feat: add hasPngSignature method

src/PngDecoder.ts

@@ -1,7 +1,8 @@
 import { IOBuffer } from 'iobuffer';
 import { inflate, Inflate as Inflator } from 'pako';
 
-import { pngSignature, crc } from './common';
+import { crc } from './common';
+import { checkSignature } from './helpers/signature';
 import {
   ColorType,
   CompressionMethod,
@@ -25,9 +26,9 @@ const uint8 = new Uint8Array(uint16.buffer);
 const osIsLittleEndian = uint8[0] === 0xff;
 
 export default class PngDecoder extends IOBuffer {
-  private _checkCrc: boolean;
-  private _inflator: Inflator;
-  private _png: DecodedPng;
+  private readonly _checkCrc: boolean;
+  private readonly _inflator: Inflator;
+  private readonly _png: DecodedPng;
   private _end: boolean;
   private _hasPalette: boolean;
   private _palette: IndexedColors;
@@ -66,25 +67,14 @@ export default class PngDecoder extends IOBuffer {
   }
 
   public decode(): DecodedPng {
-    this.decodeSignature();
+    checkSignature(this);
     while (!this._end) {
       this.decodeChunk();
     }
     this.decodeImage();
     return this._png;
   }
 
-  // https://www.w3.org/TR/PNG/#5PNG-file-signature
-  private decodeSignature(): void {
-    for (let i = 0; i < pngSignature.length; i++) {
-      if (this.readUint8() !== pngSignature[i]) {
-        throw new Error(
-          `wrong PNG signature. Byte at ${i} should be ${pngSignature[i]}.`,
-        );
-      }
-    }
-  }
-
   // https://www.w3.org/TR/PNG/#5Chunk-layout
   private decodeChunk(): void {
     const length = this.readUint32();

src/PngEncoder.ts

@@ -1,7 +1,8 @@
 import { IOBuffer } from 'iobuffer';
 import { deflate } from 'pako';
 
-import { pngSignature, crc } from './common';
+import { crc } from './common';
+import { writeSignature } from './helpers/signature';
 import {
   ColorType,
   CompressionMethod,
@@ -22,8 +23,8 @@ const defaultZlibOptions: DeflateFunctionOptions = {
 };
 
 export default class PngEncoder extends IOBuffer {
-  private _png: DecodedPng;
-  private _zlibOptions: DeflateFunctionOptions;
+  private readonly _png: DecodedPng;
+  private readonly _zlibOptions: DeflateFunctionOptions;
   private _colorType: ColorType;
 
   public constructor(data: ImageData, options: PngEncoderOptions = {}) {
@@ -35,18 +36,13 @@ export default class PngEncoder extends IOBuffer {
   }
 
   public encode(): Uint8Array {
-    this.encodeSignature();
+    writeSignature(this);
     this.encodeIHDR();
     this.encodeData();
     this.encodeIEND();
     return this.toArray();
   }
 
-  // https://www.w3.org/TR/PNG/#5PNG-file-signature
-  private encodeSignature(): void {
-    this.writeBytes(pngSignature);
-  }
-
   // https://www.w3.org/TR/PNG/#11IHDR
   private encodeIHDR(): void {
     this.writeUint32(13);

src/__tests__/decode.test.ts

@@ -88,10 +88,13 @@ describe('decode', () => {
     loadAndDecode('palette.png', { checkCrc: true });
   });
 
-  it('should throw with a non-png', () => {
-    expect(() => decode(new Uint8Array(20))).toThrow(
-      'wrong PNG signature. Byte at 0 should be 137.',
-    );
+  it.each([
+    // Enough values, last one is wrong.
+    Uint8Array.of(137, 80, 78, 71, 13, 10, 26, 9, 0, 0, 0),
+    // Not enough values.
+    Uint8Array.of(137, 80),
+  ])('should throw with a non-png', (value) => {
+    expect(() => decode(value)).toThrow('wrong PNG signature');
   });
 
   it('ICC Embeded Profile', () => {

src/common.ts

@@ -1,5 +1,3 @@
-export const pngSignature = [137, 80, 78, 71, 13, 10, 26, 10];
-
 const crcTable: number[] = [];
 for (let n = 0; n < 256; n++) {
   let c = n;

src/helpers/signature.ts

@@ -0,0 +1,27 @@
+import type { IOBuffer } from 'iobuffer';
+
+// https://www.w3.org/TR/PNG/#5PNG-file-signature
+
+const pngSignature = Uint8Array.of(137, 80, 78, 71, 13, 10, 26, 10);
+
+export function writeSignature(buffer: IOBuffer) {
+  buffer.writeBytes(pngSignature);
+}
+
+export function checkSignature(buffer: IOBuffer) {
+  if (!hasPngSignature(buffer.readBytes(pngSignature.length))) {
+    throw new Error('wrong PNG signature');
+  }
+}
+
+export function hasPngSignature(array: ArrayLike<number>) {
+  if (array.length < pngSignature.length) {
+    return false;
+  }
+  for (let i = 0; i < pngSignature.length; i++) {
+    if (array[i] !== pngSignature[i]) {
+      return false;
+    }
+  }
+  return true;
+}

src/index.ts

@@ -8,6 +8,7 @@ import {
   PngEncoderOptions,
 } from './types';
 
+export { hasPngSignature } from './helpers/signature';
 export * from './types';
 
 function decodePng(