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import { OperatorFunction } from '../types';
import { reduce } from './reduce';
/**
* Counts the number of emissions on the source and emits that number when the
* source completes.
*
* <span class="informal">Tells how many values were emitted, when the source
* completes.</span>
*
* 
*
* `count` transforms an Observable that emits values into an Observable that
* emits a single value that represents the number of values emitted by the
* source Observable. If the source Observable terminates with an error, `count`
* will pass this error notification along without emitting a value first. If
* the source Observable does not terminate at all, `count` will neither emit
* a value nor terminate. This operator takes an optional `predicate` function
* as argument, in which case the output emission will represent the number of
* source values that matched `true` with the `predicate`.
*
* ## Examples
*
* Counts how many seconds have passed before the first click happened
*
* ```ts
* import { interval, fromEvent, takeUntil, count } from 'rxjs';
*
* const seconds = interval(1000);
* const clicks = fromEvent(document, 'click');
* const secondsBeforeClick = seconds.pipe(takeUntil(clicks));
* const result = secondsBeforeClick.pipe(count());
* result.subscribe(x => console.log(x));
* ```
*
* Counts how many odd numbers are there between 1 and 7
*
* ```ts
* import { range, count } from 'rxjs';
*
* const numbers = range(1, 7);
* const result = numbers.pipe(count(i => i % 2 === 1));
* result.subscribe(x => console.log(x));
* // Results in:
* // 4
* ```
*
* @see {@link max}
* @see {@link min}
* @see {@link reduce}
*
* @param predicate A function that is used to analyze the value and the index and
* determine whether or not to increment the count. Return `true` to increment the count,
* and return `false` to keep the count the same.
* If the predicate is not provided, every value will be counted.
* @return A function that returns an Observable that emits one number that
* represents the count of emissions.
*/
export function count<T>(predicate?: (value: T, index: number) => boolean): OperatorFunction<T, number> {
return reduce((total, value, i) => (!predicate || predicate(value, i) ? total + 1 : total), 0);
}