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

namespace Chernobyl.Values
{
    /// <summary>
    /// An <see cref="IValue{T}" /> that allows you to select a value (the result)
    /// from another <see cref="IValue{T}"/> (the source) and keep it up to date
    /// even if the source value changes.
    /// </summary>
    /// <typeparam name="TSource">The type held within the <see cref="IValue{T}" />
    /// being used as the source.</typeparam>
    /// <typeparam name="TResult">The type held within this <see cref="IValue{T}" />
    /// or the result.</typeparam>
    public class Select<TSource, TResult> : HeldValue<TResult>
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="Select{TSource, TResult}" />
        /// class.
        /// </summary>
        /// <param name="source">The <see cref="IValue{T}" /> to select the result
        /// value from.</param>
        /// <param name="selector">The method to use to select the result value
        /// from <paramref name="source"/>.</param>
        public Select(IValue<TSource> source, Func<TSource, TResult> selector)
            : this(source, selector, EqualityComparer<TResult>.Default)
        {}

        /// <summary>
        /// Initializes a new instance of the <see cref="Select{TSource, TResult}" />
        /// class.
        /// </summary>
        /// <param name="source">The <see cref="IValue{T}" /> to select the result
        /// value from.</param>
        /// <param name="selector">The method to use to select the result value
        /// from <see cref="_source" />.</param>
        /// <param name="comparer">The instance used to compare new and previous
        /// values of <see cref="IValue{T}" /> to determine if 
        /// <see cref="IValue{T}.Provide"/> should be raised.</param>
        public Select(IValue<TSource> source, Func<TSource, TResult> selector, 
                      IEqualityComparer<TResult> comparer)
            : base(default(TResult), comparer)
        {
            source.ThrowIfNull("source");
            selector.ThrowIfNull("selector");

            _source = source;
            _selector = selector;

            _source.Provide += OnSourceProvide;
        }

        /// <summary>
        /// An event handler that is invoked when the value contained within
        /// <see cref="_source"/> changes. This method ensures the result value
        /// contained within <c>this</c> stays up-to-date.
        /// </summary>
        /// <param name="sender">The sender of the event.</param>
        /// <param name="e">The arguments containing information about the change.</param>
        void OnSourceProvide(object sender, ValueChangedEventArgs<TSource> e)
        {
            Value = _selector(e.NewValue);
        }

        /// <summary>
        /// The <see cref="IValue{T}"/> to select the result value from.
        /// </summary>
        readonly IValue<TSource> _source;

        /// <summary>
        /// The method to use to select the result value from <see cref="_source"/>.
        /// </summary>
        readonly Func<TSource, TResult> _selector;
    }

    /// <summary>
    /// Utility methods and extensions for <see cref="Select{TSource, TResult}"/>.
    /// </summary>
    public static class SelectExtensions
    {
        /// <summary>
        /// Creates an <see cref="IValue{T}"/> capable of selecting a result 
        /// value from another <see cref="IValue{T}"/> and keeping the result
        /// up to date.
        /// </summary>
        /// <typeparam name="TSource">The type held within the <see cref="IValue{T}" />
        /// being used as the source.</typeparam>
        /// <typeparam name="TResult">The type held within the returned 
        /// <see cref="IValue{T}" /> or the result.</typeparam>
        /// <param name="source">The <see cref="IValue{T}" /> to select the result
        /// value from.</param>
        /// <param name="selector">The method to use to select the result value
        /// from <paramref name="source"/>.</param>
        /// <returns>The instance that contains the selected result and keeps
        /// that result up to date.</returns>
        public static IValue<TResult> Select<TSource, TResult>(this IValue<TSource> source,
                                                               Func<TSource, TResult> selector)
        {
            return new Select<TSource, TResult>(source, selector);
        }
    }
}