using System;
using System.Collections.Generic;

namespace BusinessLogic.Classes
{
    public class KeyBasedEqualityComparer<T, TKey> : IEqualityComparer<T>
    {
        private readonly Func<T, TKey> _keyGetter;

        public KeyBasedEqualityComparer(Func<T, TKey> keyGetter)
        {
            if (default(T) == null)
            {
                _keyGetter = (x) => x == null ? default : keyGetter(x);
            }
            else
            {
                _keyGetter = keyGetter;
            }
        }

        public bool Equals(T x, T y)
        {
            return EqualityComparer<TKey>.Default.Equals(_keyGetter(x), _keyGetter(y));
        }

        public int GetHashCode(T obj)
        {
            TKey key = _keyGetter(obj);

            return key == null ? 0 : key.GetHashCode();
        }
    }

    public static class KeyBasedEqualityComparer<T>
    {
        public static KeyBasedEqualityComparer<T, TKey> Create<TKey>(Func<T, TKey> keyGetter)
        {
            return new KeyBasedEqualityComparer<T, TKey>(keyGetter);
        }
    }
}