import { not } from '../util/not';
import { filter } from './filter';
import { Observable } from '../Observable';
import { UnaryFunction } from '../interfaces';

/**
 * Splits the source Observable into two, one with values that satisfy a
 * predicate, and another with values that don't satisfy the predicate.
 *
 * <span class="informal">It's like {@link filter}, but returns two Observables:
 * one like the output of {@link filter}, and the other with values that did not
 * pass the condition.</span>
 *
 * <img src="./img/partition.png" width="100%">
 *
 * `partition` outputs an array with two Observables that partition the values
 * from the source Observable through the given `predicate` function. The first
 * Observable in that array emits source values for which the predicate argument
 * returns true. The second Observable emits source values for which the
 * predicate returns false. The first behaves like {@link filter} and the second
 * behaves like {@link filter} with the predicate negated.
 *
 * @example <caption>Partition click events into those on DIV elements and those elsewhere</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var parts = clicks.partition(ev => ev.target.tagName === 'DIV');
 * var clicksOnDivs = parts[0];
 * var clicksElsewhere = parts[1];
 * clicksOnDivs.subscribe(x => console.log('DIV clicked: ', x));
 * clicksElsewhere.subscribe(x => console.log('Other clicked: ', x));
 *
 * @see {@link filter}
 *
 * @param {function(value: T, index: number): boolean} predicate A function that
 * evaluates each value emitted by the source Observable. If it returns `true`,
 * the value is emitted on the first Observable in the returned array, if
 * `false` the value is emitted on the second Observable in the array. The
 * `index` parameter is the number `i` for the i-th source emission that has
 * happened since the subscription, starting from the number `0`.
 * @param {any} [thisArg] An optional argument to determine the value of `this`
 * in the `predicate` function.
 * @return {[Observable<T>, Observable<T>]} An array with two Observables: one
 * with values that passed the predicate, and another with values that did not
 * pass the predicate.
 * @method partition
 * @owner Observable
 */
export function partition<T>(predicate: (value: T, index: number) => boolean,
                             thisArg?: any): UnaryFunction<Observable<T>, [Observable<T>, Observable<T>]> {
  return (source: Observable<T>) => [
    filter(predicate, thisArg)(source),
    filter(not(predicate, thisArg) as any)(source)
  ] as [Observable<T>, Observable<T>];
}