deflate.c

给出了 zip 压缩算法的完整实现过程。

/*
  Copyright (c) 1990-2005 Info-ZIP.  All rights reserved.

  See the accompanying file LICENSE, version 2005-Feb-10 or later
  (the contents of which are also included in zip.h) for terms of use.
  If, for some reason, all these files are missing, the Info-ZIP license
  also may be found at:  ftp://ftp.info-zip.org/pub/infozip/license.html
*/
/*
 *  deflate.c by Jean-loup Gailly.
 *
 *  PURPOSE
 *
 *      Identify new text as repetitions of old text within a fixed-
 *      length sliding window trailing behind the new text.
 *
 *  DISCUSSION
 *
 *      The "deflation" process depends on being able to identify portions
 *      of the input text which are identical to earlier input (within a
 *      sliding window trailing behind the input currently being processed).
 *
 *      The most straightforward technique turns out to be the fastest for
 *      most input files: try all possible matches and select the longest.
 *      The key feature of this algorithm is that insertions into the string
 *      dictionary are very simple and thus fast, and deletions are avoided
 *      completely. Insertions are performed at each input character, whereas
 *      string matches are performed only when the previous match ends. So it
 *      is preferable to spend more time in matches to allow very fast string
 *      insertions and avoid deletions. The matching algorithm for small
 *      strings is inspired from that of Rabin & Karp. A brute force approach
 *      is used to find longer strings when a small match has been found.
 *      A similar algorithm is used in comic (by Jan-Mark Wams) and freeze
 *      (by Leonid Broukhis).
 *         A previous version of this file used a more sophisticated algorithm
 *      (by Fiala and Greene) which is guaranteed to run in linear amortized
 *      time, but has a larger average cost, uses more memory and is patented.
 *      However the F&G algorithm may be faster for some highly redundant
 *      files if the parameter max_chain_length (described below) is too large.
 *
 *  ACKNOWLEDGEMENTS
 *
 *      The idea of lazy evaluation of matches is due to Jan-Mark Wams, and
 *      I found it in 'freeze' written by Leonid Broukhis.
 *      Thanks to many info-zippers for bug reports and testing.
 *
 *  REFERENCES
 *
 *      APPNOTE.TXT documentation file in PKZIP 1.93a distribution.
 *
 *      A description of the Rabin and Karp algorithm is given in the book
 *         "Algorithms" by R. Sedgewick, Addison-Wesley, p252.
 *
 *      Fiala,E.R., and Greene,D.H.
 *         Data Compression with Finite Windows, Comm.ACM, 32,4 (1989) 490-595
 *
 *  INTERFACE
 *
 *      void lm_init (int pack_level, ush *flags)
 *          Initialize the "longest match" routines for a new file
 *
 *      ulg deflate (void)
 *          Processes a new input file and return its compressed length. Sets
 *          the compressed length, crc, deflate flags and internal file
 *          attributes.
 */

#define __DEFLATE_C

#include "zip.h"

#ifndef USE_ZLIB

/* ===========================================================================
 * Configuration parameters
 */

/* Compile with MEDIUM_MEM to reduce the memory requirements or
 * with SMALL_MEM to use as little memory as possible. Use BIG_MEM if the
 * entire input file can be held in memory (not possible on 16 bit systems).
 * Warning: defining these symbols affects HASH_BITS (see below) and thus
 * affects the compression ratio. The compressed output
 * is still correct, and might even be smaller in some cases.
 */

#ifdef SMALL_MEM
#   define HASH_BITS  13  /* Number of bits used to hash strings */
#endif
#ifdef MEDIUM_MEM
#   define HASH_BITS  14
#endif
#ifndef HASH_BITS
#   define HASH_BITS  15
   /* For portability to 16 bit machines, do not use values above 15. */
#endif

#define HASH_SIZE (unsigned)(1<<HASH_BITS)
#define HASH_MASK (HASH_SIZE-1)
#define WMASK     (WSIZE-1)
/* HASH_SIZE and WSIZE must be powers of two */

#define NIL 0
/* Tail of hash chains */

#define FAST 4
#define SLOW 2
/* speed options for the general purpose bit flag */

#ifndef TOO_FAR
#  define TOO_FAR 4096
#endif
/* Matches of length 3 are discarded if their distance exceeds TOO_FAR */

#if (defined(ASMV) && !defined(MSDOS16) && defined(DYN_ALLOC))
   error: DYN_ALLOC not yet supported in match.S or match32.asm
#endif

#ifdef MEMORY16
#  define MAXSEG_64K
#endif

/* ===========================================================================
 * Local data used by the "longest match" routines.
 */

#if defined(MMAP) || defined(BIG_MEM)
  typedef unsigned Pos; /* must be at least 32 bits */
#else
  typedef ush Pos;
#endif
typedef unsigned IPos;
/* A Pos is an index in the character window. We use short instead of int to
 * save space in the various tables. IPos is used only for parameter passing.
 */

#ifndef DYN_ALLOC
  uch    window[2L*WSIZE];
  /* Sliding window. Input bytes are read into the second half of the window,
   * and move to the first half later to keep a dictionary of at least WSIZE
   * bytes. With this organization, matches are limited to a distance of
   * WSIZE-MAX_MATCH bytes, but this ensures that IO is always
   * performed with a length multiple of the block size. Also, it limits
   * the window size to 64K, which is quite useful on MSDOS.
   * To do: limit the window size to WSIZE+CBSZ if SMALL_MEM (the code would
   * be less efficient since the data would have to be copied WSIZE/CBSZ times)
   */
  Pos    prev[WSIZE];
  /* Link to older string with same hash index. To limit the size of this
   * array to 64K, this link is maintained only for the last 32K strings.
   * An index in this array is thus a window index modulo 32K.
   */
  Pos    head[HASH_SIZE];
  /* Heads of the hash chains or NIL. If your compiler thinks that
   * HASH_SIZE is a dynamic value, recompile with -DDYN_ALLOC.
   */
#else
  uch far * near window = NULL;
  Pos far * near prev   = NULL;
  Pos far * near head;
#endif
ulg window_size;
/* window size, 2*WSIZE except for MMAP or BIG_MEM, where it is the
 * input file length plus MIN_LOOKAHEAD.
 */

long block_start;
/* window position at the beginning of the current output block. Gets
 * negative when the window is moved backwards.
 */

local int sliding;
/* Set to false when the input file is already in memory */

local unsigned ins_h;  /* hash index of string to be inserted */

#define H_SHIFT  ((HASH_BITS+MIN_MATCH-1)/MIN_MATCH)
/* Number of bits by which ins_h and del_h must be shifted at each
 * input step. It must be such that after MIN_MATCH steps, the oldest
 * byte no longer takes part in the hash key, that is:
 *   H_SHIFT * MIN_MATCH >= HASH_BITS
 */

unsigned int near prev_length;
/* Length of the best match at previous step. Matches not greater than this
 * are discarded. This is used in the lazy match evaluation.
 */

      unsigned near strstart;      /* start of string to insert */
      unsigned near match_start;   /* start of matching string */
local int           eofile;        /* flag set at end of input file */
local unsigned      lookahead;     /* number of valid bytes ahead in window */

unsigned near max_chain_length;
/* To speed up deflation, hash chains are never searched beyond this length.
 * A higher limit improves compression ratio but degrades the speed.
 */

local unsigned int max_lazy_match;
/* Attempt to find a better match only when the current match is strictly
 * smaller than this value. This mechanism is used only for compression
 * levels >= 4.
 */
#define max_insert_length  max_lazy_match
/* Insert new strings in the hash table only if the match length
 * is not greater than this length. This saves time but degrades compression.
 * max_insert_length is used only for compression levels <= 3.
 */

unsigned near good_match;
/* Use a faster search when the previous match is longer than this */

#ifdef  FULL_SEARCH
# define nice_match MAX_MATCH
#else
  int near nice_match; /* Stop searching when current match exceeds this */
#endif


/* Values for max_lazy_match, good_match, nice_match and max_chain_length,
 * depending on the desired pack level (0..9). The values given below have
 * been tuned to exclude worst case performance for pathological files.
 * Better values may be found for specific files.
 */

typedef struct config {
   ush good_length; /* reduce lazy search above this match length */
   ush max_lazy;    /* do not perform lazy search above this match length */
   ush nice_length; /* quit search above this match length */
   ush max_chain;
} config;

local config configuration_table[10] = {
/*      good lazy nice chain */
/* 0 */ {0,    0,  0,    0},  /* store only */
/* 1 */ {4,    4,  8,    4},  /* maximum speed, no lazy matches */
/* 2 */ {4,    5, 16,    8},
/* 3 */ {4,    6, 32,   32},

/* 4 */ {4,    4, 16,   16},  /* lazy matches */
/* 5 */ {8,   16, 32,   32},
/* 6 */ {8,   16, 128, 128},
/* 7 */ {8,   32, 128, 256},
/* 8 */ {32, 128, 258, 1024},
/* 9 */ {32, 258, 258, 4096}}; /* maximum compression */

/* Note: the deflate() code requires max_lazy >= MIN_MATCH and max_chain >= 4
 * For deflate_fast() (levels <= 3) good is ignored and lazy has a different
 * meaning.
 */

#define EQUAL 0
/* result of memcmp for equal strings */

/* ===========================================================================
 *  Prototypes for local functions.
 */

local void fill_window   OF((void));
local ulg deflate_fast   OF((void));

      int  longest_match OF((IPos cur_match));
#if defined(ASMV) && !defined(RISCOS)
      void match_init OF((void)); /* asm code initialization */
#endif

#ifdef DEBUG
local  void check_match OF((IPos start, IPos match, int length));
#endif

/* ===========================================================================
 * Update a hash value with the given input byte
 * IN  assertion: all calls to to UPDATE_HASH are made with consecutive
 *    input characters, so that a running hash key can be computed from the
 *    previous key instead of complete recalculation each time.
 */
#define UPDATE_HASH(h,c) (h = (((h)<<H_SHIFT) ^ (c)) & HASH_MASK)

/* ===========================================================================
 * Insert string s in the dictionary and set match_head to the previous head
 * of the hash chain (the most recent string with same hash key). Return
 * the previous length of the hash chain.
 * IN  assertion: all calls to to INSERT_STRING are made with consecutive
 *    input characters and the first MIN_MATCH bytes of s are valid
 *    (except for the last MIN_MATCH-1 bytes of the input file).
 */
#define INSERT_STRING(s, match_head) \
   (UPDATE_HASH(ins_h, window[(s) + (MIN_MATCH-1)]), \
    prev[(s) & WMASK] = match_head = head[ins_h], \
    head[ins_h] = (s))

/* ===========================================================================
 * Initialize the "longest match" routines for a new file
 *
 * IN assertion: window_size is > 0 if the input file is already read or
 *    mmap'ed in the window[] array, 0 otherwise. In the first case,
 *    window_size is sufficient to contain the whole input file plus
 *    MIN_LOOKAHEAD bytes (to avoid referencing memory beyond the end
 *    of window[] when looking for matches towards the end).
 */
void lm_init (pack_level, flags)
    int pack_level; /* 0: store, 1: best speed, 9: best compression */
    ush *flags;     /* general purpose bit flag */
{