317 lines
9.9 KiB
HTML
317 lines
9.9 KiB
HTML
<!--
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@license
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Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
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This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
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The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
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The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
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Code distributed by Google as part of the polymer project is also
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subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
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-->
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<link rel="import" href="boot.html">
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<script>
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(function() {
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'use strict';
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function newSplice(index, removed, addedCount) {
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return {
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index: index,
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removed: removed,
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addedCount: addedCount
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};
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}
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const EDIT_LEAVE = 0;
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const EDIT_UPDATE = 1;
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const EDIT_ADD = 2;
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const EDIT_DELETE = 3;
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// Note: This function is *based* on the computation of the Levenshtein
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// "edit" distance. The one change is that "updates" are treated as two
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// edits - not one. With Array splices, an update is really a delete
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// followed by an add. By retaining this, we optimize for "keeping" the
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// maximum array items in the original array. For example:
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//
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// 'xxxx123' -> '123yyyy'
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//
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// With 1-edit updates, the shortest path would be just to update all seven
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// characters. With 2-edit updates, we delete 4, leave 3, and add 4. This
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// leaves the substring '123' intact.
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function calcEditDistances(current, currentStart, currentEnd,
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old, oldStart, oldEnd) {
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// "Deletion" columns
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let rowCount = oldEnd - oldStart + 1;
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let columnCount = currentEnd - currentStart + 1;
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let distances = new Array(rowCount);
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// "Addition" rows. Initialize null column.
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for (let i = 0; i < rowCount; i++) {
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distances[i] = new Array(columnCount);
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distances[i][0] = i;
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}
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// Initialize null row
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for (let j = 0; j < columnCount; j++)
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distances[0][j] = j;
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for (let i = 1; i < rowCount; i++) {
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for (let j = 1; j < columnCount; j++) {
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if (equals(current[currentStart + j - 1], old[oldStart + i - 1]))
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distances[i][j] = distances[i - 1][j - 1];
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else {
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let north = distances[i - 1][j] + 1;
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let west = distances[i][j - 1] + 1;
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distances[i][j] = north < west ? north : west;
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}
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}
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}
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return distances;
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}
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// This starts at the final weight, and walks "backward" by finding
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// the minimum previous weight recursively until the origin of the weight
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// matrix.
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function spliceOperationsFromEditDistances(distances) {
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let i = distances.length - 1;
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let j = distances[0].length - 1;
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let current = distances[i][j];
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let edits = [];
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while (i > 0 || j > 0) {
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if (i == 0) {
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edits.push(EDIT_ADD);
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j--;
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continue;
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}
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if (j == 0) {
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edits.push(EDIT_DELETE);
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i--;
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continue;
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}
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let northWest = distances[i - 1][j - 1];
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let west = distances[i - 1][j];
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let north = distances[i][j - 1];
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let min;
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if (west < north)
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min = west < northWest ? west : northWest;
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else
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min = north < northWest ? north : northWest;
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if (min == northWest) {
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if (northWest == current) {
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edits.push(EDIT_LEAVE);
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} else {
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edits.push(EDIT_UPDATE);
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current = northWest;
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}
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i--;
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j--;
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} else if (min == west) {
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edits.push(EDIT_DELETE);
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i--;
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current = west;
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} else {
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edits.push(EDIT_ADD);
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j--;
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current = north;
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}
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}
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edits.reverse();
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return edits;
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}
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/**
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* Splice Projection functions:
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*
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* A splice map is a representation of how a previous array of items
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* was transformed into a new array of items. Conceptually it is a list of
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* tuples of
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*
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* <index, removed, addedCount>
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*
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* which are kept in ascending index order of. The tuple represents that at
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* the |index|, |removed| sequence of items were removed, and counting forward
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* from |index|, |addedCount| items were added.
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*/
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/**
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* Lacking individual splice mutation information, the minimal set of
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* splices can be synthesized given the previous state and final state of an
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* array. The basic approach is to calculate the edit distance matrix and
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* choose the shortest path through it.
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*
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* Complexity: O(l * p)
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* l: The length of the current array
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* p: The length of the old array
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*
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* @param {!Array} current The current "changed" array for which to
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* calculate splices.
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* @param {number} currentStart Starting index in the `current` array for
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* which splices are calculated.
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* @param {number} currentEnd Ending index in the `current` array for
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* which splices are calculated.
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* @param {!Array} old The original "unchanged" array to compare `current`
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* against to determine splices.
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* @param {number} oldStart Starting index in the `old` array for
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* which splices are calculated.
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* @param {number} oldEnd Ending index in the `old` array for
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* which splices are calculated.
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* @return {!Array} Returns an array of splice record objects. Each of these
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* contains: `index` the location where the splice occurred; `removed`
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* the array of removed items from this location; `addedCount` the number
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* of items added at this location.
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*/
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function calcSplices(current, currentStart, currentEnd,
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old, oldStart, oldEnd) {
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let prefixCount = 0;
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let suffixCount = 0;
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let splice;
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let minLength = Math.min(currentEnd - currentStart, oldEnd - oldStart);
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if (currentStart == 0 && oldStart == 0)
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prefixCount = sharedPrefix(current, old, minLength);
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if (currentEnd == current.length && oldEnd == old.length)
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suffixCount = sharedSuffix(current, old, minLength - prefixCount);
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currentStart += prefixCount;
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oldStart += prefixCount;
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currentEnd -= suffixCount;
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oldEnd -= suffixCount;
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if (currentEnd - currentStart == 0 && oldEnd - oldStart == 0)
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return [];
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if (currentStart == currentEnd) {
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splice = newSplice(currentStart, [], 0);
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while (oldStart < oldEnd)
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splice.removed.push(old[oldStart++]);
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return [ splice ];
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} else if (oldStart == oldEnd)
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return [ newSplice(currentStart, [], currentEnd - currentStart) ];
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let ops = spliceOperationsFromEditDistances(
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calcEditDistances(current, currentStart, currentEnd,
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old, oldStart, oldEnd));
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splice = undefined;
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let splices = [];
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let index = currentStart;
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let oldIndex = oldStart;
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for (let i = 0; i < ops.length; i++) {
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switch(ops[i]) {
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case EDIT_LEAVE:
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if (splice) {
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splices.push(splice);
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splice = undefined;
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}
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index++;
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oldIndex++;
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break;
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case EDIT_UPDATE:
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if (!splice)
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splice = newSplice(index, [], 0);
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splice.addedCount++;
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index++;
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splice.removed.push(old[oldIndex]);
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oldIndex++;
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break;
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case EDIT_ADD:
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if (!splice)
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splice = newSplice(index, [], 0);
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splice.addedCount++;
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index++;
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break;
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case EDIT_DELETE:
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if (!splice)
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splice = newSplice(index, [], 0);
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splice.removed.push(old[oldIndex]);
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oldIndex++;
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break;
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}
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}
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if (splice) {
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splices.push(splice);
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}
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return splices;
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}
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function sharedPrefix(current, old, searchLength) {
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for (let i = 0; i < searchLength; i++)
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if (!equals(current[i], old[i]))
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return i;
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return searchLength;
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}
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function sharedSuffix(current, old, searchLength) {
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let index1 = current.length;
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let index2 = old.length;
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let count = 0;
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while (count < searchLength && equals(current[--index1], old[--index2]))
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count++;
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return count;
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}
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function calculateSplices(current, previous) {
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return calcSplices(current, 0, current.length, previous, 0,
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previous.length);
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}
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function equals(currentValue, previousValue) {
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return currentValue === previousValue;
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}
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/**
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* @namespace
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* @memberof Polymer
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* @summary Module that provides utilities for diffing arrays.
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*/
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Polymer.ArraySplice = {
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/**
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* Returns an array of splice records indicating the minimum edits required
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* to transform the `previous` array into the `current` array.
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*
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* Splice records are ordered by index and contain the following fields:
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* - `index`: index where edit started
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* - `removed`: array of removed items from this index
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* - `addedCount`: number of items added at this index
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*
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* This function is based on the Levenshtein "minimum edit distance"
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* algorithm. Note that updates are treated as removal followed by addition.
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*
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* The worst-case time complexity of this algorithm is `O(l * p)`
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* l: The length of the current array
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* p: The length of the previous array
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*
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* However, the worst-case complexity is reduced by an `O(n)` optimization
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* to detect any shared prefix & suffix between the two arrays and only
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* perform the more expensive minimum edit distance calculation over the
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* non-shared portions of the arrays.
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*
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* @function
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* @memberof Polymer.ArraySplice
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* @param {!Array} current The "changed" array for which splices will be
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* calculated.
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* @param {!Array} previous The "unchanged" original array to compare
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* `current` against to determine the splices.
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* @return {!Array} Returns an array of splice record objects. Each of these
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* contains: `index` the location where the splice occurred; `removed`
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* the array of removed items from this location; `addedCount` the number
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* of items added at this location.
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*/
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calculateSplices
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};
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})();
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</script>
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