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1.1 root 1: /*
2:
3: Copyright (C) 1990-1992 Mark Adler, Richard B. Wales, Jean-loup Gailly,
4: Kai Uwe Rommel and Igor Mandrichenko.
5: Permission is granted to any individual or institution to use, copy, or
6: redistribute this software so long as all of the original files are included
7: unmodified, that it is not sold for profit, and that this copyright notice
8: is retained.
9:
10: */
11:
12: /*
13: * deflate.c by Jean-loup Gailly.
14: *
15: * PURPOSE
16: *
17: * Identify new text as repetitions of old text within a fixed-
18: * length sliding window trailing behind the new text.
19: *
20: * DISCUSSION
21: *
22: * The "deflation" process depends on being able to identify portions
23: * of the input text which are identical to earlier input (within a
24: * sliding window trailing behind the input currently being processed).
25: *
26: * The most straightforward technique turns out to be the fastest for
27: * most input files: try all possible matches and select the longest.
28: * The key feature is of this algorithm is that insertion and deletions
29: * from the string dictionary are very simple and thus fast. Insertions
30: * and deletions are performed at each input character, whereas string
31: * matches are performed only when the previous match ends. So it is
32: * preferable to spend more time in matches to allow very fast string
33: * insertions and deletions. The matching algorithm for small strings
34: * is inspired from that of Rabin & Karp. A brute force approach is
35: * used to find longer strings when a small match has been found.
36: * A similar algorithm is used in comic (by Jan-Mark Wams) and freeze
37: * (by Leonid Broukhis).
38: * A previous version of this file used a more sophisticated algorithm
39: * (by Fiala and Greene) which is guaranteed to run in linear amortized
40: * time, but has a larger average cost and uses more memory. However
41: * the F&G algorithm may be faster for some highly redundant files if
42: * the parameter max_chain_length (described below) is too large.
43: *
44: * ACKNOWLEDGEMENTS
45: *
46: * The idea of lazy evaluation of matches is due to Jan-Mark Wams, and
47: * I found it in 'freeze' written by Leonid Broukhis.
48: * Thanks to many info-zippers for bug reports and testing.
49: *
50: * REFERENCES
51: *
52: * APPNOTE.TXT documentation file in PKZIP 2.0 distribution.
53: *
54: * A description of the Rabin and Karp algorithm is given in the book
55: * "Algorithms" by R. Sedgewick, Addison-Wesley, p252.
56: *
57: * Fiala,E.R., and Greene,D.H.
58: * Data Compression with Finite Windows, Comm.ACM, 32,4 (1989) 490-595
59: *
60: * INTERFACE
61: *
62: * void lm_init (int pack_level, ush *flags)
63: * Initialize the "longest match" routines for a new file
64: *
65: * ulg deflate (void)
66: * Processes a new input file and return its compressed length. Sets
67: * the compressed length, crc, deflate flags and internal file
68: * attributes.
69: */
70:
71: #include "zip.h"
72:
73: /* ===========================================================================
74: * Configuration parameters
75: */
76:
77: /* Compile with MEDIUM_MEM to reduce the memory requirements or
78: * with SMALL_MEM to use as little memory as possible.
79: * Warning: defining these symbols affects MATCH_BUFSIZE and HASH_BITS
80: * (see below) and thus affects the compression ratio. The compressed output
81: * is still correct, and might even be smaller in some cases.
82: */
83:
84: #ifdef SMALL_MEM
85: # define HASH_BITS 13 /* Number of bits used to hash strings */
86: #else
87: #ifdef MEDIUM_MEM
88: # define HASH_BITS 14
89: #else
90: # define HASH_BITS 15
91: /* For portability to 16 bit machines, do not use values above 15. */
92: #endif
93: #endif
94:
95: #define HASH_SIZE (unsigned)(1<<HASH_BITS)
96: #define HASH_MASK (HASH_SIZE-1)
97: #define WMASK (WSIZE-1)
98: /* HASH_SIZE and WSIZE must be powers of two */
99:
100: #define NIL 0
101: /* Tail of hash chains */
102:
103: #define FAST 4
104: #define SLOW 2
105: /* speed options for the general purpose bit flag */
106:
107: #ifndef TOO_FAR
108: # define TOO_FAR 4096
109: #endif
110: /* Matches of length 3 are discarded if their distance exceeds TOO_FAR */
111:
112: /* ===========================================================================
113: * Local data used by the "longest match" routines.
114: */
115:
116: typedef ush Pos;
117: typedef unsigned IPos;
118: /* A Pos is an index in the character window. We use short instead of int to
119: * save space in the various tables. IPos is used only for parameter passing.
120: */
121:
122: #ifndef DYN_ALLOC
123: uch far window[2L*WSIZE];
124: /* Sliding window. Input bytes are read into the second half of the window,
125: * and move to the first half later to keep a dictionary of at least WSIZE
126: * bytes. With this organization, matches are limited to a distance of
127: * WSIZE-MAX_MATCH bytes, but this ensures that IO is always
128: * performed with a length multiple of the block size. Also, it limits
129: * the window size to 64K, which is quite useful on MSDOS.
130: * To do: limit the window size to WSIZE+BSZ if SMALL_MEM (the code would
131: * be less efficient since the data would have to be copied WSIZE/BSZ times)
132: */
133: Pos far prev[WSIZE];
134: /* Link to older string with same hash index. To limit the size of this
135: * array to 64K, this link is maintained only for the last 32K strings.
136: * An index in this array is thus a window index modulo 32K.
137: */
138: Pos far head[HASH_SIZE];
139: /* Heads of the hash chains or NIL */
140: #else
141: uch far * near window = NULL;
142: Pos far * near prev = NULL;
143: Pos far * near head;
144: #endif
145:
146: long block_start;
147: /* window position at the beginning of the current output block. Gets
148: * negative when the window is moved backwards.
149: */
150:
151: local unsigned near ins_h; /* hash index of string to be inserted */
152:
153: #define H_SHIFT ((HASH_BITS+MIN_MATCH-1)/MIN_MATCH)
154: /* Number of bits by which ins_h and del_h must be shifted at each
155: * input step. It must be such that after MIN_MATCH steps, the oldest
156: * byte no longer takes part in the hash key, that is:
157: * H_SHIFT * MIN_MATCH >= HASH_BITS
158: */
159:
160: unsigned int near prev_length;
161: /* Length of the best match at previous step. Matches not greater than this
162: * are discarded. This is used in the lazy match evaluation.
163: */
164:
165: unsigned near strstart; /* start of string to insert */
166: unsigned near match_start; /* start of matching string */
167: local int near eofile; /* flag set at end of input file */
168: local unsigned near lookahead; /* number of valid bytes ahead in window */
169:
170: unsigned near max_chain_length;
171: /* To speed up deflation, hash chains are never searched beyond this length.
172: * A higher limit improves compression ratio but degrades the speed.
173: */
174:
175: local unsigned int max_lazy_match;
176: /* Attempt to find a better match only when the current match is strictly
177: * smaller than this value.
178: */
179:
180: int near good_match;
181: /* Use a faster search when the previous match is longer than this */
182:
183:
184: /* Values for max_lazy_match, good_match and max_chain_length, depending on
185: * the desired pack level (0..9). The values given below have been tuned to
186: * exclude worst case performance for pathological files. Better values may be
187: * found for specific files.
188: */
189: typedef struct config {
190: int good_length;
191: int max_lazy;
192: unsigned max_chain;
193: uch flag;
194: } config;
195:
196: local config configuration_table[10] = {
197: /* good lazy chain flag */
198: /* 0 */ {0, 0, 0, 0}, /* store only */
199: /* 1 */ {4, 4, 16, FAST}, /* maximum speed */
200: /* 2 */ {6, 8, 16, 0},
201: /* 3 */ {8, 16, 32, 0},
202: /* 4 */ {8, 32, 64, 0},
203: /* 5 */ {8, 64, 128, 0},
204: /* 6 */ {8, 128, 256, 0},
205: /* 7 */ {8, 128, 512, 0},
206: /* 8 */ {32, 258, 1024, 0},
207: /* 9 */ {32, 258, 4096, SLOW}}; /* maximum compression */
208:
209: /* Note: the current code requires max_lazy >= MIN_MATCH and max_chain >= 4
210: * but these restrictions can easily be removed at a small cost.
211: */
212:
213: #define EQUAL 0
214: /* result of memcmp for equal strings */
215:
216: /* ===========================================================================
217: * Prototypes for local functions. Use asm version by default for
218: * MSDOS but not Unix. However the asm version version is recommended
219: * for 386 Unix.
220: */
221: #ifdef ATARI_ST
222: # undef MSDOS /* avoid the processor specific parts */
223: #endif
224: #if defined(MSDOS) && !defined(NO_ASM) && !defined(ASM)
225: # define ASM
226: #endif
227:
228: local void fill_window OF((void));
229: int longest_match OF((IPos cur_match));
230: #ifdef ASM
231: void match_init OF((void)); /* asm code initialization */
232: #endif
233:
234: #ifdef DEBUG
235: local void check_match OF((IPos start, IPos match, int length));
236: #endif
237:
238: #define MIN(a,b) ((a) <= (b) ? (a) : (b))
239: /* The arguments must not have side effects. */
240:
241: /* ===========================================================================
242: * Update a hash value with the given input byte
243: * IN assertion: all calls to to UPDATE_HASH are made with consecutive
244: * input characters, so that a running hash key can be computed from the
245: * previous key instead of complete recalculation each time.
246: */
247: #define UPDATE_HASH(h,c) (h = (((h)<<H_SHIFT) ^ (c)) & HASH_MASK)
248:
249: /* ===========================================================================
250: * Insert string s in the dictionary and set match_head to the previous head
251: * of the hash chain (the most recent string with same hash key). Return
252: * the previous length of the hash chain.
253: * IN assertion: all calls to to INSERT_STRING are made with consecutive
254: * input characters and the first MIN_MATCH bytes of s are valid
255: * (except for the last MIN_MATCH-1 bytes of the input file).
256: */
257: #define INSERT_STRING(s, match_head) \
258: (UPDATE_HASH(ins_h, window[(s) + MIN_MATCH-1]), \
259: prev[(s) & WMASK] = match_head = head[ins_h], \
260: head[ins_h] = (s))
261:
262: /* ===========================================================================
263: * Initialize the "longest match" routines for a new file
264: */
265: void lm_init (pack_level, flags)
266: int pack_level; /* 0: store, 1: best speed, 9: best compression */
267: ush *flags; /* general purpose bit flag */
268: {
269: register unsigned j;
270:
271: if (pack_level < 1 || pack_level > 9) error("bad pack level");
272:
273: /* Use dynamic allocation if compiler does not like big static arrays: */
274: #ifdef DYN_ALLOC
275: if (window == NULL) {
276: window = (uch far*) fcalloc(WSIZE, 2*sizeof(uch));
277: prev = (Pos far*) fcalloc(WSIZE, sizeof(Pos));
278: head = (Pos far*) fcalloc(HASH_SIZE, sizeof(Pos));
279:
280: if (window == NULL || prev == NULL || head == NULL) {
281: err(ZE_MEM, "window allocation");
282: }
283: }
284: #endif /* DYN_ALLOC */
285: #ifdef ASM
286: match_init(); /* initialize the asm code */
287: #endif
288: /* Initialize the hash table. */
289: for (j = 0; j < HASH_SIZE; j++) head[j] = NIL;
290: /* prev will be initialized on the fly */
291:
292: /* Set the default configuration parameters:
293: */
294: max_lazy_match = configuration_table[pack_level].max_lazy;
295: good_match = configuration_table[pack_level].good_length;
296: max_chain_length = configuration_table[pack_level].max_chain;
297: *flags |= configuration_table[pack_level].flag;
298: /* ??? reduce max_chain_length for binary files */
299:
300: strstart = 0;
301: block_start = 0L;
302:
303: #if defined(MSDOS) && !defined(__32BIT__)
304: /* Can't read a 64K block under MSDOS */
305: lookahead = read_buf((char*)window, (unsigned)WSIZE);
306: #else
307: lookahead = read_buf((char*)window, 2*WSIZE);
308: #endif
309: if (lookahead == 0 || lookahead == (unsigned)EOF) {
310: eofile = 1, lookahead = 0;
311: return;
312: }
313: eofile = 0;
314: /* Make sure that we always have enough lookahead. This is important
315: * if input comes from a device such as a tty.
316: */
317: while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window();
318:
319: ins_h = 0;
320: for (j=0; j<MIN_MATCH-1; j++) UPDATE_HASH(ins_h, window[j]);
321: /* If lookahead < MIN_MATCH, ins_h is garbage, but this is
322: * not important since only literal bytes will be emitted.
323: */
324: }
325:
326: /* ===========================================================================
327: * Set match_start to the longest match starting at the given string and
328: * return its length. Matches shorter or equal to prev_length are discarded,
329: * in which case the result is equal to prev_length and match_start is
330: * garbage.
331: * IN assertions: cur_match is the head of the hash chain for the current
332: * string (strstart) and its distance is <= MAX_DIST, and prev_length >= 1
333: */
334: #ifndef ASM
335: /* For MSDOS, OS/2 and 386 Unix, an optimized version is in match.asm. The code
336: * is functionally equivalent, so you can use the C version if desired.
337: */
338: int longest_match(cur_match)
339: IPos cur_match; /* current match */
340: {
341: unsigned chain_length = max_chain_length; /* max hash chain length */
342: register uch far *scan = window + strstart; /* current string */
343: register uch far *match = scan; /* matched string */
344: register int len; /* length of current match */
345: int best_len = prev_length; /* best match length so far */
346: IPos limit = strstart > (IPos)MAX_DIST ? strstart - (IPos)MAX_DIST : NIL;
347: /* Stop when cur_match becomes <= limit. To simplify the code,
348: * we prevent matches with the string of window index 0.
349: */
350: #ifdef UNALIGNED_OK
351: register ush scan_start = *(ush*)scan;
352: register ush scan_end = *(ush*)(scan+best_len-1);
353: #else
354: register uch scan_start = *scan;
355: register uch scan_end1 = scan[best_len-1];
356: register uch scan_end = scan[best_len];
357: #endif
358:
359: /* Do not waste too much time if we already have a good match: */
360: if (prev_length >= good_match) {
361: chain_length >>= 2;
362: }
363:
364: do {
365: Assert(cur_match < strstart, "no future");
366: match = window + cur_match;
367:
368: /* Skip to next match if the match length cannot increase
369: * or if the match length is less than 2:
370: */
371: #if (defined(UNALIGNED_OK) && HASH_BITS >= 8)
372: /* This code assumes sizeof(unsigned short) == 2 and
373: * sizeof(unsigned long) == 4. Do not use UNALIGNED_OK if your
374: * compiler uses different sizes.
375: */
376: if (*(ush*)(match+best_len-1) != scan_end ||
377: *(ush*)match != scan_start) continue;
378:
379: len = MIN_MATCH - 4;
380: /* It is not necessary to compare scan[2] and match[2] since they are
381: * always equal when the other bytes match, given that the hash keys
382: * are equal and that HASH_BITS >= 8.
383: */
384: do {} while ((len+=4) < MAX_MATCH-3 &&
385: *(ulg*)(scan+len) == *(ulg*)(match+len));
386: /* The funny do {} generates better code for most compilers */
387:
388: if (*(ush*)(scan+len) == *(ush*)(match+len)) len += 2;
389: if (scan[len] == match[len]) len++;
390:
391: #else /* UNALIGNED_OK */
392: if (match[best_len] != scan_end ||
393: match[best_len-1] != scan_end1 || *match != scan_start)
394: continue;
395: /* It is not necessary to compare scan[1] and match[1] since they
396: * are always equal when the other bytes match, given that
397: * the hash keys are equal and that h_shift+8 <= HASH_BITS,
398: * that is, when the last byte is entirely included in the hash key.
399: * The condition is equivalent to
400: * (HASH_BITS+2)/3 + 8 <= HASH_BITS
401: * or: HASH_BITS >= 13
402: * Also, we check for a match at best_len-1 to get rid quickly of
403: * the match with the suffix of the match made at the previous step,
404: * which is known to fail.
405: */
406: #if HASH_BITS >= 13
407: len = 1;
408: #else
409: len = 0;
410: #endif
411: do {} while (++len < MAX_MATCH && scan[len] == match[len]);
412:
413: #endif /* UNALIGNED_OK */
414:
415: if (len > best_len) {
416: match_start = cur_match;
417: best_len = len;
418: if (len == MAX_MATCH) break;
419: #ifdef UNALIGNED_OK
420: scan_end = *(ush*)(scan+best_len-1);
421: #else
422: scan_end1 = scan[best_len-1];
423: scan_end = scan[best_len];
424: #endif
425: }
426: } while (--chain_length != 0 &&
427: (cur_match = prev[cur_match & WMASK]) > limit);
428:
429: return best_len;
430: }
431: #endif /* NO_ASM */
432:
433: #ifdef DEBUG
434: /* ===========================================================================
435: * Check that the match at match_start is indeed a match.
436: */
437: local void check_match(start, match, length)
438: IPos start, match;
439: int length;
440: {
441: /* check that the match is indeed a match */
442: if (memcmp((char*)window + match,
443: (char*)window + start, length) != EQUAL) {
444: fprintf(stderr,
445: " start %d, match %d, length %d\n",
446: start, match, length);
447: error("invalid match");
448: }
449: if (verbose > 1) {
450: fprintf(stderr,"\\[%d,%d]", start-match, length);
451: do { putc(window[start++], stderr); } while (--length != 0);
452: }
453: }
454: #else
455: # define check_match(start, match, length)
456: #endif
457:
458: /* ===========================================================================
459: * Fill the window when the lookahead becomes insufficient.
460: * Updates strstart and lookahead, and sets eofile if end of input file.
461: * IN assertion: lookahead < MIN_LOOKAHEAD && strstart + lookahead > 0
462: * OUT assertions: at least one byte has been read, or eofile is set;
463: * file reads are performed for at least two bytes (required for the
464: * translate_eol option).
465: */
466: local void fill_window()
467: {
468: register unsigned n, m;
469: unsigned more = (unsigned)((ulg)2*WSIZE - (ulg)lookahead - (ulg)strstart);
470: /* Amount of free space at the end of the window. */
471:
472: /* If the window is full, move the upper half to the lower one to make
473: * room in the upper half.
474: */
475: if (more == (unsigned)EOF) {
476: /* Very unlikely, but possible on 16 bit machine if strstart == 0
477: * and lookahead == 1 (input done one byte at time)
478: */
479: more--;
480: } else if (more <= 1) {
481: /* By the IN assertion, the window is not empty so we can't confuse
482: * more == 0 with more == 64K on a 16 bit machine.
483: */
484: memcpy((char*)window, (char*)window+WSIZE, (unsigned)WSIZE);
485: match_start -= WSIZE;
486: strstart -= WSIZE;
487: /* strstart - WSIZE >= WSIZE - 1 - lookahead >= WSIZE - MIN_LOOKAHEAD
488: * so we now have strstart >= MAX_DIST:
489: */
490: Assert (strstart >= MAX_DIST, "window slide too early");
491: block_start -= (long) WSIZE;
492:
493: for (n = 0; n < HASH_SIZE; n++) {
494: m = head[n];
495: head[n] = (Pos)(m >= WSIZE ? m-WSIZE : NIL);
496: }
497: for (n = 0; n < WSIZE; n++) {
498: m = prev[n];
499: prev[n] = (Pos)(m >= WSIZE ? m-WSIZE : NIL);
500: /* If n is not on any hash chain, prev[n] is garbage but
501: * its value will never be used.
502: */
503: }
504: more += WSIZE;
505: #ifdef ZIP
506: if (verbose) putc('.', stderr);
507: #endif
508: }
509: /* At this point, more >= 2 */
510: n = read_buf((char*)window+strstart+lookahead, more);
511: if (n == 0 || n == (unsigned)EOF) {
512: eofile = 1;
513: } else {
514: lookahead += n;
515: }
516: }
517:
518: /* ===========================================================================
519: * Flush the current block, with given end-of-file flag.
520: * IN assertion: strstart is set to the end of the current match.
521: */
522: #define FLUSH_BLOCK(eof) \
523: flush_block(block_start >= 0L ? (char*)&window[block_start] : (char*)NULL,\
524: (long)strstart - block_start, (eof))
525:
526: /* ===========================================================================
527: * Processes a new input file and return its compressed length.
528: */
529: #ifdef NO_LAZY
530: ulg deflate()
531: {
532: IPos hash_head; /* head of the hash chain */
533: int flush; /* set if current block must be flushed */
534: unsigned match_length = 0; /* length of best match */
535:
536: prev_length = MIN_MATCH-1;
537: while (lookahead != 0) {
538: /* Insert the string window[strstart .. strstart+2] in the
539: * dictionary, and set hash_head to the head of the hash chain:
540: */
541: INSERT_STRING(strstart, hash_head);
542:
543: /* Find the longest match, discarding those <= prev_length.
544: * At this point we have always match_length < MIN_MATCH
545: */
546: if (hash_head != NIL && strstart - hash_head <= MAX_DIST) {
547: /* To simplify the code, we prevent matches with the string
548: * of window index 0 (in particular we have to avoid a match
549: * of the string with itself at the start of the input file).
550: */
551: match_length = longest_match (hash_head);
552: /* longest_match() sets match_start */
553: if (match_length > lookahead) match_length = lookahead;
554: }
555: if (match_length >= MIN_MATCH) {
556: check_match(strstart, match_start, match_length);
557:
558: flush = ct_tally(strstart-match_start, match_length - MIN_MATCH);
559:
560: lookahead -= match_length;
561: match_length--; /* string at strstart already in hash table */
562: do {
563: strstart++;
564: INSERT_STRING(strstart, hash_head);
565: /* strstart never exceeds WSIZE-MAX_MATCH, so there are
566: * always MIN_MATCH bytes ahead. If lookahead < MIN_MATCH
567: * these bytes are garbage, but it does not matter since the
568: * next lookahead bytes will always be emitted as literals.
569: */
570: } while (--match_length != 0);
571: } else {
572: /* No match, output a literal byte */
573: flush = ct_tally (0, window[strstart]);
574: lookahead--;
575: }
576: strstart++;
577: if (flush) FLUSH_BLOCK(0), block_start = strstart;
578:
579: /* Make sure that we always have enough lookahead, except
580: * at the end of the input file. We need MAX_MATCH bytes
581: * for the next match, plus MIN_MATCH bytes to insert the
582: * string following the next match.
583: */
584: while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window();
585:
586: }
587: return FLUSH_BLOCK(1); /* eof */
588: }
589: #else /* LAZY */
590: /* ===========================================================================
591: * Same as above, but achieves better compression. We use a lazy
592: * evaluation for matches: a match is finally adopted only if there is
593: * no better match at the next window position.
594: */
595: ulg deflate()
596: {
597: IPos hash_head; /* head of hash chain */
598: IPos prev_match; /* previous match */
599: int flush; /* set if current block must be flushed */
600: int match_available = 0; /* set if previous match exists */
601: register unsigned match_length = MIN_MATCH-1; /* length of best match */
602: #ifdef DEBUG
603: extern ulg isize; /* byte length of input file, for debug only */
604: #endif
605:
606: /* Process the input block. */
607: while (lookahead != 0) {
608: /* Insert the string window[strstart .. strstart+2] in the
609: * dictionary, and set hash_head to the head of the hash chain:
610: */
611: INSERT_STRING(strstart, hash_head);
612:
613: /* Find the longest match, discarding those <= prev_length.
614: */
615: prev_length = match_length, prev_match = match_start;
616: match_length = MIN_MATCH-1;
617:
618: if (hash_head != NIL && prev_length < max_lazy_match &&
619: strstart - hash_head <= MAX_DIST) {
620: /* To simplify the code, we prevent matches with the string
621: * of window index 0 (in particular we have to avoid a match
622: * of the string with itself at the start of the input file).
623: */
624: match_length = longest_match (hash_head);
625: /* longest_match() sets match_start */
626: if (match_length > lookahead) match_length = lookahead;
627: /* Ignore a length 3 match if it is too distant: */
628: if (match_length == MIN_MATCH && strstart-match_start > TOO_FAR){
629: /* If prev_match is also MIN_MATCH, match_start is garbage
630: * but we will ignore the current match anyway.
631: */
632: match_length--;
633: }
634: }
635: /* If there was a match at the previous step and the current
636: * match is not better, output the previous match:
637: */
638: if (prev_length >= MIN_MATCH && match_length <= prev_length) {
639:
640: check_match(strstart-1, prev_match, prev_length);
641:
642: flush = ct_tally(strstart-1-prev_match, prev_length - MIN_MATCH);
643:
644: /* Insert in hash table all strings up to the end of the match.
645: * strstart-1 and strstart are already inserted.
646: */
647: lookahead -= prev_length-1;
648: prev_length -= 2;
649: do {
650: strstart++;
651: INSERT_STRING(strstart, hash_head);
652: /* strstart never exceeds WSIZE-MAX_MATCH, so there are
653: * always MIN_MATCH bytes ahead. If lookahead < MIN_MATCH
654: * these bytes are garbage, but it does not matter since the
655: * next lookahead bytes will always be emitted as literals.
656: */
657: } while (--prev_length != 0);
658: match_available = 0;
659: match_length = MIN_MATCH-1;
660:
661: } else if (match_available) {
662: /* If there was no match at the previous position, output a
663: * single literal. If there was a match but the current match
664: * is longer, truncate the previous match to a single literal.
665: */
666: flush = ct_tally (0, window[strstart-1]);
667: Tracevv((stderr,"%c",window[strstart-1]));
668: lookahead--;
669: } else {
670: /* There is no previous match to compare with, wait for
671: * the next step to decide.
672: */
673: match_available = 1;
674: flush = 0;
675: lookahead--;
676: }
677: if (flush) FLUSH_BLOCK(0), block_start = strstart;
678: strstart++;
679: Assert (strstart <= isize && lookahead <= isize, "a bit too far");
680:
681: /* Make sure that we always have enough lookahead, except
682: * at the end of the input file. We need MAX_MATCH bytes
683: * for the next match, plus MIN_MATCH bytes to insert the
684: * string following the next match.
685: */
686: while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window();
687: }
688: if (match_available) ct_tally (0, window[strstart-1]);
689:
690: return FLUSH_BLOCK(1); /* eof */
691: }
692: #endif /* LAZY */
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