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31 lines
No EOL
1.8 KiB
Markdown
# Introduction #
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This text was written by Roger Seeck. It was copied from http://www.binaryessence.com/dct/imp/en000225.htm on November 31, 2008 for archival purposes.
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See also InfoZipAppnoteAdditions.
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# Deflate64™ (Enhanced Deflate) #
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Deflate64™ is a slightly modified variant of the Deflate procedure. The fundamental mechanisms remained completely unchanged, only the following features were improved:
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## Expansion of the dictionary on 64 Kbytes. ##
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The addressable size of the sliding dictionary is extended from 32 kbytes to 64 kbytes.
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## Expansion of the distance code ##
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The distance codes (30 and 31) not used until now are extended to address a range of 64 kbytes. According to the conventional Deflate definition these codes were not used. 14 extra bits are assigned to each of them.
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## Expansion of the length codes ##
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In contrast to Deflate, the last length code (285) will be extended by 16 extra bits. The code defines lengths in a range between 3 and 65.538 byte. The original limitation to 258 byte sequences is dropped with that.
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Deflate64™ achieves a slightly improved compression rate in comparison to Deflate at a lower processing speed. The format was specified by PKWARE, inc. and is part of the ZIP specification (compression method 9).
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Unlike all other compression methods which are specified in ZIP, Deflate64™ is not described in detail. Additionally PKWARE claims to use Deflate64™ as a trademark. The information for this article is derived from the source code of inflate.c (Info-ZIP, UnZip 5.51).
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Besides PKWARE and a large number of commercial application some freeware and open source projects support Deflate64™:
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* 7-Zip
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* Info-ZIP (Zip, UnZip)
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The zlib project does not support Deflate64™. According to the zlib FAQ, this is due to the low improvements and the decision of PKWARE to handle Deflate64™ as a proprietary format. |