trees.c

给出了 zip 压缩算法的完整实现过程。

/*
  Copyright (c) 1990-2006 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
*/
/*
 *  trees.c by Jean-loup Gailly
 *
 *  This is a new version of im_ctree.c originally written by Richard B. Wales
 *  for the defunct implosion method.
 *  The low level bit string handling routines from bits.c (originally
 *  im_bits.c written by Richard B. Wales) have been merged into this version
 *  of trees.c.
 *
 *  PURPOSE
 *
 *      Encode various sets of source values using variable-length
 *      binary code trees.
 *      Output the resulting variable-length bit strings.
 *      Compression can be done to a file or to memory.
 *
 *  DISCUSSION
 *
 *      The PKZIP "deflation" process uses several Huffman trees. The more
 *      common source values are represented by shorter bit sequences.
 *
 *      Each code tree is stored in the ZIP file in a compressed form
 *      which is itself a Huffman encoding of the lengths of
 *      all the code strings (in ascending order by source values).
 *      The actual code strings are reconstructed from the lengths in
 *      the UNZIP process, as described in the "application note"
 *      (APPNOTE.TXT) distributed as part of PKWARE's PKZIP program.
 *
 *      The PKZIP "deflate" file format interprets compressed file data
 *      as a sequence of bits.  Multi-bit strings in the file may cross
 *      byte boundaries without restriction.
 *      The first bit of each byte is the low-order bit.
 *
 *      The routines in this file allow a variable-length bit value to
 *      be output right-to-left (useful for literal values). For
 *      left-to-right output (useful for code strings from the tree routines),
 *      the bits must have been reversed first with bi_reverse().
 *
 *      For in-memory compression, the compressed bit stream goes directly
 *      into the requested output buffer. The buffer is limited to 64K on
 *      16 bit machines; flushing of the output buffer during compression
 *      process is not supported.
 *      The input data is read in blocks by the (*read_buf)() function.
 *
 *      For more details about input to and output from the deflation routines,
 *      see the actual input functions for (*read_buf)(), flush_outbuf(), and
 *      the filecompress() resp. memcompress() wrapper functions which handle
 *      the I/O setup.
 *
 *  REFERENCES
 *
 *      Lynch, Thomas J.
 *          Data Compression:  Techniques and Applications, pp. 53-55.
 *          Lifetime Learning Publications, 1985.  ISBN 0-534-03418-7.
 *
 *      Storer, James A.
 *          Data Compression:  Methods and Theory, pp. 49-50.
 *          Computer Science Press, 1988.  ISBN 0-7167-8156-5.
 *
 *      Sedgewick, R.
 *          Algorithms, p290.
 *          Addison-Wesley, 1983. ISBN 0-201-06672-6.
 *
 *  INTERFACE
 *
 *      void ct_init (ush *attr, int *method)
 *          Allocate the match buffer, initialize the various tables and save
 *          the location of the internal file attribute (ascii/binary) and
 *          method (DEFLATE/STORE)
 *
 *      void ct_tally (int dist, int lc);
 *          Save the match info and tally the frequency counts.
 *
 *      ulg flush_block (char *buf, ulg stored_len, int eof)
 *          Determine the best encoding for the current block: dynamic trees,
 *          static trees or store, and output the encoded block to the zip
 *          file. Returns the total compressed length for the file so far.
 *
 *      void bi_init (char *tgt_buf, unsigned tgt_size, int flsh_allowed)
 *          Initialize the bit string routines.
 *
 *    Most of the bit string output functions are only used internally
 *    in this source file, they are normally declared as "local" routines:
 *
 *      local void send_bits (int value, int length)
 *          Write out a bit string, taking the source bits right to
 *          left.
 *
 *      local unsigned bi_reverse (unsigned code, int len)
 *          Reverse the bits of a bit string, taking the source bits left to
 *          right and emitting them right to left.
 *
 *      local void bi_windup (void)
 *          Write out any remaining bits in an incomplete byte.
 *
 *      local void copy_block(char *buf, unsigned len, int header)
 *          Copy a stored block to the zip file, storing first the length and
 *          its one's complement if requested.
 *
 *    All output that exceeds the bitstring output buffer size (as initialized
 *    by bi_init() is fed through an externally provided transfer routine
 *    which flushes the bitstring output buffer on request and resets the
 *    buffer fill counter:
 *
 *      extern void flush_outbuf(char *o_buf, unsigned *o_idx);
 *
 */
#define __TREES_C

#include "zip.h"
#include <ctype.h>

#ifndef USE_ZLIB

/* ===========================================================================
 * Constants
 */

#define MAX_BITS 15
/* All codes must not exceed MAX_BITS bits */

#define MAX_BL_BITS 7
/* Bit length codes must not exceed MAX_BL_BITS bits */

#define LENGTH_CODES 29
/* number of length codes, not counting the special END_BLOCK code */

#define LITERALS  256
/* number of literal bytes 0..255 */

#define END_BLOCK 256
/* end of block literal code */

#define L_CODES (LITERALS+1+LENGTH_CODES)
/* number of Literal or Length codes, including the END_BLOCK code */

#define D_CODES   30
/* number of distance codes */

#define BL_CODES  19
/* number of codes used to transfer the bit lengths */


local int near extra_lbits[LENGTH_CODES] /* extra bits for each length code */
   = {0,0,0,0,0,0,0,0,1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5,5,0};

local int near extra_dbits[D_CODES] /* extra bits for each distance code */
   = {0,0,0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13};

local int near extra_blbits[BL_CODES]/* extra bits for each bit length code */
   = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,2,3,7};

#define STORED_BLOCK 0
#define STATIC_TREES 1
#define DYN_TREES    2
/* The three kinds of block type */

#ifndef LIT_BUFSIZE
#  ifdef SMALL_MEM
#    define LIT_BUFSIZE  0x2000
#  else
#  ifdef MEDIUM_MEM
#    define LIT_BUFSIZE  0x4000
#  else
#    define LIT_BUFSIZE  0x8000
#  endif
#  endif
#endif
#define DIST_BUFSIZE  LIT_BUFSIZE
/* Sizes of match buffers for literals/lengths and distances.  There are
 * 4 reasons for limiting LIT_BUFSIZE to 64K:
 *   - frequencies can be kept in 16 bit counters
 *   - if compression is not successful for the first block, all input data is
 *     still in the window so we can still emit a stored block even when input
 *     comes from standard input.  (This can also be done for all blocks if
 *     LIT_BUFSIZE is not greater than 32K.)
 *   - if compression is not successful for a file smaller than 64K, we can
 *     even emit a stored file instead of a stored block (saving 5 bytes).
 *   - creating new Huffman trees less frequently may not provide fast
 *     adaptation to changes in the input data statistics. (Take for
 *     example a binary file with poorly compressible code followed by
 *     a highly compressible string table.) Smaller buffer sizes give
 *     fast adaptation but have of course the overhead of transmitting trees
 *     more frequently.
 *   - I can't count above 4
 * The current code is general and allows DIST_BUFSIZE < LIT_BUFSIZE (to save
 * memory at the expense of compression). Some optimizations would be possible
 * if we rely on DIST_BUFSIZE == LIT_BUFSIZE.
 */

#define REP_3_6      16
/* repeat previous bit length 3-6 times (2 bits of repeat count) */

#define REPZ_3_10    17
/* repeat a zero length 3-10 times  (3 bits of repeat count) */

#define REPZ_11_138  18
/* repeat a zero length 11-138 times  (7 bits of repeat count) */

/* ===========================================================================
 * Local data
 */

/* Data structure describing a single value and its code string. */
typedef struct ct_data {
    union {
        ush  freq;       /* frequency count */
        ush  code;       /* bit string */
    } fc;
    union {
        ush  dad;        /* father node in Huffman tree */
        ush  len;        /* length of bit string */
    } dl;
} ct_data;

#define Freq fc.freq
#define Code fc.code
#define Dad  dl.dad
#define Len  dl.len

#define HEAP_SIZE (2*L_CODES+1)
/* maximum heap size */

local ct_data near dyn_ltree[HEAP_SIZE];   /* literal and length tree */
local ct_data near dyn_dtree[2*D_CODES+1]; /* distance tree */

local ct_data near static_ltree[L_CODES+2];
/* The static literal tree. Since the bit lengths are imposed, there is no
 * need for the L_CODES extra codes used during heap construction. However
 * The codes 286 and 287 are needed to build a canonical tree (see ct_init
 * below).
 */

local ct_data near static_dtree[D_CODES];
/* The static distance tree. (Actually a trivial tree since all codes use
 * 5 bits.)
 */

local ct_data near bl_tree[2*BL_CODES+1];
/* Huffman tree for the bit lengths */

typedef struct tree_desc {
    ct_data near *dyn_tree;      /* the dynamic tree */
    ct_data near *static_tree;   /* corresponding static tree or NULL */
    int     near *extra_bits;    /* extra bits for each code or NULL */
    int     extra_base;          /* base index for extra_bits */
    int     elems;               /* max number of elements in the tree */
    int     max_length;          /* max bit length for the codes */
    int     max_code;            /* largest code with non zero frequency */
} tree_desc;

local tree_desc near l_desc =
{dyn_ltree, static_ltree, extra_lbits, LITERALS+1, L_CODES, MAX_BITS, 0};

local tree_desc near d_desc =
{dyn_dtree, static_dtree, extra_dbits, 0,          D_CODES, MAX_BITS, 0};

local tree_desc near bl_desc =
{bl_tree, NULL,       extra_blbits, 0,         BL_CODES, MAX_BL_BITS, 0};


local ush near bl_count[MAX_BITS+1];
/* number of codes at each bit length for an optimal tree */

local uch near bl_order[BL_CODES]
   = {16,17,18,0,8,7,9,6,10,5,11,4,12,3,13,2,14,1,15};
/* The lengths of the bit length codes are sent in order of decreasing
 * probability, to avoid transmitting the lengths for unused bit length codes.
 */

local int near heap[2*L_CODES+1]; /* heap used to build the Huffman trees */
local int heap_len;               /* number of elements in the heap */
local int heap_max;               /* element of largest frequency */
/* The sons of heap[n] are heap[2*n] and heap[2*n+1]. heap[0] is not used.
 * The same heap array is used to build all trees.
 */

local uch near depth[2*L_CODES+1];
/* Depth of each subtree used as tie breaker for trees of equal frequency */

local uch length_code[MAX_MATCH-MIN_MATCH+1];
/* length code for each normalized match length (0 == MIN_MATCH) */

local uch dist_code[512];
/* distance codes. The first 256 values correspond to the distances
 * 3 .. 258, the last 256 values correspond to the top 8 bits of
 * the 15 bit distances.
 */

local int near base_length[LENGTH_CODES];
/* First normalized length for each code (0 = MIN_MATCH) */

local int near base_dist[D_CODES];
/* First normalized distance for each code (0 = distance of 1) */

#ifndef DYN_ALLOC
  local uch far l_buf[LIT_BUFSIZE];  /* buffer for literals/lengths */
  local ush far d_buf[DIST_BUFSIZE]; /* buffer for distances */
#else
  local uch far *l_buf;
  local ush far *d_buf;
#endif

local uch near flag_buf[(LIT_BUFSIZE/8)];
/* flag_buf is a bit array distinguishing literals from lengths in
 * l_buf, and thus indicating the presence or absence of a distance.
 */

local unsigned last_lit;    /* running index in l_buf */
local unsigned last_dist;   /* running index in d_buf */
local unsigned last_flags;  /* running index in flag_buf */
local uch flags;            /* current flags not yet saved in flag_buf */
local uch flag_bit;         /* current bit used in flags */
/* bits are filled in flags starting at bit 0 (least significant).
 * Note: these flags are overkill in the current code since we don't
 * take advantage of DIST_BUFSIZE == LIT_BUFSIZE.
 */

local ulg opt_len;        /* bit length of current block with optimal trees */
local ulg static_len;     /* bit length of current block with static trees */

local ulg cmpr_bytelen;     /* total byte length of compressed file */
local ulg cmpr_len_bits;    /* number of bits past 'cmpr_bytelen' */

#ifdef DEBUG
local ulg input_len;        /* total byte length of input file */
/* input_len is for debugging only since we can get it by other means. */
#endif

local ush *file_type;       /* pointer to UNKNOWN, BINARY or ASCII */
local int *file_method;     /* pointer to DEFLATE or STORE */

/* ===========================================================================
 * Local data used by the "bit string" routines.
 */

local int flush_flg;

#if (!defined(ASMV) || !defined(RISCOS))
local unsigned bi_buf;
#else
unsigned bi_buf;
#endif
/* Output buffer. bits are inserted starting at the bottom (least significant
 * bits). The width of bi_buf must be at least 16 bits.
 */

#define Buf_size (8 * 2*sizeof(char))
/* Number of bits used within bi_buf. (bi_buf may be implemented on
 * more than 16 bits on some systems.)
 */

#if (!defined(ASMV) || !defined(RISCOS))
local int bi_valid;
#else
int bi_valid;
#endif