using System;
using System.Collections.Generic;
using JD.Collections.Immutable;

namespace JD.Collections
{
    /// <summary>
    /// A stack implemented using a singly-linked list, allowing for easy and fast shallow cloning.
    /// </summary>
    public sealed class LinkedListStack<T> : IEnumerable<T>, ICloneable
    {
        #region Variables

        private ImmutableStack<T> m_top = new ImmutableStack<T>();

        #endregion

        #region Constructors

        /// <summary>
        /// Constructor.
        /// </summary>
        public LinkedListStack()
        {
        }

        private LinkedListStack(LinkedListStack<T> other)
        {
            m_top = other.m_top;
        }

        #endregion

        #region Properties

        /// <summary>
        /// Returns true if the <see cref="LinkedListStack{T}"/> is empty.
        /// </summary>
        public bool IsEmpty
        {
            get
            {
                return m_top.IsEmpty;
            }
        }

        #endregion

        #region Methods

        /// <summary>
        /// Inserts an object at the top of the <see cref="LinkedListStack{T}"/>.
        /// </summary>
        /// <remarks>
        /// This method is an O(1) operation.
        /// </remarks>
        /// <param name="value">The object to push onto the <see cref="LinkedListStack{T}"/>. The value can be a null reference for reference types.</param>
        public void Push(T value)
        {
            m_top = m_top.Push(value);
        }

        /// <summary>
        /// Removes and returns the object at the top of the <see cref="LinkedListStack{T}"/>.
        /// </summary>
        /// <remarks>
        /// This method is similar to the <see cref="Peek"/> method, but <see cref="Peek"/> does not modify the <see cref="LinkedListStack{T}"/>.
        /// This method is an O(1) operation.
        /// </remarks>
        /// <returns>The object removed from the top of the <see cref="LinkedListStack{T}"/>.</returns>
        public T Pop()
        {
            T current = m_top.Peek();

            m_top = m_top.Pop();

            return current;
        }

        /// <summary>
        /// Returns the object at the top of the <see cref="LinkedListStack{T}"/> without removing it.
        /// </summary>
        /// <remarks>
        /// This method is similar to the <see cref="Pop"/> method, but <see cref="Peek"/> does not modify the <see cref="LinkedListStack{T}"/>.
        /// This method is an O(1) operation.
        /// </remarks>
        /// <returns>The object at the top of the <see cref="LinkedListStack{T}"/>.</returns>
        /// <exception cref="InvalidOperationException">The Stack is empty.</exception>
        public T Peek()
        {
            return m_top.Peek();
        }

        /// <summary>
        /// Clones the current stack.
        /// </summary>
        /// <returns></returns>
        public LinkedListStack<T> Clone()
        {
            return new LinkedListStack<T>(this);
        }

        object ICloneable.Clone()
        {
            return Clone();
        }

        IEnumerator<T> IEnumerable<T>.GetEnumerator()
        {
            return m_top.GetEnumerator();
        }

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return ((IEnumerable<T>)this).GetEnumerator();
        }

        #endregion
    }
}