inflate.c

#define DEBG(x)
#define DEBG1(x)
/* inflate.c -- Not copyrighted 1992 by Mark Adler
   version c10p1, 10 January 1993 */

/* 
 * Adapted for booting Linux by Hannu Savolainen 1993
 * based on gzip-1.0.3 
 *
 * Nicolas Pitre <nico@fluxnic.net>, 1999/04/14 :
 *   Little mods for all variable to reside either into rodata or bss segments
 *   by marking constant variables with 'const' and initializing all the others
 *   at run-time only.  This allows for the kernel uncompressor to run
 *   directly from Flash or ROM memory on embedded systems.
 */

/*
   Inflate deflated (PKZIP's method 8 compressed) data.  The compression
   method searches for as much of the current string of bytes (up to a
   length of 258) in the previous 32 K bytes.  If it doesn't find any
   matches (of at least length 3), it codes the next byte.  Otherwise, it
   codes the length of the matched string and its distance backwards from
   the current position.  There is a single Huffman code that codes both
   single bytes (called "literals") and match lengths.  A second Huffman
   code codes the distance information, which follows a length code.  Each
   length or distance code actually represents a base value and a number
   of "extra" (sometimes zero) bits to get to add to the base value.  At
   the end of each deflated block is a special end-of-block (EOB) literal/
   length code.  The decoding process is basically: get a literal/length
   code; if EOB then done; if a literal, emit the decoded byte; if a
   length then get the distance and emit the referred-to bytes from the
   sliding window of previously emitted data.

   There are (currently) three kinds of inflate blocks: stored, fixed, and
   dynamic.  The compressor deals with some chunk of data at a time, and
   decides which method to use on a chunk-by-chunk basis.  A chunk might
   typically be 32 K or 64 K.  If the chunk is incompressible, then the
   "stored" method is used.  In this case, the bytes are simply stored as
   is, eight bits per byte, with none of the above coding.  The bytes are
   preceded by a count, since there is no longer an EOB code.

   If the data is compressible, then either the fixed or dynamic methods
   are used.  In the dynamic method, the compressed data is preceded by
   an encoding of the literal/length and distance Huffman codes that are
   to be used to decode this block.  The representation is itself Huffman
   coded, and so is preceded by a description of that code.  These code
   descriptions take up a little space, and so for small blocks, there is
   a predefined set of codes, called the fixed codes.  The fixed method is
   used if the block codes up smaller that way (usually for quite small
   chunks), otherwise the dynamic method is used.  In the latter case, the
   codes are customized to the probabilities in the current block, and so
   can code it much better than the pre-determined fixed codes.
 
   The Huffman codes themselves are decoded using a multi-level table
   lookup, in order to maximize the speed of decoding plus the speed of
   building the decoding tables.  See the comments below that precede the
   lbits and dbits tuning parameters.
 */


/*
   Notes beyond the 1.93a appnote.txt:

   1. Distance pointers never point before the beginning of the output
      stream.
   2. Distance pointers can point back across blocks, up to 32k away.
   3. There is an implied maximum of 7 bits for the bit length table and
      15 bits for the actual data.
   4. If only one code exists, then it is encoded using one bit.  (Zero
      would be more efficient, but perhaps a little confusing.)  If two