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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]
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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.
-
-
-
-
-
-
-
-
-
-
-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]
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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.
-
-
-
-
-
-
-
-
-
-
-
-
-Deutsch                      Informational                      [Page 7]
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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]++;
-                    }
-
-
-
-Deutsch                      Informational                      [Page 8]
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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.
-
-
-
-
-
-Deutsch                      Informational                      [Page 9]
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-RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
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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
-
-
-
-Deutsch                      Informational                     [Page 10]
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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:
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-Deutsch                      Informational                     [Page 11]
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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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-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
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-   specification per se, and a compressor need not follow it in order to
-   be compliant.
-
-   The compressor terminates a block when it determines that starting a
-   new block with fresh trees would be useful, or when the block size
-   fills up the compressor's block buffer.
-
-   The compressor uses a chained hash table to find duplicated strings,
-   using a hash function that operates on 3-byte sequences.  At any
-   given point during compression, let XYZ be the next 3 input bytes to
-   be examined (not necessarily all different, of course).  First, the
-   compressor examines the hash chain for XYZ.  If the chain is empty,
-   the compressor simply writes out X as a literal byte and advances one
-   byte in the input.  If the hash chain is not empty, indicating that
-   the sequence XYZ (or, if we are unlucky, some other 3 bytes with the
-   same hash function value) has occurred recently, the compressor
-   compares all strings on the XYZ hash chain with the actual input data
-   sequence starting at the current point, and selects the longest
-   match.
-
-   The compressor searches the hash chains starting with the most recent
-   strings, to favor small distances and thus take advantage of the
-   Huffman encoding.  The hash chains are singly linked. There are no
-   deletions from the hash chains; the algorithm simply discards matches
-   that are too old.  To avoid a worst-case situation, very long hash
-   chains are arbitrarily truncated at a certain length, determined by a
-   run-time parameter.
-
-   To improve overall compression, the compressor optionally defers the
-   selection of matches ("lazy matching"): after a match of length N has
-   been found, the compressor searches for a longer match starting at
-   the next input byte.  If it finds a longer match, it truncates the
-   previous match to a length of one (thus producing a single literal
-   byte) and then emits the longer match.  Otherwise, it emits the
-   original match, and, as described above, advances N bytes before
-   continuing.
-
-   Run-time parameters also control this "lazy match" procedure.  If
-   compression ratio is most important, the compressor attempts a
-   complete second search regardless of the length of the first match.
-   In the normal case, if the current match is "long enough", the
-   compressor reduces the search for a longer match, thus speeding up
-   the process.  If speed is most important, the compressor inserts new
-   strings in the hash table only when no match was found, or when the
-   match is not "too long".  This degrades the compression ratio but
-   saves time since there are both fewer insertions and fewer searches.
-
-
-
-
-
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-RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
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-
-5. References
-
-   [1] Huffman, D. A., "A Method for the Construction of Minimum
-       Redundancy Codes", Proceedings of the Institute of Radio
-       Engineers, September 1952, Volume 40, Number 9, pp. 1098-1101.
-
-   [2] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data
-       Compression", IEEE Transactions on Information Theory, Vol. 23,
-       No. 3, pp. 337-343.
-
-   [3] Gailly, J.-L., and Adler, M., ZLIB documentation and sources,
-       available in ftp://ftp.uu.net/pub/archiving/zip/doc/
-
-   [4] Gailly, J.-L., and Adler, M., GZIP documentation and sources,
-       available as gzip-*.tar in ftp://prep.ai.mit.edu/pub/gnu/
-
-   [5] Schwartz, E. S., and Kallick, B. "Generating a canonical prefix
-       encoding." Comm. ACM, 7,3 (Mar. 1964), pp. 166-169.
-
-   [6] Hirschberg and Lelewer, "Efficient decoding of prefix codes,"
-       Comm. ACM, 33,4, April 1990, pp. 449-459.
-
-6. Security Considerations
-
-   Any data compression method involves the reduction of redundancy in
-   the data.  Consequently, any corruption of the data is likely to have
-   severe effects and be difficult to correct.  Uncompressed text, on
-   the other hand, will probably still be readable despite the presence
-   of some corrupted bytes.
-
-   It is recommended that systems using this data format provide some
-   means of validating the integrity of the compressed data.  See
-   reference [3], for example.
-
-7. Source code
-
-   Source code for a C language implementation of a "deflate" compliant
-   compressor and decompressor is available within the zlib package at
-   ftp://ftp.uu.net/pub/archiving/zip/zlib/.
-
-8. Acknowledgements
-
-   Trademarks cited in this document are the property of their
-   respective owners.
-
-   Phil Katz designed the deflate format.  Jean-Loup Gailly and Mark
-   Adler wrote the related software described in this specification.
-   Glenn Randers-Pehrson converted this document to RFC and HTML format.
-
-
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-RFC 1951      DEFLATE Compressed Data Format Specification      May 1996
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-9. Author's Address
-
-   L. Peter Deutsch
-   Aladdin Enterprises
-   203 Santa Margarita Ave.
-   Menlo Park, CA 94025
-
-   Phone: (415) 322-0103 (AM only)
-   FAX:   (415) 322-1734
-   EMail: <ghost@aladdin.com>
-
-   Questions about the technical content of this specification can be
-   sent by email to:
-
-   Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
-   Mark Adler <madler@alumni.caltech.edu>
-
-   Editorial comments on this specification can be sent by email to:
-
-   L. Peter Deutsch <ghost@aladdin.com> and
-   Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
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