import { scanInternals } from './scanInternals';
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import { OperatorFunction } from '../types';
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import { operate } from '../util/lift';
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export function reduce<V, A = V>(accumulator: (acc: A | V, value: V, index: number) => A): OperatorFunction<V, V | A>;
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export function reduce<V, A>(accumulator: (acc: A, value: V, index: number) => A, seed: A): OperatorFunction<V, A>;
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export function reduce<V, A, S = A>(accumulator: (acc: A | S, value: V, index: number) => A, seed: S): OperatorFunction<V, A>;
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/**
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* Applies an accumulator function over the source Observable, and returns the
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* accumulated result when the source completes, given an optional seed value.
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*
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* <span class="informal">Combines together all values emitted on the source,
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* using an accumulator function that knows how to join a new source value into
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* the accumulation from the past.</span>
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*
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* 
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*
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* Like
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* [Array.prototype.reduce()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce),
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* `reduce` applies an `accumulator` function against an accumulation and each
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* value of the source Observable (from the past) to reduce it to a single
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* value, emitted on the output Observable. Note that `reduce` will only emit
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* one value, only when the source Observable completes. It is equivalent to
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* applying operator {@link scan} followed by operator {@link last}.
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*
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* Returns an Observable that applies a specified `accumulator` function to each
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* item emitted by the source Observable. If a `seed` value is specified, then
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* that value will be used as the initial value for the accumulator. If no seed
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* value is specified, the first item of the source is used as the seed.
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*
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* ## Example
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*
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* Count the number of click events that happened in 5 seconds
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*
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* ```ts
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* import { fromEvent, takeUntil, interval, map, reduce } from 'rxjs';
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*
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* const clicksInFiveSeconds = fromEvent(document, 'click')
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* .pipe(takeUntil(interval(5000)));
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*
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* const ones = clicksInFiveSeconds.pipe(map(() => 1));
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* const seed = 0;
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* const count = ones.pipe(reduce((acc, one) => acc + one, seed));
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*
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* count.subscribe(x => console.log(x));
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* ```
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*
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* @see {@link count}
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* @see {@link expand}
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* @see {@link mergeScan}
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* @see {@link scan}
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*
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* @param {function(acc: A, value: V, index: number): A} accumulator The accumulator function
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* called on each source value.
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* @param {A} [seed] The initial accumulation value.
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* @return A function that returns an Observable that emits a single value that
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* is the result of accumulating the values emitted by the source Observable.
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*/
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export function reduce<V, A>(accumulator: (acc: V | A, value: V, index: number) => A, seed?: any): OperatorFunction<V, V | A> {
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return operate(scanInternals(accumulator, seed, arguments.length >= 2, false, true));
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}
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