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import { scanInternals } from './scanInternals';
import { OperatorFunction } from '../types';
import { operate } from '../util/lift';
export function reduce<V, A = V>(accumulator: (acc: A | V, value: V, index: number) => A): OperatorFunction<V, V | A>;
export function reduce<V, A>(accumulator: (acc: A, value: V, index: number) => A, seed: A): OperatorFunction<V, A>;
export function reduce<V, A, S = A>(accumulator: (acc: A | S, value: V, index: number) => A, seed: S): OperatorFunction<V, A>;
/**
* Applies an accumulator function over the source Observable, and returns the
* accumulated result when the source completes, given an optional seed value.
*
* <span class="informal">Combines together all values emitted on the source,
* using an accumulator function that knows how to join a new source value into
* the accumulation from the past.</span>
*
* 
*
* Like
* [Array.prototype.reduce()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce),
* `reduce` applies an `accumulator` function against an accumulation and each
* value of the source Observable (from the past) to reduce it to a single
* value, emitted on the output Observable. Note that `reduce` will only emit
* one value, only when the source Observable completes. It is equivalent to
* applying operator {@link scan} followed by operator {@link last}.
*
* Returns an Observable that applies a specified `accumulator` function to each
* item emitted by the source Observable. If a `seed` value is specified, then
* that value will be used as the initial value for the accumulator. If no seed
* value is specified, the first item of the source is used as the seed.
*
* ## Example
*
* Count the number of click events that happened in 5 seconds
*
* ```ts
* import { fromEvent, takeUntil, interval, map, reduce } from 'rxjs';
*
* const clicksInFiveSeconds = fromEvent(document, 'click')
* .pipe(takeUntil(interval(5000)));
*
* const ones = clicksInFiveSeconds.pipe(map(() => 1));
* const seed = 0;
* const count = ones.pipe(reduce((acc, one) => acc + one, seed));
*
* count.subscribe(x => console.log(x));
* ```
*
* @see {@link count}
* @see {@link expand}
* @see {@link mergeScan}
* @see {@link scan}
*
* @param {function(acc: A, value: V, index: number): A} accumulator The accumulator function
* called on each source value.
* @param {A} [seed] The initial accumulation value.
* @return A function that returns an Observable that emits a single value that
* is the result of accumulating the values emitted by the source Observable.
*/
export function reduce<V, A>(accumulator: (acc: V | A, value: V, index: number) => A, seed?: any): OperatorFunction<V, V | A> {
return operate(scanInternals(accumulator, seed, arguments.length >= 2, false, true));
}