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