Fix fixUint8Array for cases where view is not the whole buffer

Author: webmaster128

packages/encoding/src/uint8array.spec.ts

@@ -9,11 +9,34 @@ describe("fixUint8Array", () => {
     expect(fixed.buffer).toBe(input.buffer); // no copy must be performed in this case
   });
 
+  it("works for Uint8Array<ArrayBuffer> where data is not using all of the buffer", () => {
+    const buffer = new Uint8Array([0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a]).buffer;
+    expect(buffer.byteLength).toEqual(11);
+    const original = new Uint8Array(buffer, 1, 5);
+
+    const fixed = fixUint8Array(original);
+    expect(fixed.length).toEqual(5);
+    expect(fixed).toEqual(new Uint8Array([0x01, 0x02, 0x03, 0x04, 0x05]));
+    expect(fixed.buffer).toBe(buffer); // no copy must be performed in this case
+  });
+
   it("works for Uint8Array<SharedArrayBuffer>", () => {
     const sharedBuffer = new SharedArrayBuffer(8);
     const input: Uint8Array<SharedArrayBuffer> = new Uint8Array(sharedBuffer);
     const fixed = fixUint8Array(input);
     expect(fixed).toEqual(input);
     expect(fixed.buffer).toBeInstanceOf(ArrayBuffer);
   });
+
+  it("works for Uint8Array<SharedArrayBuffer> where data is not using all of the buffer", () => {
+    const sharedBuffer = new SharedArrayBuffer(8);
+    const input: Uint8Array<SharedArrayBuffer> = new Uint8Array(sharedBuffer, 0, 3);
+    input[0] = 0xaa;
+    input[1] = 0xbb;
+    input[2] = 0xcc;
+    const fixed = fixUint8Array(input);
+    expect(fixed.buffer).toBeInstanceOf(ArrayBuffer);
+    expect(fixed.length).toEqual(3);
+    expect(fixed).toEqual(new Uint8Array([0xaa, 0xbb, 0xcc]));
+  });
 });

packages/encoding/src/uint8array.ts

@@ -7,10 +7,11 @@
 export function fixUint8Array<T extends ArrayBufferLike>(source: Uint8Array<T>): Uint8Array<ArrayBuffer> {
   const buffer = source.buffer;
   if (buffer instanceof ArrayBuffer) {
-    return new Uint8Array(buffer); // new instance just to make TS happy without unsafe cast
+    return new Uint8Array(buffer, source.byteOffset, source.length); // new instance just to make TS happy without unsafe cast
   }
 
-  const copy = new ArrayBuffer(buffer.byteLength);
-  new Uint8Array(copy).set(new Uint8Array(buffer));
-  return new Uint8Array(copy);
+  const copy = new ArrayBuffer(source.byteLength); // allocate memory
+  const out = new Uint8Array(copy);
+  out.set(source); // copy data
+  return out;
 }