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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.
+
+
+
+
+
+
+
+
+Deutsch                      Informational                      [Page 1]
+
+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;
+
+
+
+Deutsch                      Informational                      [Page 2]
+
+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
+
+
+
+Deutsch                      Informational                      [Page 3]
+
+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.
+
+
+
+
+
+
+
+
+
+
+Deutsch                      Informational                      [Page 4]
+
+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
+
+
+
+Deutsch                      Informational                      [Page 5]
+
+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:
+
+
+
+
+
+
+
+
+
+
+
+Deutsch                      Informational                      [Page 6]
+
+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.
+
+
+
+
+
+
+
+
+
+
+
+
+Deutsch                      Informational                      [Page 7]
+
+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]++;
+                    }
+
+
+
+Deutsch                      Informational                      [Page 8]
+
+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.
+
+
+
+
+
+Deutsch                      Informational                      [Page 9]
+
+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
+
+
+
+Deutsch                      Informational                     [Page 10]
+
+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:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Deutsch                      Informational                     [Page 11]
+
+RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
+
+
+                 Extra               Extra               Extra
+            Code Bits Length(s) Code Bits Lengths   Code Bits Length(s)
+            ---- ---- ------     ---- ---- -------   ---- ---- -------
+             257   0     3       267   1   15,16     277   4   67-82
+             258   0     4       268   1   17,18     278   4   83-98
+             259   0     5       269   2   19-22     279   4   99-114
+             260   0     6       270   2   23-26     280   4  115-130
+             261   0     7       271   2   27-30     281   5  131-162
+             262   0     8       272   2   31-34     282   5  163-194
+             263   0     9       273   3   35-42     283   5  195-226
+             264   0    10       274   3   43-50     284   5  227-257
+             265   1  11,12      275   3   51-58     285   0    258
+             266   1  13,14      276   3   59-66
+
+         The extra bits should be interpreted as a machine integer
+         stored with the most-significant bit first, e.g., bits 1110
+         represent the value 14.
+
+                  Extra           Extra               Extra
+             Code Bits Dist  Code Bits   Dist     Code Bits Distance
+             ---- ---- ----  ---- ----  ------    ---- ---- --------
+               0   0    1     10   4     33-48    20    9   1025-1536
+               1   0    2     11   4     49-64    21    9   1537-2048
+               2   0    3     12   5     65-96    22   10   2049-3072
+               3   0    4     13   5     97-128   23   10   3073-4096
+               4   1   5,6    14   6    129-192   24   11   4097-6144
+               5   1   7,8    15   6    193-256   25   11   6145-8192
+               6   2   9-12   16   7    257-384   26   12  8193-12288
+               7   2  13-16   17   7    385-512   27   12 12289-16384
+               8   3  17-24   18   8    513-768   28   13 16385-24576
+               9   3  25-32   19   8   769-1024   29   13 24577-32768
+
+      3.2.6. Compression with fixed Huffman codes (BTYPE=01)
+
+         The Huffman codes for the two alphabets are fixed, and are not
+         represented explicitly in the data.  The Huffman code lengths
+         for the literal/length alphabet are:
+
+                   Lit Value    Bits        Codes
+                   ---------    ----        -----
+                     0 - 143     8          00110000 through
+                                            10111111
+                   144 - 255     9          110010000 through
+                                            111111111
+                   256 - 279     7          0000000 through
+                                            0010111
+                   280 - 287     8          11000000 through
+                                            11000111
+
+
+
+Deutsch                      Informational                     [Page 12]
+
+RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
+
+
+         The code lengths are sufficient to generate the actual codes,
+         as described above; we show the codes in the table for added
+         clarity.  Literal/length values 286-287 will never actually
+         occur in the compressed data, but participate in the code
+         construction.
+
+         Distance codes 0-31 are represented by (fixed-length) 5-bit
+         codes, with possible additional bits as shown in the table
+         shown in Paragraph 3.2.5, above.  Note that distance codes 30-
+         31 will never actually occur in the compressed data.
+
+      3.2.7. Compression with dynamic Huffman codes (BTYPE=10)
+
+         The Huffman codes for the two alphabets appear in the block
+         immediately after the header bits and before the actual
+         compressed data, first the literal/length code and then the
+         distance code.  Each code is defined by a sequence of code
+         lengths, as discussed in Paragraph 3.2.2, above.  For even
+         greater compactness, the code length sequences themselves are
+         compressed using a Huffman code.  The alphabet for code lengths
+         is as follows:
+
+               0 - 15: Represent code lengths of 0 - 15
+                   16: Copy the previous code length 3 - 6 times.
+                       The next 2 bits indicate repeat length
+                             (0 = 3, ... , 3 = 6)
+                          Example:  Codes 8, 16 (+2 bits 11),
+                                    16 (+2 bits 10) will expand to
+                                    12 code lengths of 8 (1 + 6 + 5)
+                   17: Repeat a code length of 0 for 3 - 10 times.
+                       (3 bits of length)
+                   18: Repeat a code length of 0 for 11 - 138 times
+                       (7 bits of length)
+
+         A code length of 0 indicates that the corresponding symbol in
+         the literal/length or distance alphabet will not occur in the
+         block, and should not participate in the Huffman code
+         construction algorithm given earlier.  If only one distance
+         code is used, it is encoded using one bit, not zero bits; in
+         this case there is a single code length of one, with one unused
+         code.  One distance code of zero bits means that there are no
+         distance codes used at all (the data is all literals).
+
+         We can now define the format of the block:
+
+               5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286)
+               5 Bits: HDIST, # of Distance codes - 1        (1 - 32)
+               4 Bits: HCLEN, # of Code Length codes - 4     (4 - 19)
+
+
+
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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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+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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