type EventListener<Events, EventType extends keyof Events> = Events[EventType];

type EventListeners<Events, EventType extends keyof Events> = Array<{
  listener: EventListener<Events, EventType>;
  once?: boolean;
}>;

export type EventParameters<Events, EventType extends keyof Events> = {
  [Event in EventType]: EventListener<Events, EventType> extends (...args: infer P) => any ? P : never;
}[EventType];

export class EventEmitter<EventTypes extends Record<string, (...args: any) => any>> {
  #listeners: {
    [Event in keyof EventTypes]?: EventListeners<EventTypes, Event>;
  } = {};

  /**
   * Adds the listener function to the end of the listeners array for the event.
   * No checks are made to see if the listener has already been added. Multiple calls passing
   * the same combination of event and listener will result in the listener being added, and
   * called, multiple times.
   * @returns this, so that calls can be chained
   */
  on<Event extends keyof EventTypes>(event: Event, listener: EventListener<EventTypes, Event>): this {
    const listeners: EventListeners<EventTypes, Event> =
      this.#listeners[event] || (this.#listeners[event] = []);
    listeners.push({ listener });
    return this;
  }

  /**
   * Removes the specified listener from the listener array for the event.
   * off() will remove, at most, one instance of a listener from the listener array. If any single
   * listener has been added multiple times to the listener array for the specified event, then
   * off() must be called multiple times to remove each instance.
   * @returns this, so that calls can be chained
   */
  off<Event extends keyof EventTypes>(event: Event, listener: EventListener<EventTypes, Event>): this {
    const listeners = this.#listeners[event];
    if (!listeners) return this;
    const index = listeners.findIndex((l) => l.listener === listener);
    if (index >= 0) listeners.splice(index, 1);
    return this;
  }

  /**
   * Adds a one-time listener function for the event. The next time the event is triggered,
   * this listener is removed and then invoked.
   * @returns this, so that calls can be chained
   */
  once<Event extends keyof EventTypes>(event: Event, listener: EventListener<EventTypes, Event>): this {
    const listeners: EventListeners<EventTypes, Event> =
      this.#listeners[event] || (this.#listeners[event] = []);
    listeners.push({ listener, once: true });
    return this;
  }

  /**
   * This is similar to `.once()`, but returns a Promise that resolves the next time
   * the event is triggered, instead of calling a listener callback.
   * @returns a Promise that resolves the next time given event is triggered,
   * or rejects if an error is emitted.  (If you request the 'error' event,
   * returns a promise that resolves with the error).
   *
   * Example:
   *
   *   const message = await stream.emitted('message') // rejects if the stream errors
   */
  emitted<Event extends keyof EventTypes>(
    event: Event,
  ): Promise<
    EventParameters<EventTypes, Event> extends [infer Param] ? Param
    : EventParameters<EventTypes, Event> extends [] ? void
    : EventParameters<EventTypes, Event>
  > {
    return new Promise((resolve, reject) => {
      // TODO: handle errors
      this.once(event, resolve as any);
    });
  }

  protected _emit<Event extends keyof EventTypes>(
    this: EventEmitter<EventTypes>,
    event: Event,
    ...args: EventParameters<EventTypes, Event>
  ) {
    const listeners: EventListeners<EventTypes, Event> | undefined = this.#listeners[event];
    if (listeners) {
      this.#listeners[event] = listeners.filter((l) => !l.once) as any;
      listeners.forEach(({ listener }: any) => listener(...(args as any)));
    }
  }

  protected _hasListener(event: keyof EventTypes): boolean {
    const listeners = this.#listeners[event];
    return listeners && listeners.length > 0;
  }
}