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+import { MonoTypeOperatorFunction, ObservableInput } from '../types';
+import { operate } from '../util/lift';
+import { createOperatorSubscriber } from './OperatorSubscriber';
+import { noop } from '../util/noop';
+import { innerFrom } from '../observable/innerFrom';
+
+/**
+ * Returns an Observable that emits all items emitted by the source Observable that are distinct by comparison from previous items.
+ *
+ * If a `keySelector` function is provided, then it will project each value from the source observable into a new value that it will
+ * check for equality with previously projected values. If the `keySelector` function is not provided, it will use each value from the
+ * source observable directly with an equality check against previous values.
+ *
+ * In JavaScript runtimes that support `Set`, this operator will use a `Set` to improve performance of the distinct value checking.
+ *
+ * In other runtimes, this operator will use a minimal implementation of `Set` that relies on an `Array` and `indexOf` under the
+ * hood, so performance will degrade as more values are checked for distinction. Even in newer browsers, a long-running `distinct`
+ * use might result in memory leaks. To help alleviate this in some scenarios, an optional `flushes` parameter is also provided so
+ * that the internal `Set` can be "flushed", basically clearing it of values.
+ *
+ * ## Examples
+ *
+ * A simple example with numbers
+ *
+ * ```ts
+ * import { of, distinct } from 'rxjs';
+ *
+ * of(1, 1, 2, 2, 2, 1, 2, 3, 4, 3, 2, 1)
+ *   .pipe(distinct())
+ *   .subscribe(x => console.log(x));
+ *
+ * // Outputs
+ * // 1
+ * // 2
+ * // 3
+ * // 4
+ * ```
+ *
+ * An example using the `keySelector` function
+ *
+ * ```ts
+ * import { of, distinct } from 'rxjs';
+ *
+ * of(
+ *   { age: 4, name: 'Foo'},
+ *   { age: 7, name: 'Bar'},
+ *   { age: 5, name: 'Foo'}
+ * )
+ * .pipe(distinct(({ name }) => name))
+ * .subscribe(x => console.log(x));
+ *
+ * // Outputs
+ * // { age: 4, name: 'Foo' }
+ * // { age: 7, name: 'Bar' }
+ * ```
+ * @see {@link distinctUntilChanged}
+ * @see {@link distinctUntilKeyChanged}
+ *
+ * @param keySelector Optional `function` to select which value you want to check as distinct.
+ * @param flushes Optional `ObservableInput` for flushing the internal HashSet of the operator.
+ * @return A function that returns an Observable that emits items from the
+ * source Observable with distinct values.
+ */
+export function distinct<T, K>(keySelector?: (value: T) => K, flushes?: ObservableInput<any>): MonoTypeOperatorFunction<T> {
+  return operate((source, subscriber) => {
+    const distinctKeys = new Set();
+    source.subscribe(
+      createOperatorSubscriber(subscriber, (value) => {
+        const key = keySelector ? keySelector(value) : value;
+        if (!distinctKeys.has(key)) {
+          distinctKeys.add(key);
+          subscriber.next(value);
+        }
+      })
+    );
+
+    flushes && innerFrom(flushes).subscribe(createOperatorSubscriber(subscriber, () => distinctKeys.clear(), noop));
+  });
+}