diff_match_patch.prototype.match_bitap_ = function()

in pontoon/base/static/js/lib/diff.js [1437:1543]

• 75 lines of code
• 15 McCabe index (conditional complexity)

diff_match_patch.prototype.match_bitap_ = function(text, pattern, loc) {
  if (pattern.length > this.Match_MaxBits) {
    throw new Error('Pattern too long for this browser.');
  }

  // Initialise the alphabet.
  var s = this.match_alphabet_(pattern);

  var dmp = this;  // 'this' becomes 'window' in a closure.

  /**
   * Compute and return the score for a match with e errors and x location.
   * Accesses loc and pattern through being a closure.
   * @param {number} e Number of errors in match.
   * @param {number} x Location of match.
   * @return {number} Overall score for match (0.0 = good, 1.0 = bad).
   * @private
   */
  function match_bitapScore_(e, x) {
    var accuracy = e / pattern.length;
    var proximity = Math.abs(loc - x);
    if (!dmp.Match_Distance) {
      // Dodge divide by zero error.
      return proximity ? 1.0 : accuracy;
    }
    return accuracy + (proximity / dmp.Match_Distance);
  }

  // Highest score beyond which we give up.
  var score_threshold = this.Match_Threshold;
  // Is there a nearby exact match? (speedup)
  var best_loc = text.indexOf(pattern, loc);
  if (best_loc != -1) {
    score_threshold = Math.min(match_bitapScore_(0, best_loc), score_threshold);
    // What about in the other direction? (speedup)
    best_loc = text.lastIndexOf(pattern, loc + pattern.length);
    if (best_loc != -1) {
      score_threshold =
          Math.min(match_bitapScore_(0, best_loc), score_threshold);
    }
  }

  // Initialise the bit arrays.
  var matchmask = 1 << (pattern.length - 1);
  best_loc = -1;

  var bin_min, bin_mid;
  var bin_max = pattern.length + text.length;
  var last_rd;
  for (var d = 0; d < pattern.length; d++) {
    // Scan for the best match; each iteration allows for one more error.
    // Run a binary search to determine how far from 'loc' we can stray at this
    // error level.
    bin_min = 0;
    bin_mid = bin_max;
    while (bin_min < bin_mid) {
      if (match_bitapScore_(d, loc + bin_mid) <= score_threshold) {
        bin_min = bin_mid;
      } else {
        bin_max = bin_mid;
      }
      bin_mid = Math.floor((bin_max - bin_min) / 2 + bin_min);
    }
    // Use the result from this iteration as the maximum for the next.
    bin_max = bin_mid;
    var start = Math.max(1, loc - bin_mid + 1);
    var finish = Math.min(loc + bin_mid, text.length) + pattern.length;

    var rd = Array(finish + 2);
    rd[finish + 1] = (1 << d) - 1;
    for (var j = finish; j >= start; j--) {
      // The alphabet (s) is a sparse hash, so the following line generates
      // warnings.
      var charMatch = s[text.charAt(j - 1)];
      if (d === 0) {  // First pass: exact match.
        rd[j] = ((rd[j + 1] << 1) | 1) & charMatch;
      } else {  // Subsequent passes: fuzzy match.
        rd[j] = (((rd[j + 1] << 1) | 1) & charMatch) |
                (((last_rd[j + 1] | last_rd[j]) << 1) | 1) |
                last_rd[j + 1];
      }
      if (rd[j] & matchmask) {
        var score = match_bitapScore_(d, j - 1);
        // This match will almost certainly be better than any existing match.
        // But check anyway.
        if (score <= score_threshold) {
          // Told you so.
          score_threshold = score;
          best_loc = j - 1;
          if (best_loc > loc) {
            // When passing loc, don't exceed our current distance from loc.
            start = Math.max(1, 2 * loc - best_loc);
          } else {
            // Already passed loc, downhill from here on in.
            break;
          }
        }
      }
    }
    // No hope for a (better) match at greater error levels.
    if (match_bitapScore_(d + 1, loc) > score_threshold) {
      break;
    }
    last_rd = rd;
  }
  return best_loc;
};