import LinkedList from '../linked-list/LinkedList';

export default class Stack {
 constructor() {
 // We're going to implement Stack based on LinkedList since these
 // structures are quite similar. Compare push/pop operations of the Stack
 // with prepend/deleteHead operations of LinkedList.
 this.linkedList = new LinkedList();
 }

 /**
 * @return {boolean}
 */
 isEmpty() {
 // The stack is empty if its linked list doesn't have a head.
 return !this.linkedList.head;
 }

 /**
 * @return {*}
 */
 peek() {
 if (this.isEmpty()) {
 // If the linked list is empty then there is nothing to peek from.
 return null;
 }

 // Just read the value from the start of linked list without deleting it.
 return this.linkedList.head.value;
 }

 /**
 * @param {*} value
 */
 push(value) {
 // Pushing means to lay the value on top of the stack. Therefore let's just add
 // the new value at the start of the linked list.
 this.linkedList.prepend(value);
 }

 /**
 * @return {*}
 */
 pop() {
 // Let's try to delete the first node (the head) from the linked list.
 // If there is no head (the linked list is empty) just return null.
 const removedHead = this.linkedList.deleteHead();
 return removedHead ? removedHead.value : null;
 }

 /**
 * @return {*[]}
 */
 toArray() {
 return this.linkedList
 .toArray()
 .map((linkedListNode) => linkedListNode.value);
 }

 /**
 * @param {function} [callback]
 * @return {string}
 */
 toString(callback) {
 return this.linkedList.toString(callback);
 }
}