Function longest_match

Picviewer CE+/dist.user.js:6364–6464

(s, cur_match)

 * OUT assertion: the match length is not greater than s->lookahead.
 */
function longest_match(s, cur_match) {
  var chain_length = s.max_chain_length;      /* max hash chain length */
  var scan = s.strstart; /* current string */
  var match;                       /* matched string */
  var len;                           /* length of current match */
  var best_len = s.prev_length;              /* best match length so far */
  var nice_match = s.nice_match;             /* stop if match long enough */
  var limit = (s.strstart > (s.w_size - MIN_LOOKAHEAD)) ?
      s.strstart - (s.w_size - MIN_LOOKAHEAD) : 0/*NIL*/;

  var _win = s.window; // shortcut

  var wmask = s.w_mask;
  var prev  = s.prev;

  /* Stop when cur_match becomes <= limit. To simplify the code,
   * we prevent matches with the string of window index 0.
   */

  var strend = s.strstart + MAX_MATCH;
  var scan_end1  = _win[scan + best_len - 1];
  var scan_end   = _win[scan + best_len];

  /* 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.
   */
  // Assert(s->hash_bits >= 8 && MAX_MATCH == 258, "Code too clever");

  /* Do not waste too much time if we already have a good match: */
  if (s.prev_length >= s.good_match) {
    chain_length >>= 2;
  }
  /* Do not look for matches beyond the end of the input. This is necessary
   * to make deflate deterministic.
   */
  if (nice_match > s.lookahead) { nice_match = s.lookahead; }

  // Assert((ulg)s->strstart <= s->window_size-MIN_LOOKAHEAD, "need lookahead");

  do {
    // Assert(cur_match < s->strstart, "no future");
    match = cur_match;

    /* Skip to next match if the match length cannot increase
     * or if the match length is less than 2.  Note that the checks below
     * for insufficient lookahead only occur occasionally for performance
     * reasons.  Therefore uninitialized memory will be accessed, and
     * conditional jumps will be made that depend on those values.
     * However the length of the match is limited to the lookahead, so
     * the output of deflate is not affected by the uninitialized values.
     */

    if (_win[match + best_len]     !== scan_end  ||
        _win[match + best_len - 1] !== scan_end1 ||
        _win[match]                !== _win[scan] ||
        _win[++match]              !== _win[scan + 1]) {
      continue;
    }