Attempt at performance improvements in recalculateVminsumForAffectedPixels

Replaced marked with a weird LRU-esque set thing. This array gets to be
enormous but only the last few items are ever relevant.

Removed enqueued, this also gets enormous and we can just rely on marked
stopping these pixels getting re-processed.

Author: Alligator

SeamCarver.js

@@ -5,6 +5,24 @@ const GREEN = 1;
 const BLUE = 2;
 const BORDER_ENERGY = 1000;
 
+class LRUCache {
+    constructor(max = 10) {
+        this.max = max;
+        this.cache = new Map();
+    }
+
+    has(key) { return this.cache.has(key); }
+    set(key) {
+        if (this.cache.size > this.max) {
+            this.cache.delete(this.cache.keys().next().value);
+        }
+        if (this.cache.has(key)) {
+            return;
+        }
+        this.cache.set(key, 1);
+    }
+}
+
 /** Seam carver removes low energy seams in an image from HTML5 canvas. */
 class SeamCarver {
 
@@ -310,9 +328,7 @@ class SeamCarver {
      * Recalculate vminsum for affected pixels
      */
     recalculateVminsumForAffectedPixels(queue) {
-        var marked = {};
-        var enqueued = {};
-        var maxRow = -1;
+        var marked = new LRUCache(16);
         // start at second to last row
         var row = this.height - 2;
         var enqueuedCols = queue[row];
@@ -327,9 +343,11 @@ class SeamCarver {
             if (enqueuedCols.length === 0) enqueuedCols = queue[--row];
 
             // already explored this pixel
-            if (marked[pixelIndex]) continue;
+            if (marked.has(pixelIndex)) {
+                continue;
+            }
 
-            marked[pixelIndex] = true;
+            marked.set(pixelIndex);
 
             var nodeEnergy = this.energyMatrix[col][row];
             var oldVminsum = this.minsumMatrix[col][row];
@@ -355,11 +373,7 @@ class SeamCarver {
 
             // enqueue three affected children from row above
             for (var i = Math.max(col - 1, 0); i < Math.min(col + 2, lastCol + 1); i ++) {
-                var childIndex = this.pixelToIndex(i, row - 1);
-                if (!enqueued[childIndex]) {
-                    enqueued[childIndex] = true;
-                    queue[row - 1].push(i);
-                }
+                queue[row - 1].push(i);
             }
         }
     }