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Speed up the semantic alignment loop in Javascript #103

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Speed up the semantic alignment loop in Javascript #103

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75 changes: 39 additions & 36 deletions javascript/diff_match_patch_uncompressed.js
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Original file line number Diff line number Diff line change
Expand Up @@ -874,17 +874,17 @@ diff_match_patch.prototype.diff_cleanupSemantic = function(diffs) {
*/
diff_match_patch.prototype.diff_cleanupSemanticLossless = function(diffs) {
/**
* Given two strings, compute a score representing whether the internal
* Given a string and a boundary, compute a score representing whether the
* boundary falls on logical boundaries.
* Scores range from 6 (best) to 0 (worst).
* Closure, but does not reference any external variables.
* @param {string} one First string.
* @param {string} two Second string.
* @param {string} buffer String containing the boundary and surrounding text.
* @param {number} index Index of the boundary.
* @return {number} The score.
* @private
*/
function diff_cleanupSemanticScore_(one, two) {
if (!one || !two) {
function diff_cleanupSemanticScore_(buffer, index) {
if (index === 0 || index === buffer.length) {
// Edges are the best.
return 6;
}
Expand All @@ -894,8 +894,8 @@ diff_match_patch.prototype.diff_cleanupSemanticLossless = function(diffs) {
// 'whitespace'. Since this function's purpose is largely cosmetic,
// the choice has been made to use each language's native features
// rather than force total conformity.
var char1 = one.charAt(one.length - 1);
var char2 = two.charAt(0);
var char1 = buffer.charAt(index - 1);
var char2 = buffer.charAt(index);
var nonAlphaNumeric1 = char1.match(diff_match_patch.nonAlphaNumericRegex_);
var nonAlphaNumeric2 = char2.match(diff_match_patch.nonAlphaNumericRegex_);
var whitespace1 = nonAlphaNumeric1 &&
Expand All @@ -907,9 +907,11 @@ diff_match_patch.prototype.diff_cleanupSemanticLossless = function(diffs) {
var lineBreak2 = whitespace2 &&
char2.match(diff_match_patch.linebreakRegex_);
var blankLine1 = lineBreak1 &&
one.match(diff_match_patch.blanklineEndRegex_);
buffer.substring(index - diff_match_patch.blanklineEndRegexMaxLength_, index)
.match(diff_match_patch.blanklineEndRegex_);
var blankLine2 = lineBreak2 &&
two.match(diff_match_patch.blanklineStartRegex_);
buffer.substring(index, index + diff_match_patch.blanklineStartRegexMaxLength_)
.match(diff_match_patch.blanklineStartRegex_);

if (blankLine1 || blankLine2) {
// Five points for blank lines.
Expand Down Expand Up @@ -939,48 +941,45 @@ diff_match_patch.prototype.diff_cleanupSemanticLossless = function(diffs) {
var equality1 = diffs[pointer - 1][1];
var edit = diffs[pointer][1];
var equality2 = diffs[pointer + 1][1];
var buffer = equality1 + edit + equality2;

// First, shift the edit as far left as possible.
var commonOffset = this.diff_commonSuffix(equality1, edit);
if (commonOffset) {
var commonString = edit.substring(edit.length - commonOffset);
equality1 = equality1.substring(0, equality1.length - commonOffset);
edit = commonString + edit.substring(0, edit.length - commonOffset);
equality2 = commonString + equality2;
}
var offsetLeft = this.diff_commonSuffix(equality1, edit);
var offsetRight = this.diff_commonPrefix(edit, equality2);
var originalEditStart = equality1.length;
var editStart = originalEditStart - offsetLeft;
var maxEditStart = originalEditStart + offsetRight;
var editEnd = editStart + edit.length;

// Second, step character by character right, looking for the best fit.
var bestEquality1 = equality1;
var bestEdit = edit;
var bestEquality2 = equality2;
var bestScore = diff_cleanupSemanticScore_(equality1, edit) +
diff_cleanupSemanticScore_(edit, equality2);
while (edit.charAt(0) === equality2.charAt(0)) {
equality1 += edit.charAt(0);
edit = edit.substring(1) + equality2.charAt(0);
equality2 = equality2.substring(1);
var score = diff_cleanupSemanticScore_(equality1, edit) +
diff_cleanupSemanticScore_(edit, equality2);
var bestEditStart = editStart;
var bestEditEnd = editEnd;
var bestScore = diff_cleanupSemanticScore_(buffer, editStart) +
diff_cleanupSemanticScore_(buffer, editEnd);
while (editStart < maxEditStart) {
editStart += 1;
editEnd += 1;
var score = diff_cleanupSemanticScore_(buffer, editStart) +
diff_cleanupSemanticScore_(buffer, editEnd);
// The >= encourages trailing rather than leading whitespace on edits.
if (score >= bestScore) {
bestScore = score;
bestEquality1 = equality1;
bestEdit = edit;
bestEquality2 = equality2;
bestEditStart = editStart;
bestEditEnd = editEnd;
}
}

if (diffs[pointer - 1][1] != bestEquality1) {
if (bestEditStart != originalEditStart) {
// We have an improvement, save it back to the diff.
if (bestEquality1) {
diffs[pointer - 1][1] = bestEquality1;
if (bestEditStart > 0) {
diffs[pointer - 1][1] = buffer.substring(0, bestEditStart);
} else {
diffs.splice(pointer - 1, 1);
pointer--;
}
diffs[pointer][1] = bestEdit;
if (bestEquality2) {
diffs[pointer + 1][1] = bestEquality2;
diffs[pointer][1] = buffer.substring(bestEditStart, bestEditEnd);
if (bestEditEnd < buffer.length) {
diffs[pointer + 1][1] = buffer.substring(bestEditEnd);
} else {
diffs.splice(pointer + 1, 1);
pointer--;
Expand All @@ -998,6 +997,10 @@ diff_match_patch.linebreakRegex_ = /[\r\n]/;
diff_match_patch.blanklineEndRegex_ = /\n\r?\n$/;
diff_match_patch.blanklineStartRegex_ = /^\r?\n\r?\n/;

// Maximum length of a match for blank line regexes
diff_match_patch.blanklineEndRegexMaxLength_ = 3;
diff_match_patch.blanklineStartRegexMaxLength_ = 4;

/**
* Reduce the number of edits by eliminating operationally trivial equalities.
* @param {!Array.<!diff_match_patch.Diff>} diffs Array of diff tuples.
Expand Down