(fix) Search acTL only among chunk types

Byte sequence `acTL` can be encountered in other parts of PNG,
that are not chunk type (e.g. in chunk data or crc),
so we should check only chunk types to avoid false positive detection.

src/index.test.js

@@ -23,43 +23,63 @@ test('returns false on JPG', (t) => {
   t.false(check(dir + '/images/static.jpg'))
 })
 
+test('returns false on static PNG with `acTL` text in metadata', (t) => {
+  t.false(check(dir + '/images/staticWithMetadata.png'))
+})
+
 test('returns true when IDAT follows acTL', (t) => {
   t.true(
     isApng(
       new Uint8Array([
         // PNG header
         0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a,
-        // acTL
+        // Chunk length: 0
+        0x00, 0x00, 0x00, 0x00,
+        // Chunk type: acTL
         0x61, 0x63, 0x54, 0x4c,
-        // IDAT
+        // Chunk CRC
+        0x00, 0x00, 0x00, 0x00,
+        // Chunk length: 0
+        0x00, 0x00, 0x00, 0x00,
+        // Chunk type: IDAT
         0x49, 0x44, 0x41, 0x54,
       ]),
     ),
   )
 })
 
-test('returns false when IDAT precedes acTL', (t) => {
+test('returns false when acTL is not a chunk type', (t) => {
   t.false(
     isApng(
       new Uint8Array([
         // PNG header
         0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a,
-        // IDAT
-        0x49, 0x44, 0x41, 0x54,
-        // acTL
+        // Chunk length: 4
+        0x00, 0x00, 0x00, 0x04,
+        // Chunk type: any
+        0x00, 0x00, 0x00, 0x01,
+        // Chunk data: acTL
         0x61, 0x63, 0x54, 0x4c,
       ]),
     ),
   )
 })
 
-test('returns false when missing IDAT', (t) => {
+test('returns false when IDAT precedes acTL', (t) => {
   t.false(
     isApng(
       new Uint8Array([
         // PNG header
         0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a,
-        // acTL
+        // Chunk length: 0
+        0x00, 0x00, 0x00, 0x00,
+        // Chunk type: IDAT
+        0x49, 0x44, 0x41, 0x54,
+        // Chunk CRC
+        0x00, 0x00, 0x00, 0x00,
+        // Chunk length: 0
+        0x00, 0x00, 0x00, 0x00,
+        // Chunk type: acTL
         0x61, 0x63, 0x54, 0x4c,
       ]),
     ),
@@ -72,10 +92,10 @@ test('chunks should be found when preceded by a partial of themselves', (t) => {
       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,
+        // Chunk length: a
+        0x00, 0x00, 0x00, 0x61,
+        // Chunk type: acTL
+        0x61, 0x63, 0x54, 0x4c,
       ]),
     ),
   )

src/index.ts

@@ -1,7 +1,34 @@
+function convertToInt(bytes: Uint8Array) {
+  return bytes.reduce((value, byte) => (value << 8) + byte)
+}
+
+function isEqual(first, second) {
+  return (
+    first[0] === second[0] &&
+    first[1] === second[1] &&
+    first[2] === second[2] &&
+    first[3] === second[3]
+  )
+}
+
 const encoder = new TextEncoder()
-const sequences = {
-  animationControlChunk: encoder.encode('acTL'),
-  imageDataChunk: encoder.encode('IDAT'),
+const chunkTypes = {
+  animationControl: encoder.encode('acTL'),
+  imageData: encoder.encode('IDAT'),
+}
+
+/**
+ * @see http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html
+ */
+const headerSizes = {
+  /** Number of bytes reserved for PNG signature */
+  SIGNATURE: 8,
+  /** Number of bytes reserved for chunk type */
+  LENGTH: 4,
+  /** Number of bytes reserved for chunk type */
+  TYPE: 4,
+  /** Number of bytes reserved for chunk CRC */
+  CRC: 4,
 }
 
 export default function isApng(buffer: Buffer | Uint8Array): boolean {
@@ -33,32 +60,28 @@ export default function isApng(buffer: Buffer | Uint8Array): boolean {
   // APNGs have an animation control chunk (acTL) preceding any IDAT(s).
   // See: https://en.wikipedia.org/wiki/APNG#File_format
 
-  buffer = buffer.subarray(8)
+  buffer = buffer.subarray(headerSizes.SIGNATURE)
 
-  let firstIndex = 0
-  let secondIndex = 0
-  for (let i = 0; i < buffer.length; i++) {
-    if (buffer[i] !== sequences.animationControlChunk[firstIndex]) {
-      firstIndex = 0
-    }
+  const minBufferSize = headerSizes.LENGTH + headerSizes.TYPE
+  while (buffer.length >= minBufferSize) {
+    const chunkLength = convertToInt(buffer.subarray(0, headerSizes.LENGTH))
+    const chunkType = buffer.subarray(
+      headerSizes.LENGTH,
+      headerSizes.LENGTH + headerSizes.TYPE,
+    )
 
-    if (buffer[i] === sequences.animationControlChunk[firstIndex]) {
-      firstIndex++
-      if (firstIndex === sequences.animationControlChunk.length) {
-        return true
-      }
+    if (isEqual(chunkType, chunkTypes.animationControl)) {
+      return true
     }
 
-    if (buffer[i] !== sequences.imageDataChunk[secondIndex]) {
-      secondIndex = 0
+    if (isEqual(chunkType, chunkTypes.imageData)) {
+      return false
     }
 
-    if (buffer[i] === sequences.imageDataChunk[secondIndex]) {
-      secondIndex++
-      if (secondIndex === sequences.imageDataChunk.length) {
-        return false
-      }
-    }
+    const nextChunkPosition =
+      headerSizes.LENGTH + headerSizes.TYPE + chunkLength + headerSizes.CRC
+
+    buffer = buffer.subarray(nextChunkPosition)
   }
 
   return false