let pubSubInstance;

/**
 * The PubSub class servers as a singleton,
 * it's used by the EventDispatcher class
 * (don't instantiate this class directly)
 */
class PubSub {
  constructor() {
    if (!pubSubInstance) {
      this.subjects = {};
      this.hOP = this.subjects.hasOwnProperty;
      pubSubInstance = this;
    }

    return pubSubInstance;
  }

  /**
   *
   * @param {string} topic The event name
   * @param {function} callback Function to bind to the event
   * @return {PubSub} The PubSub singleton instance
   */
  on(topic, callback) {
    const { subjects, hOP } = this;
    if (!hOP.call(subjects, topic)) {
      subjects[topic] = [];
    }

    subjects[topic].push(callback);

    return this;
  }

  /**
   *
   * @param {string} topic The event name
   * @param {function} callback Function to un-bind from the event
   * @return {PubSub} The PubSub singleton instance
   */
  off(topic, callback) {
    const { subjects } = this;
    if (subjects[topic] && subjects[topic].length) {
      const idx = subjects[topic].indexOf(callback);
      if (idx !== -1) {
        subjects[topic].splice(idx, 1);
      }
    }

    return this;
  }

  /**
   *
   * @param {string} topic The event name
   * @param {object} data Extra data to pass with the event
   * @return {PubSub|null} The PubSub singleton instance
   */
  trigger(topic, data) {
    const { subjects, hOP } = this;
    if (!hOP.call(subjects, topic)) {
      return null;
    }

    subjects[topic].forEach((entry) => {
      if (entry) {
        entry(data !== undefined ? data : {});
      }
    });

    return this;
  }
}

/**
 * The EventDispatcher class servers as a global event dispatcher,
 * it uses the PubSub singleton class
 */
export default class EventDispatcher {
  constructor() {
    this.pubSub = new PubSub();
  }

  /**
   *
   * Please read the {@link PubSub.on} specs
   */
  on(...args) {
    return this.pubSub.on(...args);
  }

  /**
   *
   * Please read the {@link PubSub.off} specs
   */
  off(...args) {
    return this.pubSub.off(...args);
  }

  /**
   *
   * Please read the {@link PubSub.trigger} specs
   */
  trigger(...args) {
    return this.pubSub.trigger(...args);
  }
}