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

namespace System.Linq
{
	class OrderByEnumerable<TElement, TKey> : IOrderedEnumerable<TElement>
	{
		public readonly IEnumerable<TElement> Source;
		public readonly Func<TElement, TKey> KeySelector;
		public readonly IComparer<TKey> Comparer;
		public readonly bool Descending;

		public OrderByEnumerable (IEnumerable<TElement> source, Func<TElement, TKey> keySelector, IComparer<TKey> comparer, bool descending)
		{
			if (source == null) throw new ArgumentNullException ("source");
			if (keySelector == null) throw new ArgumentNullException ("keySelector");

			Source = source;
			KeySelector = keySelector;
			Comparer = comparer ?? Comparer<TKey>.Default;
			Descending = descending;
		}

		public IOrderedEnumerable<TElement> CreateOrderedEnumerable<TNewKey> (
			Func<TElement, TNewKey> keySelector,
			IComparer<TNewKey> comparer,
			bool descending)
		{
			return new ThenByEnumerable<TElement, TNewKey, TKey> (this, keySelector, comparer, descending);
		}

		internal virtual int CompareElements (TElement e1, TElement e2)   // ThenByEnumerable will override this method.
		{
			int result = Comparer.Compare (KeySelector (e1), KeySelector (e2));
			return Descending ? -result : result;
		}

		internal virtual IEnumerable<TElement> GetElementsToSort ()   // ThenByEnumerable will override this method.
		{
			return Source;
		}		

		public IEnumerator<TElement> GetEnumerator ()
		{
			TElement [] array = GetElementsToSort ().ToArray();
			Array.Sort (array, CompareElements);
			foreach (TElement element in array)
				yield return element;
		}

		System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator ()
		{
			return GetEnumerator();
		}
	}

}