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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@]

下面只是rfc文档中的第1,2,3,节,其余部分请参照原文。
1. Introduction

  1.1. Purpose

 本规范是用来定义一个无损的数据压缩格式:
  The purpose of this specification is to define a loSSLess
  compressed data format that:
 它不依赖于cpu类型,操作系统类型,文件系统类型,字符集。因此
可以被用来进行交换
  * Is independent of CPU type, operating system, file system,
  and character set, and hence can be used for interchange;
 能够被生成或是去除,甚至是使用一个预先确定的中间存贮器的边界
长度,对一个任意长的连续输入数据流进行操作。因此可以被用在数据通信及相似的
结构中,如unix过滤器。
  * 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;
 其压缩效率可与目前的可用的,最普遍的压缩方法相比拟。而且在
一些方面还要好于压缩程序
  * Compresses data with efficiency comparable to the best
  currently available general-purpose compression methods,
  and in particular considerably better than the "compress"
  program;
 
  * 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 gzip utility, in that confoRming decompressors
  will be able to read data produced by the existing gzip
  compressor.

 这篇文档所指的数据格式不包括:
  The data format defined by this specification does not attempt to:

 允许随机存取压缩数据。
  * Allow random access to compressed data;
 压缩专门的数据。
  * 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
 这篇文档被用来实现软件压缩数据为“deflate”格式或从“deflate”解压数据。
  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. SCOpe
 这篇文档说明了一种方法来将一个字节序列描述成一个位序列,还说明了一种方
法来将位序列组合成字节序列。
  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 confoRM to all the specifications presented
  here.

  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

 一个压缩的数据集包含一系列的块,与输入数据的块相对应。块的大小是任意
的,但是非压缩的块要在65535字节之内。

  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.

 每一块的压缩都使用了LZ77法则和Huffman编码方法。每块的Huffman树都
和它的前一块及后一块无关。LZ77法则可以参考所复制的前一个串中的前32K内容。
  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.

 每个块包含了两部分:1)一对Huffman编码树,它描述了压缩内容的表示方法。
2)还有就是压缩数据的部分。(其中的Huffman树也是经过了Huffman编码的)压缩后的
数据包括了两种类型。1)文字字节(是字符串的一部分,这个字符串不是所复制的
前32K的内容。2)指向复制的字符串的指针,这个指针的形式是一个<长度,向后的距
离>对。这个使用在"deflate"格式中的形式限定了距离为32K及长度为258字节。但是
没有限制块的大小(除了没有压缩的数据以外)。

  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.

 压缩数据中的每种类型值(文字,距离及长度)都使用Huffman编码方法,使用一
个编码树编码文字和长度,使用另一个独立的编码树来编码距离。每一块的编码树都以
压缩的方式存在于那一块中压缩的数据之前的位置。
  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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