FreeBSD/Linux Kernel Cross Reference
sys/contrib/zlib/doc/rfc1951.txt

     
     
     
     
     
     
     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
   
   
   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
   
   
   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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