source: trip-planner-front/node_modules/rxjs/src/internal/operators/distinct.ts

import { Observable } from '../Observable';
import { Operator } from '../Operator';
import { Subscriber } from '../Subscriber';
import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
import { SimpleOuterSubscriber, innerSubscribe, SimpleInnerSubscriber } from '../innerSubscribe';

/**
 * 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 a 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 } from 'rxjs';
 * import { distinct } from 'rxjs/operators';
 *
 * of(1, 1, 2, 2, 2, 1, 2, 3, 4, 3, 2, 1).pipe(
 *     distinct(),
 *   )
 *   .subscribe(x => console.log(x)); // 1, 2, 3, 4
 * ```
 *
 * An example using a keySelector function
 * ```typescript
 * import { of } from 'rxjs';
 * import { distinct } from 'rxjs/operators';
 *
 * interface Person {
 *    age: number,
 *    name: string
 * }
 *
 * of<Person>(
 *     { age: 4, name: 'Foo'},
 *     { age: 7, name: 'Bar'},
 *     { age: 5, name: 'Foo'},
 *   ).pipe(
 *     distinct((p: Person) => p.name),
 *   )
 *   .subscribe(x => console.log(x));
 *
 * // displays:
 * // { age: 4, name: 'Foo' }
 * // { age: 7, name: 'Bar' }
 * ```
 * @see {@link distinctUntilChanged}
 * @see {@link distinctUntilKeyChanged}
 *
 * @param {function} [keySelector] Optional function to select which value you want to check as distinct.
 * @param {Observable} [flushes] Optional Observable for flushing the internal HashSet of the operator.
 * @return {Observable} An Observable that emits items from the source Observable with distinct values.
 * @method distinct
 * @owner Observable
 */
export function distinct<T, K>(keySelector?: (value: T) => K,
                               flushes?: Observable<any>): MonoTypeOperatorFunction<T> {
  return (source: Observable<T>) => source.lift(new DistinctOperator(keySelector, flushes));
}

class DistinctOperator<T, K> implements Operator<T, T> {
  constructor(private keySelector?: (value: T) => K, private flushes?: Observable<any>) {
  }

  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
    return source.subscribe(new DistinctSubscriber(subscriber, this.keySelector, this.flushes));
  }
}

/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
export class DistinctSubscriber<T, K> extends SimpleOuterSubscriber<T, T> {
  private values = new Set<K>();

  constructor(destination: Subscriber<T>, private keySelector?: (value: T) => K, flushes?: Observable<any>) {
    super(destination);

    if (flushes) {
      this.add(innerSubscribe(flushes, new SimpleInnerSubscriber(this)));
    }
  }

  notifyNext(): void {
    this.values.clear();
  }

  notifyError(error: any): void {
    this._error(error);
  }

  protected _next(value: T): void {
    if (this.keySelector) {
      this._useKeySelector(value);
    } else {
      this._finalizeNext(value, value);
    }
  }

  private _useKeySelector(value: T): void {
    let key: K;
    const { destination } = this;
    try {
      key = this.keySelector!(value);
    } catch (err) {
      destination.error!(err);
      return;
    }
    this._finalizeNext(key, value);
  }

  private _finalizeNext(key: K|T, value: T) {
    const { values } = this;
    if (!values.has(<K>key)) {
      values.add(<K>key);
      this.destination.next!(value);
    }
  }

}