function longest_match()

in TestApplications/JDIChromePlugin/DevPanel/libs/jszip/jszip.js [4155:4255]

• 52 lines of code
• 22 McCabe index (conditional complexity)

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;
    }

    /* 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++;
    // Assert(*scan == *match, "match[2]?");

    /* We check for insufficient lookahead only every 8th comparison;
     * the 256th check will be made at strstart+258.
     */
    do {
      /*jshint noempty:false*/
    } while (_win[++scan] === _win[++match] && _win[++scan] === _win[++match] &&
             _win[++scan] === _win[++match] && _win[++scan] === _win[++match] &&
             _win[++scan] === _win[++match] && _win[++scan] === _win[++match] &&
             _win[++scan] === _win[++match] && _win[++scan] === _win[++match] &&
             scan < strend);

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

    len = MAX_MATCH - (strend - scan);
    scan = strend - MAX_MATCH;

    if (len > best_len) {
      s.match_start = cur_match;
      best_len = len;
      if (len >= nice_match) {
        break;
      }
      scan_end1  = _win[scan + best_len - 1];
      scan_end   = _win[scan + best_len];
    }
  } while ((cur_match = prev[cur_match & wmask]) > limit && --chain_length !== 0);

  if (best_len <= s.lookahead) {
    return best_len;
  }
  return s.lookahead;
}