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deflate_fast.c
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deflate_fast.c
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/* deflate_fast.c -- compress data using the fast strategy of deflation algorithm
*
* Copyright (C) 1995-2024 Jean-loup Gailly and Mark Adler
* For conditions of distribution and use, see copyright notice in zlib.h
*/
#include "zbuild.h"
#include "deflate.h"
#include "deflate_p.h"
#include "functable.h"
/* ===========================================================================
* Compress as much as possible from the input stream, return the current
* block state.
* This function does not perform lazy evaluation of 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.
*/
Z_INTERNAL block_state deflate_fast(deflate_state *s, int flush) {
Pos hash_head; /* head of the hash chain */
int bflush = 0; /* set if current block must be flushed */
int64_t dist;
uint32_t match_len = 0;
for (;;) {
/* Make sure that we always have enough lookahead, except
* at the end of the input file. We need STD_MAX_MATCH bytes
* for the next match, plus WANT_MIN_MATCH bytes to insert the
* string following the next match.
*/
if (s->lookahead < MIN_LOOKAHEAD) {
PREFIX(fill_window)(s);
if (UNLIKELY(s->lookahead < MIN_LOOKAHEAD && flush == Z_NO_FLUSH)) {
return need_more;
}
if (UNLIKELY(s->lookahead == 0))
break; /* flush the current block */
}
/* Insert the string window[strstart .. strstart+2] in the
* dictionary, and set hash_head to the head of the hash chain:
*/
if (s->lookahead >= WANT_MIN_MATCH) {
hash_head = quick_insert_string(s, s->strstart);
dist = (int64_t)s->strstart - hash_head;
/* Find the longest match, discarding those <= prev_length.
* At this point we have always match length < WANT_MIN_MATCH
*/
if (dist <= MAX_DIST(s) && dist > 0 && hash_head != 0) {
/* 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_len = FUNCTABLE_CALL(longest_match)(s, hash_head);
/* longest_match() sets match_start */
}
}
if (match_len >= WANT_MIN_MATCH) {
Assert(s->strstart <= UINT16_MAX, "strstart should fit in uint16_t");
Assert(s->match_start <= UINT16_MAX, "match_start should fit in uint16_t");
check_match(s, (Pos)s->strstart, (Pos)s->match_start, match_len);
bflush = zng_tr_tally_dist(s, s->strstart - s->match_start, match_len - STD_MIN_MATCH);
s->lookahead -= match_len;
/* Insert new strings in the hash table only if the match length
* is not too large. This saves time but degrades compression.
*/
if (match_len <= s->max_insert_length && s->lookahead >= WANT_MIN_MATCH) {
match_len--; /* string at strstart already in table */
s->strstart++;
insert_string(s, s->strstart, match_len);
s->strstart += match_len;
} else {
s->strstart += match_len;
quick_insert_string(s, s->strstart + 2 - STD_MIN_MATCH);
/* If lookahead < STD_MIN_MATCH, ins_h is garbage, but it does not
* matter since it will be recomputed at next deflate call.
*/
}
match_len = 0;
} else {
/* No match, output a literal byte */
bflush = zng_tr_tally_lit(s, s->window[s->strstart]);
s->lookahead--;
s->strstart++;
}
if (UNLIKELY(bflush))
FLUSH_BLOCK(s, 0);
}
s->insert = s->strstart < (STD_MIN_MATCH - 1) ? s->strstart : (STD_MIN_MATCH - 1);
if (UNLIKELY(flush == Z_FINISH)) {
FLUSH_BLOCK(s, 1);
return finish_done;
}
if (UNLIKELY(s->sym_next))
FLUSH_BLOCK(s, 0);
return block_done;
}