/**
 * Copyright 2018 Google Inc. All Rights Reserved.
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *     http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import { Observable } from "../types.js";

export function amb<T>(...os: Array<Observable<T>>): Observable<T> {
  return new ReadableStream({
    async start(controller) {
      const readers = os.map(o => o.getReader());
      const reads = readers.map<Promise<[ReadableStreamReadResult<T>, number]>>(
        async (r, i) => [await r.read(), i]
      );
      let [{ value, done }, i] = await Promise.race(reads);
      reads
        .filter((_, j) => j !== i)
        .map(async (r, j) => {
          await r;
          readers[j].releaseLock();
        });
      const [fastestObs] = readers.slice(i, i + 1);
      while (true) {
        if (done) {
          return controller.close();
        }
        controller.enqueue(value);
        ({ value, done } = await fastestObs.read());
      }
    }
  });
}