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/* 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
#include "gzip.h"
#include "lzw.h" /* just for consistency checking */
/* ===========================================================================
* 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.
*/
#ifndef HASH_BITS
#define HASH_BITS 15 /* hash */
/* 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-1
error: cannot overlay head with tab_prefix1
#endif
#define HASH_SIZE (unsigned)(1<= HASH_BITS
*/
unsigned int near prev_length = 0;
/* 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 = 0; /* start of string to insert */
unsigned near match_start = 0; /* start of matching string */
local int eofile = 0; /* flag set at end of input file */
local unsigned lookahead = 0; /* number of valid bytes ahead in window */
unsigned near max_chain_length = 0;
/* 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 = 0;
/* 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.
*/
/* compression level (1..9) */
unsigned near good_match = 0;
/* 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;
int near nice_match = 0; /* Stop searching when current match exceeds this */
/* good lazy nice chain */
local config configuration_table[10] = {
{ 0, 0, 0, 0}, /* store only *//* 0 */
{ 4, 4, 8, 4}, /* maximum speed, no lazy matches */ /* 1 */
{ 4, 5, 16, 8}, /* 2 */
{ 4, 6, 32, 32}, /* 3 */
{ 4, 4, 16, 16}, /* lazy matches *//* 4 */
{ 8, 16, 32, 32},/* 5 */
{ 8, 16, 128, 128},/* 6 */
{ 8, 32, 128, 256},/* 7 */
{ 32, 128, 258, 1024},/* 8 */
{ 32, 258, 258, 4096}}; /* maximum compression *//* 9 */
/* 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 ));
int longest_match OF(( IPos cur_match ));
/* ===========================================================================
* 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)<= 1
*/
/* 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( IPos cur_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.
*/
register uch * strend = window + strstart + MAX_MATCH;
register uch scan_end1 = scan[best_len - 1];
register uch scan_end = scan[best_len];
if ( prev_length >= good_match ) {
/* Do not waste too much time if we already have a good match: */
chain_length >>= 2;
}
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 ( 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;
if ( len > best_len ) {
match_start = cur_match;
best_len = len;
if ( len >= nice_match ) break;
scan_end1 = scan[best_len - 1];
scan_end = scan[best_len];
}
} while (( cur_match = prev[cur_match & WMASK] ) > limit
&& -- chain_length != 0 );
return best_len;
}
#define check_match(start, match, length)
/* ===========================================================================
* 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 )- 1 ) {
/* 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 -= ( int ) 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 )- 1 ) {
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))
/* ===========================================================================
* 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 */
/* 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; /* -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 */
}