using System;
using System.Collections.Generic;

namespace Fedortsov.Lab7Exc1
{
    public class DoublyLinkedList<T>
    {
        private class Node
        {
            public T Value { get; set; }
            public Node Next { get; set; }
            public Node Previous { get; set; }

            public Node(T value)
            {
                Value = value;
                Next = null;
                Previous = null;
            }
        }

        private Node head;
        private Node tail;
        public int Count { get; private set; }

        public DoublyLinkedList()
        {
            head = null;
            tail = null;
            Count = 0;
        }

        public void AddFirst(T value)
        {
            Node newNode = new Node(value);

            if (head == null)
            {
                head = newNode;
                tail = newNode;
            }
            else
            {
                newNode.Next = head;
                head.Previous = newNode;
                head = newNode;
            }

            Count++;
        }

        public void AddLast(T value)
        {
            Node newNode = new Node(value);

            if (tail == null)
            {
                head = newNode;
                tail = newNode;
            }
            else
            {
                newNode.Previous = tail;
                tail.Next = newNode;
                tail = newNode;
            }

            Count++;
        }

        public void RemoveFirst()
        {
            if (head == null)
            {
                throw new InvalidOperationException("List is empty");
            }

            if (head == tail)
            {
                head = null;
                tail = null;
            }
            else
            {
                head = head.Next;
                head.Previous = null;
            }

            Count--;
        }

        public void RemoveLast()
        {
            if (tail == null)
            {
                throw new InvalidOperationException("List is empty");
            }

            if (head == tail)
            {
                head = null;
                tail = null;
            }
            else
            {
                tail = tail.Previous;
                tail.Next = null;
            }

            Count--;
        }

        public bool Contains(T value)
        {
            Node current = head;

            while (current != null)
            {
                if (current.Value.Equals(value))
                {
                    return true;
                }

                current = current.Next;
            }

            return false;
        }

        public void Clear()
        {
            head = null;
            tail = null;
            Count = 0;
        }

        public IEnumerator<T> GetEnumerator()
        {
            Node current = head;

            while (current != null)
            {
                yield return current.Value;
                current = current.Next;
            }
        }
    }
}