Support APNGs in Sticker Creator

ts/util/sniffImageMimeType.ts

@@ -0,0 +1,127 @@
+import {
+  IMAGE_BMP,
+  IMAGE_GIF,
+  IMAGE_ICO,
+  IMAGE_JPEG,
+  IMAGE_PNG,
+  IMAGE_WEBP,
+  MIMEType,
+} from '../types/MIME';
+
+/**
+ * This follows the [MIME Sniffing Standard for images][0].
+ *
+ * [0]: https://mimesniff.spec.whatwg.org/#matching-an-image-type-pattern
+ */
+export function sniffImageMimeType(
+  bytes: ArrayBuffer | Uint8Array
+): undefined | MIMEType {
+  const asTypedArray = new Uint8Array(bytes);
+  for (let i = 0; i < TYPES.length; i += 1) {
+    const type = TYPES[i];
+    if (matchesType(asTypedArray, type)) {
+      return type.mimeType;
+    }
+  }
+  return undefined;
+}
+
+interface Type {
+  mimeType: MIMEType;
+  bytePattern: Uint8Array;
+  patternMask?: Uint8Array;
+}
+const TYPES: Array<Type> = [
+  {
+    mimeType: IMAGE_ICO,
+    bytePattern: new Uint8Array([0x00, 0x00, 0x01, 0x00]),
+  },
+  {
+    mimeType: IMAGE_ICO,
+    bytePattern: new Uint8Array([0x00, 0x00, 0x02, 0x00]),
+  },
+  {
+    mimeType: IMAGE_BMP,
+    bytePattern: new Uint8Array([0x42, 0x4d]),
+  },
+  {
+    mimeType: IMAGE_GIF,
+    bytePattern: new Uint8Array([0x47, 0x49, 0x46, 0x38, 0x37, 0x61]),
+  },
+  {
+    mimeType: IMAGE_GIF,
+    bytePattern: new Uint8Array([0x47, 0x49, 0x46, 0x38, 0x39, 0x61]),
+  },
+  {
+    mimeType: IMAGE_WEBP,
+    bytePattern: new Uint8Array([
+      0x52,
+      0x49,
+      0x46,
+      0x46,
+      0x00,
+      0x00,
+      0x00,
+      0x00,
+      0x57,
+      0x45,
+      0x42,
+      0x50,
+      0x56,
+      0x50,
+    ]),
+    patternMask: new Uint8Array([
+      0xff,
+      0xff,
+      0xff,
+      0xff,
+      0x00,
+      0x00,
+      0x00,
+      0x00,
+      0xff,
+      0xff,
+      0xff,
+      0xff,
+      0xff,
+      0xff,
+    ]),
+  },
+  {
+    mimeType: IMAGE_PNG,
+    bytePattern: new Uint8Array([
+      0x89,
+      0x50,
+      0x4e,
+      0x47,
+      0x0d,
+      0x0a,
+      0x1a,
+      0x0a,
+    ]),
+  },
+  {
+    mimeType: IMAGE_JPEG,
+    bytePattern: new Uint8Array([0xff, 0xd8, 0xff]),
+  },
+];
+
+// This follows the [pattern matching algorithm in the spec][1].
+// [1]: https://mimesniff.spec.whatwg.org/#pattern-matching-algorithm
+function matchesType(input: Uint8Array, type: Type): boolean {
+  if (input.byteLength < type.bytePattern.byteLength) {
+    return false;
+  }
+
+  for (let p = 0; p < type.bytePattern.length; p += 1) {
+    const mask = type.patternMask ? type.patternMask[p] : 0xff;
+    // We need to use a bitwise operator here, per the spec.
+    // eslint-disable-next-line no-bitwise
+    const maskedData = input[p] & mask;
+    if (maskedData !== type.bytePattern[p]) {
+      return false;
+    }
+  }
+
+  return true;
+}