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C/C++ Source or Header | 1993-05-25 | 27.2 KB | 731 lines

/* 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 <stdio.h> #include "tailor.h" #include "gzip.h" #include "lzw.h" /* just for consistency checking */ #ifndef lint static char rcsid[] = "$Id: deflate.c,v 0.13 1993/05/25 16:25:40 jloup Exp $"; #endif /* =========================================================================== * 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. */ #ifdef SMALL_MEM # define HASH_BITS 13 /* Number of bits used to hash strings */ #endif #ifdef MEDIUM_MEM # define HASH_BITS 14 #endif #ifndef HASH_BITS # define HASH_BITS 15 /* 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 error: cannot overlay window with tab_suffix and prev with tab_prefix0 #endif #if HASH_BITS > BITS-1 error: cannot overlay head with tab_prefix1 #endif #define HASH_SIZE (unsigned)(1<<HASH_BITS) #define HASH_MASK (HASH_SIZE-1) #define WMASK (WSIZE-1) /* HASH_SIZE and WSIZE must be powers of two */ #define NIL 0 /* Tail of hash chains */ #define FAST 4 #define SLOW 2 /* speed options for the general purpose bit flag */ #ifndef TOO_FAR # define TOO_FAR 4096 #endif /* Matches of length 3 are discarded if their distance exceeds TOO_FAR */ /* =========================================================================== * Local data used by the "longest match" routines. */ typedef ush Pos; typedef unsigned IPos; /* A Pos is an index in the character window. We use short instead of int to * save space in the various tables. IPos is used only for parameter passing. */ /* DECLARE(uch, window, 2L*WSIZE); */ /* Sliding window. Input bytes are read into the second half of the window, * and move to the first half later to keep a dictionary of at least WSIZE * bytes. With this organization, matches are limited to a distance of * WSIZE-MAX_MATCH bytes, but this ensures that IO is always * performed with a length multiple of the block size. Also, it limits * the window size to 64K, which is quite useful on MSDOS. * To do: limit the window size to WSIZE+BSZ if SMALL_MEM (the code would * be less efficient). */ /* DECLARE(Pos, prev, WSIZE); */ /* Link to older string with same hash index. To limit the size of this * array to 64K, this link is maintained only for the last 32K strings. * An index in this array is thus a window index modulo 32K. */ /* DECLARE(Pos, head, 1<<HASH_BITS); */ /* Heads of the hash chains or NIL. */ ulg window_size = (ulg)2*WSIZE; /* window size, 2*WSIZE except for MMAP or BIG_MEM, where it is the * input file length plus MIN_LOOKAHEAD. */ long block_start; /* window position at the beginning of the current output block. Gets * negative when the window is moved backwards. */ local unsigned ins_h; /* hash index of string to be inserted */ #define H_SHIFT ((HASH_BITS+MIN_MATCH-1)/MIN_MATCH) /* Number of bits by which ins_h and del_h must be shifted at each * input step. It must be such that after MIN_MATCH steps, the oldest * byte no longer takes part in the hash key, that is: * H_SHIFT * MIN_MATCH >= HASH_BITS */ unsigned int near prev_length; /* 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; /* start of string to insert */ unsigned near match_start; /* start of matching string */ local int eofile; /* flag set at end of input file */ local unsigned lookahead; /* number of valid bytes ahead in window */ unsigned near max_chain_length; /* 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; /* Attempt to find a better match only when the current match is strictly * smaller than this value. */ int near good_match; /* 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; #ifdef FULL_SEARCH # define nice_match MAX_MATCH #else int near nice_match; /* Stop searching when current match exceeds this */ #endif local config configuration_table[10] = { /* good lazy nice chain */ /* 0 */ {0, 0, 0, 0}, /* store only */ /* 1 */ {4, 4, 16, 16}, /* maximum speed */ /* 2 */ {6, 8, 16, 16}, /* 3 */ {8, 16, 32, 32}, /* 4 */ {8, 16, 64, 64}, /* 5 */ {8, 16, 128, 128}, /* 6 */ {8, 32, 128, 256}, /* 7 */ {8, 64, 128, 512}, /* 8 */ {32, 128, 258, 1024}, /* 9 */ {32, 258, 258, 4096}}; /* maximum compression */ /* Note: the current code requires max_lazy >= MIN_MATCH and max_chain >= 4 * but these restrictions can easily be removed at a small cost. */ #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)); #ifdef ASMV void match_init OF((void)); /* asm code initialization */ #endif #ifdef DEBUG local void check_match OF((IPos start, IPos match, int length)); #endif /* =========================================================================== * 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)<<H_SHIFT) ^ (c)) & HASH_MASK) /* =========================================================================== * Insert string s in the dictionary and set match_head to the previous head * of the hash chain (the most recent string with same hash key). Return * the previous length of the hash chain. * IN assertion: all calls to to INSERT_STRING are made with consecutive * input characters and the first MIN_MATCH bytes of s are valid * (except for the last MIN_MATCH-1 bytes of the input file). */ #define INSERT_STRING(s, match_head) \ (UPDATE_HASH(ins_h, window[(s) + MIN_MATCH-1]), \ prev[(s) & WMASK] = match_head = head[ins_h], \ head[ins_h] = (s)) /* =========================================================================== * Initialize the "longest match" routines for a new file */ void lm_init (pack_level, flags) int pack_level; /* 0: store, 1: best speed, 9: best compression */ ush *flags; /* general purpose bit flag */ { register unsigned j; if (pack_level < 1 || pack_level > 9) error("bad pack level"); /* Initialize the hash table. */ #if defined(MAXSEG_64K) && HASH_BITS == 15 for (j = 0; j < HASH_SIZE; j++) head[j] = NIL; #else memzero((char*)head, HASH_SIZE*sizeof(*head)); #endif /* prev will be initialized on the fly */ /* Set the default configuration parameters: */ max_lazy_match = configuration_table[pack_level].max_lazy; good_match = configuration_table[pack_level].good_length; #ifndef FULL_SEARCH nice_match = configuration_table[pack_level].nice_length; #endif max_chain_length = configuration_table[pack_level].max_chain; if (pack_level == 1) { *flags |= FAST; } else if (pack_level == 9) { *flags |= SLOW; } /* ??? reduce max_chain_length for binary files */ strstart = 0; block_start = 0L; #ifdef ASMV match_init(); /* initialize the asm code */ #endif lookahead = read_buf((char*)window, sizeof(int) <= 2 ? (unsigned)WSIZE : 2*WSIZE); if (lookahead == 0 || lookahead == (unsigned)EOF) { eofile = 1, lookahead = 0; return; } eofile = 0; /* Make sure that we always have enough lookahead. This is important * if input comes from a device such as a tty. */ while (lookahead < MIN_LOOKAHEAD && !eofile) fill_window(); ins_h = 0; for (j=0; j<MIN_MATCH-1; j++) UPDATE_HASH(ins_h, window[j]); /* If lookahead < MIN_MATCH, ins_h is garbage, but this is * not important since only literal bytes will be emitted. */ } /* =========================================================================== * Set match_start to the longest match starting at the given string and * return its length. Matches shorter or equal to prev_length are discarded, * in which case the result is equal to prev_length and match_start is * garbage. * IN assertions: cur_match is the head of the hash chain for the current * string (strstart) and its distance is <= MAX_DIST, and prev_length >= 1 */ #ifndef ASMV /* 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(cur_match) IPos cur_match; /* current 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. */ #if HASH_BITS < 8 || MAX_MATCH != 258 error: Code too clever #endif #ifdef UNALIGNED_OK /* Compare two bytes at a time. Note: this is not always beneficial. * Try with and without -DUNALIGNED_OK to check. */ register uch *strend = window + strstart + MAX_MATCH - 1; register ush scan_start = *(ush*)scan; register ush scan_end = *(ush*)(scan+best_len-1); #else register uch *strend = window + strstart + MAX_MATCH; register uch scan_end1 = scan[best_len-1]; register uch scan_end = scan[best_len]; #endif /* Do not waste too much time if we already have a good match: */ if (prev_length >= good_match) { chain_length >>= 2; } Assert(strstart <= window_size-MIN_LOOKAHEAD, "insufficient lookahead"); 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 (defined(UNALIGNED_OK) && MAX_MATCH == 258) /* This code assumes sizeof(unsigned short) == 2. Do not use * UNALIGNED_OK if your compiler uses a different size. */ if (*(ush*)(match+best_len-1) != scan_end || *(ush*)match != scan_start) continue; /* 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. Compare 2 bytes at a time at * strstart+3, +5, ... up to strstart+257. We check for insufficient * lookahead only every 4th comparison; the 128th check will be made * at strstart+257. If MAX_MATCH-2 is not a multiple of 8, it is * necessary to put more guard bytes at the end of the window, or * to check more often for insufficient lookahead. */ scan++, match++; do { } while (*(ush*)(scan+=2) == *(ush*)(match+=2) && *(ush*)(scan+=2) == *(ush*)(match+=2) && *(ush*)(scan+=2) == *(ush*)(match+=2) && *(ush*)(scan+=2) == *(ush*)(match+=2) && scan < strend); /* The funny "do {}" generates better code on most compilers */ /* Here, scan <= window+strstart+257 */ Assert(scan <= window+(unsigned)(window_size-1), "wild scan"); if (*scan == *match) scan++; len = (MAX_MATCH - 1) - (int)(strend-scan); scan = strend - (MAX_MATCH-1); #else /* UNALIGNED_OK */ 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; #endif /* UNALIGNED_OK */ if (len > best_len) { match_start = cur_match; best_len = len; if (len >= nice_match) break; #ifdef UNALIGNED_OK scan_end = *(ush*)(scan+best_len-1); #else scan_end1 = scan[best_len-1]; scan_end = scan[best_len]; #endif } } while ((cur_match = prev[cur_match & WMASK]) > limit && --chain_length != 0); return best_len; } #endif /* ASMV */ #ifdef DEBUG /* =========================================================================== * Check that the match at match_start is indeed a match. */ 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. */ #ifdef NO_LAZY ulg deflate() { 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; 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 always be emitted as literals. */ } while (--match_length != 0); } else { /* No match, output a literal byte */ 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 */ } #else /* LAZY */ /* =========================================================================== * 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 /* 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 */ } #endif /* LAZY */