词条 | Lempel–Ziv–Stac | ||||||||||||||||||||
释义 |
Lempel–Ziv–Stac (LZS, or Stac compression) is a lossless data compression algorithm that uses a combination of the LZ77 sliding-window compression algorithm and fixed Huffman coding. It was originally developed by Stac Electronics for tape compression, and subsequently adapted for hard disk compression and sold as the Stacker disk compression software. It was later specified as a compression algorithm for various network protocols. LZS is specified in the Cisco IOS stack. StandardsLZS compression is standardised as an INCITS (previously ANSI) standard.[1] LZS compression is specified for various Internet protocols:
AlgorithmLZS compression and decompression uses an LZ77 type algorithm. It uses the last 2 KB of uncompressed data as a sliding-window dictionary. An LZS compressor looks for matches between the data to be compressed and the last 2 KB of data. If it finds a match, it encodes an offset/length reference to the dictionary. If no match is found, the next data byte is encoded as a "literal" byte. The compressed data stream ends with an end-marker. Compressed data formatData is encoded into a stream of variable-bit-width tokens. Literal byteA literal byte is encoded as a '0' bit followed by the 8 bits of the byte. Offset/length referenceAn offset/length reference is encoded as a '1' bit followed by the encoded offset, followed by the encoded length. One exceptional encoding is an end marker, described below. An offset can have a minimum value of 1 and a maximum value of 2047. A value of 1 refers to the most recent byte in the history buffer, immediately preceding the next data byte to be processed. An offset is encoded as:
A length is encoded as:
End markerAn end marker is encoded as the 9-bit token 110000000. Following the end marker, 0 to 7 extra '0' bits are appended as needed, to pad the stream to the next byte boundary. PatentsStac Electronics' spin-off Hifn has held several patents for LZS compression.[2][3] These patents lapsed due to non-payment of fees and attempts to reinstate them in 2007 failed. In 1993–94, Stac Electronics successfully sued Microsoft for infringement of LZS patents in the DoubleSpace disk compression program included with MS-DOS 6.0.[4] See also
References1. ^INCITS/ANSI X3.241-1994 - Data Compression Method – Adaptive Coding with Sliding Window for Information Interchange {{Compression Methods}}{{DEFAULTSORT:Lempel-Ziv-Stac}}2. ^{{Cite web | last = Friend | first = Robert C. | title = Hifn's Statement about IPR claimed in draft-friend-tls-lzs-compression, RFC1967, RFC1974, RFC2118, RFC2395, and RFC3078 | url=http://www.ietf.org/ietf-ftp/IPR/hifn-ipr-draft-friend-tls-lzs-compression.txt | accessdate = 21 July 2010 }} 3. ^{{Cite web | last = Friend | first = Robert | title = Hifn's Statement on IPR Claimed in LZS and MPPC compression algorithms | url=http://www.ietf.org/ietf-ftp/IPR/hifn-ipr-multiple-rfcs.txt | accessdate = 21 July 2010 }} 4. ^Complaint for patent infringement and Demand for jury trial {{webarchive|url=https://web.archive.org/web/20070509205650/http://www.vaxxine.com/lawyers/articles/stac.html |date=2007-05-09 }} by Stac Electronics v Microsoft Corporation 1 : Lossless compression algorithms |
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