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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.png)
+ *
+ * `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);
+}