feat(streams): ByteSliceStream

streams/buffer.ts

@@ -232,3 +232,43 @@ export class LimitedTransformStream<T> extends TransformStream<T, T> {
     });
   }
 }
+
+/**
+ * A transform stream that only transforms from the zero-indexed `start` and `end` bytes (both inclusive).
+ *
+ * @example
+ * ```ts
+ * import { RangedByteTransformStream } from "https://deno.land/std@$STD_VERSION/streams/buffer.ts";
+ * const response = await fetch("https://example.com");
+ * const rangedStream = response.body!
+ *   .pipeThrough(new RangedByteTransformStream(3, 8));
+ * ```
+ */
+export class RangedByteTransformStream
+  extends TransformStream<Uint8Array, Uint8Array> {
+  #offset = 0;
+
+  constructor(start: number, end = Infinity) {
+    super({
+      start: () => {
+        end += 1;
+      },
+      transform: (chunk, controller) => {
+        this.#offset += chunk.byteLength;
+        if (this.#offset >= start + 1) {
+          if (start - (this.#offset - chunk.byteLength) >= 0) {
+            chunk = chunk.slice(start - (this.#offset - chunk.byteLength));
+          }
+
+          if (end <= this.#offset) {
+            chunk = chunk.slice(0, chunk.byteLength - (this.#offset - end));
+            controller.enqueue(chunk);
+            controller.terminate();
+          }
+
+          controller.enqueue(chunk);
+        }
+      },
+    });
+  }
+}

streams/buffer_test.ts

@@ -5,6 +5,7 @@ import {
   Buffer,
   LimitedBytesTransformStream,
   LimitedTransformStream,
+  RangedByteTransformStream,
 } from "./buffer.ts";
 
 Deno.test("[streams] Buffer Write & Read", async function () {
@@ -128,3 +129,28 @@ Deno.test("[streams] LimitedTransformStream error", async function () {
     }
   }, RangeError);
 });
+
+Deno.test("[streams] RangedByteTransformStream", async function () {
+  const stream = new ReadableStream({
+    start(controller) {
+      controller.enqueue(new Uint8Array([1, 2, 3]));
+      controller.enqueue(new Uint8Array([4, 5, 6]));
+      controller.enqueue(new Uint8Array([7, 8, 9]));
+      controller.enqueue(new Uint8Array([10, 11, 12]));
+      controller.enqueue(new Uint8Array([13, 14, 15]));
+      controller.enqueue(new Uint8Array([16, 17, 18]));
+      controller.close();
+    },
+  }).pipeThrough(new RangedByteTransformStream(2, 10));
+
+  const chunks = [];
+  for await (const chunk of stream) {
+    chunks.push(chunk);
+  }
+  assertEquals(chunks, [
+    new Uint8Array([3]),
+    new Uint8Array([4, 5, 6]),
+    new Uint8Array([7, 8, 9]),
+    new Uint8Array([10, 11]),
+  ]);
+});