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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


     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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


        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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


                         /\              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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


        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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


               }

        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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


        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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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


        The code lengths are sufficient to generate the actual codes,
        as described above; we show the codes in the table for added
        clarity.  Literal/length values 286-287 will never actually
        occur in the compressed data, but participate in the code
        construction.

        Distance codes 0-31 are represented by (fixed-length) 5-bit
        codes, with possible additional bits as shown in the table
        shown in Paragraph 3.2.5, above.  Note that distance codes 30-
        31 will never actually occur in the compressed data.

     3.2.7. Compression with dynamic Huffman codes (BTYPE=10)

        The Huffman codes for the two alphabets appear in the block
        immediately after the header bits and before the actual
        compressed data, first the literal/length code and then the
        distance code.  Each code is defined by a sequence of code
        lengths, as discussed in Paragraph 3.2.2, above.  For even
        greater compactness, the code length sequences themselves are
        compressed using a Huffman code.  The alphabet for code lengths
        is as follows:

              0 - 15: Represent code lengths of 0 - 15
                  16: Copy the previous code length 3 - 6 times.
                      The next 2 bits indicate repeat length
                            (0 = 3, ... , 3 = 6)
                         Example:  Codes 8, 16 (+2 bits 11),
                                   16 (+2 bits 10) will expand to
                                   12 code lengths of 8 (1 + 6 + 5)
                  17: Repeat a code length of 0 for 3 - 10 times.
                      (3 bits of length)
                  18: Repeat a code length of 0 for 11 - 138 times
                      (7 bits of length)

        A code length of 0 indicates that the corresponding symbol in
        the literal/length or distance alphabet will not occur in the
        block, and should not participate in the Huffman code
        construction algorithm given earlier.  If only one distance
        code is used, it is encoded using one bit, not zero bits; in
        this case there is a single code length of one, with one unused
        code.  One distance code of zero bits means that there are no
        distance codes used at all (the data is all literals).

        We can now define the format of the block:

              5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286)
              5 Bits: HDIST, # of Distance codes - 1        (1 - 32)
              4 Bits: HCLEN, # of Code Length codes - 4     (4 - 19)



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RFC 1951      DEFLATE Compressed Data Format Specification      May 1996


              (HCLEN + 4) x 3 bits: code lengths for the code length
                 alphabet given just above, in the order: 16, 17, 18,
                 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15

                 These code lengths are interpreted as 3-bit integers
                 (0-7); as above, a code length of 0 means the
                 corresponding symbol (literal/length or distance code
                 length) is not used.

              HLIT + 257 code lengths for the literal/length alphabet,
                 encoded using the code length Huffman code

              HDIST + 1 code lengths for the distance alphabet,
                 encoded using the code length Huffman code

              The actual compressed data of the block,
                 encoded using the literal/length and distance Huffman
                 codes

              The literal/length symbol 256 (end of data),
                 encoded using the literal/length Huffman code

        The code length repeat codes can cross from HLIT + 257 to the
        HDIST + 1 code lengths.  In other words, all code lengths form
        a single sequence of HLIT + HDIST + 258 values.

  3.3. Compliance

     A compressor may limit further the ranges of values specified in
     the previous section and still be compliant; for example, it may
     limit the range of backward pointers to some value smaller than
     32K.  Similarly, a compressor may limit the size of blocks so that
     a compressible block fits in memory.

     A compliant decompressor must accept the full range of possible
     values defined in the previous section, and must accept blocks of
     arbitrary size.

4. Compression algorithm details

  While it is the intent of this document to define the "deflate"
  compressed data format without reference to any particular
  compression algorithm, the format is related to the compressed
  formats produced by LZ77 (Lempel-Ziv 1977, see reference [2] below);
  since many variations of LZ77 are patented, it is strongly
  recommended that the implementor of a compressor follow the general
  algorithm presented here, which is known not to be patented per se.
  The material in this section is not part of the definition of the



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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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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: <[email protected]>

  Questions about the technical content of this specification can be
  sent by email to:

  Jean-Loup Gailly <[email protected]> and
  Mark Adler <[email protected]>

  Editorial comments on this specification can be sent by email to:

  L. Peter Deutsch <[email protected]> and
  Glenn Randers-Pehrson <[email protected]>






























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