webcomponentsjs/src/ShadowDOM/ArraySplice.js

// Copyright 2013 The Polymer Authors. All rights reserved.
// Use of this source code is goverened by a BSD-style
// license that can be found in the LICENSE file.
(function(scope) {
  'use strict';

  function newSplice(index, removed, addedCount) {
    return {
      index: index,
      removed: removed,
      addedCount: addedCount
    };
  }

	var EDIT_LEAVE = 0;
	var EDIT_UPDATE = 1;
	var EDIT_ADD = 2;
	var EDIT_DELETE = 3;

	function ArraySplice() {}

	ArraySplice.prototype = {

	  // Note: This function is *based* on the computation of the Levenshtein
	  // "edit" distance. The one change is that "updates" are treated as two
	  // edits - not one. With Array splices, an update is really a delete
	  // followed by an add. By retaining this, we optimize for "keeping" the
	  // maximum array items in the original array. For example:
	  //
	  //   'xxxx123' -> '123yyyy'
	  //
	  // With 1-edit updates, the shortest path would be just to update all seven
	  // characters. With 2-edit updates, we delete 4, leave 3, and add 4. This
	  // leaves the substring '123' intact.
	  calcEditDistances: function(current, currentStart, currentEnd,
	                              old, oldStart, oldEnd) {
	    // "Deletion" columns
	    var rowCount = oldEnd - oldStart + 1;
	    var columnCount = currentEnd - currentStart + 1;
	    var distances = new Array(rowCount);

	    // "Addition" rows. Initialize null column.
	    for (var i = 0; i < rowCount; i++) {
	      distances[i] = new Array(columnCount);
	      distances[i][0] = i;
	    }

	    // Initialize null row
	    for (var j = 0; j < columnCount; j++)
	      distances[0][j] = j;

	    for (var i = 1; i < rowCount; i++) {
	      for (var j = 1; j < columnCount; j++) {
	        if (this.equals(current[currentStart + j - 1], old[oldStart + i - 1]))
	          distances[i][j] = distances[i - 1][j - 1];
	        else {
	          var north = distances[i - 1][j] + 1;
	          var west = distances[i][j - 1] + 1;
	          distances[i][j] = north < west ? north : west;
	        }
	      }
	    }

	    return distances;
	  },

	  // This starts at the final weight, and walks "backward" by finding
	  // the minimum previous weight recursively until the origin of the weight
	  // matrix.
	  spliceOperationsFromEditDistances: function(distances) {
	    var i = distances.length - 1;
	    var j = distances[0].length - 1;
	    var current = distances[i][j];
	    var edits = [];
	    while (i > 0 || j > 0) {
	      if (i == 0) {
	        edits.push(EDIT_ADD);
	        j--;
	        continue;
	      }
	      if (j == 0) {
	        edits.push(EDIT_DELETE);
	        i--;
	        continue;
	      }
	      var northWest = distances[i - 1][j - 1];
	      var west = distances[i - 1][j];
	      var north = distances[i][j - 1];

	      var min;
	      if (west < north)
	        min = west < northWest ? west : northWest;
	      else
	        min = north < northWest ? north : northWest;

	      if (min == northWest) {
	        if (northWest == current) {
	          edits.push(EDIT_LEAVE);
	        } else {
	          edits.push(EDIT_UPDATE);
	          current = northWest;
	        }
	        i--;
	        j--;
	      } else if (min == west) {
	        edits.push(EDIT_DELETE);
	        i--;
	        current = west;
	      } else {
	        edits.push(EDIT_ADD);
	        j--;
	        current = north;
	      }
	    }

	    edits.reverse();
	    return edits;
	  },

	  /**
	   * Splice Projection functions:
	   *
	   * A splice map is a representation of how a previous array of items
	   * was transformed into a new array of items. Conceptually it is a list of
	   * tuples of
	   *
	   *   <index, removed, addedCount>
	   *
	   * which are kept in ascending index order of. The tuple represents that at
	   * the |index|, |removed| sequence of items were removed, and counting forward
	   * from |index|, |addedCount| items were added.
	   */

	  /**
	   * Lacking individual splice mutation information, the minimal set of
	   * splices can be synthesized given the previous state and final state of an
	   * array. The basic approach is to calculate the edit distance matrix and
	   * choose the shortest path through it.
	   *
	   * Complexity: O(l * p)
	   *   l: The length of the current array
	   *   p: The length of the old array
	   */
	  calcSplices: function(current, currentStart, currentEnd,
	                        old, oldStart, oldEnd) {
	    var prefixCount = 0;
	    var suffixCount = 0;

	    var minLength = Math.min(currentEnd - currentStart, oldEnd - oldStart);
	    if (currentStart == 0 && oldStart == 0)
	      prefixCount = this.sharedPrefix(current, old, minLength);

	    if (currentEnd == current.length && oldEnd == old.length)
	      suffixCount = this.sharedSuffix(current, old, minLength - prefixCount);

	    currentStart += prefixCount;
	    oldStart += prefixCount;
	    currentEnd -= suffixCount;
	    oldEnd -= suffixCount;

	    if (currentEnd - currentStart == 0 && oldEnd - oldStart == 0)
	      return [];

	    if (currentStart == currentEnd) {
	      var splice = newSplice(currentStart, [], 0);
	      while (oldStart < oldEnd)
	        splice.removed.push(old[oldStart++]);

	      return [ splice ];
	    } else if (oldStart == oldEnd)
	      return [ newSplice(currentStart, [], currentEnd - currentStart) ];

	    var ops = this.spliceOperationsFromEditDistances(
	        this.calcEditDistances(current, currentStart, currentEnd,
	                               old, oldStart, oldEnd));

	    var splice = undefined;
	    var splices = [];
	    var index = currentStart;
	    var oldIndex = oldStart;
	    for (var i = 0; i < ops.length; i++) {
	      switch(ops[i]) {
	        case EDIT_LEAVE:
	          if (splice) {
	            splices.push(splice);
	            splice = undefined;
	          }

	          index++;
	          oldIndex++;
	          break;
	        case EDIT_UPDATE:
	          if (!splice)
	            splice = newSplice(index, [], 0);

	          splice.addedCount++;
	          index++;

	          splice.removed.push(old[oldIndex]);
	          oldIndex++;
	          break;
	        case EDIT_ADD:
	          if (!splice)
	            splice = newSplice(index, [], 0);

	          splice.addedCount++;
	          index++;
	          break;
	        case EDIT_DELETE:
	          if (!splice)
	            splice = newSplice(index, [], 0);

	          splice.removed.push(old[oldIndex]);
	          oldIndex++;
	          break;
	      }
	    }

	    if (splice) {
	      splices.push(splice);
	    }
	    return splices;
	  },

	  sharedPrefix: function(current, old, searchLength) {
	    for (var i = 0; i < searchLength; i++)
	      if (!this.equals(current[i], old[i]))
	        return i;
	    return searchLength;
	  },

	  sharedSuffix: function(current, old, searchLength) {
	    var index1 = current.length;
	    var index2 = old.length;
	    var count = 0;
	    while (count < searchLength && this.equals(current[--index1], old[--index2]))
	      count++;

	    return count;
	  },

	  calculateSplices: function(current, previous) {
	    return this.calcSplices(current, 0, current.length, previous, 0,
	                            previous.length);
	  },

	  equals: function(currentValue, previousValue) {
	    return currentValue === previousValue;
	  }
	};

	// exports
	scope.ArraySplice = ArraySplice;

})(window.ShadowDOMPolyfill);