use copyWithin in removeSeam

pretty nice speed-up:

    image  | before | after
    -------|--------|------
    castle |  13.4s | 8.5s
    dog    |   8.6s | 5.1s

Author: Alligator

.gitattributes

@@ -0,0 +1 @@
+build/index.js -diff

build/index.js

@@ -26,15 +26,12 @@ function removeSeam(data, width, height, remove) {
     let offset = 0;
     for (let y = 0; y < height; y++) {
         let removex = remove[y];
-        for (let x = 0; x < width; x++) {
-            if (x === removex) {
-                // pixels been removed, skip over it
-                offset++;
-            }
-            if (x < width && offset > 0) {
-                data32[y * (width - 1) + x] = data32[y * (width - 1) + x + offset];
-            }
-        }
+        // copy left half
+        data32.copyWithin(y * (width - 1), y * (width - 1) + offset, y * (width - 1) + offset + removex);
+        // skip removex
+        offset++;
+        // copy right half
+        data32.copyWithin(y * (width - 1) + removex, y * (width - 1) + offset + removex, y * (width - 1) + (width - 1) + offset);
     }
     return data;
 }
@@ -119,7 +116,7 @@ function recalcEnergy(data, width, height, energy, removed) {
 }
 function calcEnergySum(energySum, energy, width, height) {
     // populate the first row in energySum
-    for (let j = 0; j < energy.length; j++) {
+    for (let j = 0; j < width; j++) {
         energySum[j] = energy[j];
     }
     // populate the rest of the rows

index.ts

@@ -39,15 +39,20 @@ function removeSeam(
   let offset = 0;
   for (let y = 0; y < height; y++) {
     let removex = remove[y];
-    for (let x = 0; x < width; x++) {
-      if (x === removex) {
-        // pixels been removed, skip over it
-        offset++;
-      }
-      if (x < width && offset > 0) {
-        data32[y * (width - 1) + x] = data32[y * (width - 1) + x + offset];
-      }
-    }
+    // copy left half
+    data32.copyWithin(
+      y * (width - 1),
+      y * (width - 1) + offset,
+      y * (width - 1) + offset + removex,
+    );
+    // skip removex
+    offset++;
+    // copy right half
+    data32.copyWithin(
+      y * (width - 1) + removex,
+      y * (width - 1) + offset + removex,
+      y * (width - 1) + (width - 1) + offset,
+    );
   }
 
   return data;
@@ -158,7 +163,7 @@ function calcEnergySum(
   height: number,
 ) {
   // populate the first row in energySum
-  for (let j = 0; j < energy.length; j++) {
+  for (let j = 0; j < width; j++) {
     energySum[j] = energy[j];
   }
 
@@ -252,4 +257,4 @@ function dbgEnergySum(energySum: Uint32Array) {
   }
 
   return newData;
-}
+}