/**
 * List of unique values (Set)
 */
class UniqueList {
  constructor() {
    this.list = {}
  }

  add(key) {
    this.list[key] = 1
  }

  toArray() {
    return Object.keys(this.list)
  }

  static filter(value, index, self) {
    return self.indexOf(value) === index
  }
}