(fix) Chunk matching

Fixes bug where chunk would not be found if preceded by a part of itself

package.json

@@ -72,7 +72,7 @@
   "scripts": {
     "lint": "eslint ./src/",
     "lint:fix": "eslint ./src/ --fix",
-    "build:watch": "cross-env NODE_ENV=development rollup --config rollup.config.js --watch --sourcemap inline",
+    "build:watch": "cross-env NODE_ENV=development rollup --config rollup.config.js --sourcemap inline --watch",
     "build:dev": "cross-env NODE_ENV=development rollup --config rollup.config.js --sourcemap inline",
     "build": "cross-env NODE_ENV=production rollup --config rollup.config.js",
     "test": "ava",

src/index.test.js

@@ -65,3 +65,18 @@ test('returns false when missing IDAT', (t) => {
     ),
   )
 })
+
+test('chunks should be found when preceded by a partial of themselves', (t) => {
+  t.true(
+    isApng(
+      new Uint8Array([
+        // PNG header
+        0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a,
+        // a acTL
+        0x61, 0x61, 0x63, 0x54, 0x4c,
+        // I IDAT
+        0x49, 0x49, 0x44, 0x41, 0x54,
+      ]),
+    ),
+  )
+})

src/index.ts

@@ -39,22 +39,29 @@ export default function isApng(buffer: Buffer | Uint8Array): boolean {
   let firstIndex = 0
   let secondIndex = 0
   for (let i = 0; i < buffer.length; i++) {
-    if (buffer[i] === sequences.animationControlChunk[firstIndex]) {
+    if (
+      !foundFirst &&
+      (buffer[i] === sequences.animationControlChunk[firstIndex] ||
+        (firstIndex > 0 &&
+          ((firstIndex = 0) ||
+            buffer[i] === sequences.animationControlChunk[firstIndex])))
+    ) {
       firstIndex++
       if (firstIndex === sequences.animationControlChunk.length) {
         foundFirst = true
       }
-    } else {
-      firstIndex = 0
     }
 
-    if (buffer[i] === sequences.imageDataChunk[secondIndex]) {
+    if (
+      buffer[i] === sequences.imageDataChunk[secondIndex] ||
+      (secondIndex > 0 &&
+        ((secondIndex = 0) ||
+          buffer[i] === sequences.imageDataChunk[secondIndex]))
+    ) {
       secondIndex++
       if (secondIndex === sequences.imageDataChunk.length) {
         return foundFirst
       }
-    } else {
-      secondIndex = 0
     }
   }