using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MyCollections
{
    public class MyQueue<T> : IEnumerable<T>
    {
        private const int _startCapacity = 8;
        private T[] _items;
        private int _capacity;
        private int _head;
        private int _tail;
        public int Count { get; private set; }

        public MyQueue()
        {
            _items = new T[_startCapacity];
            _capacity = _startCapacity;
        }

        public void Enqueue(T item)
        {
            if (Count == _capacity)
                ExtendInnerArray();
            _items[_head] = item;
            _head++;
            Count++;
            if (_head == _capacity)
                _head = 0;
        }
        private void ExtendInnerArray()
        {
            if (_capacity == Array.MaxLength)
                throw new OutOfMemoryException();
            int newCapacity = 2 * _capacity;
            if ((uint)newCapacity > Array.MaxLength)
                newCapacity = Array.MaxLength;
            T[] newItems = new T[newCapacity];
            if (_tail < _head)
            {
                Array.Copy(_items, _head, newItems, 0, Count);
            }
            else
            {
                Array.Copy(_items, _tail, newItems, 0, _capacity - _tail);
                Array.Copy(_items, 0, newItems, _capacity - _tail, _head);
            }
            _items = newItems;
            _tail = 0;
            _head = _capacity;
            _capacity = newCapacity;
        }
        public T Dequeue()
        {
            if (Count == 0)
                throw new InvalidOperationException("Queue is empty");
            T returnValue = _items[_tail];
            Count--;
            _tail++;
            if (_tail == _capacity)
                _tail = 0;
            return returnValue;
        }
        public T Peek()
        {
            if (Count == 0)
                throw new InvalidOperationException("Queue is empty");
            return _items[_tail];
        }

        public IEnumerator<T> GetEnumerator()
        {
            int index = _tail;
            for (int i = 0; i < Count; i++)
            {
                yield return _items[index];
                index++;
                if (index == _capacity)
                    index = 0;
            }
            yield break;
        }
        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}