using System;
using System.Collections;
using System.Collections.Generic;

namespace Chernobyl.Collections.Generic
{
    /// <summary>
    /// A special enumerator that moves over lists of 
    /// <see cref="WeakReference{T}"/> types. This enumerator shields the user 
    /// from the <see cref="WeakReference{T}"/> instance and skips over items 
    /// in the list that are not alive (see <see cref="WeakReference.IsAlive"/>).
    /// </summary>
    /// <typeparam name="T">The type that is held within the 
    /// <see cref="WeakReference{T}"/> that will be iterated over with this 
    /// <see cref="IEnumerator{T}"/>. This type must be a reference type because 
    /// this is required by <see cref="WeakReference{T}"/></typeparam>
    public class WeakReferenceEnumerator<T> : IEnumerator<T> where T : class
    {
        /// <summary>
        /// Initializes a new instance of the 
        /// <see cref="WeakReferenceEnumerator{ResourceType}"/> class.
        /// </summary>
        /// <param name="enumerator">The .</param>
        public WeakReferenceEnumerator(IEnumerator<WeakReference<T>> enumerator)
        {
            TheEnumerator = enumerator;
        }

        /// <summary>
        /// Gets the element in the collection at the current position of the 
        /// enumerator.
        /// </summary>
        /// <returns>The element in the collection at the current position of 
        /// the enumerator.</returns>
        public T Current
        {
            get { return TheEnumerator.Current.Target; }
        }

        /// <summary>
        /// Performs application-defined tasks associated with freeing, 
        /// releasing, or resetting unmanaged resources.
        /// </summary>
        public void Dispose()
        {
            TheEnumerator.Dispose();
        }

        /// <summary>
        /// Gets the element in the collection at the current position of the 
        /// enumerator.
        /// </summary>
        /// <value></value>
        /// <returns>The element in the collection at the current position of 
        /// the enumerator.</returns>
        object IEnumerator.Current
        {
            get { return TheEnumerator.Current.Target; }
        }

        /// <summary>
        /// Advances the enumerator to the next element of the collection.
        /// </summary>
        /// <returns>
        /// true if the enumerator was successfully advanced to the next element; 
        /// false if the enumerator has passed the end of the collection or if 
        /// no alive objects exist.
        /// </returns>
        /// <exception cref="InvalidOperationException">The collection 
        /// was modified after the enumerator was created. </exception>
        public bool MoveNext()
        {
            // keep going until we run out of items to move through or until 
            // we find an alive item
            bool result = false;
            do
            {
                result = TheEnumerator.MoveNext();
            } while (result == true && TheEnumerator.Current.IsAlive == false);

            return result;
        }

        /// <summary>
        /// Sets the enumerator to its initial position, which is before the 
        /// first element in the collection.
        /// </summary>
        /// <exception cref="T:System.InvalidOperationException">The collection 
        /// was modified after the enumerator was created. </exception>
        public void Reset()
        {
            TheEnumerator.Reset();
        }

        /// <summary>
        /// The <see cref="IEnumerator{T}"/> used to iterate through the 
        /// items in the list.
        /// </summary>
        IEnumerator<WeakReference<T>> TheEnumerator { get; set; }
    }
}