add encodeInto to TextEncoder (#2558)

js/text_encoding.ts

@@ -461,6 +461,11 @@ export class TextDecoder {
   }
 }
 
+interface TextEncoderEncodeIntoResult {
+  read: number;
+  written: number;
+}
+
 export class TextEncoder {
   /** Returns "utf-8". */
   readonly encoding = "utf-8";
@@ -484,6 +489,36 @@ export class TextEncoder {
 
     return new Uint8Array(output);
   }
+  encodeInto(input: string, dest: Uint8Array): TextEncoderEncodeIntoResult {
+    const encoder = new UTF8Encoder();
+    const inputStream = new Stream(stringToCodePoints(input));
+
+    let written = 0;
+    let read = 0;
+    while (true) {
+      const result = encoder.handler(inputStream.read());
+      if (result === FINISHED) {
+        break;
+      }
+      read++;
+      if (Array.isArray(result)) {
+        dest.set(result, written);
+        written += result.length;
+        if (result.length > 3) {
+          // increment read a second time if greater than U+FFFF
+          read++;
+        }
+      } else {
+        dest[written] = result;
+        written++;
+      }
+    }
+
+    return {
+      read,
+      written
+    };
+  }
   get [Symbol.toStringTag](): string {
     return "TextEncoder";
   }

js/text_encoding_test.ts

@@ -91,7 +91,7 @@ test(function textDecoderErrorEncoding(): void {
   assert(didThrow);
 });
 
-test(function textEncoder2(): void {
+test(function textEncoder(): void {
   const fixture = "𝓽𝓮𝔁𝓽";
   const encoder = new TextEncoder();
   // prettier-ignore
@@ -103,6 +103,35 @@ test(function textEncoder2(): void {
   ]);
 });
 
+test(function textEncodeInto(): void {
+  const fixture = "text";
+  const encoder = new TextEncoder();
+  const bytes = new Uint8Array(5);
+  const result = encoder.encodeInto(fixture, bytes);
+  assertEquals(result.read, 4);
+  assertEquals(result.written, 4);
+  // prettier-ignore
+  assertEquals(Array.from(bytes), [
+    0x74, 0x65, 0x78, 0x74, 0x00,
+  ]);
+});
+
+test(function textEncodeInto2(): void {
+  const fixture = "𝓽𝓮𝔁𝓽";
+  const encoder = new TextEncoder();
+  const bytes = new Uint8Array(17);
+  const result = encoder.encodeInto(fixture, bytes);
+  assertEquals(result.read, 8);
+  assertEquals(result.written, 16);
+  // prettier-ignore
+  assertEquals(Array.from(bytes), [
+    0xf0, 0x9d, 0x93, 0xbd,
+    0xf0, 0x9d, 0x93, 0xae,
+    0xf0, 0x9d, 0x94, 0x81,
+    0xf0, 0x9d, 0x93, 0xbd, 0x00,
+  ]);
+});
+
 test(function textDecoderSharedUint8Array(): void {
   const ab = new SharedArrayBuffer(6);
   const dataView = new DataView(ab);