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