From 55acf4f97ca446603235b3a282c66a05df60c2a4 Mon Sep 17 00:00:00 2001
From: Yegor Pelykh <yegor.dev@gmail.com>
Date: Fri, 12 Apr 2024 23:17:07 +0300
Subject: [PATCH] fix: Improve decoding performance of animated GIF

---
 src/formats/gif-decoder.ts       | 40 +++++++++++++++++++++-----------
 src/formats/gif/gif-color-map.ts | 17 +++++++++-----
 2 files changed, 38 insertions(+), 19 deletions(-)

diff --git a/src/formats/gif-decoder.ts b/src/formats/gif-decoder.ts
index cf3e2a2..22407b8 100644
--- a/src/formats/gif-decoder.ts
+++ b/src/formats/gif-decoder.ts
@@ -723,19 +723,33 @@ export class GifDecoder implements Decoder {
       if (frame.disposal === 2) {
         nextImage.clear(colorMap.getColor(this._info.backgroundColor!.r));
       } else if (frame.disposal !== 3) {
-        const nextBytes = nextImage.toUint8Array();
-        const lastBytes = lastImage.toUint8Array();
-        const lp = lastImage.palette!;
-        for (let i = 0, l = nextBytes.length; i < l; ++i) {
-          const lc = lastBytes[i];
-          const nc = colorMap.findColor(
-            lp.getRed(lc),
-            lp.getGreen(lc),
-            lp.getBlue(lc),
-            lp.getAlpha(lc)
-          );
-          if (nc !== -1) {
-            nextBytes[i] = nc;
+        if (
+          frame.x !== 0 ||
+          frame.y !== 0 ||
+          frame.width !== lastImage.width ||
+          frame.height !== lastImage.height
+        ) {
+          if (frame.colorMap !== undefined) {
+            const lp = lastImage.palette!;
+            const remapColors = new Map<number, number | undefined>();
+            for (let ci = 0; ci < colorMap.numColors; ++ci) {
+              const nc = colorMap.findColor(
+                lp.getRed(ci),
+                lp.getGreen(ci),
+                lp.getBlue(ci),
+                lp.getAlpha(ci)
+              );
+              remapColors.set(ci, nc);
+            }
+            const nextBytes = nextImage.toUint8Array();
+            const lastBytes = lastImage.toUint8Array();
+            for (let i = 0, l = nextBytes.length; i < l; ++i) {
+              const lc = lastBytes[i];
+              const nc = remapColors.get(lc);
+              if (nc !== undefined) {
+                nextBytes[i] = nc;
+              }
+            }
           }
         }
       }
diff --git a/src/formats/gif/gif-color-map.ts b/src/formats/gif/gif-color-map.ts
index 5393913..da2082b 100644
--- a/src/formats/gif/gif-color-map.ts
+++ b/src/formats/gif/gif-color-map.ts
@@ -62,18 +62,23 @@ export class GifColorMap {
     this._palette.setRgb(index, r, g, b);
   }
 
-  public findColor(r: number, g: number, b: number, a: number): number {
+  public findColor(
+    r: number,
+    g: number,
+    b: number,
+    a: number
+  ): number | undefined {
     for (let i = 0; i < this._numColors; ++i) {
       if (
-        this._palette.getRed(i) == r &&
-        this._palette.getGreen(i) == g &&
-        this._palette.getBlue(i) == b &&
-        this._palette.getAlpha(i) == a
+        this._palette.getRed(i) === r &&
+        this._palette.getGreen(i) === g &&
+        this._palette.getBlue(i) === b &&
+        this._palette.getAlpha(i) === a
       ) {
         return i;
       }
     }
-    return -1;
+    return undefined;
   }
 
   public getRed(color: number): number {