function createListener(fn, that, type, namespace, once) {
  return {
    fn, that, type, namespace, once,
    bound: that ? fn.bind(that) : fn,
  }
}

export function EventEmitterMixin(Superclass) {

  return class EventEmitter extends Superclass {

    constructor(...args) {
      super(...args);
      this.$$events = {};
      this.$$namespaces = {};
    }

    addListener(...args) { return this.on(...args); }

    // Notify ALL the listeners!
    emit(...args) {
      const type = args[0];
      const listeners = this.lookupEmitListeners(type);
      if (0 === listeners.length) {
        return;
      }

      for (const {once, type, namespace, fn, bound, that} of listeners) {
        const offset = type !== '*' ? 1 : 0;

        if (once) {
          this.removeListener(`${type}.${namespace}`, fn);
        }

        // Fast path...
        if (args.length === offset) {
          bound()
        }

        else if (args.length === offset + 1) {
          bound(
            args[offset + 0]
          );
        }

        else if (args.length === offset + 2) {
          bound(
            args[offset + 0],
            args[offset + 1]
          )
        }

        else if (args.length === offset + 3) {
          bound(
            args[offset + 0],
            args[offset + 1],
            args[offset + 2]
          )
        }

        else if (args.length === offset + 4) {
          bound(
            args[offset + 0],
            args[offset + 1],
            args[offset + 2],
            args[offset + 3]
          )
        }

        else if (args.length === offset + 5) {
          bound(
            args[offset + 0],
            args[offset + 1],
            args[offset + 2],
            args[offset + 3],
            args[offset + 4]
          )
        }

        // Slow path...
        else {
          fn.apply(that, args.slice(offset));
        }
      }
    }

    lookupEmitListeners(type) {
      return ['*', type].reduce((r, type) => {
        if (type in this.$$events) {
          r.push(...this.$$events[type]);
        }
        return r;
      }, []);
    }

    off(typesOrType, fn) {
      parseTypes(typesOrType).forEach((typeOrCompositeType) => {
        this.offSingleEvent(typeOrCompositeType, fn);
      });
      return this;
    }

    offSingleEvent(typeOrCompositeType, fn) {
      const [type, namespace] = decomposeType(typeOrCompositeType);

      // Function.
      if ('function' === typeof fn) {
        const lists = [];

        if ((type in this.$$events)) {
          lists.push(this.$$events);
        }

        if (
          (namespace in this.$$namespaces)
          && (type in this.$$namespaces[namespace])
        ) {
          lists.push(this.$$namespaces[namespace]);
        }

        lists.forEach((listeners) => {
          listeners[type] = listeners[type].filter((listener) => {
            return listener.fn !== fn;
          });
        });

        return;
      }

      // Only type.
      if (0 === namespace.length) {

        if (type in this.$$events) {
          delete this.$$events[type];
        }

        for (const namespace in this.$$namespaces) {
          const namespaceEvents = this.$$namespaces[namespace];
          if (type in namespaceEvents) {
            delete namespaceEvents[type];
          }
        }

        return;
      }

      // Only namespace.
      if (!(namespace in this.$$namespaces)) {
        return;
      }
      if (0 === type.length) {
        for (const type in this.$$namespaces[namespace]) {
          this.removeEventListenersFor(type, namespace);
        }
        delete this.$$namespaces[namespace];
      }

      // Type & namespace.
      else if (type in this.$$namespaces[namespace]) {
        this.removeEventListenersFor(type, namespace);
        delete this.$$namespaces[namespace][type];
      }
    }

    on(types, fn, that = undefined) {
      this._on(types, fn, that, false);
      return this;
    }

    _on(typesOrType, fn, that, once) {
      parseTypes(typesOrType).forEach((typeOrCompositeType) => {
        this.onSingleEvent(typeOrCompositeType, fn, that, once);
      });
    }

    once(types, fn, that = undefined) {
      this._on(types, fn, that, true);
      return this;
    }

    onSingleEvent(typeOrCompositeType, fn, that, once) {
      const [type, namespace] = decomposeType(typeOrCompositeType);
      const listener = createListener(fn, that, type, namespace, once);

      if (!(type in this.$$events)) {
        this.$$events[type] = [];
      }
      this.$$events[type].push(listener);

      if (!(namespace in this.$$namespaces)) {
        this.$$namespaces[namespace] = {};
      }
      if (!(type in this.$$namespaces[namespace])) {
        this.$$namespaces[namespace][type] = [];
      }
      this.$$namespaces[namespace][type].push(listener);
    }

    removeEventListenersFor(type, namespace) {

      for (const {fn} of this.$$namespaces[namespace][type]) {
        this.$$events[type] = this.$$events[type].filter((listener) => {
          return listener.fn !== fn;
        });
      }

      if (0 === this.$$events[type].length) {
        delete this.$$events[type];
      }
    }

    removeListener(...args) { return this.off(...args); }

  }
}

export function decomposeType(typeOrCompositeType) {
  const index = typeOrCompositeType.indexOf('.');
  const isCompositeType = -1 !== index;
  if (isCompositeType) {
    return [
      typeOrCompositeType.substr(0, index),
      typeOrCompositeType.substr(index + 1),
    ];
  }
  return [typeOrCompositeType, ''];
}

// Split, flatten, and trim.
export function parseTypes(typesOrType) {
  const types = Array.isArray(typesOrType) ? typesOrType : [typesOrType];
  return types.map((type) => {
    return type.split(' ');

  }).reduce((r, types) => {
    r.push(...types);
    return r;

  }, []).map((type) => {
    return type.trim();

  });
}