scan.ts

import { Observable } from 'rxjs';
import { scan as higherOrderScan } from 'rxjs/operators';

/* tslint:disable:max-line-length */
export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed: R): Observable<R>;
export function scan<T>(this: Observable<T>, accumulator: (acc: T, value: T, index: number) => T, seed?: T): Observable<T>;
export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R): Observable<R>;
/* tslint:enable:max-line-length */

/**
 * Applies an accumulator function over the source Observable, and returns each
 * intermediate result, with an optional seed value.
 *
 * <span class="informal">It's like {@link reduce}, but emits the current
 * accumulation whenever the source emits a value.</span>
 *
 * <img src="./img/scan.png" width="100%">
 *
 * 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. Is similar to {@link reduce}, but emits the intermediate
 * accumulations.
 *
 * 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 <caption>Count the number of click events</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var ones = clicks.mapTo(1);
 * var seed = 0;
 * var count = ones.scan((acc, one) => acc + one, seed);
 * count.subscribe(x => console.log(x));
 *
 * @see {@link expand}
 * @see {@link mergeScan}
 * @see {@link reduce}
 *
 * @param {function(acc: R, value: T, index: number): R} accumulator
 * The accumulator function called on each source value.
 * @param {T|R} [seed] The initial accumulation value.
 * @return {Observable<R>} An observable of the accumulated values.
 * @method scan
 * @owner Observable
 */
export function scan<T, R>(this: Observable<T>,
                           accumulator: (acc: T | R, value: T, index: number) => T | R,
                           seed?: T | R): Observable<T | R> {
  if (arguments.length >= 2) {
    return higherOrderScan(accumulator, seed)(this);
  }
  return higherOrderScan<T, T | R>(accumulator)(this);
}