using System;
using System.Collections.Generic;
using System.Linq;
using Chernobyl.Collections.Generic.Event;
using Chernobyl.Event;
using Chernobyl.Utility;

namespace Chernobyl.Values
{
    /// <summary>
    /// Holds the first item of an <see cref="IEventEnumerable{T}"/> or
    /// <c>default(T)</c>. If the first item in the <see cref="IEventEnumerable{T}"/> 
    /// changes, then the value held by this instance will be changed.
    /// </summary>
    /// <typeparam name="T">The type being held within this instance.</typeparam>
    public class First<T> : HeldValue<T>
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="First{T}" /> class.
        /// </summary>
        /// <param name="enumerable">The <see cref="IEventEnumerable{T}" /> that
        /// is to be watched for changes to its first item.</param>
        /// <exception cref="ArgumentNullException">Thrown if
        /// <paramref name="enumerable" /> is null.</exception>
        public First(IEventEnumerable<T> enumerable)
            : this(enumerable, EqualityComparer<T>.Default)
        {}

        /// <summary>
        /// Initializes a new instance of the <see cref="First{T}" /> class.
        /// </summary>
        /// <param name="enumerable">The <see cref="IEventEnumerable{T}" /> that
        /// is to be watched for changes to its first item.</param>
        /// <param name="comparer">The instance that is used to check if the
        /// <paramref name="enumerable"/> instance's latest first is equal to it's
        /// previous first and therefore results in a change.</param>
        /// <exception cref="ArgumentNullException">Thrown if
        /// <paramref name="enumerable" /> or <paramref name="comparer"/> is null.</exception>
        public First(IEventEnumerable<T> enumerable, IEqualityComparer<T> comparer)
            : base(default(T), comparer)
        {
            enumerable.ThrowIfNull("enumerable");

            _enumerable = enumerable;
            Value = _enumerable.FirstOrDefault();
            _enumerable.ItemsAdded += OnItemsAdded;
            _enumerable.ItemsRemoved += OnItemsRemoved;
        }

        /// <summary>
        /// An event that is invoked when an item is added to 
        /// <see cref="_enumerable"/>. This method invokes <see cref="Update()"/>.
        /// </summary>
        /// <param name="sender">The sender of the event.</param>
        /// <param name="e">The instance containing the event data.</param>
        void OnItemsAdded(object sender, ItemsEventArgs<T> e)
        {
            Update();
        }

        /// <summary>
        /// An event that is invoked when an item is removed from
        /// <see cref="_enumerable"/>. This method invokes <see cref="Update()"/>.
        /// </summary>
        /// <param name="sender">The sender of the event.</param>
        /// <param name="e">The instance containing the event data.</param>
        void OnItemsRemoved(object sender, ItemsEventArgs<T> e)
        {
            Update();
        }

        /// <summary>
        /// Checks if the first item in the <see cref="_enumerable"/> is
        /// different.
        /// </summary>
        void Update()
        {
            Value = _enumerable.FirstOrDefault();
        }

        /// <summary>
        /// The <see cref="IEventEnumerable{T}"/> that is to be watched for
        /// changes to its first item.
        /// </summary>
        readonly IEventEnumerable<T> _enumerable;
    }

    /// <summary>
    /// Utility methods for <see cref="First{T}"/> or any code that uses
    /// <see cref="First{T}"/>.
    /// </summary>
    public static class FirstExtensions
    {
        /// <summary>
        /// Returns an instance that holds or will hold the first item of an 
        /// <see cref="IEventEnumerable{T}"/>.
        /// </summary>
        /// <typeparam name="T">The type being held within this instance.</typeparam>
        /// <param name="enumerable">The <see cref="IEventEnumerable{T}" /> that
        /// is to be watched for changes to its first item.</param>
        /// <returns>The instance that contains or will contain the first item.</returns>
        public static IValue<T> First<T>(this IEventEnumerable<T> enumerable)
        {
            return new First<T>(enumerable);
        }
    }
}