Poor Man's LINQ in Visual Studio 2005

// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
//
// Authors:
//        Alejandro Serrano "Serras" (trupill@yahoo.es)
//

using System;
using System.Collections.Generic;

namespace Compatibility.Linq
{
        internal class InternalOrderedSequence<T, K> : OrderedSequence<T>
        {
                IEnumerable<T> source;
                Func<T, K> key_selector;
                IComparer<K> comparer;
                bool descending;
                OrderedSequence<T> previous;
                
                internal InternalOrderedSequence (IEnumerable<T> source, Func<T, K> keySelector,
                                                  IComparer<K> comparer, bool descending, OrderedSequence<T> previous)
                {
                        this.source = source;
                        this.key_selector = keySelector;
                        this.comparer = comparer;
                        this.descending = descending;
                        this.previous = previous;
                }
                
                protected override IEnumerator<T> EnumeratorImplementation ()
                {
                        return Sort (source).GetEnumerator ();
                }
                
                protected internal override IEnumerable<T> Sort (IEnumerable<T> previousList)
                {
                        if (previous == null)
                                return PerformSort (previousList);
                        else
                                return previous.Sort (PerformSort (previousList));
                }
                
                List<T> source_list;
                K[] keys;
                int[] indexes;
                
                private IEnumerable<T> PerformSort (IEnumerable<T> items)
                {
                        // It first enumerates source, collecting all elements
                        source_list = new List<T> (items);
                        
                        // If the source contains just zero or one element, there's no need to sort
                        if (source_list.Count <= 1)
                                return source_list;
                        
                        // Then evaluate the keySelector function for each element,
                        // collecting the key values
                        keys = new K [source_list.Count];
                        for (int i = 0; i < source_list.Count; i++)
                                keys[i] = key_selector(source_list [i]);
                        
                        // Then sorts the elements according to the collected
                        // key values and the selected ordering
                        indexes = new int [keys.Length];
                        for (int i = 0; i < indexes.Length; i++)
                                indexes [i] = i;
                        
                        QuickSort(indexes, 0, indexes.Length - 1);
                        
                        // Return the values as IEnumerable<T>
                        T[] orderedList = new T [indexes.Length];
                        for (int i = 0; i < indexes.Length; i++)
                                orderedList [i] = source_list [indexes [i]];
                        return orderedList;
                }
                
                private int CompareItems (int firstIndex, int secondIndex)
                {
                        int comparison;
                        
                        if (comparer == null)
                                comparison = ((IComparable<K>)keys [firstIndex]).CompareTo (keys [secondIndex]);
                        else
                                comparison =  comparer.Compare (keys [firstIndex], keys [secondIndex]);
                       
                        // If descending, return the opposite comparison
                        return (descending ? -comparison : comparison);
                }
                
                /** QuickSort implementation
                    Based on implementation found in Wikipedia 
                    http://en.wikipedia.org/wiki/Quicksort_implementations
                    that was released under the GNU Free Documentation License **/
               
                private void QuickSort (int[] array, int left, int right)
                {
                        int lhold = left;
                        int rhold = right;
                        Random random = new Random ();
                        int pivot = random.Next (left, right);
                        Swap (array, pivot, left);
                        pivot = left;
                        left++;
                        
                        while (right >= left) {
                                int leftComparison = CompareItems (indexes [left], indexes [pivot]);
                                int rightComparison = CompareItems (indexes [right], indexes [pivot]);
                                if (leftComparison >= 0 && rightComparison < 0)
                                        Swap (array, left, right);
                                else if (leftComparison >= 0)
                                        right--;
                                else if (rightComparison < 0)
                                        left++;
                                else {
                                        right--;
                                        left++;
                                }
                        }
                        
                        Swap (array, pivot, right);
                        pivot = right;
                        if (pivot > lhold)
                                QuickSort (array, lhold, pivot);
                        if (rhold > pivot + 1)
                                QuickSort (array, pivot + 1, rhold);
                }
                
                private void Swap (int[] items, int left, int right)
                {
                        int temp = items [right];
                        items [right] = items [left];
                        items [left] = temp;
                }
                
        }
}