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-Network Working Group P. Deutsch
-Request for Comments: 1951 Aladdin Enterprises
-Category: Informational May 1996
-
-
- DEFLATE Compressed Data Format Specification version 1.3
-
-Status of This Memo
-
- This memo provides information for the Internet community. This memo
- does not specify an Internet standard of any kind. Distribution of
- this memo is unlimited.
-
-IESG Note:
-
- The IESG takes no position on the validity of any Intellectual
- Property Rights statements contained in this document.
-
-Notices
-
- Copyright (c) 1996 L. Peter Deutsch
-
- Permission is granted to copy and distribute this document for any
- purpose and without charge, including translations into other
- languages and incorporation into compilations, provided that the
- copyright notice and this notice are preserved, and that any
- substantive changes or deletions from the original are clearly
- marked.
-
- A pointer to the latest version of this and related documentation in
- HTML format can be found at the URL
- <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
-
-Abstract
-
- This specification defines a lossless compressed data format that
- compresses data using a combination of the LZ77 algorithm and Huffman
- coding, with efficiency comparable to the best currently available
- general-purpose compression methods. The data 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. The format can be implemented readily in a manner not
- covered by patents.
-
-
-
-
-
-
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-
-Table of Contents
-
- 1. Introduction ................................................... 2
- 1.1. Purpose ................................................... 2
- 1.2. Intended audience ......................................... 3
- 1.3. Scope ..................................................... 3
- 1.4. Compliance ................................................ 3
- 1.5. Definitions of terms and conventions used ................ 3
- 1.6. Changes from previous versions ............................ 4
- 2. Compressed representation overview ............................. 4
- 3. Detailed specification ......................................... 5
- 3.1. Overall conventions ....................................... 5
- 3.1.1. Packing into bytes .................................. 5
- 3.2. Compressed block format ................................... 6
- 3.2.1. Synopsis of prefix and Huffman coding ............... 6
- 3.2.2. Use of Huffman coding in the "deflate" format ....... 7
- 3.2.3. Details of block format ............................. 9
- 3.2.4. Non-compressed blocks (BTYPE=00) ................... 11
- 3.2.5. Compressed blocks (length and distance codes) ...... 11
- 3.2.6. Compression with fixed Huffman codes (BTYPE=01) .... 12
- 3.2.7. Compression with dynamic Huffman codes (BTYPE=10) .. 13
- 3.3. Compliance ............................................... 14
- 4. Compression algorithm details ................................. 14
- 5. References .................................................... 16
- 6. Security Considerations ....................................... 16
- 7. Source code ................................................... 16
- 8. Acknowledgements .............................................. 16
- 9. Author's Address .............................................. 17
-
-1. Introduction
-
- 1.1. Purpose
-
- The purpose of this specification is to define a lossless
- 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 considerably better than the "compress"
- program;
- * Can be implemented readily in a manner not covered by
- patents, and hence can be practiced freely;
-
-
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- * 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.
-
- The data format defined by this specification does not attempt to:
-
- * Allow random 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. Scope
-
- 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
-
-
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-
- 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 <length, backward distance>. 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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-
-3. Detailed specification
-
- 3.1. Overall conventions In the diagrams below, a box like this:
-
- +---+
- | | <-- the vertical bars might be missing
- +---+
-
- represents one byte; a box like this:
-
- +==============+
- | |
- +==============+
-
- represents a variable number of bytes.
-
- Bytes stored within a computer do not have a "bit order", since
- they are always treated as a unit. However, a byte considered as
- an integer between 0 and 255 does have a most- and least-
- significant bit, and since we write numbers with the most-
- significant digit on the left, we also write bytes with the most-
- significant bit on the left. In the diagrams below, we number the
- bits of a byte so that bit 0 is the least-significant bit, i.e.,
- the bits are numbered:
-
- +--------+
- |76543210|
- +--------+
-
- Within a computer, a number may occupy multiple bytes. All
- multi-byte numbers in the format described here are stored with
- the least-significant byte first (at the lower memory address).
- For example, the decimal number 520 is stored as:
-
- 0 1
- +--------+--------+
- |00001000|00000010|
- +--------+--------+
- ^ ^
- | |
- | + more significant byte = 2 x 256
- + less significant byte = 8
-
- 3.1.1. Packing into bytes
-
- This document does not address the issue of the order in which
- bits of a byte are transmitted on a bit-sequential medium,
- since the final data format described here is byte- rather than
-
-
-
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-
- bit-oriented. However, we describe the compressed block format
- in below, as a sequence of data elements of various bit
- lengths, not a sequence of bytes. We must therefore specify
- how to pack these data elements into bytes to form the final
- compressed byte sequence:
-
- * Data elements are packed into bytes in order of
- increasing bit number within the byte, i.e., starting
- with the least-significant bit of the byte.
- * Data elements other than Huffman codes are packed
- starting with the least-significant bit of the data
- element.
- * Huffman codes are packed starting with the most-
- significant bit of the code.
-
- In other words, if one were to print out the compressed data as
- a sequence of bytes, starting with the first byte at the
- *right* margin and proceeding to the *left*, with the most-
- significant bit of each byte on the left as usual, one would be
- able to parse the result from right to left, with fixed-width
- elements in the correct MSB-to-LSB order and Huffman codes in
- bit-reversed order (i.e., with the first bit of the code in the
- relative LSB position).
-
- 3.2. Compressed block format
-
- 3.2.1. Synopsis of prefix and Huffman coding
-
- Prefix coding represents symbols from an a priori known
- alphabet by bit sequences (codes), one code for each symbol, in
- a manner such that different symbols may be represented by bit
- sequences of different lengths, but a parser can always parse
- an encoded string unambiguously symbol-by-symbol.
-
- We define a prefix code in terms of a binary tree in which the
- two edges descending from each non-leaf node are labeled 0 and
- 1 and in which the leaf nodes correspond one-for-one with (are
- labeled with) the symbols of the alphabet; then the code for a
- symbol is the sequence of 0's and 1's on the edges leading from
- the root to the leaf labeled with that symbol. For example:
-
-
-
-
-
-
-
-
-
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-
- /\ Symbol Code
- 0 1 ------ ----
- / \ A 00
- /\ B B 1
- 0 1 C 011
- / \ D 010
- A /\
- 0 1
- / \
- D C
-
- A parser can decode the next symbol from an encoded input
- stream by walking down the tree from the root, at each step
- choosing the edge corresponding to the next input bit.
-
- Given an alphabet with known symbol frequencies, the Huffman
- algorithm allows the construction of an optimal prefix code
- (one which represents strings with those symbol frequencies
- using the fewest bits of any possible prefix codes for that
- alphabet). Such a code is called a Huffman code. (See
- reference [1] in Chapter 5, references for additional
- information on Huffman codes.)
-
- Note that in the "deflate" format, the Huffman codes for the
- various alphabets must not exceed certain maximum code lengths.
- This constraint complicates the algorithm for computing code
- lengths from symbol frequencies. Again, see Chapter 5,
- references for details.
-
- 3.2.2. Use of Huffman coding in the "deflate" format
-
- The Huffman codes used for each alphabet in the "deflate"
- format have two additional rules:
-
- * All codes of a given bit length have lexicographically
- consecutive values, in the same order as the symbols
- they represent;
-
- * Shorter codes lexicographically precede longer codes.
-
-
-
-
-
-
-
-
-
-
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-
- We could recode the example above to follow this rule as
- follows, assuming that the order of the alphabet is ABCD:
-
- Symbol Code
- ------ ----
- A 10
- B 0
- C 110
- D 111
-
- I.e., 0 precedes 10 which precedes 11x, and 110 and 111 are
- lexicographically consecutive.
-
- Given this rule, we can define the Huffman code for an alphabet
- just by giving the bit lengths of the codes for each symbol of
- the alphabet in order; this is sufficient to determine the
- actual codes. In our example, the code is completely defined
- by the sequence of bit lengths (2, 1, 3, 3). The following
- algorithm generates the codes as integers, intended to be read
- from most- to least-significant bit. The code lengths are
- initially in tree[I].Len; the codes are produced in
- tree[I].Code.
-
- 1) Count the number of codes for each code length. Let
- bl_count[N] be the number of codes of length N, N >= 1.
-
- 2) Find the numerical value of the smallest code for each
- code length:
-
- code = 0;
- bl_count[0] = 0;
- for (bits = 1; bits <= MAX_BITS; bits++) {
- code = (code + bl_count[bits-1]) << 1;
- next_code[bits] = code;
- }
-
- 3) Assign numerical values to all codes, using consecutive
- values for all codes of the same length with the base
- values determined at step 2. Codes that are never used
- (which have a bit length of zero) must not be assigned a
- value.
-
- for (n = 0; n <= max_code; n++) {
- len = tree[n].Len;
- if (len != 0) {
- tree[n].Code = next_code[len];
- next_code[len]++;
- }
-
-
-
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-
- }
-
- Example:
-
- Consider the alphabet ABCDEFGH, with bit lengths (3, 3, 3, 3,
- 3, 2, 4, 4). After step 1, we have:
-
- N bl_count[N]
- - -----------
- 2 1
- 3 5
- 4 2
-
- Step 2 computes the following next_code values:
-
- N next_code[N]
- - ------------
- 1 0
- 2 0
- 3 2
- 4 14
-
- Step 3 produces the following code values:
-
- Symbol Length Code
- ------ ------ ----
- A 3 010
- B 3 011
- C 3 100
- D 3 101
- E 3 110
- F 2 00
- G 4 1110
- H 4 1111
-
- 3.2.3. Details of block format
-
- Each block of compressed data begins with 3 header bits
- containing the following data:
-
- first bit BFINAL
- next 2 bits BTYPE
-
- Note that the header bits do not necessarily begin on a byte
- boundary, since a block does not necessarily occupy an integral
- number of bytes.
-
-
-
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- BFINAL is set if and only if this is the last block of the data
- set.
-
- BTYPE specifies how the data are compressed, as follows:
-
- 00 - no compression
- 01 - compressed with fixed Huffman codes
- 10 - compressed with dynamic Huffman codes
- 11 - reserved (error)
-
- The only difference between the two compressed cases is how the
- Huffman codes for the literal/length and distance alphabets are
- defined.
-
- In all cases, the decoding algorithm for the actual data is as
- follows:
-
- do
- read block header from input stream.
- if stored with no compression
- skip any remaining bits in current partially
- processed byte
- read LEN and NLEN (see next section)
- copy LEN bytes of data to output
- otherwise
- if compressed with dynamic Huffman codes
- read representation of code trees (see
- subsection below)
- loop (until end of block code recognized)
- decode literal/length value from input stream
- if value < 256
- copy value (literal byte) to output stream
- otherwise
- if value = end of block (256)
- break from loop
- otherwise (value = 257..285)
- decode distance from input stream
-
- move backwards distance bytes in the output
- stream, and copy length bytes from this
- position to the output stream.
- end loop
- while not last block
-
- Note that a duplicated string reference may refer to a string
- in a previous block; i.e., the backward distance may cross one
- or more block boundaries. However a distance cannot refer past
- the beginning of the output stream. (An application using a
-
-
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-
- preset dictionary might discard part of the output stream; a
- distance can refer to that part of the output stream anyway)
- Note also that the referenced string may overlap the current
- position; for example, if the last 2 bytes decoded have values
- X and Y, a string reference with <length = 5, distance = 2>
- adds X,Y,X,Y,X to the output stream.
-
- We now specify each compression method in turn.
-
- 3.2.4. Non-compressed blocks (BTYPE=00)
-
- Any bits of input up to the next byte boundary are ignored.
- The rest of the block consists of the following information:
-
- 0 1 2 3 4...
- +---+---+---+---+================================+
- | LEN | NLEN |... LEN bytes of literal data...|
- +---+---+---+---+================================+
-
- 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:
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
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-
- 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
-
-
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- 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)
-
-
-
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- (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
-
-
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- 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.
-
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-RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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-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.
-
-
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-RFC 1951 DEFLATE Compressed Data Format Specification May 1996
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-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>
-
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