ifthe current match is long enough, zip reduces the search for a longermatch, thus speeding up the whole process. If compression ratio is moreimportant than speed, zip attempts a complete second search even ifthe first match is already long enough.The lazy match evaluation is no performed for the fastest compressionmodes (speed options -1 to -3). For these fast modes, new stringsare inserted in the hash table only when no match was found, orwhen the match is not too long. This degrades the compression ratiobut saves time since there are both fewer insertions and fewer searches.Jean-loup Gaillyjloup@chorus.frReferences:[LZ77] Ziv J., Lempel A., "A Universal Algorithm for Sequential DataCompression", IEEE Transactions on Information Theory", Vol. 23, No. 3,pp. 337-343.APPNOTE.TXT documentation file in PKZIP 1.93a. It is available byftp in ftp.cso.uiuc.edu:/pc/exec-pc/pkz193a.exe [128.174.5.59]