initial commit: scaffolding

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+import { Subscriber } from '../Subscriber';
+import { MonoTypeOperatorFunction, ObservableInput } from '../types';
+import { operate } from '../util/lift';
+import { noop } from '../util/noop';
+import { createOperatorSubscriber } from './OperatorSubscriber';
+import { innerFrom } from '../observable/innerFrom';
+
+/**
+ * Emits a notification from the source Observable only after a particular time span
+ * determined by another Observable has passed without another source emission.
+ *
+ * <span class="informal">It's like {@link debounceTime}, but the time span of
+ * emission silence is determined by a second Observable.</span>
+ *
+ * ![](debounce.svg)
+ *
+ * `debounce` delays notifications emitted by the source Observable, but drops previous
+ * pending delayed emissions if a new notification arrives on the source Observable.
+ * This operator keeps track of the most recent notification from the source
+ * Observable, and spawns a duration Observable by calling the
+ * `durationSelector` function. The notification is emitted only when the duration
+ * Observable emits a next notification, and if no other notification was emitted on
+ * the source Observable since the duration Observable was spawned. If a new
+ * notification appears before the duration Observable emits, the previous notification will
+ * not be emitted and a new duration is scheduled from `durationSelector` is scheduled.
+ * If the completing event happens during the scheduled duration the last cached notification
+ * is emitted before the completion event is forwarded to the output observable.
+ * If the error event happens during the scheduled duration or after it only the error event is
+ * forwarded to the output observable. The cache notification is not emitted in this case.
+ *
+ * Like {@link debounceTime}, this is a rate-limiting operator, and also a
+ * delay-like operator since output emissions do not necessarily occur at the
+ * same time as they did on the source Observable.
+ *
+ * ## Example
+ *
+ * Emit the most recent click after a burst of clicks
+ *
+ * ```ts
+ * import { fromEvent, scan, debounce, interval } from 'rxjs';
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const result = clicks.pipe(
+ *   scan(i => ++i, 1),
+ *   debounce(i => interval(200 * i))
+ * );
+ * result.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link audit}
+ * @see {@link auditTime}
+ * @see {@link debounceTime}
+ * @see {@link delay}
+ * @see {@link sample}
+ * @see {@link sampleTime}
+ * @see {@link throttle}
+ * @see {@link throttleTime}
+ *
+ * @param durationSelector A function
+ * that receives a value from the source Observable, for computing the timeout
+ * duration for each source value, returned as an Observable or a Promise.
+ * @return A function that returns an Observable that delays the emissions of
+ * the source Observable by the specified duration Observable returned by
+ * `durationSelector`, and may drop some values if they occur too frequently.
+ */
+export function debounce<T>(durationSelector: (value: T) => ObservableInput<any>): MonoTypeOperatorFunction<T> {
+  return operate((source, subscriber) => {
+    let hasValue = false;
+    let lastValue: T | null = null;
+    // The subscriber/subscription for the current debounce, if there is one.
+    let durationSubscriber: Subscriber<any> | null = null;
+
+    const emit = () => {
+      // Unsubscribe any current debounce subscription we have,
+      // we only cared about the first notification from it, and we
+      // want to clean that subscription up as soon as possible.
+      durationSubscriber?.unsubscribe();
+      durationSubscriber = null;
+      if (hasValue) {
+        // We have a value! Free up memory first, then emit the value.
+        hasValue = false;
+        const value = lastValue!;
+        lastValue = null;
+        subscriber.next(value);
+      }
+    };
+
+    source.subscribe(
+      createOperatorSubscriber(
+        subscriber,
+        (value: T) => {
+          // Cancel any pending debounce duration. We don't
+          // need to null it out here yet tho, because we're just going
+          // to create another one in a few lines.
+          durationSubscriber?.unsubscribe();
+          hasValue = true;
+          lastValue = value;
+          // Capture our duration subscriber, so we can unsubscribe it when we're notified
+          // and we're going to emit the value.
+          durationSubscriber = createOperatorSubscriber(subscriber, emit, noop);
+          // Subscribe to the duration.
+          innerFrom(durationSelector(value)).subscribe(durationSubscriber);
+        },
+        () => {
+          // Source completed.
+          // Emit any pending debounced values then complete
+          emit();
+          subscriber.complete();
+        },
+        // Pass all errors through to consumer
+        undefined,
+        () => {
+          // Finalization.
+          lastValue = durationSubscriber = null;
+        }
+      )
+    );
+  });
+}