// Copyright 2018-2026 the Deno authors. MIT license.
// This module is browser compatible.
/**
* A transform stream that only transforms from the zero-indexed `start` and
* `end` bytes (both inclusive).
*
* @example Basic usage
* ```ts
* import { ByteSliceStream } from "@std/streams/byte-slice-stream";
* import { assertEquals } from "@std/assert";
*
* const stream = ReadableStream.from([
* new Uint8Array([0, 1]),
* new Uint8Array([2, 3, 4]),
* ]);
* const slicedStream = stream.pipeThrough(new ByteSliceStream(1, 3));
*
* assertEquals(
* await Array.fromAsync(slicedStream),
* [new Uint8Array([1]), new Uint8Array([2, 3])]
* );
* ```
*
* @example Get a range of bytes from a fetch response body
* ```ts
* import { ByteSliceStream } from "@std/streams/byte-slice-stream";
* import { assertEquals } from "@std/assert";
*
* const response = await fetch("https://example.com");
* const rangedStream = response.body!
* .pipeThrough(new ByteSliceStream(3, 8));
* const collected = await Array.fromAsync(rangedStream);
* assertEquals(collected[0]?.length, 6);
* ```
*/
export class ByteSliceStream extends TransformStream<Uint8Array, Uint8Array> {
#offsetStart = 0;
#offsetEnd = 0;
/**
* Constructs a new instance.
*
* @param start The zero-indexed byte index to start reading from.
* @param end The zero-indexed byte index to stop reading at. Inclusive.
*/
constructor(start = 0, end: number = Infinity) {
super({
start: () => {
if (start < 0) {
throw new RangeError(
`Cannot construct ByteSliceStream as start must be >= 0: received ${start}`,
);
}
end += 1;
},
transform: (chunk, controller) => {
this.#offsetStart = this.#offsetEnd;
this.#offsetEnd += chunk.byteLength;
if (this.#offsetEnd > start) {
if (this.#offsetStart < start) {
chunk = chunk.slice(start - this.#offsetStart);
}
if (this.#offsetEnd >= end) {
chunk = chunk.slice(0, chunk.byteLength - this.#offsetEnd + end);
controller.enqueue(chunk);
controller.terminate();
} else {
controller.enqueue(chunk);
}
}
},
});
}
}
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