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.











 
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




 
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.