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import { Observable } from '../Observable';
import { scan } from './scan';
import { takeLast } from './takeLast';
import { defaultIfEmpty } from './defaultIfEmpty';
import { OperatorFunction, MonoTypeOperatorFunction } from '../types';
import { pipe } from '../util/pipe';
/* tslint:disable:max-line-length */
export function reduce<T>(accumulator: (acc: T, value: T, index: number) => T, seed?: T): MonoTypeOperatorFunction<T>;
export function reduce<T>(accumulator: (acc: T[], value: T, index: number) => T[], seed: T[]): OperatorFunction<T, T[]>;
export function reduce<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed?: R): OperatorFunction<T, R>;
/* tslint:enable:max-line-length */
/**
* 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
* ```javascript
* const clicksInFiveSeconds = fromEvent(document, 'click').pipe(
* takeUntil(interval(5000)),
* );
* const ones = clicksInFiveSeconds.pipe(mapTo(1));
* const seed = 0;
* const count = ones.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: R, value: T, index: number): R} accumulator The accumulator function
* called on each source value.
* @param {R} [seed] The initial accumulation value.
* @return {Observable<R>} An Observable that emits a single value that is the
* result of accumulating the values emitted by the source Observable.
* @method reduce
* @owner Observable
*/
export function reduce<T, R>(accumulator: (acc: R, value: T, index?: number) => R, seed?: R): OperatorFunction<T, R> {
// providing a seed of `undefined` *should* be valid and trigger
// hasSeed! so don't use `seed !== undefined` checks!
// For this reason, we have to check it here at the original call site
// otherwise inside Operator/Subscriber we won't know if `undefined`
// means they didn't provide anything or if they literally provided `undefined`
if (arguments.length >= 2) {
return function reduceOperatorFunctionWithSeed(source: Observable<T>): Observable<R> {
return pipe(scan(accumulator, seed), takeLast(1), defaultIfEmpty(seed))(source);
};
}
return function reduceOperatorFunction(source: Observable<T>): Observable<R> {
return pipe(
scan((acc: R, value: T, index: number): R => accumulator(acc, value, index + 1)),
takeLast(1),
)(source);
};
}
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