File: deflate.c

package info (click to toggle)
gzip 1.2.4-28
  • links: PTS
  • area: main
  • in suites: slink
  • size: 984 kB
  • ctags: 1,024
  • sloc: ansic: 5,833; asm: 458; makefile: 263; sh: 31; perl: 11
file content (763 lines) | stat: -rw-r--r-- 29,179 bytes parent folder | download | duplicates (10)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
/* deflate.c -- compress data using the deflation algorithm
 * Copyright (C) 1992-1993 Jean-loup Gailly
 * This is free software; you can redistribute it and/or modify it under the
 * terms of the GNU General Public License, see the file COPYING.
 */

/*
 *  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.
 */

#include <stdio.h>

#include "tailor.h"
#include "gzip.h"
#include "lzw.h" /* just for consistency checking */

#ifdef RCSID
static char rcsid[] = "$Id: deflate.c,v 0.15 1993/06/24 10:53:53 jloup Exp $";
#endif

/* ===========================================================================
 * 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

/* To save space (see unlzw.c), we overlay prev+head with tab_prefix and
 * window with tab_suffix. Check that we can do this:
 */
#if (WSIZE<<1) > (1<<BITS)
   error: cannot overlay window with tab_suffix and prev with tab_prefix0
#endif
#if HASH_BITS > BITS-1
   error: cannot overlay head with tab_prefix1
#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 */

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

typedef ush Pos;
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.
 */

/* DECLARE(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+BSZ if SMALL_MEM (the code would
 * be less efficient).
 */

/* DECLARE(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.
 */

/* DECLARE(Pos, head, 1<<HASH_BITS); */
/* Heads of the hash chains or NIL. */

ulg window_size = (ulg)2*WSIZE;
/* 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 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.
 */

local int compr_level;
/* compression level (1..9) */

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


/* Values for max_lazy_match, good_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;

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

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));
#ifdef ASMV
      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
 */
void lm_init (pack_level, flags)
    int pack_level; /* 0: store, 1: best speed, 9: best compression */
    ush *flags;     /* general purpose bit flag */
{
    register unsigned j;

    if (pack_level < 1 || pack_level > 9) error("bad pack level");
    compr_level = pack_level;

    /* Initialize the hash table. */
#if defined(MAXSEG_64K) && HASH_BITS == 15
    for (j = 0;  j < HASH_SIZE; j++) head[j] = NIL;
#else
    memzero((char*)head, HASH_SIZE*sizeof(*head));
#endif
    /* prev will be initialized on the fly */

    /* Set the default configuration parameters:
     */
    max_lazy_match   = configuration_table[pack_level].max_lazy;
    good_match       = configuration_table[pack_level].good_length;
#ifndef FULL_SEARCH
    nice_match       = configuration_table[pack_level].nice_length;
#endif
    max_chain_length = configuration_table[pack_level].max_chain;
    if (pack_level == 1) {
       *flags |= FAST;
    } else if (pack_level == 9) {
       *flags |= SLOW;
    }
    /* ??? reduce max_chain_length for binary files */

    strstart = 0;
    block_start = 0L;
#ifdef ASMV
    match_init(); /* initialize the asm code */
#endif

    lookahead = read_buf((char*)window,
			 sizeof(int) <= 2 ? (unsigned)WSIZE : 2*WSIZE);

    if (lookahead == 0 || lookahead == (unsigned)EOF) {
       eofile = 1, lookahead = 0;
       return;
    }
    eofile = 0;
    /* Make sure that we always have enough lookahead. This is important
     * if input comes from a device such as a tty.
     */
    while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window();

    ins_h = 0;
    for (j=0; j<MIN_MATCH-1; j++) UPDATE_HASH(ins_h, window[j]);
    /* If lookahead < MIN_MATCH, ins_h is garbage, but this is
     * not important since only literal bytes will be emitted.
     */
}

/* ===========================================================================
 * Set match_start to the longest match starting at the given string and
 * return its length. Matches shorter or equal to prev_length are discarded,
 * in which case the result is equal to prev_length and match_start is
 * garbage.
 * IN assertions: cur_match is the head of the hash chain for the current
 *   string (strstart) and its distance is <= MAX_DIST, and prev_length >= 1
 */
#ifndef ASMV
/* For MSDOS, OS/2 and 386 Unix, an optimized version is in match.asm or
 * match.s. The code is functionally equivalent, so you can use the C version
 * if desired.
 */
int longest_match(cur_match)
    IPos cur_match;                             /* current match */
{
    unsigned chain_length = max_chain_length;   /* max hash chain length */
    register uch *scan = window + strstart;     /* current string */
    register uch *match;                        /* matched string */
    register int len;                           /* length of current match */
    int best_len = prev_length;                 /* best match length so far */
    IPos limit = strstart > (IPos)MAX_DIST ? strstart - (IPos)MAX_DIST : NIL;
    /* Stop when cur_match becomes <= limit. To simplify the code,
     * we prevent matches with the string of window index 0.
     */

/* The code is optimized for HASH_BITS >= 8 and MAX_MATCH-2 multiple of 16.
 * It is easy to get rid of this optimization if necessary.
 */
#if HASH_BITS < 8 || MAX_MATCH != 258
   error: Code too clever
#endif

#ifdef UNALIGNED_OK
    /* Compare two bytes at a time. Note: this is not always beneficial.
     * Try with and without -DUNALIGNED_OK to check.
     */
    register uch *strend = window + strstart + MAX_MATCH - 1;
    register ush scan_start = *(ush*)scan;
    register ush scan_end   = *(ush*)(scan+best_len-1);
#else
    register uch *strend = window + strstart + MAX_MATCH;
    register uch scan_end1  = scan[best_len-1];
    register uch scan_end   = scan[best_len];
#endif

    /* Do not waste too much time if we already have a good match: */
    if (prev_length >= good_match) {
        chain_length >>= 2;
    }
    Assert(strstart <= window_size-MIN_LOOKAHEAD, "insufficient lookahead");

    do {
        Assert(cur_match < strstart, "no future");
        match = window + cur_match;

        /* Skip to next match if the match length cannot increase
         * or if the match length is less than 2:
         */
#if (defined(UNALIGNED_OK) && MAX_MATCH == 258)
        /* This code assumes sizeof(unsigned short) == 2. Do not use
         * UNALIGNED_OK if your compiler uses a different size.
         */
        if (*(ush*)(match+best_len-1) != scan_end ||
            *(ush*)match != scan_start) continue;

        /* It is not necessary to compare scan[2] and match[2] since they are
         * always equal when the other bytes match, given that the hash keys
         * are equal and that HASH_BITS >= 8. Compare 2 bytes at a time at
         * strstart+3, +5, ... up to strstart+257. We check for insufficient
         * lookahead only every 4th comparison; the 128th check will be made
         * at strstart+257. If MAX_MATCH-2 is not a multiple of 8, it is
         * necessary to put more guard bytes at the end of the window, or
         * to check more often for insufficient lookahead.
         */
        scan++, match++;
        do {
        } while (*(ush*)(scan+=2) == *(ush*)(match+=2) &&
                 *(ush*)(scan+=2) == *(ush*)(match+=2) &&
                 *(ush*)(scan+=2) == *(ush*)(match+=2) &&
                 *(ush*)(scan+=2) == *(ush*)(match+=2) &&
                 scan < strend);
        /* The funny "do {}" generates better code on most compilers */

        /* Here, scan <= window+strstart+257 */
        Assert(scan <= window+(unsigned)(window_size-1), "wild scan");
        if (*scan == *match) scan++;

        len = (MAX_MATCH - 1) - (int)(strend-scan);
        scan = strend - (MAX_MATCH-1);

#else /* UNALIGNED_OK */

        if (match[best_len]   != scan_end  ||
            match[best_len-1] != scan_end1 ||
            *match            != *scan     ||
            *++match          != scan[1])      continue;

        /* The check at best_len-1 can be removed because it will be made
         * again later. (This heuristic is not always a win.)
         * It is not necessary to compare scan[2] and match[2] since they
         * are always equal when the other bytes match, given that
         * the hash keys are equal and that HASH_BITS >= 8.
         */
        scan += 2, match++;

        /* We check for insufficient lookahead only every 8th comparison;
         * the 256th check will be made at strstart+258.
         */
        do {
        } while (*++scan == *++match && *++scan == *++match &&
                 *++scan == *++match && *++scan == *++match &&
                 *++scan == *++match && *++scan == *++match &&
                 *++scan == *++match && *++scan == *++match &&
                 scan < strend);

        len = MAX_MATCH - (int)(strend - scan);
        scan = strend - MAX_MATCH;

#endif /* UNALIGNED_OK */

        if (len > best_len) {
            match_start = cur_match;
            best_len = len;
            if (len >= nice_match) break;
#ifdef UNALIGNED_OK
            scan_end = *(ush*)(scan+best_len-1);
#else
            scan_end1  = scan[best_len-1];
            scan_end   = scan[best_len];
#endif
        }
    } while ((cur_match = prev[cur_match & WMASK]) > limit
	     && --chain_length != 0);

    return best_len;
}
#endif /* ASMV */

#ifdef DEBUG
/* ===========================================================================
 * Check that the match at match_start is indeed a match.
 */
local void check_match(start, match, length)
    IPos start, match;
    int length;
{
    /* check that the match is indeed a match */
    if (memcmp((char*)window + match,
                (char*)window + start, length) != EQUAL) {
        fprintf(stderr,
            " start %d, match %d, length %d\n",
            start, match, length);
        error("invalid match");
    }
    if (verbose > 1) {
        fprintf(stderr,"\\[%d,%d]", start-match, length);
        do { putc(window[start++], stderr); } while (--length != 0);
    }
}
#else
#  define check_match(start, match, length)
#endif

/* ===========================================================================
 * Fill the window when the lookahead becomes insufficient.
 * Updates strstart and lookahead, and sets eofile if end of input file.
 * IN assertion: lookahead < MIN_LOOKAHEAD && strstart + lookahead > 0
 * OUT assertions: at least one byte has been read, or eofile is set;
 *    file reads are performed for at least two bytes (required for the
 *    translate_eol option).
 */
local void fill_window()
{
    register unsigned n, m;
    unsigned more = (unsigned)(window_size - (ulg)lookahead - (ulg)strstart);
    /* Amount of free space at the end of the window. */

    /* If the window is almost full and there is insufficient lookahead,
     * move the upper half to the lower one to make room in the upper half.
     */
    if (more == (unsigned)EOF) {
        /* Very unlikely, but possible on 16 bit machine if strstart == 0
         * and lookahead == 1 (input done one byte at time)
         */
        more--;
    } else if (strstart >= WSIZE+MAX_DIST) {
        /* By the IN assertion, the window is not empty so we can't confuse
         * more == 0 with more == 64K on a 16 bit machine.
         */
        Assert(window_size == (ulg)2*WSIZE, "no sliding with BIG_MEM");

        memcpy((char*)window, (char*)window+WSIZE, (unsigned)WSIZE);
        match_start -= WSIZE;
        strstart    -= WSIZE; /* we now have strstart >= MAX_DIST: */

        block_start -= (long) WSIZE;

        for (n = 0; n < HASH_SIZE; n++) {
            m = head[n];
            head[n] = (Pos)(m >= WSIZE ? m-WSIZE : NIL);
        }
        for (n = 0; n < WSIZE; n++) {
            m = prev[n];
            prev[n] = (Pos)(m >= WSIZE ? m-WSIZE : NIL);
            /* If n is not on any hash chain, prev[n] is garbage but
             * its value will never be used.
             */
        }
        more += WSIZE;
    }
    /* At this point, more >= 2 */
    if (!eofile) {
        n = read_buf((char*)window+strstart+lookahead, more);
        if (n == 0 || n == (unsigned)EOF) {
            eofile = 1;
        } else {
            lookahead += n;
        }
    }
}

/* ===========================================================================
 * Flush the current block, with given end-of-file flag.
 * IN assertion: strstart is set to the end of the current match.
 */
#define FLUSH_BLOCK(eof) \
   flush_block(block_start >= 0L ? (char*)&window[(unsigned)block_start] : \
                (char*)NULL, (long)strstart - block_start, (eof))

/* ===========================================================================
 * Processes a new input file and return its compressed length. This
 * function does not perform lazy evaluationof matches and inserts
 * new strings in the dictionary only for unmatched strings or for short
 * matches. It is used only for the fast compression options.
 */
local ulg deflate_fast()
{
    IPos hash_head; /* head of the hash chain */
    int flush;      /* set if current block must be flushed */
    unsigned match_length = 0;  /* length of best match */

    prev_length = MIN_MATCH-1;
    while (lookahead != 0) {
        /* Insert the string window[strstart .. strstart+2] in the
         * dictionary, and set hash_head to the head of the hash chain:
         */
        INSERT_STRING(strstart, hash_head);

        /* Find the longest match, discarding those <= prev_length.
         * At this point we have always match_length < MIN_MATCH
         */
        if (hash_head != NIL && strstart - hash_head <= MAX_DIST) {
            /* To simplify the code, we prevent matches with the string
             * of window index 0 (in particular we have to avoid a match
             * of the string with itself at the start of the input file).
             */
            match_length = longest_match (hash_head);
            /* longest_match() sets match_start */
            if (match_length > lookahead) match_length = lookahead;
        }
        if (match_length >= MIN_MATCH) {
            check_match(strstart, match_start, match_length);

            flush = ct_tally(strstart-match_start, match_length - MIN_MATCH);

            lookahead -= match_length;

	    /* Insert new strings in the hash table only if the match length
             * is not too large. This saves time but degrades compression.
             */
            if (match_length <= max_insert_length) {
                match_length--; /* string at strstart already in hash table */
                do {
                    strstart++;
                    INSERT_STRING(strstart, hash_head);
                    /* strstart never exceeds WSIZE-MAX_MATCH, so there are
                     * always MIN_MATCH bytes ahead. If lookahead < MIN_MATCH
                     * these bytes are garbage, but it does not matter since
                     * the next lookahead bytes will be emitted as literals.
                     */
                } while (--match_length != 0);
	        strstart++; 
            } else {
	        strstart += match_length;
	        match_length = 0;
	        ins_h = window[strstart];
	        UPDATE_HASH(ins_h, window[strstart+1]);
#if MIN_MATCH != 3
                Call UPDATE_HASH() MIN_MATCH-3 more times
#endif
            }
        } else {
            /* No match, output a literal byte */
            Tracevv((stderr,"%c",window[strstart]));
            flush = ct_tally (0, window[strstart]);
            lookahead--;
	    strstart++; 
        }
        if (flush) FLUSH_BLOCK(0), block_start = strstart;

        /* Make sure that we always have enough lookahead, except
         * at the end of the input file. We need MAX_MATCH bytes
         * for the next match, plus MIN_MATCH bytes to insert the
         * string following the next match.
         */
        while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window();

    }
    return FLUSH_BLOCK(1); /* eof */
}

/* ===========================================================================
 * Same as above, but achieves better compression. We use a lazy
 * evaluation for matches: a match is finally adopted only if there is
 * no better match at the next window position.
 */
ulg deflate()
{
    IPos hash_head;          /* head of hash chain */
    IPos prev_match;         /* previous match */
    int flush;               /* set if current block must be flushed */
    int match_available = 0; /* set if previous match exists */
    register unsigned match_length = MIN_MATCH-1; /* length of best match */
#ifdef DEBUG
    extern long isize;        /* byte length of input file, for debug only */
#endif

    if (compr_level <= 3) return deflate_fast(); /* optimized for speed */

    /* Process the input block. */
    while (lookahead != 0) {
        /* Insert the string window[strstart .. strstart+2] in the
         * dictionary, and set hash_head to the head of the hash chain:
         */
        INSERT_STRING(strstart, hash_head);

        /* Find the longest match, discarding those <= prev_length.
         */
        prev_length = match_length, prev_match = match_start;
        match_length = MIN_MATCH-1;

        if (hash_head != NIL && prev_length < max_lazy_match &&
            strstart - hash_head <= MAX_DIST) {
            /* To simplify the code, we prevent matches with the string
             * of window index 0 (in particular we have to avoid a match
             * of the string with itself at the start of the input file).
             */
            match_length = longest_match (hash_head);
            /* longest_match() sets match_start */
            if (match_length > lookahead) match_length = lookahead;

            /* Ignore a length 3 match if it is too distant: */
            if (match_length == MIN_MATCH && strstart-match_start > TOO_FAR){
                /* If prev_match is also MIN_MATCH, match_start is garbage
                 * but we will ignore the current match anyway.
                 */
                match_length--;
            }
        }
        /* If there was a match at the previous step and the current
         * match is not better, output the previous match:
         */
        if (prev_length >= MIN_MATCH && match_length <= prev_length) {

            check_match(strstart-1, prev_match, prev_length);

            flush = ct_tally(strstart-1-prev_match, prev_length - MIN_MATCH);

            /* Insert in hash table all strings up to the end of the match.
             * strstart-1 and strstart are already inserted.
             */
            lookahead -= prev_length-1;
            prev_length -= 2;
            do {
                strstart++;
                INSERT_STRING(strstart, hash_head);
                /* strstart never exceeds WSIZE-MAX_MATCH, so there are
                 * always MIN_MATCH bytes ahead. If lookahead < MIN_MATCH
                 * these bytes are garbage, but it does not matter since the
                 * next lookahead bytes will always be emitted as literals.
                 */
            } while (--prev_length != 0);
            match_available = 0;
            match_length = MIN_MATCH-1;
            strstart++;
            if (flush) FLUSH_BLOCK(0), block_start = strstart;

        } else if (match_available) {
            /* If there was no match at the previous position, output a
             * single literal. If there was a match but the current match
             * is longer, truncate the previous match to a single literal.
             */
            Tracevv((stderr,"%c",window[strstart-1]));
            if (ct_tally (0, window[strstart-1])) {
                FLUSH_BLOCK(0), block_start = strstart;
            }
            strstart++;
            lookahead--;
        } else {
            /* There is no previous match to compare with, wait for
             * the next step to decide.
             */
            match_available = 1;
            strstart++;
            lookahead--;
        }
        Assert (strstart <= isize && lookahead <= isize, "a bit too far");

        /* Make sure that we always have enough lookahead, except
         * at the end of the input file. We need MAX_MATCH bytes
         * for the next match, plus MIN_MATCH bytes to insert the
         * string following the next match.
         */
        while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window();
    }
    if (match_available) ct_tally (0, window[strstart-1]);

    return FLUSH_BLOCK(1); /* eof */
}