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RFC1951的部分翻译及原文(1/2) (转)

原创 网络安全 作者:themoney 时间:2007-10-01 11:32:21 0 删除 编辑
RFC1951的部分翻译及原文(1/2) (转)[@more@]

1. Introduction

  1.1. Purpose

  The purpose of this specification is to define a loess
  compressed data format that:
  * Is independent of CPU type, operating system, file system,
  and character set, and hence can be used for interchange;
  * Can be produced or consumed, even for an arbitrarily long
  sequentially presented input data stream, using only an a
  priori bounded amount of intermediate storage, and hence
  can be used in data communications or similar structures
  such as Unix filters;
  * Compresses data with efficiency comparable to the best
  currently available general-purpose compression methods,
  and in particular consrably better than the "compress"
  * Can be implemented readily in a manner not covered by
  patents, and hence can be practiced freely;

  * Is compatible with the file format produced by the current
  widely used gzutility, in that confong decompressors
  will be able to read data produced by the existing

  The data format defined by this specification does not attempt to:

  * Allow ran access to compressed data;
  * Compress specialized data (e.g., raster graphics) as well
  as the best currently available specialized algorithms.

  A simple counting argument shows that no lossless compression
  algorithm can compress every possible input data set.  For the
  format defined here, the worst case expansion is 5 bytes per 32K-
  byte block, i.e., a size increase of 0.015% for large data sets.
  English text usually compresses by a factor of 2.5 to 3;
  executable files usually compress somewhat less; graphical data
  such as raster images may compress much more.

  1.2. Intended audience
  This specification is intended for use by implementors of software
  to compress data into "deflate" format and/or decompress data from
  "deflate" format.
  The text of the specification assumes a basic background in
  programming at the level of bits and other primitive data
  representations.  Familiarity with the technique of Huffman coding
  is helpful but not required.

  1.3. pe
  The specification specifies a method for representing a sequence
  of bytes as a (usually shorter) sequence of bits, and a method for
  packing the latter bit sequence into bytes.

  1.4. Compliance

  Unless otherwise indicated below, a compliant decompressor must be
  able to accept and decompress any data set that conforms to all
  the specifications presented here; a compliant compressor must
  produce data sets that confoto all the specifications presented

  1.5.  Definitions of terms and conventions used

  Byte: 8 bits stored or transmitted as a unit (same as an octet).
  For this specification, a byte is exactly 8 bits, even on machines

  which store a character on a number of bits different from eight.
  See below, for the numbering of bits within a byte.

  String: a sequence of arbitrary bytes.

  1.6. Changes from previous versions

  There have been no technical changes to the deflate format since
  version 1.1 of this specification.  In version 1.2, some
  terminology was changed.  Version 1.3 is a conversion of the
  specification to RFC style.

2. Compressed representation overview


  A compressed data set consists of a series of blocks, corresponding
  to successive blocks of input data.  The block sizes are arbitrary,
  except that non-compressible blocks are limited to 65,535 bytes.

  Each block is compressed using a combination of the LZ77 algorithm
  and Huffman coding. The Huffman trees for each block are independent
  of those for previous or subsequent blocks; the LZ77 algorithm may
  use a reference to a duplicated string occurring in a previous block,
  up to 32K input bytes before.


  Each block consists of two parts: a pair of Huffman code trees that
  describe the representation of the compressed data part, and a
  compressed data part.  (The Huffman trees themselves are compressed
  using Huffman encoding.)  The compressed data consists of a series of
  elements of two types: literal bytes (of strings that have not been
  detected as duplicated within the previous 32K input bytes), and
  pointers to duplicated strings, where a pointer is represented as a
  pair .  The representation used in the
  "deflate" format limits distances to 32K bytes and lengths to 258
  bytes, but does not limit the size of a block, except for
  uncompressible blocks, which are limited as noted above.

  Each type of value (literals, distances, and lengths) in the
  compressed data is represented using a Huffman code, using one code
  tree for literals and lengths and a separate code tree for distances.
  The code trees for each block appear in a compact form just before
  the compressed data for that block.


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