feat(mergeMap): add higher-order lettable version of mergeMap

- Note: typings incompatibility forced an `any` in the instance version. Very strange

src/operator/mergeMap.ts

@@ -5,6 +5,7 @@ import { Subscription } from '../Subscription';
 import { subscribeToResult } from '../util/subscribeToResult';
 import { OuterSubscriber } from '../OuterSubscriber';
 import { InnerSubscriber } from '../InnerSubscriber';
+import { mergeMap as higherOrderMergeMap } from '../operators';
 
 /* tslint:disable:max-line-length */
 export function mergeMap<T, R>(this: Observable<T>, project: (value: T, index: number) => ObservableInput<R>, concurrent?: number): Observable<R>;
@@ -72,11 +73,7 @@ export function mergeMap<T, I, R>(this: Observable<T>, project: (value: T, index
 export function mergeMap<T, I, R>(this: Observable<T>, project: (value: T, index: number) => ObservableInput<I>,
                                   resultSelector?: ((outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R) | number,
                                   concurrent: number = Number.POSITIVE_INFINITY): Observable<I | R> {
-  if (typeof resultSelector === 'number') {
-    concurrent = <number>resultSelector;
-    resultSelector = null;
-  }
-  return this.lift(new MergeMapOperator(project, <any>resultSelector, concurrent));
+  return higherOrderMergeMap(project, <any>resultSelector, concurrent)(this);
 }
 
 export class MergeMapOperator<T, I, R> implements Operator<T, I> {

src/operators/OperatorFunction.ts

@@ -0,0 +1,5 @@
+import { Observable } from '../Observable';
+
+export interface OperatorFunction<T, R> {
+  (source: Observable<T>): Observable<R>;
+}

src/operators/index.ts

@@ -1 +1,3 @@
-export { map, OperatorFunction } from './map';
+export { OperatorFunction } from './OperatorFunction';
+export { map } from './map';
+export { mergeMap } from './mergeMap';

src/operators/map.ts

@@ -1,10 +1,7 @@
 import { Operator } from '../Operator';
 import { Subscriber } from '../Subscriber';
 import { Observable } from '../Observable';
-
-export interface OperatorFunction<T, R> {
-  (source: Observable<T>): Observable<R>;
-}
+import { OperatorFunction } from './OperatorFunction';
 
 /**
  * Applies a given `project` function to each value emitted by the source

src/operators/mergeMap.ts

@@ -0,0 +1,179 @@
+import { Observable, ObservableInput } from '../Observable';
+import { Operator } from '../Operator';
+import { Subscriber } from '../Subscriber';
+import { Subscription } from '../Subscription';
+import { subscribeToResult } from '../util/subscribeToResult';
+import { OuterSubscriber } from '../OuterSubscriber';
+import { InnerSubscriber } from '../InnerSubscriber';
+import { OperatorFunction } from './OperatorFunction';
+
+/* tslint:disable:max-line-length */
+export function mergeMap<T, R>(project: (value: T, index: number) => ObservableInput<R>, concurrent?: number): OperatorFunction<T, R>;
+export function mergeMap<T, I, R>(project: (value: T, index: number) => ObservableInput<I>, resultSelector: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R, concurrent?: number): OperatorFunction<T, R>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Projects each source value to an Observable which is merged in the output
+ * Observable.
+ *
+ * <span class="informal">Maps each value to an Observable, then flattens all of
+ * these inner Observables using {@link mergeAll}.</span>
+ *
+ * <img src="./img/mergeMap.png" width="100%">
+ *
+ * Returns an Observable that emits items based on applying a function that you
+ * supply to each item emitted by the source Observable, where that function
+ * returns an Observable, and then merging those resulting Observables and
+ * emitting the results of this merger.
+ *
+ * @example <caption>Map and flatten each letter to an Observable ticking every 1 second</caption>
+ * var letters = Rx.Observable.of('a', 'b', 'c');
+ * var result = letters.mergeMap(x =>
+ *   Rx.Observable.interval(1000).map(i => x+i)
+ * );
+ * result.subscribe(x => console.log(x));
+ *
+ * // Results in the following:
+ * // a0
+ * // b0
+ * // c0
+ * // a1
+ * // b1
+ * // c1
+ * // continues to list a,b,c with respective ascending integers
+ *
+ * @see {@link concatMap}
+ * @see {@link exhaustMap}
+ * @see {@link merge}
+ * @see {@link mergeAll}
+ * @see {@link mergeMapTo}
+ * @see {@link mergeScan}
+ * @see {@link switchMap}
+ *
+ * @param {function(value: T, ?index: number): ObservableInput} project A function
+ * that, when applied to an item emitted by the source Observable, returns an
+ * Observable.
+ * @param {function(outerValue: T, innerValue: I, outerIndex: number, innerIndex: number): any} [resultSelector]
+ * A function to produce the value on the output Observable based on the values
+ * and the indices of the source (outer) emission and the inner Observable
+ * emission. The arguments passed to this function are:
+ * - `outerValue`: the value that came from the source
+ * - `innerValue`: the value that came from the projected Observable
+ * - `outerIndex`: the "index" of the value that came from the source
+ * - `innerIndex`: the "index" of the value from the projected Observable
+ * @param {number} [concurrent=Number.POSITIVE_INFINITY] Maximum number of input
+ * Observables being subscribed to concurrently.
+ * @return {Observable} An Observable that emits the result of applying the
+ * projection function (and the optional `resultSelector`) to each item emitted
+ * by the source Observable and merging the results of the Observables obtained
+ * from this transformation.
+ * @method mergeMap
+ * @owner Observable
+ */
+export function mergeMap<T, I, R>(project: (value: T, index: number) => ObservableInput<I>,
+                                  resultSelector?: ((outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R) | number,
+                                  concurrent: number = Number.POSITIVE_INFINITY): OperatorFunction<T, I|R> {
+  return function mergeMapOperatorFunction(source: Observable<T>) {
+    if (typeof resultSelector === 'number') {
+      concurrent = <number>resultSelector;
+      resultSelector = null;
+    }
+    return source.lift(new MergeMapOperator(project, <any>resultSelector, concurrent));
+  };
+}
+
+export class MergeMapOperator<T, I, R> implements Operator<T, I> {
+  constructor(private project: (value: T, index: number) => ObservableInput<I>,
+              private resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R,
+              private concurrent: number = Number.POSITIVE_INFINITY) {
+  }
+
+  call(observer: Subscriber<I>, source: any): any {
+    return source.subscribe(new MergeMapSubscriber(
+      observer, this.project, this.resultSelector, this.concurrent
+    ));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+export class MergeMapSubscriber<T, I, R> extends OuterSubscriber<T, I> {
+  private hasCompleted: boolean = false;
+  private buffer: T[] = [];
+  private active: number = 0;
+  protected index: number = 0;
+
+  constructor(destination: Subscriber<I>,
+              private project: (value: T, index: number) => ObservableInput<I>,
+              private resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R,
+              private concurrent: number = Number.POSITIVE_INFINITY) {
+    super(destination);
+  }
+
+  protected _next(value: T): void {
+    if (this.active < this.concurrent) {
+      this._tryNext(value);
+    } else {
+      this.buffer.push(value);
+    }
+  }
+
+  protected _tryNext(value: T) {
+    let result: ObservableInput<I>;
+    const index = this.index++;
+    try {
+      result = this.project(value, index);
+    } catch (err) {
+      this.destination.error(err);
+      return;
+    }
+    this.active++;
+    this._innerSub(result, value, index);
+  }
+
+  private _innerSub(ish: ObservableInput<I>, value: T, index: number): void {
+    this.add(subscribeToResult<T, I>(this, ish, value, index));
+  }
+
+  protected _complete(): void {
+    this.hasCompleted = true;
+    if (this.active === 0 && this.buffer.length === 0) {
+      this.destination.complete();
+    }
+  }
+
+  notifyNext(outerValue: T, innerValue: I,
+             outerIndex: number, innerIndex: number,
+             innerSub: InnerSubscriber<T, I>): void {
+    if (this.resultSelector) {
+      this._notifyResultSelector(outerValue, innerValue, outerIndex, innerIndex);
+    } else {
+      this.destination.next(innerValue);
+    }
+  }
+
+  private _notifyResultSelector(outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) {
+    let result: R;
+    try {
+      result = this.resultSelector(outerValue, innerValue, outerIndex, innerIndex);
+    } catch (err) {
+      this.destination.error(err);
+      return;
+    }
+    this.destination.next(result);
+  }
+
+  notifyComplete(innerSub: Subscription): void {
+    const buffer = this.buffer;
+    this.remove(innerSub);
+    this.active--;
+    if (buffer.length > 0) {
+      this._next(buffer.shift());
+    } else if (this.active === 0 && this.hasCompleted) {
+      this.destination.complete();
+    }
+  }
+}