From 8a2109f3f423f0506086e92507aef8c2e8f80bcf Mon Sep 17 00:00:00 2001
From: Philip <17368112+vHeemstra@users.noreply.github.com>
Date: Mon, 12 Aug 2024 12:47:58 +0200
Subject: [PATCH] perf: Some refactoring

---
 src/index.ts | 65 +++++++++++++++++++++++-----------------------------
 1 file changed, 29 insertions(+), 36 deletions(-)

diff --git a/src/index.ts b/src/index.ts
index 6c58e4e..0a3ef0e 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,23 +2,18 @@ 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 chunkTypes = {
-  animationControl: encoder.encode('acTL'),
-  imageData: encoder.encode('IDAT'),
+function isEqual(first, second, length = 4) {
+  while (length > 0) {
+    length--
+    if (first[length] !== second[length]) {
+      return false
+    }
+  }
+  return true;
 }
 
 /**
- * @see http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html
+ * @see https://www.w3.org/TR/png/#5DataRep
  */
 const headerSizes = {
   /** Number of bytes reserved for PNG signature */
@@ -31,55 +26,53 @@ const headerSizes = {
   CRC: 4,
 }
 
+const chunkTypes = {
+  signature: [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a],
+  animationControl: [0x61, 0x63, 0x54, 0x4c], // 'acTL'
+  imageData: [0x49, 0x44, 0x41, 0x54], // 'IDAT'
+}
+
 export default function isApng(buffer: Buffer | Uint8Array): boolean {
+  const minChunkSize = headerSizes.LENGTH + headerSizes.TYPE + headerSizes.CRC
+  
   if (
     !buffer ||
     !(
       (typeof Buffer !== 'undefined' && Buffer.isBuffer(buffer)) ||
       buffer instanceof Uint8Array
     ) ||
-    buffer.length < 16
+    buffer.length < headerSizes.SIGNATURE + minChunkSize
   ) {
     return false
   }
 
-  const isPNG =
-    buffer[0] === 0x89 &&
-    buffer[1] === 0x50 &&
-    buffer[2] === 0x4e &&
-    buffer[3] === 0x47 &&
-    buffer[4] === 0x0d &&
-    buffer[5] === 0x0a &&
-    buffer[6] === 0x1a &&
-    buffer[7] === 0x0a
-
-  if (!isPNG) {
+  /** Check for PNG signature */
+  if (!isEqual(buffer, chunkTypes.signature, headerSizes.SIGNATURE)) {
     return false
   }
 
-  // APNGs have an animation control chunk (acTL) preceding any IDAT(s).
-  // See: https://en.wikipedia.org/wiki/APNG#File_format
-
   buffer = buffer.subarray(headerSizes.SIGNATURE)
 
-  const minBufferSize = headerSizes.LENGTH + headerSizes.TYPE
-  while (buffer.length >= minBufferSize) {
-    const chunkLength = convertToInt(buffer.subarray(0, headerSizes.LENGTH))
+  /**
+   * APNGs have an animation control (acTL) chunk preceding any image data (IDAT) chunks.
+   * @see: https://www.w3.org/TR/png/#5ChunkOrdering
+   */
+
+  while (buffer.length >= minChunkSize) {
     const chunkType = buffer.subarray(
       headerSizes.LENGTH,
       headerSizes.LENGTH + headerSizes.TYPE,
     )
 
-    if (isEqual(chunkType, chunkTypes.animationControl)) {
+    if (isEqual(chunkType, chunkTypes.animationControl, headerSizes.TYPE)) {
       return true
     }
 
-    if (isEqual(chunkType, chunkTypes.imageData)) {
+    if (isEqual(chunkType, chunkTypes.imageData, headerSizes.TYPE)) {
       return false
     }
 
-    const nextChunkPosition =
-      headerSizes.LENGTH + headerSizes.TYPE + chunkLength + headerSizes.CRC
+    const nextChunkPosition = minChunkSize + convertToInt(buffer.subarray(0, headerSizes.LENGTH))
 
     buffer = buffer.subarray(nextChunkPosition)
   }