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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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 RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
 
 
 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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 RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
 
 
           * 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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 RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
 
 
          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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 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
 
 
 
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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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 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.
 
 
 
 
 
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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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