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