Implement ignoreBOM option of UTF8Decoder in text_encoding (#3040)

js/lib.deno_runtime.d.ts

@@ -2372,7 +2372,7 @@ declare namespace textEncoding {
   }
   export interface TextDecoderOptions {
     fatal?: boolean;
-    ignoreBOM?: false;
+    ignoreBOM?: boolean;
   }
   export class TextDecoder {
     private _encoding;

js/text_encoding.ts

@@ -59,11 +59,13 @@ class UTF8Decoder implements Decoder {
   private _bytesSeen = 0;
   private _bytesNeeded = 0;
   private _fatal: boolean;
+  private _ignoreBOM: boolean;
   private _lowerBoundary = 0x80;
   private _upperBoundary = 0xbf;
 
   constructor(options: DecoderOptions) {
     this._fatal = options.fatal || false;
+    this._ignoreBOM = options.ignoreBOM || false;
   }
 
   handler(stream: Stream, byte: number): number | null {
@@ -76,6 +78,26 @@ class UTF8Decoder implements Decoder {
       return FINISHED;
     }
 
+    if (this._ignoreBOM) {
+      if (
+        (this._bytesSeen === 0 && byte !== 0xef) ||
+        (this._bytesSeen === 1 && byte !== 0xbb)
+      ) {
+        this._ignoreBOM = false;
+      }
+
+      if (this._bytesSeen === 2) {
+        this._ignoreBOM = false;
+        if (byte === 0xbf) {
+          //Ignore BOM
+          this._codePoint = 0;
+          this._bytesNeeded = 0;
+          this._bytesSeen = 0;
+          return CONTINUE;
+        }
+      }
+    }
+
     if (this._bytesNeeded === 0) {
       if (isASCIIByte(byte)) {
         // Single byte code point
@@ -225,6 +247,7 @@ export function btoa(s: string): string {
 
 interface DecoderOptions {
   fatal?: boolean;
+  ignoreBOM?: boolean;
 }
 
 interface Decoder {
@@ -240,6 +263,9 @@ class SingleByteDecoder implements Decoder {
   private _fatal: boolean;
 
   constructor(index: number[], options: DecoderOptions) {
+    if (options.ignoreBOM) {
+      throw new TypeError("Ignoring the BOM is available only with utf-8.");
+    }
     this._fatal = options.fatal || false;
     this._index = index;
   }
@@ -367,7 +393,7 @@ export interface TextDecodeOptions {
 
 export interface TextDecoderOptions {
   fatal?: boolean;
-  ignoreBOM?: false;
+  ignoreBOM?: boolean;
 }
 
 type EitherArrayBuffer = SharedArrayBuffer | ArrayBuffer;
@@ -387,11 +413,11 @@ export class TextDecoder {
   /** Returns `true` if error mode is "fatal", and `false` otherwise. */
   readonly fatal: boolean = false;
   /** Returns `true` if ignore BOM flag is set, and `false` otherwise. */
-  readonly ignoreBOM = false;
+  readonly ignoreBOM: boolean = false;
 
   constructor(label = "utf-8", options: TextDecoderOptions = { fatal: false }) {
     if (options.ignoreBOM) {
-      throw new TypeError("Ignoring the BOM not supported.");
+      this.ignoreBOM = true;
     }
     if (options.fatal) {
       this.fatal = true;
@@ -435,7 +461,10 @@ export class TextDecoder {
       bytes = new Uint8Array(0);
     }
 
-    const decoder = decoders.get(this._encoding)!({ fatal: this.fatal });
+    const decoder = decoders.get(this._encoding)!({
+      fatal: this.fatal,
+      ignoreBOM: this.ignoreBOM
+    });
     const inputStream = new Stream(bytes);
     const output: number[] = [];
 

js/text_encoding_test.ts

@@ -74,6 +74,32 @@ test(function textDecoder2(): void {
   assertEquals(decoder.decode(fixture), "𝓽𝓮𝔁𝓽");
 });
 
+test(function textDecoderIgnoreBOM(): void {
+  // prettier-ignore
+  const fixture = new Uint8Array([
+    0xef, 0xbb, 0xbf,
+    0xf0, 0x9d, 0x93, 0xbd,
+    0xf0, 0x9d, 0x93, 0xae,
+    0xf0, 0x9d, 0x94, 0x81,
+    0xf0, 0x9d, 0x93, 0xbd
+  ]);
+  const decoder = new TextDecoder("utf-8", { ignoreBOM: true });
+  assertEquals(decoder.decode(fixture), "𝓽𝓮𝔁𝓽");
+});
+
+test(function textDecoderNotBOM(): void {
+  // prettier-ignore
+  const fixture = new Uint8Array([
+    0xef, 0xbb, 0x89,
+    0xf0, 0x9d, 0x93, 0xbd,
+    0xf0, 0x9d, 0x93, 0xae,
+    0xf0, 0x9d, 0x94, 0x81,
+    0xf0, 0x9d, 0x93, 0xbd
+  ]);
+  const decoder = new TextDecoder("utf-8", { ignoreBOM: true });
+  assertEquals(decoder.decode(fixture), "ﻉ𝓽𝓮𝔁𝓽");
+});
+
 test(function textDecoderASCII(): void {
   const fixture = new Uint8Array([0x89, 0x95, 0x9f, 0xbf]);
   const decoder = new TextDecoder("ascii");