import { OperatorFunction } from '../types';
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import { operate } from '../util/lift';
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import { createOperatorSubscriber } from './OperatorSubscriber';
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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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*
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* On every click (starting from the second), emit the relative distance to the previous click
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*
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* ```ts
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* import { fromEvent, pairwise, map } from 'rxjs';
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*
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* const clicks = fromEvent<PointerEvent>(document, 'click');
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* const pairs = clicks.pipe(pairwise());
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* const distance = pairs.pipe(
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* map(([first, second]) => {
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* const x0 = first.clientX;
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* const y0 = first.clientY;
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* const x1 = second.clientX;
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* const y1 = second.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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*
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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 A function that returns an Observable of pairs (as arrays) of
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* consecutive values from the source Observable.
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*/
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export function pairwise<T>(): OperatorFunction<T, [T, T]> {
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return operate((source, subscriber) => {
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let prev: T;
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let hasPrev = false;
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source.subscribe(
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createOperatorSubscriber(subscriber, (value) => {
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const p = prev;
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prev = value;
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hasPrev && subscriber.next([p, value]);
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hasPrev = true;
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})
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);
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});
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}
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