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deflate.c

/*
  Copyright (c) 1990-2005 Info-ZIP.  All rights reserved.

  See the accompanying file LICENSE, version 2004-May-22 or later
  (the contents of which are also included in zip.h) for terms of use.
  If, for some reason, both of these files are missing, the Info-ZIP license
  also may be found at:  ftp://ftp.info-zip.org/pub/infozip/license.html
*/
/*
 *  deflate.c by Jean-loup Gailly.
 *
 *  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.
 */

#define __DEFLATE_C

#include "zip.h"

#ifndef USE_ZLIB

/* ===========================================================================
 * 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

#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 */

#if (defined(ASMV) && !defined(MSDOS16) && defined(DYN_ALLOC))
   error: DYN_ALLOC not yet supported in match.S or match32.asm
#endif

#ifdef MEMORY16
#  define MAXSEG_64K
#endif

/* ===========================================================================
 * Local data used by the "longest match" routines.
 */

#if defined(MMAP) || defined(BIG_MEM)
  typedef unsigned Pos; /* must be at least 32 bits */
#else
  typedef ush Pos;
#endif
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.
 */

#ifndef DYN_ALLOC
  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+CBSZ if SMALL_MEM (the code would
   * be less efficient since the data would have to be copied WSIZE/CBSZ times)
   */
  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.
   */
  Pos    head[HASH_SIZE];
  /* Heads of the hash chains or NIL. If your compiler thinks that
   * HASH_SIZE is a dynamic value, recompile with -DDYN_ALLOC.
   */
#else
  uch far * near window = NULL;
  Pos far * near prev   = NULL;
  Pos far * near head;
#endif
ulg window_size;
/* 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 int sliding;
/* Set to false when the input file is already in memory */

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. 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.
 */

unsigned near good_match;
/* Use a faster search when the previous match is longer than this */

#ifdef  FULL_SEARCH
# define nice_match MAX_MATCH
#else
  int near nice_match; /* Stop searching when current match exceeds this */
#endif


/* Values for max_lazy_match, good_match, nice_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;

local config configuration_table[10] = {
/*      good lazy nice chain */
/* 0 */ {0,    0,  0,    0},  /* store only */
/* 1 */ {4,    4,  8,    4},  /* maximum speed, no lazy matches */
/* 2 */ {4,    5, 16,    8},
/* 3 */ {4,    6, 32,   32},

/* 4 */ {4,    4, 16,   16},  /* lazy matches */
/* 5 */ {8,   16, 32,   32},
/* 6 */ {8,   16, 128, 128},
/* 7 */ {8,   32, 128, 256},
/* 8 */ {32, 128, 258, 1024},
/* 9 */ {32, 258, 258, 4096}}; /* maximum compression */

/* 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));
local ulg deflate_fast   OF((void));

      int  longest_match OF((IPos cur_match));
#if defined(ASMV) && !defined(RISCOS)
      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
 *
 * IN assertion: window_size is > 0 if the input file is already read or
 *    mmap'ed in the window[] array, 0 otherwise. In the first case,
 *    window_size is sufficient to contain the whole input file plus
 *    MIN_LOOKAHEAD bytes (to avoid referencing memory beyond the end
 *    of window[] when looking for matches towards the end).
 */
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");

    /* Do not slide the window if the whole input is already in memory
     * (window_size > 0)
     */
    sliding = 0;
    if (window_size == 0L) {
        sliding = 1;
        window_size = (ulg)2L*WSIZE;
    }

    /* Use dynamic allocation if compiler does not like big static arrays: */
#ifdef DYN_ALLOC
    if (window == NULL) {
        window = (uch far *) zcalloc(WSIZE,   2*sizeof(uch));
        if (window == NULL) ziperr(ZE_MEM, "window allocation");
    }
    if (prev == NULL) {
        prev   = (Pos far *) zcalloc(WSIZE,     sizeof(Pos));
        head   = (Pos far *) zcalloc(HASH_SIZE, sizeof(Pos));
        if (prev == NULL || head == NULL) {
            ziperr(ZE_MEM, "hash table allocation");
        }
    }
#endif /* DYN_ALLOC */

    /* Initialize the hash table (avoiding 64K overflow for 16 bit systems).
     * prev[] will be initialized on the fly.
     */
    head[HASH_SIZE-1] = NIL;
    memset((char*)head, NIL, (unsigned)(HASH_SIZE-1)*sizeof(*head));

    /* 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 <= 2) {
       *flags |= FAST;
    } else if (pack_level >= 8) {
       *flags |= SLOW;
    }
    /* ??? reduce max_chain_length for binary files */

    strstart = 0;
    block_start = 0L;
#if defined(ASMV) && !defined(RISCOS)
    match_init(); /* initialize the asm code */
#endif

    j = WSIZE;
#ifndef MAXSEG_64K
    if (sizeof(int) > 2) j <<= 1; /* Can read 64K in one step */
#endif
    lookahead = (*read_buf)((char*)window, j);

    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.
     */
    if (lookahead < MIN_LOOKAHEAD) 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.
     */
}

/* ===========================================================================
 * Free the window and hash table
 */
void lm_free()
{
#ifdef DYN_ALLOC
    if (window != NULL) {
        zcfree(window);
        window = NULL;
    }
    if (prev != NULL) {
        zcfree(prev);
        zcfree(head);
        prev = head = NULL;
    }
#endif /* DYN_ALLOC */
}

/* ===========================================================================
 * 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 80x86 and 680x0 and ARM, 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 far *scan = window + strstart; /* current string */
    register uch far *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 far *strend = window + strstart + MAX_MATCH - 1;
    register ush scan_start = *(ush far *)scan;
    register ush scan_end   = *(ush far *)(scan+best_len-1);
#else
    register uch far *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 far *)(match+best_len-1) != scan_end ||
            *(ush far *)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 far *)(scan+=2) == *(ush far *)(match+=2) &&
                 *(ush far *)(scan+=2) == *(ush far *)(match+=2) &&
                 *(ush far *)(scan+=2) == *(ush far *)(match+=2) &&
                 *(ush far *)(scan+=2) == *(ush far *)(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);

        Assert(scan <= window+(unsigned)(window_size-1), "wild scan");

        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 far *)(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);
#ifndef WINDLL
        do { putc(window[start++], stderr); } while (--length != 0);
#else
        do { fprintf(stdout,"%c",window[start++]); } while (--length != 0);
#endif
    }
}
#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: strstart <= window_size-MIN_LOOKAHEAD
 *    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;    /* Amount of free space at the end of the window. */

    do {
        more = (unsigned)(window_size - (ulg)lookahead - (ulg)strstart);

        /* 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--;

        /* For MMAP or BIG_MEM, the whole input file is already in memory so
         * we must not perform sliding. We must however call (*read_buf)() in
         * order to compute the crc, update lookahead and possibly set eofile.
         */
        } else if (strstart >= WSIZE+MAX_DIST && sliding) {

            /* By the IN assertion, the window is not empty so we can't confuse
             * more == 0 with more == 64K on a 16 bit machine.
             */
            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;
#ifndef WINDLL
            if (verbose) putc('.', stderr);
#else
            if (verbose) fprintf(stdout,"%c",'.');
#endif
        }
        if (eofile) return;

        /* If there was no sliding:
         *    strstart <= WSIZE+MAX_DIST-1 && lookahead <= MIN_LOOKAHEAD - 1 &&
         *    more == window_size - lookahead - strstart
         * => more >= window_size - (MIN_LOOKAHEAD-1 + WSIZE + MAX_DIST-1)
         * => more >= window_size - 2*WSIZE + 2
         * In the MMAP or BIG_MEM case (not yet supported in gzip),
         *   window_size == input_size + MIN_LOOKAHEAD  &&
         *   strstart + lookahead <= input_size => more >= MIN_LOOKAHEAD.
         * Otherwise, window_size == 2*WSIZE so more >= 2.
         * If there was sliding, more >= WSIZE. So in all cases, more >= 2.
         */
        Assert(more >= 2, "more < 2");

        n = (*read_buf)((char*)window+strstart+lookahead, more);
        if (n == 0 || n == (unsigned)EOF) {
            eofile = 1;
        } else {
            lookahead += n;
        }
    } while (lookahead < MIN_LOOKAHEAD && !eofile);
}

/* ===========================================================================
 * 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 evaluation of matches and inserts
 * new strings in the dictionary only for unmatched strings or for short
 * matches. It is used only for the fast compression options.
 */
local ulg deflate_fast()
{
    IPos hash_head = NIL;       /* 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:
         */
#ifndef DEFL_UNDETERM
        if (lookahead >= MIN_MATCH)
#endif
        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).
             */
#ifndef HUFFMAN_ONLY
#  ifndef DEFL_UNDETERM
            /* Do not look for matches beyond the end of the input.
             * This is necessary to make deflate deterministic.
             */
            if ((unsigned)nice_match > lookahead) nice_match = (int)lookahead;
#  endif
            match_length = longest_match (hash_head);
            /* longest_match() sets match_start */
            if (match_length > lookahead) match_length = lookahead;
#endif
        }
        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
#ifndef DEFL_UNDETERM
                && lookahead >= MIN_MATCH
#endif
                                                 ) {
                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.
                     */
#ifdef DEFL_UNDETERM
                    /* If lookahead < MIN_MATCH these bytes are garbage,
                     * but it does not matter since the next lookahead bytes
                     * will be emitted as literals.
                     */
#endif
                } 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.
         */
        if (lookahead < MIN_LOOKAHEAD) 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 = NIL;       /* 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 ulg isize;           /* byte length of input file, for debug only */
#endif

    if (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:
         */
#ifndef DEFL_UNDETERM
        if (lookahead >= MIN_MATCH)
#endif
        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).
             */
#ifndef HUFFMAN_ONLY
#  ifndef DEFL_UNDETERM
            /* Do not look for matches beyond the end of the input.
             * This is necessary to make deflate deterministic.
             */
            if ((unsigned)nice_match > lookahead) nice_match = (int)lookahead;
#  endif
            match_length = longest_match (hash_head);
            /* longest_match() sets match_start */
            if (match_length > lookahead) match_length = lookahead;
#endif

#ifdef FILTERED
            /* Ignore matches of length <= 5 */
            if (match_length <= 5) {
#else
            /* Ignore a length 3 match if it is too distant: */
            if (match_length == MIN_MATCH && strstart-match_start > TOO_FAR){
#endif
                /* If prev_match is also MIN_MATCH, match_start is garbage
                 * but we will ignore the current match anyway.
                 */
                match_length = MIN_MATCH-1;
            }
        }
        /* 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) {
#ifndef DEFL_UNDETERM
            unsigned max_insert = strstart + lookahead - MIN_MATCH;

#endif
            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;
#ifndef DEFL_UNDETERM
            do {
                if (++strstart <= max_insert) {
                    INSERT_STRING(strstart, hash_head);
                    /* strstart never exceeds WSIZE-MAX_MATCH, so there are
                     * always MIN_MATCH bytes ahead.
                     */
                }
            } while (--prev_length != 0);
            strstart++;
#else /* DEFL_UNDETERM */
            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);
            strstart++;
#endif /* ?DEFL_UNDETERM */
            match_available = 0;
            match_length = MIN_MATCH-1;

            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.
         */
        if (lookahead < MIN_LOOKAHEAD) fill_window();
    }
    if (match_available) ct_tally (0, window[strstart-1]);

    return FLUSH_BLOCK(1); /* eof */
}
#endif /* !USE_ZLIB */

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