import { Observable } from '../Observable';
import { mergeScan as higherOrder } from '../operators/mergeScan';

/**
 * Applies an accumulator function over the source Observable where the
 * accumulator function itself returns an Observable, then each intermediate
 * Observable returned is merged into the output Observable.
 *
 * <span class="informal">It's like {@link scan}, but the Observables returned
 * by the accumulator are merged into the outer Observable.</span>
 *
 * @example <caption>Count the number of click events</caption>
 * const click$ = Rx.Observable.fromEvent(document, 'click');
 * const one$ = click$.mapTo(1);
 * const seed = 0;
 * const count$ = one$.mergeScan((acc, one) => Rx.Observable.of(acc + one), seed);
 * count$.subscribe(x => console.log(x));
 *
 * // Results:
 * 1
 * 2
 * 3
 * 4
 * // ...and so on for each click
 *
 * @param {function(acc: R, value: T): Observable<R>} accumulator
 * The accumulator function called on each source value.
 * @param seed The initial accumulation value.
 * @param {number} [concurrent=Number.POSITIVE_INFINITY] Maximum number of
 * input Observables being subscribed to concurrently.
 * @return {Observable<R>} An observable of the accumulated values.
 * @method mergeScan
 * @owner Observable
 */
export function mergeScan<T, R>(this: Observable<T>,
                                accumulator: (acc: R, value: T) => Observable<R>,
                                seed: R,
                                concurrent: number = Number.POSITIVE_INFINITY): Observable<R> {
  return higherOrder(accumulator, seed, concurrent)(this);
}