956 lines
36 KiB
Plaintext
956 lines
36 KiB
Plaintext
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Network Working Group P. Deutsch
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Request for Comments: 1951 Aladdin Enterprises
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Category: Informational May 1996
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DEFLATE Compressed Data Format Specification version 1.3
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Status of This Memo
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This memo provides information for the Internet community. This memo
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does not specify an Internet standard of any kind. Distribution of
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this memo is unlimited.
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IESG Note:
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The IESG takes no position on the validity of any Intellectual
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Property Rights statements contained in this document.
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Notices
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Copyright (c) 1996 L. Peter Deutsch
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Permission is granted to copy and distribute this document for any
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purpose and without charge, including translations into other
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languages and incorporation into compilations, provided that the
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copyright notice and this notice are preserved, and that any
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substantive changes or deletions from the original are clearly
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marked.
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A pointer to the latest version of this and related documentation in
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HTML format can be found at the URL
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<ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
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Abstract
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This specification defines a lossless compressed data format that
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compresses data using a combination of the LZ77 algorithm and Huffman
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coding, with efficiency comparable to the best currently available
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general-purpose compression methods. The data can be produced or
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consumed, even for an arbitrarily long sequentially presented input
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data stream, using only an a priori bounded amount of intermediate
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storage. The format can be implemented readily in a manner not
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covered by patents.
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Deutsch Informational [Page 1]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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Table of Contents
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1. Introduction ................................................... 2
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1.1. Purpose ................................................... 2
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1.2. Intended audience ......................................... 3
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1.3. Scope ..................................................... 3
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1.4. Compliance ................................................ 3
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1.5. Definitions of terms and conventions used ................ 3
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1.6. Changes from previous versions ............................ 4
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2. Compressed representation overview ............................. 4
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3. Detailed specification ......................................... 5
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3.1. Overall conventions ....................................... 5
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3.1.1. Packing into bytes .................................. 5
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3.2. Compressed block format ................................... 6
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3.2.1. Synopsis of prefix and Huffman coding ............... 6
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3.2.2. Use of Huffman coding in the "deflate" format ....... 7
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3.2.3. Details of block format ............................. 9
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3.2.4. Non-compressed blocks (BTYPE=00) ................... 11
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3.2.5. Compressed blocks (length and distance codes) ...... 11
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3.2.6. Compression with fixed Huffman codes (BTYPE=01) .... 12
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3.2.7. Compression with dynamic Huffman codes (BTYPE=10) .. 13
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3.3. Compliance ............................................... 14
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4. Compression algorithm details ................................. 14
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5. References .................................................... 16
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6. Security Considerations ....................................... 16
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7. Source code ................................................... 16
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8. Acknowledgements .............................................. 16
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9. Author's Address .............................................. 17
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1. Introduction
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1.1. Purpose
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The purpose of this specification is to define a lossless
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compressed data format that:
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* Is independent of CPU type, operating system, file system,
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and character set, and hence can be used for interchange;
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* Can be produced or consumed, even for an arbitrarily long
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sequentially presented input data stream, using only an a
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priori bounded amount of intermediate storage, and hence
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can be used in data communications or similar structures
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such as Unix filters;
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* Compresses data with efficiency comparable to the best
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currently available general-purpose compression methods,
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and in particular considerably better than the "compress"
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program;
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* Can be implemented readily in a manner not covered by
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patents, and hence can be practiced freely;
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Deutsch Informational [Page 2]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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* Is compatible with the file format produced by the current
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widely used gzip utility, in that conforming decompressors
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will be able to read data produced by the existing gzip
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compressor.
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The data format defined by this specification does not attempt to:
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* Allow random access to compressed data;
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* Compress specialized data (e.g., raster graphics) as well
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as the best currently available specialized algorithms.
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A simple counting argument shows that no lossless compression
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algorithm can compress every possible input data set. For the
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format defined here, the worst case expansion is 5 bytes per 32K-
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byte block, i.e., a size increase of 0.015% for large data sets.
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English text usually compresses by a factor of 2.5 to 3;
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executable files usually compress somewhat less; graphical data
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such as raster images may compress much more.
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1.2. Intended audience
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This specification is intended for use by implementors of software
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to compress data into "deflate" format and/or decompress data from
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"deflate" format.
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The text of the specification assumes a basic background in
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programming at the level of bits and other primitive data
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representations. Familiarity with the technique of Huffman coding
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is helpful but not required.
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1.3. Scope
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The specification specifies a method for representing a sequence
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of bytes as a (usually shorter) sequence of bits, and a method for
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packing the latter bit sequence into bytes.
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1.4. Compliance
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Unless otherwise indicated below, a compliant decompressor must be
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able to accept and decompress any data set that conforms to all
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the specifications presented here; a compliant compressor must
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produce data sets that conform to all the specifications presented
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here.
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1.5. Definitions of terms and conventions used
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Byte: 8 bits stored or transmitted as a unit (same as an octet).
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For this specification, a byte is exactly 8 bits, even on machines
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Deutsch Informational [Page 3]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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which store a character on a number of bits different from eight.
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See below, for the numbering of bits within a byte.
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String: a sequence of arbitrary bytes.
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1.6. Changes from previous versions
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There have been no technical changes to the deflate format since
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version 1.1 of this specification. In version 1.2, some
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terminology was changed. Version 1.3 is a conversion of the
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specification to RFC style.
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2. Compressed representation overview
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A compressed data set consists of a series of blocks, corresponding
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to successive blocks of input data. The block sizes are arbitrary,
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except that non-compressible blocks are limited to 65,535 bytes.
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Each block is compressed using a combination of the LZ77 algorithm
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and Huffman coding. The Huffman trees for each block are independent
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of those for previous or subsequent blocks; the LZ77 algorithm may
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use a reference to a duplicated string occurring in a previous block,
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up to 32K input bytes before.
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Each block consists of two parts: a pair of Huffman code trees that
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describe the representation of the compressed data part, and a
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compressed data part. (The Huffman trees themselves are compressed
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using Huffman encoding.) The compressed data consists of a series of
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elements of two types: literal bytes (of strings that have not been
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detected as duplicated within the previous 32K input bytes), and
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pointers to duplicated strings, where a pointer is represented as a
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pair <length, backward distance>. The representation used in the
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"deflate" format limits distances to 32K bytes and lengths to 258
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bytes, but does not limit the size of a block, except for
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uncompressible blocks, which are limited as noted above.
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Each type of value (literals, distances, and lengths) in the
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compressed data is represented using a Huffman code, using one code
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tree for literals and lengths and a separate code tree for distances.
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The code trees for each block appear in a compact form just before
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the compressed data for that block.
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Deutsch Informational [Page 4]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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3. Detailed specification
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3.1. Overall conventions In the diagrams below, a box like this:
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+---+
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| | <-- the vertical bars might be missing
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+---+
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represents one byte; a box like this:
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+==============+
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| |
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+==============+
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represents a variable number of bytes.
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Bytes stored within a computer do not have a "bit order", since
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they are always treated as a unit. However, a byte considered as
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an integer between 0 and 255 does have a most- and least-
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significant bit, and since we write numbers with the most-
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significant digit on the left, we also write bytes with the most-
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significant bit on the left. In the diagrams below, we number the
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bits of a byte so that bit 0 is the least-significant bit, i.e.,
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the bits are numbered:
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+--------+
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|76543210|
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+--------+
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Within a computer, a number may occupy multiple bytes. All
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multi-byte numbers in the format described here are stored with
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the least-significant byte first (at the lower memory address).
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For example, the decimal number 520 is stored as:
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0 1
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+--------+--------+
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|00001000|00000010|
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+--------+--------+
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^ ^
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| |
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| + more significant byte = 2 x 256
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+ less significant byte = 8
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3.1.1. Packing into bytes
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This document does not address the issue of the order in which
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bits of a byte are transmitted on a bit-sequential medium,
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since the final data format described here is byte- rather than
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Deutsch Informational [Page 5]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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bit-oriented. However, we describe the compressed block format
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in below, as a sequence of data elements of various bit
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lengths, not a sequence of bytes. We must therefore specify
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how to pack these data elements into bytes to form the final
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compressed byte sequence:
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* Data elements are packed into bytes in order of
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increasing bit number within the byte, i.e., starting
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with the least-significant bit of the byte.
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* Data elements other than Huffman codes are packed
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starting with the least-significant bit of the data
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element.
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* Huffman codes are packed starting with the most-
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significant bit of the code.
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In other words, if one were to print out the compressed data as
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a sequence of bytes, starting with the first byte at the
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*right* margin and proceeding to the *left*, with the most-
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significant bit of each byte on the left as usual, one would be
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able to parse the result from right to left, with fixed-width
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elements in the correct MSB-to-LSB order and Huffman codes in
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bit-reversed order (i.e., with the first bit of the code in the
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relative LSB position).
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3.2. Compressed block format
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3.2.1. Synopsis of prefix and Huffman coding
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Prefix coding represents symbols from an a priori known
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alphabet by bit sequences (codes), one code for each symbol, in
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a manner such that different symbols may be represented by bit
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sequences of different lengths, but a parser can always parse
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an encoded string unambiguously symbol-by-symbol.
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We define a prefix code in terms of a binary tree in which the
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two edges descending from each non-leaf node are labeled 0 and
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1 and in which the leaf nodes correspond one-for-one with (are
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labeled with) the symbols of the alphabet; then the code for a
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symbol is the sequence of 0's and 1's on the edges leading from
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the root to the leaf labeled with that symbol. For example:
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Deutsch Informational [Page 6]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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/\ Symbol Code
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0 1 ------ ----
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/ \ A 00
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/\ B B 1
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0 1 C 011
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/ \ D 010
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A /\
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0 1
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/ \
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D C
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A parser can decode the next symbol from an encoded input
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stream by walking down the tree from the root, at each step
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choosing the edge corresponding to the next input bit.
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Given an alphabet with known symbol frequencies, the Huffman
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algorithm allows the construction of an optimal prefix code
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(one which represents strings with those symbol frequencies
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using the fewest bits of any possible prefix codes for that
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alphabet). Such a code is called a Huffman code. (See
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reference [1] in Chapter 5, references for additional
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information on Huffman codes.)
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Note that in the "deflate" format, the Huffman codes for the
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various alphabets must not exceed certain maximum code lengths.
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This constraint complicates the algorithm for computing code
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lengths from symbol frequencies. Again, see Chapter 5,
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references for details.
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3.2.2. Use of Huffman coding in the "deflate" format
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The Huffman codes used for each alphabet in the "deflate"
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format have two additional rules:
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* All codes of a given bit length have lexicographically
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consecutive values, in the same order as the symbols
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they represent;
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* Shorter codes lexicographically precede longer codes.
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Deutsch Informational [Page 7]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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We could recode the example above to follow this rule as
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follows, assuming that the order of the alphabet is ABCD:
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Symbol Code
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------ ----
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A 10
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B 0
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C 110
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D 111
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I.e., 0 precedes 10 which precedes 11x, and 110 and 111 are
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lexicographically consecutive.
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Given this rule, we can define the Huffman code for an alphabet
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just by giving the bit lengths of the codes for each symbol of
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the alphabet in order; this is sufficient to determine the
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actual codes. In our example, the code is completely defined
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by the sequence of bit lengths (2, 1, 3, 3). The following
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algorithm generates the codes as integers, intended to be read
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from most- to least-significant bit. The code lengths are
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initially in tree[I].Len; the codes are produced in
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tree[I].Code.
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1) Count the number of codes for each code length. Let
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bl_count[N] be the number of codes of length N, N >= 1.
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2) Find the numerical value of the smallest code for each
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code length:
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code = 0;
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bl_count[0] = 0;
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for (bits = 1; bits <= MAX_BITS; bits++) {
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code = (code + bl_count[bits-1]) << 1;
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next_code[bits] = code;
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}
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3) Assign numerical values to all codes, using consecutive
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values for all codes of the same length with the base
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values determined at step 2. Codes that are never used
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(which have a bit length of zero) must not be assigned a
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value.
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for (n = 0; n <= max_code; n++) {
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len = tree[n].Len;
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if (len != 0) {
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tree[n].Code = next_code[len];
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next_code[len]++;
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}
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Deutsch Informational [Page 8]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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}
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Example:
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Consider the alphabet ABCDEFGH, with bit lengths (3, 3, 3, 3,
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3, 2, 4, 4). After step 1, we have:
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N bl_count[N]
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- -----------
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2 1
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3 5
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4 2
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Step 2 computes the following next_code values:
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N next_code[N]
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- ------------
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1 0
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2 0
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3 2
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4 14
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Step 3 produces the following code values:
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Symbol Length Code
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------ ------ ----
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A 3 010
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B 3 011
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C 3 100
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D 3 101
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E 3 110
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F 2 00
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G 4 1110
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H 4 1111
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3.2.3. Details of block format
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Each block of compressed data begins with 3 header bits
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containing the following data:
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first bit BFINAL
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next 2 bits BTYPE
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Note that the header bits do not necessarily begin on a byte
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boundary, since a block does not necessarily occupy an integral
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number of bytes.
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Deutsch Informational [Page 9]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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BFINAL is set if and only if this is the last block of the data
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set.
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BTYPE specifies how the data are compressed, as follows:
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00 - no compression
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01 - compressed with fixed Huffman codes
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10 - compressed with dynamic Huffman codes
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11 - reserved (error)
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The only difference between the two compressed cases is how the
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Huffman codes for the literal/length and distance alphabets are
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defined.
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In all cases, the decoding algorithm for the actual data is as
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follows:
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do
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read block header from input stream.
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if stored with no compression
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skip any remaining bits in current partially
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processed byte
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read LEN and NLEN (see next section)
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copy LEN bytes of data to output
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otherwise
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if compressed with dynamic Huffman codes
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read representation of code trees (see
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subsection below)
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loop (until end of block code recognized)
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decode literal/length value from input stream
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if value < 256
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copy value (literal byte) to output stream
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otherwise
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if value = end of block (256)
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break from loop
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otherwise (value = 257..285)
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decode distance from input stream
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move backwards distance bytes in the output
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stream, and copy length bytes from this
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position to the output stream.
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end loop
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while not last block
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Note that a duplicated string reference may refer to a string
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in a previous block; i.e., the backward distance may cross one
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or more block boundaries. However a distance cannot refer past
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the beginning of the output stream. (An application using a
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Deutsch Informational [Page 10]
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RFC 1951 DEFLATE Compressed Data Format Specification May 1996
|
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preset dictionary might discard part of the output stream; a
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distance can refer to that part of the output stream anyway)
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Note also that the referenced string may overlap the current
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position; for example, if the last 2 bytes decoded have values
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X and Y, a string reference with <length = 5, distance = 2>
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adds X,Y,X,Y,X to the output stream.
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We now specify each compression method in turn.
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3.2.4. Non-compressed blocks (BTYPE=00)
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Any bits of input up to the next byte boundary are ignored.
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The rest of the block consists of the following information:
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0 1 2 3 4...
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+---+---+---+---+================================+
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| LEN | NLEN |... LEN bytes of literal data...|
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+---+---+---+---+================================+
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LEN is the number of data bytes in the block. NLEN is the
|
||
one's complement of LEN.
|
||
|
||
3.2.5. Compressed blocks (length and distance codes)
|
||
|
||
As noted above, encoded data blocks in the "deflate" format
|
||
consist of sequences of symbols drawn from three conceptually
|
||
distinct alphabets: either literal bytes, from the alphabet of
|
||
byte values (0..255), or <length, backward distance> pairs,
|
||
where the length is drawn from (3..258) and the distance is
|
||
drawn from (1..32,768). In fact, the literal and length
|
||
alphabets are merged into a single alphabet (0..285), where
|
||
values 0..255 represent literal bytes, the value 256 indicates
|
||
end-of-block, and values 257..285 represent length codes
|
||
(possibly in conjunction with extra bits following the symbol
|
||
code) as follows:
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
Deutsch Informational [Page 11]
|
||
|
||
RFC 1951 DEFLATE Compressed Data Format Specification May 1996
|
||
|
||
|
||
Extra Extra Extra
|
||
Code Bits Length(s) Code Bits Lengths Code Bits Length(s)
|
||
---- ---- ------ ---- ---- ------- ---- ---- -------
|
||
257 0 3 267 1 15,16 277 4 67-82
|
||
258 0 4 268 1 17,18 278 4 83-98
|
||
259 0 5 269 2 19-22 279 4 99-114
|
||
260 0 6 270 2 23-26 280 4 115-130
|
||
261 0 7 271 2 27-30 281 5 131-162
|
||
262 0 8 272 2 31-34 282 5 163-194
|
||
263 0 9 273 3 35-42 283 5 195-226
|
||
264 0 10 274 3 43-50 284 5 227-257
|
||
265 1 11,12 275 3 51-58 285 0 258
|
||
266 1 13,14 276 3 59-66
|
||
|
||
The extra bits should be interpreted as a machine integer
|
||
stored with the most-significant bit first, e.g., bits 1110
|
||
represent the value 14.
|
||
|
||
Extra Extra Extra
|
||
Code Bits Dist Code Bits Dist Code Bits Distance
|
||
---- ---- ---- ---- ---- ------ ---- ---- --------
|
||
0 0 1 10 4 33-48 20 9 1025-1536
|
||
1 0 2 11 4 49-64 21 9 1537-2048
|
||
2 0 3 12 5 65-96 22 10 2049-3072
|
||
3 0 4 13 5 97-128 23 10 3073-4096
|
||
4 1 5,6 14 6 129-192 24 11 4097-6144
|
||
5 1 7,8 15 6 193-256 25 11 6145-8192
|
||
6 2 9-12 16 7 257-384 26 12 8193-12288
|
||
7 2 13-16 17 7 385-512 27 12 12289-16384
|
||
8 3 17-24 18 8 513-768 28 13 16385-24576
|
||
9 3 25-32 19 8 769-1024 29 13 24577-32768
|
||
|
||
3.2.6. Compression with fixed Huffman codes (BTYPE=01)
|
||
|
||
The Huffman codes for the two alphabets are fixed, and are not
|
||
represented explicitly in the data. The Huffman code lengths
|
||
for the literal/length alphabet are:
|
||
|
||
Lit Value Bits Codes
|
||
--------- ---- -----
|
||
0 - 143 8 00110000 through
|
||
10111111
|
||
144 - 255 9 110010000 through
|
||
111111111
|
||
256 - 279 7 0000000 through
|
||
0010111
|
||
280 - 287 8 11000000 through
|
||
11000111
|
||
|
||
|
||
|
||
Deutsch Informational [Page 12]
|
||
|
||
RFC 1951 DEFLATE Compressed Data Format Specification May 1996
|
||
|
||
|
||
The code lengths are sufficient to generate the actual codes,
|
||
as described above; we show the codes in the table for added
|
||
clarity. Literal/length values 286-287 will never actually
|
||
occur in the compressed data, but participate in the code
|
||
construction.
|
||
|
||
Distance codes 0-31 are represented by (fixed-length) 5-bit
|
||
codes, with possible additional bits as shown in the table
|
||
shown in Paragraph 3.2.5, above. Note that distance codes 30-
|
||
31 will never actually occur in the compressed data.
|
||
|
||
3.2.7. Compression with dynamic Huffman codes (BTYPE=10)
|
||
|
||
The Huffman codes for the two alphabets appear in the block
|
||
immediately after the header bits and before the actual
|
||
compressed data, first the literal/length code and then the
|
||
distance code. Each code is defined by a sequence of code
|
||
lengths, as discussed in Paragraph 3.2.2, above. For even
|
||
greater compactness, the code length sequences themselves are
|
||
compressed using a Huffman code. The alphabet for code lengths
|
||
is as follows:
|
||
|
||
0 - 15: Represent code lengths of 0 - 15
|
||
16: Copy the previous code length 3 - 6 times.
|
||
The next 2 bits indicate repeat length
|
||
(0 = 3, ... , 3 = 6)
|
||
Example: Codes 8, 16 (+2 bits 11),
|
||
16 (+2 bits 10) will expand to
|
||
12 code lengths of 8 (1 + 6 + 5)
|
||
17: Repeat a code length of 0 for 3 - 10 times.
|
||
(3 bits of length)
|
||
18: Repeat a code length of 0 for 11 - 138 times
|
||
(7 bits of length)
|
||
|
||
A code length of 0 indicates that the corresponding symbol in
|
||
the literal/length or distance alphabet will not occur in the
|
||
block, and should not participate in the Huffman code
|
||
construction algorithm given earlier. If only one distance
|
||
code is used, it is encoded using one bit, not zero bits; in
|
||
this case there is a single code length of one, with one unused
|
||
code. One distance code of zero bits means that there are no
|
||
distance codes used at all (the data is all literals).
|
||
|
||
We can now define the format of the block:
|
||
|
||
5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286)
|
||
5 Bits: HDIST, # of Distance codes - 1 (1 - 32)
|
||
4 Bits: HCLEN, # of Code Length codes - 4 (4 - 19)
|
||
|
||
|
||
|
||
Deutsch Informational [Page 13]
|
||
|
||
RFC 1951 DEFLATE Compressed Data Format Specification May 1996
|
||
|
||
|
||
(HCLEN + 4) x 3 bits: code lengths for the code length
|
||
alphabet given just above, in the order: 16, 17, 18,
|
||
0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15
|
||
|
||
These code lengths are interpreted as 3-bit integers
|
||
(0-7); as above, a code length of 0 means the
|
||
corresponding symbol (literal/length or distance code
|
||
length) is not used.
|
||
|
||
HLIT + 257 code lengths for the literal/length alphabet,
|
||
encoded using the code length Huffman code
|
||
|
||
HDIST + 1 code lengths for the distance alphabet,
|
||
encoded using the code length Huffman code
|
||
|
||
The actual compressed data of the block,
|
||
encoded using the literal/length and distance Huffman
|
||
codes
|
||
|
||
The literal/length symbol 256 (end of data),
|
||
encoded using the literal/length Huffman code
|
||
|
||
The code length repeat codes can cross from HLIT + 257 to the
|
||
HDIST + 1 code lengths. In other words, all code lengths form
|
||
a single sequence of HLIT + HDIST + 258 values.
|
||
|
||
3.3. Compliance
|
||
|
||
A compressor may limit further the ranges of values specified in
|
||
the previous section and still be compliant; for example, it may
|
||
limit the range of backward pointers to some value smaller than
|
||
32K. Similarly, a compressor may limit the size of blocks so that
|
||
a compressible block fits in memory.
|
||
|
||
A compliant decompressor must accept the full range of possible
|
||
values defined in the previous section, and must accept blocks of
|
||
arbitrary size.
|
||
|
||
4. Compression algorithm details
|
||
|
||
While it is the intent of this document to define the "deflate"
|
||
compressed data format without reference to any particular
|
||
compression algorithm, the format is related to the compressed
|
||
formats produced by LZ77 (Lempel-Ziv 1977, see reference [2] below);
|
||
since many variations of LZ77 are patented, it is strongly
|
||
recommended that the implementor of a compressor follow the general
|
||
algorithm presented here, which is known not to be patented per se.
|
||
The material in this section is not part of the definition of the
|
||
|
||
|
||
|
||
Deutsch Informational [Page 14]
|
||
|
||
RFC 1951 DEFLATE Compressed Data Format Specification May 1996
|
||
|
||
|
||
specification per se, and a compressor need not follow it in order to
|
||
be compliant.
|
||
|
||
The compressor terminates a block when it determines that starting a
|
||
new block with fresh trees would be useful, or when the block size
|
||
fills up the compressor's block buffer.
|
||
|
||
The compressor uses a chained hash table to find duplicated strings,
|
||
using a hash function that operates on 3-byte sequences. At any
|
||
given point during compression, let XYZ be the next 3 input bytes to
|
||
be examined (not necessarily all different, of course). First, the
|
||
compressor examines the hash chain for XYZ. If the chain is empty,
|
||
the compressor simply writes out X as a literal byte and advances one
|
||
byte in the input. If the hash chain is not empty, indicating that
|
||
the sequence XYZ (or, if we are unlucky, some other 3 bytes with the
|
||
same hash function value) has occurred recently, the compressor
|
||
compares all strings on the XYZ hash chain with the actual input data
|
||
sequence starting at the current point, and selects the longest
|
||
match.
|
||
|
||
The compressor searches the hash chains starting with the most recent
|
||
strings, to favor small distances and thus take advantage of the
|
||
Huffman encoding. The hash chains are singly linked. There are no
|
||
deletions from the hash chains; the algorithm simply discards matches
|
||
that are too old. To avoid a worst-case situation, very long hash
|
||
chains are arbitrarily truncated at a certain length, determined by a
|
||
run-time parameter.
|
||
|
||
To improve overall compression, the compressor optionally defers the
|
||
selection of matches ("lazy matching"): after a match of length N has
|
||
been found, the compressor searches for a longer match starting at
|
||
the next input byte. If it finds a longer match, it truncates the
|
||
previous match to a length of one (thus producing a single literal
|
||
byte) and then emits the longer match. Otherwise, it emits the
|
||
original match, and, as described above, advances N bytes before
|
||
continuing.
|
||
|
||
Run-time parameters also control this "lazy match" procedure. If
|
||
compression ratio is most important, the compressor attempts a
|
||
complete second search regardless of the length of the first match.
|
||
In the normal case, if the current match is "long enough", the
|
||
compressor reduces the search for a longer match, thus speeding up
|
||
the process. If speed is most important, the compressor inserts new
|
||
strings in the hash table only when no match was found, or when the
|
||
match is not "too long". This degrades the compression ratio but
|
||
saves time since there are both fewer insertions and fewer searches.
|
||
|
||
|
||
|
||
|
||
|
||
Deutsch Informational [Page 15]
|
||
|
||
RFC 1951 DEFLATE Compressed Data Format Specification May 1996
|
||
|
||
|
||
5. References
|
||
|
||
[1] Huffman, D. A., "A Method for the Construction of Minimum
|
||
Redundancy Codes", Proceedings of the Institute of Radio
|
||
Engineers, September 1952, Volume 40, Number 9, pp. 1098-1101.
|
||
|
||
[2] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data
|
||
Compression", IEEE Transactions on Information Theory, Vol. 23,
|
||
No. 3, pp. 337-343.
|
||
|
||
[3] Gailly, J.-L., and Adler, M., ZLIB documentation and sources,
|
||
available in ftp://ftp.uu.net/pub/archiving/zip/doc/
|
||
|
||
[4] Gailly, J.-L., and Adler, M., GZIP documentation and sources,
|
||
available as gzip-*.tar in ftp://prep.ai.mit.edu/pub/gnu/
|
||
|
||
[5] Schwartz, E. S., and Kallick, B. "Generating a canonical prefix
|
||
encoding." Comm. ACM, 7,3 (Mar. 1964), pp. 166-169.
|
||
|
||
[6] Hirschberg and Lelewer, "Efficient decoding of prefix codes,"
|
||
Comm. ACM, 33,4, April 1990, pp. 449-459.
|
||
|
||
6. Security Considerations
|
||
|
||
Any data compression method involves the reduction of redundancy in
|
||
the data. Consequently, any corruption of the data is likely to have
|
||
severe effects and be difficult to correct. Uncompressed text, on
|
||
the other hand, will probably still be readable despite the presence
|
||
of some corrupted bytes.
|
||
|
||
It is recommended that systems using this data format provide some
|
||
means of validating the integrity of the compressed data. See
|
||
reference [3], for example.
|
||
|
||
7. Source code
|
||
|
||
Source code for a C language implementation of a "deflate" compliant
|
||
compressor and decompressor is available within the zlib package at
|
||
ftp://ftp.uu.net/pub/archiving/zip/zlib/.
|
||
|
||
8. Acknowledgements
|
||
|
||
Trademarks cited in this document are the property of their
|
||
respective owners.
|
||
|
||
Phil Katz designed the deflate format. Jean-Loup Gailly and Mark
|
||
Adler wrote the related software described in this specification.
|
||
Glenn Randers-Pehrson converted this document to RFC and HTML format.
|
||
|
||
|
||
|
||
Deutsch Informational [Page 16]
|
||
|
||
RFC 1951 DEFLATE Compressed Data Format Specification May 1996
|
||
|
||
|
||
9. Author's Address
|
||
|
||
L. Peter Deutsch
|
||
Aladdin Enterprises
|
||
203 Santa Margarita Ave.
|
||
Menlo Park, CA 94025
|
||
|
||
Phone: (415) 322-0103 (AM only)
|
||
FAX: (415) 322-1734
|
||
EMail: <ghost@aladdin.com>
|
||
|
||
Questions about the technical content of this specification can be
|
||
sent by email to:
|
||
|
||
Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
|
||
Mark Adler <madler@alumni.caltech.edu>
|
||
|
||
Editorial comments on this specification can be sent by email to:
|
||
|
||
L. Peter Deutsch <ghost@aladdin.com> and
|
||
Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
Deutsch Informational [Page 17]
|
||
|