// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.

/**
 * Merge multiple streams into a single one, taking order into account, and each
 * stream will wait for a chunk to enqueue before the next stream can append
 * another chunk.
 *
 * If a stream ends before other ones, the others will be cancelled after the
 * last chunk of said stream is read. See the examples below for more
 * comprehensible information. If you want to continue reading the other streams
 * even after one of them ends, use {@linkcode zipReadableStreams}.
 *
 * @typeparam T The type of the chunks in the input streams.
 * @returns A `ReadableStream` that will emit the zipped chunks
 *
 * @example Zip 2 streams with the same length
 * ```ts
 * import { earlyZipReadableStreams } from "@std/streams/early-zip-readable-streams";
 * import { assertEquals } from "@std/assert/assert-equals";
 *
 * const stream1 = ReadableStream.from(["1", "2", "3"]);
 * const stream2 = ReadableStream.from(["a", "b", "c"]);
 * const zippedStream = earlyZipReadableStreams(stream1, stream2);
 *
 * assertEquals(
 *   await Array.fromAsync(zippedStream),
 *   ["1", "a", "2", "b", "3", "c"],
 * );
 * ```
 *
 * @example Zip 2 streams with different length (first one is shorter)
 * ```ts
 * import { earlyZipReadableStreams } from "@std/streams/early-zip-readable-streams";
 * import { assertEquals } from "@std/assert/assert-equals";
 *
 * const stream1 = ReadableStream.from(["1", "2"]);
 * const stream2 = ReadableStream.from(["a", "b", "c", "d"]);
 * const zippedStream = earlyZipReadableStreams(stream1, stream2);
 *
 * // The first stream ends before the second one. When the first stream ends,
 * // the second one is cancelled and no more data is read or added to the
 * // zipped stream.
 * assertEquals(
 *   await Array.fromAsync(zippedStream),
 *   ["1", "a", "2", "b"],
 * );
 * ```
 *
 * @example Zip 2 streams with different length (first one is longer)
 * ```ts
 * import { earlyZipReadableStreams } from "@std/streams/early-zip-readable-streams";
 * import { assertEquals } from "@std/assert/assert-equals";
 *
 * const stream1 = ReadableStream.from(["1", "2", "3", "4"]);
 * const stream2 = ReadableStream.from(["a", "b"]);
 * const zippedStream = earlyZipReadableStreams(stream1, stream2);
 *
 * // The second stream ends before the first one. When the second stream ends,
 * // the first one is cancelled, but the chunk of "3" is already read so it
 * // is added to the zipped stream.
 * assertEquals(
 *   await Array.fromAsync(zippedStream),
 *   ["1", "a", "2", "b", "3"],
 * );
 * ```
 *
 * @example Zip 3 streams
 * ```ts
 * import { earlyZipReadableStreams } from "@std/streams/early-zip-readable-streams";
 * import { assertEquals } from "@std/assert/assert-equals";
 *
 * const stream1 = ReadableStream.from(["1"]);
 * const stream2 = ReadableStream.from(["a", "b"]);
 * const stream3 = ReadableStream.from(["A", "B", "C"]);
 * const zippedStream = earlyZipReadableStreams(stream1, stream2, stream3);
 *
 * assertEquals(
 *   await Array.fromAsync(zippedStream),
 *   ["1", "a", "A"],
 * );
 * ```
 */
export function earlyZipReadableStreams<T>(
  ...streams: ReadableStream<T>[]
): ReadableStream<T> {
  const readers = streams.map((s) => s.getReader());
  return new ReadableStream<T>({
    async start(controller) {
      try {
        loop:
        while (true) {
          for (const reader of readers) {
            const { value, done } = await reader.read();
            if (!done) {
              controller.enqueue(value!);
            } else {
              await Promise.all(readers.map((reader) => reader.cancel()));
              break loop;
            }
          }
        }
        controller.close();
      } catch (e) {
        controller.error(e);
      }
    },
  });
}

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