import { ObservableInput, OperatorFunction } from '../types';
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import { operate } from '../util/lift';
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import { mergeInternals } from './mergeInternals';
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/**
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* Applies an accumulator function over the source Observable where the
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* accumulator function itself returns an Observable, then each intermediate
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* Observable returned is merged into the output Observable.
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*
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* <span class="informal">It's like {@link scan}, but the Observables returned
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* by the accumulator are merged into the outer Observable.</span>
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*
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* The first parameter of the `mergeScan` is an `accumulator` function which is
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* being called every time the source Observable emits a value. `mergeScan` will
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* subscribe to the value returned by the `accumulator` function and will emit
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* values to the subscriber emitted by inner Observable.
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*
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* The `accumulator` function is being called with three parameters passed to it:
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* `acc`, `value` and `index`. The `acc` parameter is used as the state parameter
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* whose value is initially set to the `seed` parameter (the second parameter
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* passed to the `mergeScan` operator).
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*
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* `mergeScan` internally keeps the value of the `acc` parameter: as long as the
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* source Observable emits without inner Observable emitting, the `acc` will be
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* set to `seed`. The next time the inner Observable emits a value, `mergeScan`
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* will internally remember it and it will be passed to the `accumulator`
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* function as `acc` parameter the next time source emits.
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*
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* The `value` parameter of the `accumulator` function is the value emitted by the
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* source Observable, while the `index` is a number which represent the order of the
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* current emission by the source Observable. It starts with 0.
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*
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* The last parameter to the `mergeScan` is the `concurrent` value which defaults
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* to Infinity. It represents the maximum number of inner Observable subscriptions
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* at a time.
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*
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* ## Example
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*
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* Count the number of click events
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*
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* ```ts
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* import { fromEvent, map, mergeScan, of } from 'rxjs';
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*
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* const click$ = fromEvent(document, 'click');
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* const one$ = click$.pipe(map(() => 1));
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* const seed = 0;
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* const count$ = one$.pipe(
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* mergeScan((acc, one) => of(acc + one), seed)
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* );
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*
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* count$.subscribe(x => console.log(x));
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*
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* // Results:
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* // 1
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* // 2
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* // 3
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* // 4
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* // ...and so on for each click
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* ```
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*
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* @see {@link scan}
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* @see {@link switchScan}
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*
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* @param {function(acc: R, value: T): Observable<R>} accumulator
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* The accumulator function called on each source value.
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* @param seed The initial accumulation value.
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* @param {number} [concurrent=Infinity] Maximum number of
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* input Observables being subscribed to concurrently.
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* @return A function that returns an Observable of the accumulated values.
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*/
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export function mergeScan<T, R>(
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accumulator: (acc: R, value: T, index: number) => ObservableInput<R>,
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seed: R,
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concurrent = Infinity
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): OperatorFunction<T, R> {
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return operate((source, subscriber) => {
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// The accumulated state.
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let state = seed;
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return mergeInternals(
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source,
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subscriber,
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(value, index) => accumulator(state, value, index),
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concurrent,
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(value) => {
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state = value;
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},
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false,
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undefined,
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() => (state = null!)
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);
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});
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}
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