From e3ca928636832814999d1e5daa89e7cd451d0e2d Mon Sep 17 00:00:00 2001
From: Markus Ewald <cygon@nuclex.org>
Date: Wed, 2 Nov 2022 22:27:51 +0000
Subject: [PATCH] Implemented a messy QuickSort method for IList, appears to be
 working but needs to be cleaned and optimized still

git-svn-id: file:///srv/devel/repo-conversion/nusu@338 d2e56fa2-650e-0410-a79f-9358c0239efd
---
 Source/Collections/IListExtensions.Test.cs | 15 ++++
 Source/Collections/IListExtensions.cs      | 84 ++++++++++++++++++++--
 2 files changed, 93 insertions(+), 6 deletions(-)

diff --git a/Source/Collections/IListExtensions.Test.cs b/Source/Collections/IListExtensions.Test.cs
index ae1b27e..215089b 100644
--- a/Source/Collections/IListExtensions.Test.cs
+++ b/Source/Collections/IListExtensions.Test.cs
@@ -60,6 +60,21 @@ namespace Nuclex.Support.Collections {
       );
     }
 
+    /// <summary>Tests whether the insertion sort algorithm can be applied to 'Text' property works as expected</summary>
+    [Test]
+    public void QuickSortCanSortWholeList() {
+      var testList = new List<int>(capacity: 5) { 1, 5, 2, 4, 3 };
+      var testListAsIList = (IList<int>)testList;
+      
+      testListAsIList.QuickSort(Comparer<int>.Default);
+
+      CollectionAssert.AreEqual(
+        new List<int>(capacity: 5) { 1, 2, 3, 4, 5 },
+        testList
+      );
+    }
+
+
   }
 
 } // namespace Nuclex.Support.Collections
diff --git a/Source/Collections/IListExtensions.cs b/Source/Collections/IListExtensions.cs
index c00e025..5891a90 100644
--- a/Source/Collections/IListExtensions.cs
+++ b/Source/Collections/IListExtensions.cs
@@ -38,13 +38,10 @@ namespace Nuclex.Support.Collections {
     public static void InsertionSort<TElement>(
       this IList<TElement> list, int startIndex, int count, IComparer<TElement> comparer
     ) {
-      int index = startIndex;
-      int endIndex = startIndex + count - 1;
+      int rightIndex = startIndex;
 
-      while(index < endIndex) {
-        int rightIndex = index;
-        
-        ++index;
+      int lastIndex = startIndex + count - 1;
+      for(int index = startIndex + 1; index <= lastIndex; ++index) {
         TElement temp = list[index];
 
         while(rightIndex >= startIndex) {
@@ -57,6 +54,8 @@ namespace Nuclex.Support.Collections {
         }
 
         list[rightIndex + 1] = temp;
+
+        rightIndex = index;
       }
     }
 
@@ -81,6 +80,79 @@ namespace Nuclex.Support.Collections {
       InsertionSort(list, Comparer<TElement>.Default);
     }
 
+    /// <summary>
+    ///   Sorts all the elements in an IList&lt;T&gt; using the quicksort algorithm
+    /// </summary>
+    /// <typeparam name="TElement">Type of elements the list contains</typeparam>
+    /// <param name="list">List in which a subset will be sorted</param>
+    /// <param name="comparer">Comparison function to use for comparing list elements</param>
+    public static void QuickSort<TElement>(
+      this IList<TElement> list, IComparer<TElement> comparer
+    ) {
+      int rightIndex = list.Count - 1;
+
+      var spans = new Stack<int>();
+      spans.Push(0);
+      spans.Push(rightIndex);
+
+      while(spans.Count > 0) {
+        int partitionRightEndIndex = spans.Pop();
+        int partitionLeftEndIndex = spans.Pop();
+
+        int leftIndex = partitionLeftEndIndex + 1;
+        int pivotIndex = partitionLeftEndIndex;
+        rightIndex = partitionRightEndIndex;
+
+        TElement pivot = list[pivotIndex];
+
+        if(leftIndex > rightIndex)
+          continue;
+
+        while(leftIndex < rightIndex) {
+
+          while(leftIndex <= rightIndex) {
+            if(comparer.Compare(list[leftIndex], pivot) > 0) {
+              break;
+            }
+            ++leftIndex;
+          }
+
+          while(leftIndex <= rightIndex) {
+            if(comparer.Compare(list[rightIndex], pivot) < 0) {
+              break;
+            }
+
+            --rightIndex;
+          }
+
+          if(rightIndex >= leftIndex) {
+            TElement temp = list[leftIndex];
+            list[leftIndex] = list[rightIndex];
+            list[rightIndex] = temp;
+          }
+
+        }
+
+        if(pivotIndex <= rightIndex) {
+          if(comparer.Compare(list[pivotIndex], list[rightIndex]) > 0) {
+            TElement temp = list[pivotIndex];
+            list[pivotIndex] = list[rightIndex];
+            list[rightIndex] = temp;
+          }
+        }
+
+        if(partitionLeftEndIndex < rightIndex) {
+          spans.Push(partitionLeftEndIndex);
+          spans.Push(rightIndex - 1);
+        }
+
+        if(partitionRightEndIndex > rightIndex) {
+          spans.Push(rightIndex + 1);
+          spans.Push(partitionRightEndIndex);
+        }
+      }
+    }
+
   }
 
 } // namespace Nuclex.Support.Collections