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/* deflate_slow.c -- compress data using the slow 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"
/* ===========================================================================
* Same as deflate_medium, 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.
*/
Z_INTERNAL block_state deflate_slow(deflate_state *s, int flush) {
Pos hash_head; /* head of hash chain */
int bflush; /* set if current block must be flushed */
int64_t dist;
uint32_t match_len;
match_func longest_match;
if (s->max_chain_length <= 1024)
longest_match = FUNCTABLE_FPTR(longest_match);
else
longest_match = FUNCTABLE_FPTR(longest_match_slow);
/* Process the input block. */
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:
*/
hash_head = 0;
if (LIKELY(s->lookahead >= WANT_MIN_MATCH)) {
hash_head = s->quick_insert_string(s, s->strstart);
}
/* Find the longest match, discarding those <= prev_length.
*/
s->prev_match = (Pos)s->match_start;
match_len = STD_MIN_MATCH - 1;
dist = (int64_t)s->strstart - hash_head;
if (dist <= MAX_DIST(s) && dist > 0 && s->prev_length < s->max_lazy_match && 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 = longest_match(s, hash_head);
/* longest_match() sets match_start */
if (match_len <= 5 && (s->strategy == Z_FILTERED)) {
/* If prev_match is also WANT_MIN_MATCH, match_start is garbage
* but we will ignore the current match anyway.
*/
match_len = STD_MIN_MATCH - 1;
}
}
/* If there was a match at the previous step and the current
* match is not better, output the previous match:
*/
if (s->prev_length >= STD_MIN_MATCH && match_len <= s->prev_length) {
unsigned int max_insert = s->strstart + s->lookahead - STD_MIN_MATCH;
/* Do not insert strings in hash table beyond this. */
Assert((s->strstart-1) <= UINT16_MAX, "strstart-1 should fit in uint16_t");
check_match(s, (Pos)(s->strstart - 1), s->prev_match, s->prev_length);
bflush = zng_tr_tally_dist(s, s->strstart -1 - s->prev_match, s->prev_length - STD_MIN_MATCH);
/* Insert in hash table all strings up to the end of the match.
* strstart-1 and strstart are already inserted. If there is not
* enough lookahead, the last two strings are not inserted in
* the hash table.
*/
s->prev_length -= 1;
s->lookahead -= s->prev_length;
unsigned int mov_fwd = s->prev_length - 1;
if (max_insert > s->strstart) {
unsigned int insert_cnt = mov_fwd;
if (UNLIKELY(insert_cnt > max_insert - s->strstart))
insert_cnt = max_insert - s->strstart;
s->insert_string(s, s->strstart + 1, insert_cnt);
}
s->prev_length = 0;
s->match_available = 0;
s->strstart += mov_fwd + 1;
if (UNLIKELY(bflush))
FLUSH_BLOCK(s, 0);
} else if (s->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.
*/
bflush = zng_tr_tally_lit(s, s->window[s->strstart-1]);
if (UNLIKELY(bflush))
FLUSH_BLOCK_ONLY(s, 0);
s->prev_length = match_len;
s->strstart++;
s->lookahead--;
if (UNLIKELY(s->strm->avail_out == 0))
return need_more;
} else {
/* There is no previous match to compare with, wait for
* the next step to decide.
*/
s->prev_length = match_len;
s->match_available = 1;
s->strstart++;
s->lookahead--;
}
}
Assert(flush != Z_NO_FLUSH, "no flush?");
if (UNLIKELY(s->match_available)) {
Z_UNUSED(zng_tr_tally_lit(s, s->window[s->strstart-1]));
s->match_available = 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;
}
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