export default class Queue {
  constructor() {
    this._first = null
    this._last = null
  }

  // Shows us the first item in the list
  first() {
    return this._first && this._first.value
  }

  // Shows us the last item in the list
  last() {
    return this._last && this._last.value
  }

  push(value) {
    // An item stores an id and the provided value
    const item =
      typeof value.next !== 'undefined'
        ? value
        : { value: value, next: null, prev: null }

    // Deal with the queue being empty or populated
    if (this._last) {
      item.prev = this._last
      this._last.next = item
      this._last = item
    } else {
      this._last = item
      this._first = item
    }

    // Return the current item
    return item
  }

  // Removes the item that was returned from the push
  remove(item) {
    // Relink the previous item
    if (item.prev) item.prev.next = item.next
    if (item.next) item.next.prev = item.prev
    if (item === this._last) this._last = item.prev
    if (item === this._first) this._first = item.next

    // Invalidate item
    item.prev = null
    item.next = null
  }

  shift() {
    // Check if we have a value
    const remove = this._first
    if (!remove) return null

    // If we do, remove it and relink things
    this._first = remove.next
    if (this._first) this._first.prev = null
    this._last = this._first ? this._last : null
    return remove.value
  }
}