class Node {
  
  constructor(data){
    this.data = data;
    this.prev = null;
    this.next = null;
  }
  
}


class DoublyLinkedList{
  
  constructor(){
    this.head = null;
    this.tail = null;
    this.length = 0;
  }
  
  push(data){
    
    const node = new Node(data);
    
    if(!this.head){
      this.head = node;
      this.tail = node;
    }else{
       node.prev = this.tail; 
      this.tail.next = node;
     
      this.tail = node;
      
    }
    
    this.length++;
  }
  
  
  pop(){
    
    if(!this.head) return null
    
    // tail is the last node so that we take the 
    // prev property from the tail
    const prevNode = this.tail.prev

    if(prevNode){
       prevNode.next = null;
       this.tail = prevNode; // updating the tail
    }else{
      // if  prev property is null 
      // it means there is only single node
      this.head = null;
      this.tail = null;
    }
     this.length--; //decrement the length
  }
  
  
  insertBeginning(data){
    
    // new node is created
    const node = new Node(data);
    
    // if there is no nodes
    if(!this.head) {
      this.head = node;
      this.tail = node;
    }else{
      // update the head.prev to the new node
      // take the new node.next property and link it to the
      // head property
      this.head.prev = node
      node.next = this.head;
      this.head = node;
    }
    // increment the length
    this.length++;
    
  }
  
  removeFirst(){
    
    if(!this.head) return null
    
    const node = this.head.next;
    
    if(node){
      node.prev = null
      this.head = node
    }else{
      this.head = null
      this.tail = null
    }
    
     this.length--;
    
  }
  
  
}



mocha.setup('bdd');


var assert = chai.assert;

describe('Doubly Linked list',()=>{
  
  it("should add the new node at the end",()=>{
    
    const list = new DoublyLinkedList();
    list.push(1)
    list.push(2)
    list.push(3)
    list.push(4)
    
    assert.equal(4,list.length)
      
  })

     it("should remove the node at the end",()=>{
    const list = new DoublyLinkedList();
    list.push("A")
    list.push("B")
    list.push("C")
    list.push("D")
     
       list.pop();
       list.pop();
    
    assert.equal(2,list.length)
    assert.equal("B",list.tail.data)
  })
  
  
    it("should insert the new node at the beginning",()=>{
    const list = new DoublyLinkedList();
    list.push("C")
    list.push("D")
    list.insertBeginning("B");
    list.insertBeginning("A");
      
      
    assert.equal(4,list.length);
    assert.equal("A",list.head.data)  
      
  })
  
  
   it("should remove the node at the beginning",()=>{
    const list = new DoublyLinkedList();
    list.push("C")
    list.push("D")
    list.insertBeginning("B");
    list.insertBeginning("A");
    list.removeFirst()  
      
    assert.equal(3,list.length);
    assert.equal("B",list.head.data)  
      
  })
  
})



mocha.run()