import { Observable, ObservableInput } from 'rxjs';
import { exhaustMap as higherOrder } from 'rxjs/operators';
/**
 * Projects each source value to an Observable which is merged in the output
 * Observable only if the previous projected Observable has completed.
 *
 * <span class="informal">Maps each value to an Observable, then flattens all of
 * these inner Observables using {@link exhaust}.</span>
 *
 * <img src="./img/exhaustMap.png" width="100%">
 *
 * Returns an Observable that emits items based on applying a function that you
 * supply to each item emitted by the source Observable, where that function
 * returns an (so-called "inner") Observable. When it projects a source value to
 * an Observable, the output Observable begins emitting the items emitted by
 * that projected Observable. However, `exhaustMap` ignores every new projected
 * Observable if the previous projected Observable has not yet completed. Once
 * that one completes, it will accept and flatten the next projected Observable
 * and repeat this process.
 *
 * @example <caption>Run a finite timer for each click, only if there is no currently active timer</caption>
 * var clicks = fromEvent(document, 'click');
 * var result = clicks.pipe(exhaustMap((ev) => Rx.Observable.interval(1000).take(5)));
 * result.subscribe(x => console.log(x));
 *
 * @see {@link concatMap}
 * @see {@link exhaust}
 * @see {@link mergeMap}
 * @see {@link switchMap}
 *
 * @param {function(value: T, ?index: number): ObservableInput} project A function
 * that, when applied to an item emitted by the source Observable, returns an
 * Observable.
 * @return {Observable} An Observable containing projected Observables
 * of each item of the source, ignoring projected Observables that start before
 * their preceding Observable has completed.
 */
export function exhaustMap<T, R>(
  this: Observable<T>,
  project: (value: T, index: number) => ObservableInput<R>
): Observable<R> {
  return higherOrder(project)(this);
}