import { Operator } from '../Operator';
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import { Observable } from '../Observable';
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import { Subscriber } from '../Subscriber';
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import { OperatorFunction } from '../types';
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/**
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* Groups pairs of consecutive emissions together and emits them as an array of
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* two values.
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*
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* <span class="informal">Puts the current value and previous value together as
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* an array, and emits that.</span>
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*
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* 
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*
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* The Nth emission from the source Observable will cause the output Observable
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* to emit an array [(N-1)th, Nth] of the previous and the current value, as a
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* pair. For this reason, `pairwise` emits on the second and subsequent
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* emissions from the source Observable, but not on the first emission, because
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* there is no previous value in that case.
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*
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* ## Example
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* On every click (starting from the second), emit the relative distance to the previous click
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* ```ts
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* import { fromEvent } from 'rxjs';
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* import { pairwise, map } from 'rxjs/operators';
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*
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* const clicks = fromEvent(document, 'click');
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* const pairs = clicks.pipe(pairwise());
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* const distance = pairs.pipe(
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* map(pair => {
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* const x0 = pair[0].clientX;
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* const y0 = pair[0].clientY;
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* const x1 = pair[1].clientX;
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* const y1 = pair[1].clientY;
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* return Math.sqrt(Math.pow(x0 - x1, 2) + Math.pow(y0 - y1, 2));
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* }),
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* );
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* distance.subscribe(x => console.log(x));
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* ```
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*
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* @see {@link buffer}
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* @see {@link bufferCount}
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*
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* @return {Observable<Array<T>>} An Observable of pairs (as arrays) of
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* consecutive values from the source Observable.
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* @method pairwise
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* @owner Observable
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*/
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export function pairwise<T>(): OperatorFunction<T, [T, T]> {
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return (source: Observable<T>) => source.lift(new PairwiseOperator());
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}
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class PairwiseOperator<T> implements Operator<T, [T, T]> {
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call(subscriber: Subscriber<[T, T]>, source: any): any {
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return source.subscribe(new PairwiseSubscriber(subscriber));
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}
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}
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/**
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* We need this JSDoc comment for affecting ESDoc.
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* @ignore
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* @extends {Ignored}
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*/
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class PairwiseSubscriber<T> extends Subscriber<T> {
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private prev: T;
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private hasPrev: boolean = false;
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constructor(destination: Subscriber<[T, T]>) {
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super(destination);
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}
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_next(value: T): void {
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let pair: [T, T] | undefined;
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if (this.hasPrev) {
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pair = [this.prev, value];
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} else {
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this.hasPrev = true;
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}
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this.prev = value;
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if (pair) {
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this.destination.next(pair);
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}
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}
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}
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