mirror of https://github.com/madler/zlib.git
Mark Adler
13 years ago
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1. Compression algorithm (deflate) |
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The deflation algorithm used by zlib (also zip and gzip) is a variation of |
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LZ77 (Lempel-Ziv 1977, see reference below). It finds duplicated strings in |
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the input data. The second occurrence of a string is replaced by a |
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pointer to the previous string, in the form of a pair (distance, |
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length). Distances are limited to 32K bytes, and lengths are limited |
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to 258 bytes. When a string does not occur anywhere in the previous |
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32K bytes, it is emitted as a sequence of literal bytes. (In this |
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description, 'string' must be taken as an arbitrary sequence of bytes, |
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and is not restricted to printable characters.) |
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Literals or match lengths are compressed with one Huffman tree, and |
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match distances are compressed with another tree. The trees are stored |
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in a compact form at the start of each block. The blocks can have any |
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size (except that the compressed data for one block must fit in |
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available memory). A block is terminated when deflate() determines that |
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it would be useful to start another block with fresh trees. (This is |
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somewhat similar to compress.) |
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Duplicated strings are found using a hash table. All input strings of |
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length 3 are inserted in the hash table. A hash index is computed for |
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the next 3 bytes. If the hash chain for this index is not empty, all |
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strings in the chain are compared with the current input string, and |
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the longest match is selected. |
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The hash chains are searched starting with the most recent strings, to |
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favor small distances and thus take advantage of the Huffman encoding. |
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The hash chains are singly linked. There are no deletions from the |
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hash chains, the algorithm simply discards matches that are too old. |
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To avoid a worst-case situation, very long hash chains are arbitrarily |
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truncated at a certain length, determined by a runtime option (level |
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parameter of deflateInit). So deflate() does not always find the longest |
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possible match but generally finds a match which is long enough. |
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deflate() also defers the selection of matches with a lazy evaluation |
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mechanism. After a match of length N has been found, deflate() searches for a |
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longer match at the next input byte. If a longer match is found, the |
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previous match is truncated to a length of one (thus producing a single |
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literal byte) and the longer match is emitted afterwards. Otherwise, |
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the original match is kept, and the next match search is attempted only |
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N steps later. |
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The lazy match evaluation is also subject to a runtime parameter. If |
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the current match is long enough, deflate() reduces the search for a longer |
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match, thus speeding up the whole process. If compression ratio is more |
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important than speed, deflate() attempts a complete second search even if |
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the first match is already long enough. |
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The lazy match evaluation is not performed for the fastest compression |
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modes (level parameter 1 to 3). For these fast modes, new strings |
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are inserted in the hash table only when no match was found, or |
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when the match is not too long. This degrades the compression ratio |
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but saves time since there are both fewer insertions and fewer searches. |
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2. Decompression algorithm (inflate) |
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The real question is given a Huffman tree, how to decode fast. The most |
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important realization is that shorter codes are much more common than |
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longer codes, so pay attention to decoding the short codes fast, and let |
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the long codes take longer to decode. |
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inflate() sets up a first level table that covers some number of bits of |
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input less than the length of longest code. It gets that many bits from the |
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stream, and looks it up in the table. The table will tell if the next |
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code is that many bits or less and how many, and if it is, it will tell |
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the value, else it will point to the next level table for which inflate() |
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grabs more bits and tries to decode a longer code. |
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How many bits to make the first lookup is a tradeoff between the time it |
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takes to decode and the time it takes to build the table. If building the |
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table took no time (and if you had infinite memory), then there would only |
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be a first level table to cover all the way to the longest code. However, |
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building the table ends up taking a lot longer for more bits since short |
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codes are replicated many times in such a table. What inflate() does is |
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simply to make the number of bits in the first table a variable, and set it |
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for the maximum speed. |
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inflate() sends new trees relatively often, so it is possibly set for a |
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smaller first level table than an application that has only one tree for |
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all the data. For inflate, which has 286 possible codes for the |
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literal/length tree, the size of the first table is nine bits. Also the |
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distance trees have 30 possible values, and the size of the first table is |
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six bits. Note that for each of those cases, the table ended up one bit |
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longer than the "average" code length, i.e. the code length of an |
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approximately flat code which would be a little more than eight bits for |
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286 symbols and a little less than five bits for 30 symbols. It would be |
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interesting to see if optimizing the first level table for other |
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applications gave values within a bit or two of the flat code size. |
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Jean-loup Gailly Mark Adler |
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gzip@prep.ai.mit.edu madler@alumni.caltech.edu |
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References: |
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[LZ77] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data |
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Compression", IEEE Transactions on Information Theory", Vol. 23, No. 3, |
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pp. 337-343. |
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"DEFLATE Compressed Data Format Specification" available in |
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ftp://ds.internic.net/rfc/rfc1951.txt |
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LIBRARY "zlib" |
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DESCRIPTION '"""zlib data compression library"""' |
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EXETYPE NT |
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SUBSYSTEM WINDOWS |
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STUB 'WINSTUB.EXE' |
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VERSION 1.0.2 |
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CODE EXECUTE READ |
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DATA READ WRITE |
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HEAPSIZE 1048576,4096 |
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EXPORTS |
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zlibVersion |
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deflate |
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deflateEnd |
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inflate |
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inflateEnd |
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deflateSetDictionary |
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deflateCopy |
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deflateReset |
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deflateParams |
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inflateSetDictionary |
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inflateSync |
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inflateReset |
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compress |
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uncompress |
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gzopen |
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gzdopen |
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gzread |
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gzwrite |
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gzflush |
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gzclose |
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gzerror |
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adler32 |
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crc32 |
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deflateInit_ |
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inflateInit_ |
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deflateInit2_ |
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inflateInit2_ |
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@ -0,0 +1,32 @@ |
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#include <windows.h> |
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#define IDR_VERSION1 1 |
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IDR_VERSION1 VERSIONINFO MOVEABLE IMPURE LOADONCALL DISCARDABLE |
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FILEVERSION 1,0,2,0 |
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PRODUCTVERSION 1,0,2,0 |
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FILEFLAGSMASK VS_FFI_FILEFLAGSMASK |
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FILEFLAGS 0 |
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FILEOS VOS_DOS_WINDOWS32 |
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FILETYPE VFT_DLL |
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FILESUBTYPE 0 // not used |
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BEGIN |
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BLOCK "StringFileInfo" |
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BEGIN |
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BLOCK "040904E4" |
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//language ID = U.S. English, char set = Windows, Multilingual |
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BEGIN |
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VALUE "FileDescription", "zlib data compression library\0" |
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VALUE "FileVersion", "1.0.2\0" |
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VALUE "InternalName", "zlib\0" |
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VALUE "OriginalFilename", "zlib.lib\0" |
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VALUE "ProductName", "ZLib.DLL\0" |
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VALUE "Comments", "DLL support by Alessandro Iacopetti\0" |
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VALUE "LegalCopyright", "(C) 1995-1996 Jean-loup Gailly & Mark Adler\0" |
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END |
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END |
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BLOCK "VarFileInfo" |
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BEGIN |
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VALUE "Translation", 0x0409, 1252 |
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END |
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END |
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