initial commit: scaffolding

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+import { innerFrom } from '../observable/innerFrom';
+import { Subject } from '../Subject';
+import { SafeSubscriber } from '../Subscriber';
+import { Subscription } from '../Subscription';
+import { MonoTypeOperatorFunction, SubjectLike, ObservableInput } from '../types';
+import { operate } from '../util/lift';
+
+export interface ShareConfig<T> {
+  /**
+   * The factory used to create the subject that will connect the source observable to
+   * multicast consumers.
+   */
+  connector?: () => SubjectLike<T>;
+  /**
+   * If `true`, the resulting observable will reset internal state on error from source and return to a "cold" state. This
+   * allows the resulting observable to be "retried" in the event of an error.
+   * If `false`, when an error comes from the source it will push the error into the connecting subject, and the subject
+   * will remain the connecting subject, meaning the resulting observable will not go "cold" again, and subsequent retries
+   * or resubscriptions will resubscribe to that same subject. In all cases, RxJS subjects will emit the same error again, however
+   * {@link ReplaySubject} will also push its buffered values before pushing the error.
+   * It is also possible to pass a notifier factory returning an `ObservableInput` instead which grants more fine-grained
+   * control over how and when the reset should happen. This allows behaviors like conditional or delayed resets.
+   */
+  resetOnError?: boolean | ((error: any) => ObservableInput<any>);
+  /**
+   * If `true`, the resulting observable will reset internal state on completion from source and return to a "cold" state. This
+   * allows the resulting observable to be "repeated" after it is done.
+   * If `false`, when the source completes, it will push the completion through the connecting subject, and the subject
+   * will remain the connecting subject, meaning the resulting observable will not go "cold" again, and subsequent repeats
+   * or resubscriptions will resubscribe to that same subject.
+   * It is also possible to pass a notifier factory returning an `ObservableInput` instead which grants more fine-grained
+   * control over how and when the reset should happen. This allows behaviors like conditional or delayed resets.
+   */
+  resetOnComplete?: boolean | (() => ObservableInput<any>);
+  /**
+   * If `true`, when the number of subscribers to the resulting observable reaches zero due to those subscribers unsubscribing, the
+   * internal state will be reset and the resulting observable will return to a "cold" state. This means that the next
+   * time the resulting observable is subscribed to, a new subject will be created and the source will be subscribed to
+   * again.
+   * If `false`, when the number of subscribers to the resulting observable reaches zero due to unsubscription, the subject
+   * will remain connected to the source, and new subscriptions to the result will be connected through that same subject.
+   * It is also possible to pass a notifier factory returning an `ObservableInput` instead which grants more fine-grained
+   * control over how and when the reset should happen. This allows behaviors like conditional or delayed resets.
+   */
+  resetOnRefCountZero?: boolean | (() => ObservableInput<any>);
+}
+
+export function share<T>(): MonoTypeOperatorFunction<T>;
+
+export function share<T>(options: ShareConfig<T>): MonoTypeOperatorFunction<T>;
+
+/**
+ * Returns a new Observable that multicasts (shares) the original Observable. As long as there is at least one
+ * Subscriber this Observable will be subscribed and emitting data. When all subscribers have unsubscribed it will
+ * unsubscribe from the source Observable. Because the Observable is multicasting it makes the stream `hot`.
+ * This is an alias for `multicast(() => new Subject()), refCount()`.
+ *
+ * The subscription to the underlying source Observable can be reset (unsubscribe and resubscribe for new subscribers),
+ * if the subscriber count to the shared observable drops to 0, or if the source Observable errors or completes. It is
+ * possible to use notifier factories for the resets to allow for behaviors like conditional or delayed resets. Please
+ * note that resetting on error or complete of the source Observable does not behave like a transparent retry or restart
+ * of the source because the error or complete will be forwarded to all subscribers and their subscription will be
+ * closed. Only new subscribers after a reset on error or complete happened will cause a fresh subscription to the
+ * source. To achieve transparent retries or restarts pipe the source through appropriate operators before sharing.
+ *
+ * ![](share.png)
+ *
+ * ## Example
+ *
+ * Generate new multicast Observable from the `source` Observable value
+ *
+ * ```ts
+ * import { interval, tap, map, take, share } from 'rxjs';
+ *
+ * const source = interval(1000).pipe(
+ *   tap(x => console.log('Processing: ', x)),
+ *   map(x => x * x),
+ *   take(6),
+ *   share()
+ * );
+ *
+ * source.subscribe(x => console.log('subscription 1: ', x));
+ * source.subscribe(x => console.log('subscription 2: ', x));
+ *
+ * // Logs:
+ * // Processing: 0
+ * // subscription 1: 0
+ * // subscription 2: 0
+ * // Processing: 1
+ * // subscription 1: 1
+ * // subscription 2: 1
+ * // Processing: 2
+ * // subscription 1: 4
+ * // subscription 2: 4
+ * // Processing: 3
+ * // subscription 1: 9
+ * // subscription 2: 9
+ * // Processing: 4
+ * // subscription 1: 16
+ * // subscription 2: 16
+ * // Processing: 5
+ * // subscription 1: 25
+ * // subscription 2: 25
+ * ```
+ *
+ * ## Example with notifier factory: Delayed reset
+ *
+ * ```ts
+ * import { interval, take, share, timer } from 'rxjs';
+ *
+ * const source = interval(1000).pipe(
+ *   take(3),
+ *   share({
+ *     resetOnRefCountZero: () => timer(1000)
+ *   })
+ * );
+ *
+ * const subscriptionOne = source.subscribe(x => console.log('subscription 1: ', x));
+ * setTimeout(() => subscriptionOne.unsubscribe(), 1300);
+ *
+ * setTimeout(() => source.subscribe(x => console.log('subscription 2: ', x)), 1700);
+ *
+ * setTimeout(() => source.subscribe(x => console.log('subscription 3: ', x)), 5000);
+ *
+ * // Logs:
+ * // subscription 1:  0
+ * // (subscription 1 unsubscribes here)
+ * // (subscription 2 subscribes here ~400ms later, source was not reset)
+ * // subscription 2:  1
+ * // subscription 2:  2
+ * // (subscription 2 unsubscribes here)
+ * // (subscription 3 subscribes here ~2000ms later, source did reset before)
+ * // subscription 3:  0
+ * // subscription 3:  1
+ * // subscription 3:  2
+ * ```
+ *
+ * @see {@link shareReplay}
+ *
+ * @return A function that returns an Observable that mirrors the source.
+ */
+export function share<T>(options: ShareConfig<T> = {}): MonoTypeOperatorFunction<T> {
+  const { connector = () => new Subject<T>(), resetOnError = true, resetOnComplete = true, resetOnRefCountZero = true } = options;
+  // It's necessary to use a wrapper here, as the _operator_ must be
+  // referentially transparent. Otherwise, it cannot be used in calls to the
+  // static `pipe` function - to create a partial pipeline.
+  //
+  // The _operator function_ - the function returned by the _operator_ - will
+  // not be referentially transparent - as it shares its source - but the
+  // _operator function_ is called when the complete pipeline is composed via a
+  // call to a source observable's `pipe` method - not when the static `pipe`
+  // function is called.
+  return (wrapperSource) => {
+    let connection: SafeSubscriber<T> | undefined;
+    let resetConnection: Subscription | undefined;
+    let subject: SubjectLike<T> | undefined;
+    let refCount = 0;
+    let hasCompleted = false;
+    let hasErrored = false;
+
+    const cancelReset = () => {
+      resetConnection?.unsubscribe();
+      resetConnection = undefined;
+    };
+    // Used to reset the internal state to a "cold"
+    // state, as though it had never been subscribed to.
+    const reset = () => {
+      cancelReset();
+      connection = subject = undefined;
+      hasCompleted = hasErrored = false;
+    };
+    const resetAndUnsubscribe = () => {
+      // We need to capture the connection before
+      // we reset (if we need to reset).
+      const conn = connection;
+      reset();
+      conn?.unsubscribe();
+    };
+
+    return operate<T, T>((source, subscriber) => {
+      refCount++;
+      if (!hasErrored && !hasCompleted) {
+        cancelReset();
+      }
+
+      // Create the subject if we don't have one yet. Grab a local reference to
+      // it as well, which avoids non-null assertions when using it and, if we
+      // connect to it now, then error/complete need a reference after it was
+      // reset.
+      const dest = (subject = subject ?? connector());
+
+      // Add the finalization directly to the subscriber - instead of returning it -
+      // so that the handling of the subscriber's unsubscription will be wired
+      // up _before_ the subscription to the source occurs. This is done so that
+      // the assignment to the source connection's `closed` property will be seen
+      // by synchronous firehose sources.
+      subscriber.add(() => {
+        refCount--;
+
+        // If we're resetting on refCount === 0, and it's 0, we only want to do
+        // that on "unsubscribe", really. Resetting on error or completion is a different
+        // configuration.
+        if (refCount === 0 && !hasErrored && !hasCompleted) {
+          resetConnection = handleReset(resetAndUnsubscribe, resetOnRefCountZero);
+        }
+      });
+
+      // The following line adds the subscription to the subscriber passed.
+      // Basically, `subscriber === dest.subscribe(subscriber)` is `true`.
+      dest.subscribe(subscriber);
+
+      if (
+        !connection &&
+        // Check this shareReplay is still activate - it can be reset to 0
+        // and be "unsubscribed" _before_ it actually subscribes.
+        // If we were to subscribe then, it'd leak and get stuck.
+        refCount > 0
+      ) {
+        // We need to create a subscriber here - rather than pass an observer and
+        // assign the returned subscription to connection - because it's possible
+        // for reentrant subscriptions to the shared observable to occur and in
+        // those situations we want connection to be already-assigned so that we
+        // don't create another connection to the source.
+        connection = new SafeSubscriber({
+          next: (value) => dest.next(value),
+          error: (err) => {
+            hasErrored = true;
+            cancelReset();
+            resetConnection = handleReset(reset, resetOnError, err);
+            dest.error(err);
+          },
+          complete: () => {
+            hasCompleted = true;
+            cancelReset();
+            resetConnection = handleReset(reset, resetOnComplete);
+            dest.complete();
+          },
+        });
+        innerFrom(source).subscribe(connection);
+      }
+    })(wrapperSource);
+  };
+}
+
+function handleReset<T extends unknown[] = never[]>(
+  reset: () => void,
+  on: boolean | ((...args: T) => ObservableInput<any>),
+  ...args: T
+): Subscription | undefined {
+  if (on === true) {
+    reset();
+    return;
+  }
+
+  if (on === false) {
+    return;
+  }
+
+  const onSubscriber = new SafeSubscriber({
+    next: () => {
+      onSubscriber.unsubscribe();
+      reset();
+    },
+  });
+
+  return innerFrom(on(...args)).subscribe(onSubscriber);
+}