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