/**
 * Copyright 2019 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 { Transform } from "../types.ts";

export type ScanFunc<U, T> = (acc: U, v: T) => U;

/**
 * Reduces the original observable with `f`, emitting every intermediate result
 * not including the initial value.
 *
 * @typeparam U Type of result returned from the accumulator.
 * @typeparam T Type of items emitted by the observable.
 * @param f Reduce function called with the accumulated value so far and the
 * current item. Should return a new accumulated value.
 * @param v0 Initial value.
 * @returns Transform that emits accumulated values produced by `f`.
 */
export function scan<U, T>(f: ScanFunc<U, T>, v0: U): Transform<T, U> {
  return new TransformStream<T, U>(
    {
      transform(chunk, controller) {
        v0 = f(v0, chunk);
        controller.enqueue(v0);
      },
    },
    { highWaterMark: 1 },
    { highWaterMark: 0 },
  );
}