refactor: efficiency

packages/topdown/tiles.js

@@ -33,19 +33,28 @@ class TilesBase {
 
   slice(rectangle) {
     const tilesRectangle = this.rectangle;
+    // Get intersection.
     if (!Rectangle.intersects(rectangle, tilesRectangle)) {
       return [];
     }
-    let [x, y, width, height] = Rectangle.intersection(rectangle, tilesRectangle);
-    const rowWidth = tilesRectangle[2];
-    const slice = new Array(width * height);
-    for (let j = 0; j < height; ++j) {
-      for (let i = 0; i < width; ++i) {
-        slice[y * width + x] = this.data[y * rowWidth + x];
+    let [x, y, sliceWidth, sliceHeight] = Rectangle.intersection(
+      rectangle,
+      tilesRectangle,
+    );
+    // No muls in the loop.
+    let sliceRow = y * sliceWidth;
+    const dataWidth = this.width;
+    let dataRow = y * dataWidth;
+    // Copy slice.
+    const slice = new Array(sliceWidth * sliceHeight);
+    for (let j = 0; j < sliceHeight; ++j) {
+      for (let i = 0; i < sliceWidth; ++i) {
+        slice[sliceRow + x] = this.data[dataRow + x];
         x++;
       }
-      y++;
-      x -= width;
+      sliceRow += sliceWidth;
+      dataRow += dataWidth;
+      x -= sliceWidth;
     }
     return slice;
   }