764 lines
28 KiB
C
764 lines
28 KiB
C
/* deflate.c -- compress data using the deflation algorithm
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* Copyright (C) 1992-1993 Jean-loup Gailly
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* This is free software; you can redistribute it and/or modify it under the
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* terms of the GNU General Public License, see the file COPYING.
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*/
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/*
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* PURPOSE
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*
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* Identify new text as repetitions of old text within a fixed-
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* length sliding window trailing behind the new text.
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*
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* DISCUSSION
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*
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* The "deflation" process depends on being able to identify portions
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* of the input text which are identical to earlier input (within a
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* sliding window trailing behind the input currently being processed).
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*
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* The most straightforward technique turns out to be the fastest for
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* most input files: try all possible matches and select the longest.
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* The key feature of this algorithm is that insertions into the string
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* dictionary are very simple and thus fast, and deletions are avoided
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* completely. Insertions are performed at each input character, whereas
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* string matches are performed only when the previous match ends. So it
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* is preferable to spend more time in matches to allow very fast string
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* insertions and avoid deletions. The matching algorithm for small
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* strings is inspired from that of Rabin & Karp. A brute force approach
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* is used to find longer strings when a small match has been found.
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* A similar algorithm is used in comic (by Jan-Mark Wams) and freeze
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* (by Leonid Broukhis).
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* A previous version of this file used a more sophisticated algorithm
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* (by Fiala and Greene) which is guaranteed to run in linear amortized
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* time, but has a larger average cost, uses more memory and is patented.
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* However the F&G algorithm may be faster for some highly redundant
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* files if the parameter max_chain_length (described below) is too large.
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*
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* ACKNOWLEDGEMENTS
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*
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* The idea of lazy evaluation of matches is due to Jan-Mark Wams, and
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* I found it in 'freeze' written by Leonid Broukhis.
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* Thanks to many info-zippers for bug reports and testing.
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*
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* REFERENCES
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*
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* APPNOTE.TXT documentation file in PKZIP 1.93a distribution.
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*
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* A description of the Rabin and Karp algorithm is given in the book
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* "Algorithms" by R. Sedgewick, Addison-Wesley, p252.
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*
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* Fiala,E.R., and Greene,D.H.
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* Data Compression with Finite Windows, Comm.ACM, 32,4 (1989) 490-595
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*
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* INTERFACE
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*
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* void lm_init (int pack_level, ush *flags)
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* Initialize the "longest match" routines for a new file
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*
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* ulg deflate (void)
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* Processes a new input file and return its compressed length. Sets
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* the compressed length, crc, deflate flags and internal file
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* attributes.
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*/
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#include <stdio.h>
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#include "tailor.h"
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#include "gzip.h"
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#include "lzw.h" /* just for consistency checking */
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#ifdef RCSID
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static char rcsid[] = "$Id: deflate.c,v 0.15 1993/06/24 10:53:53 jloup Exp $";
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#endif
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/* ===========================================================================
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* Configuration parameters
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*/
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/* Compile with MEDIUM_MEM to reduce the memory requirements or
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* with SMALL_MEM to use as little memory as possible. Use BIG_MEM if the
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* entire input file can be held in memory (not possible on 16 bit systems).
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* Warning: defining these symbols affects HASH_BITS (see below) and thus
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* affects the compression ratio. The compressed output
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* is still correct, and might even be smaller in some cases.
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*/
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#ifdef SMALL_MEM
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# define HASH_BITS 13 /* Number of bits used to hash strings */
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#endif
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#ifdef MEDIUM_MEM
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# define HASH_BITS 14
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#endif
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#ifndef HASH_BITS
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# define HASH_BITS 15
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/* For portability to 16 bit machines, do not use values above 15. */
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#endif
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/* To save space (see unlzw.c), we overlay prev+head with tab_prefix and
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* window with tab_suffix. Check that we can do this:
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*/
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#if (WSIZE<<1) > (1<<BITS)
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error: cannot overlay window with tab_suffix and prev with tab_prefix0
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#endif
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#if HASH_BITS > BITS-1
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error: cannot overlay head with tab_prefix1
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#endif
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#define HASH_SIZE (unsigned)(1<<HASH_BITS)
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#define HASH_MASK (HASH_SIZE-1)
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#define WMASK (WSIZE-1)
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/* HASH_SIZE and WSIZE must be powers of two */
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#define NIL 0
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/* Tail of hash chains */
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#define FAST 4
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#define SLOW 2
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/* speed options for the general purpose bit flag */
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#ifndef TOO_FAR
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# define TOO_FAR 4096
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#endif
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/* Matches of length 3 are discarded if their distance exceeds TOO_FAR */
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/* ===========================================================================
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* Local data used by the "longest match" routines.
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*/
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typedef ush Pos;
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typedef unsigned IPos;
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/* A Pos is an index in the character window. We use short instead of int to
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* save space in the various tables. IPos is used only for parameter passing.
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*/
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/* DECLARE(uch, window, 2L*WSIZE); */
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/* Sliding window. Input bytes are read into the second half of the window,
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* and move to the first half later to keep a dictionary of at least WSIZE
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* bytes. With this organization, matches are limited to a distance of
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* WSIZE-MAX_MATCH bytes, but this ensures that IO is always
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* performed with a length multiple of the block size. Also, it limits
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* the window size to 64K, which is quite useful on MSDOS.
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* To do: limit the window size to WSIZE+BSZ if SMALL_MEM (the code would
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* be less efficient).
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*/
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/* DECLARE(Pos, prev, WSIZE); */
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/* Link to older string with same hash index. To limit the size of this
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* array to 64K, this link is maintained only for the last 32K strings.
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* An index in this array is thus a window index modulo 32K.
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*/
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/* DECLARE(Pos, head, 1<<HASH_BITS); */
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/* Heads of the hash chains or NIL. */
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ulg window_size = (ulg)2*WSIZE;
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/* window size, 2*WSIZE except for MMAP or BIG_MEM, where it is the
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* input file length plus MIN_LOOKAHEAD.
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*/
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long block_start;
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/* window position at the beginning of the current output block. Gets
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* negative when the window is moved backwards.
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*/
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local unsigned ins_h; /* hash index of string to be inserted */
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#define H_SHIFT ((HASH_BITS+MIN_MATCH-1)/MIN_MATCH)
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/* Number of bits by which ins_h and del_h must be shifted at each
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* input step. It must be such that after MIN_MATCH steps, the oldest
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* byte no longer takes part in the hash key, that is:
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* H_SHIFT * MIN_MATCH >= HASH_BITS
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*/
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unsigned int near prev_length;
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/* Length of the best match at previous step. Matches not greater than this
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* are discarded. This is used in the lazy match evaluation.
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*/
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unsigned near strstart; /* start of string to insert */
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unsigned near match_start; /* start of matching string */
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local int eofile; /* flag set at end of input file */
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local unsigned lookahead; /* number of valid bytes ahead in window */
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unsigned near max_chain_length;
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/* To speed up deflation, hash chains are never searched beyond this length.
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* A higher limit improves compression ratio but degrades the speed.
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*/
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local unsigned int max_lazy_match;
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/* Attempt to find a better match only when the current match is strictly
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* smaller than this value. This mechanism is used only for compression
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* levels >= 4.
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*/
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#define max_insert_length max_lazy_match
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/* Insert new strings in the hash table only if the match length
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* is not greater than this length. This saves time but degrades compression.
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* max_insert_length is used only for compression levels <= 3.
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*/
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local int compr_level;
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/* compression level (1..9) */
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unsigned near good_match;
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/* Use a faster search when the previous match is longer than this */
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/* Values for max_lazy_match, good_match and max_chain_length, depending on
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* the desired pack level (0..9). The values given below have been tuned to
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* exclude worst case performance for pathological files. Better values may be
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* found for specific files.
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*/
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typedef struct config {
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ush good_length; /* reduce lazy search above this match length */
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ush max_lazy; /* do not perform lazy search above this match length */
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ush nice_length; /* quit search above this match length */
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ush max_chain;
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} config;
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#ifdef FULL_SEARCH
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# define nice_match MAX_MATCH
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#else
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int near nice_match; /* Stop searching when current match exceeds this */
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#endif
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local config configuration_table[10] = {
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/* good lazy nice chain */
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/* 0 */ {0, 0, 0, 0}, /* store only */
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/* 1 */ {4, 4, 8, 4}, /* maximum speed, no lazy matches */
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/* 2 */ {4, 5, 16, 8},
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/* 3 */ {4, 6, 32, 32},
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/* 4 */ {4, 4, 16, 16}, /* lazy matches */
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/* 5 */ {8, 16, 32, 32},
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/* 6 */ {8, 16, 128, 128},
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/* 7 */ {8, 32, 128, 256},
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/* 8 */ {32, 128, 258, 1024},
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/* 9 */ {32, 258, 258, 4096}}; /* maximum compression */
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/* Note: the deflate() code requires max_lazy >= MIN_MATCH and max_chain >= 4
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* For deflate_fast() (levels <= 3) good is ignored and lazy has a different
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* meaning.
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*/
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#define EQUAL 0
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/* result of memcmp for equal strings */
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/* ===========================================================================
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* Prototypes for local functions.
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*/
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local void fill_window OF((void));
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local ulg deflate_fast OF((void));
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int longest_match OF((IPos cur_match));
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#ifdef ASMV
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void match_init OF((void)); /* asm code initialization */
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#endif
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#ifdef DEBUG
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local void check_match OF((IPos start, IPos match, int length));
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#endif
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/* ===========================================================================
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* Update a hash value with the given input byte
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* IN assertion: all calls to to UPDATE_HASH are made with consecutive
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* input characters, so that a running hash key can be computed from the
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* previous key instead of complete recalculation each time.
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*/
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#define UPDATE_HASH(h,c) (h = (((h)<<H_SHIFT) ^ (c)) & HASH_MASK)
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/* ===========================================================================
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* Insert string s in the dictionary and set match_head to the previous head
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* of the hash chain (the most recent string with same hash key). Return
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* the previous length of the hash chain.
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* IN assertion: all calls to to INSERT_STRING are made with consecutive
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* input characters and the first MIN_MATCH bytes of s are valid
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* (except for the last MIN_MATCH-1 bytes of the input file).
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*/
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#define INSERT_STRING(s, match_head) \
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(UPDATE_HASH(ins_h, window[(s) + MIN_MATCH-1]), \
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prev[(s) & WMASK] = match_head = head[ins_h], \
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head[ins_h] = (s))
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/* ===========================================================================
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* Initialize the "longest match" routines for a new file
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*/
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void lm_init (pack_level, flags)
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int pack_level; /* 0: store, 1: best speed, 9: best compression */
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ush *flags; /* general purpose bit flag */
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{
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register unsigned j;
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if (pack_level < 1 || pack_level > 9) error("bad pack level");
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compr_level = pack_level;
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/* Initialize the hash table. */
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#if defined(MAXSEG_64K) && HASH_BITS == 15
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for (j = 0; j < HASH_SIZE; j++) head[j] = NIL;
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#else
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memzero((char*)head, HASH_SIZE*sizeof(*head));
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#endif
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/* prev will be initialized on the fly */
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/* Set the default configuration parameters:
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*/
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max_lazy_match = configuration_table[pack_level].max_lazy;
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good_match = configuration_table[pack_level].good_length;
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#ifndef FULL_SEARCH
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nice_match = configuration_table[pack_level].nice_length;
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#endif
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max_chain_length = configuration_table[pack_level].max_chain;
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if (pack_level == 1) {
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*flags |= FAST;
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} else if (pack_level == 9) {
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*flags |= SLOW;
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}
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/* ??? reduce max_chain_length for binary files */
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strstart = 0;
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block_start = 0L;
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#ifdef ASMV
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match_init(); /* initialize the asm code */
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#endif
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lookahead = read_buf((char*)window,
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sizeof(int) <= 2 ? (unsigned)WSIZE : 2*WSIZE);
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if (lookahead == 0 || lookahead == (unsigned)EOF) {
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eofile = 1, lookahead = 0;
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return;
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}
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eofile = 0;
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/* Make sure that we always have enough lookahead. This is important
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* if input comes from a device such as a tty.
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*/
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while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window();
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ins_h = 0;
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for (j=0; j<MIN_MATCH-1; j++) UPDATE_HASH(ins_h, window[j]);
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/* If lookahead < MIN_MATCH, ins_h is garbage, but this is
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* not important since only literal bytes will be emitted.
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*/
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}
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/* ===========================================================================
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* Set match_start to the longest match starting at the given string and
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* return its length. Matches shorter or equal to prev_length are discarded,
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* in which case the result is equal to prev_length and match_start is
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* garbage.
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* IN assertions: cur_match is the head of the hash chain for the current
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* string (strstart) and its distance is <= MAX_DIST, and prev_length >= 1
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*/
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#ifndef ASMV
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/* For MSDOS, OS/2 and 386 Unix, an optimized version is in match.asm or
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* match.s. The code is functionally equivalent, so you can use the C version
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* if desired.
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*/
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int longest_match(cur_match)
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IPos cur_match; /* current match */
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{
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unsigned chain_length = max_chain_length; /* max hash chain length */
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register uch *scan = window + strstart; /* current string */
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register uch *match; /* matched string */
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register int len; /* length of current match */
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int best_len = prev_length; /* best match length so far */
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IPos limit = strstart > (IPos)MAX_DIST ? strstart - (IPos)MAX_DIST : NIL;
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/* Stop when cur_match becomes <= limit. To simplify the code,
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* we prevent matches with the string of window index 0.
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*/
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/* The code is optimized for HASH_BITS >= 8 and MAX_MATCH-2 multiple of 16.
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* It is easy to get rid of this optimization if necessary.
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*/
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#if HASH_BITS < 8 || MAX_MATCH != 258
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error: Code too clever
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#endif
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#ifdef UNALIGNED_OK
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/* Compare two bytes at a time. Note: this is not always beneficial.
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* Try with and without -DUNALIGNED_OK to check.
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*/
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register uch *strend = window + strstart + MAX_MATCH - 1;
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register ush scan_start = *(ush*)scan;
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register ush scan_end = *(ush*)(scan+best_len-1);
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#else
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register uch *strend = window + strstart + MAX_MATCH;
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register uch scan_end1 = scan[best_len-1];
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register uch scan_end = scan[best_len];
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#endif
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/* Do not waste too much time if we already have a good match: */
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if (prev_length >= good_match) {
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chain_length >>= 2;
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}
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Assert(strstart <= window_size-MIN_LOOKAHEAD, "insufficient lookahead");
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do {
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Assert(cur_match < strstart, "no future");
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match = window + cur_match;
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/* Skip to next match if the match length cannot increase
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* or if the match length is less than 2:
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*/
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#if (defined(UNALIGNED_OK) && MAX_MATCH == 258)
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/* This code assumes sizeof(unsigned short) == 2. Do not use
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* UNALIGNED_OK if your compiler uses a different size.
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*/
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if (*(ush*)(match+best_len-1) != scan_end ||
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*(ush*)match != scan_start) continue;
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/* It is not necessary to compare scan[2] and match[2] since they are
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* always equal when the other bytes match, given that the hash keys
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* are equal and that HASH_BITS >= 8. Compare 2 bytes at a time at
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* strstart+3, +5, ... up to strstart+257. We check for insufficient
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* lookahead only every 4th comparison; the 128th check will be made
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* at strstart+257. If MAX_MATCH-2 is not a multiple of 8, it is
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* necessary to put more guard bytes at the end of the window, or
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* to check more often for insufficient lookahead.
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*/
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scan++, match++;
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do {
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} while (*(ush*)(scan+=2) == *(ush*)(match+=2) &&
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*(ush*)(scan+=2) == *(ush*)(match+=2) &&
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*(ush*)(scan+=2) == *(ush*)(match+=2) &&
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*(ush*)(scan+=2) == *(ush*)(match+=2) &&
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scan < strend);
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/* The funny "do {}" generates better code on most compilers */
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/* Here, scan <= window+strstart+257 */
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Assert(scan <= window+(unsigned)(window_size-1), "wild scan");
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if (*scan == *match) scan++;
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len = (MAX_MATCH - 1) - (int)(strend-scan);
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scan = strend - (MAX_MATCH-1);
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#else /* UNALIGNED_OK */
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if (match[best_len] != scan_end ||
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match[best_len-1] != scan_end1 ||
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*match != *scan ||
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*++match != scan[1]) continue;
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/* The check at best_len-1 can be removed because it will be made
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* again later. (This heuristic is not always a win.)
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* It is not necessary to compare scan[2] and match[2] since they
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* are always equal when the other bytes match, given that
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* the hash keys are equal and that HASH_BITS >= 8.
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*/
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scan += 2, match++;
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/* We check for insufficient lookahead only every 8th comparison;
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* the 256th check will be made at strstart+258.
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*/
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do {
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} while (*++scan == *++match && *++scan == *++match &&
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*++scan == *++match && *++scan == *++match &&
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*++scan == *++match && *++scan == *++match &&
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*++scan == *++match && *++scan == *++match &&
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scan < strend);
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len = MAX_MATCH - (int)(strend - scan);
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scan = strend - MAX_MATCH;
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#endif /* UNALIGNED_OK */
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if (len > best_len) {
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match_start = cur_match;
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best_len = len;
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if (len >= nice_match) break;
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#ifdef UNALIGNED_OK
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scan_end = *(ush*)(scan+best_len-1);
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#else
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scan_end1 = scan[best_len-1];
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scan_end = scan[best_len];
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#endif
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}
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} while ((cur_match = prev[cur_match & WMASK]) > limit
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&& --chain_length != 0);
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return best_len;
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}
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#endif /* ASMV */
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#ifdef DEBUG
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/* ===========================================================================
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* Check that the match at match_start is indeed a match.
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*/
|
|
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 */
|
|
}
|