Fix Sort-Object -Stable -Unique to actually do stable sorting (PowerShell#17189)

Author: m1k0net

src/Microsoft.PowerShell.Commands.Utility/commands/utility/Sort-Object.cs

@@ -147,7 +147,7 @@ private int Heapify(List<OrderByPropertyEntry> dataToSort, OrderByPropertyCompar
 
             // Tracking the index is necessary so that unsortable items can be output at the end, in the order
             // in which they were received.
-            for (int dataIndex = 0, discardedDuplicates = 0; dataIndex < dataToSort.Count - discardedDuplicates; dataIndex++)
+            for (int dataIndex = 0, discardedDuplicates = 0; dataIndex + discardedDuplicates < dataToSort.Count; dataIndex++)
             {
                 // Min-heap: if the heap is full and the root item is larger than the entry, discard the entry
                 // Max-heap: if the heap is full and the root item is smaller than the entry, discard the entry
@@ -157,20 +157,19 @@ private int Heapify(List<OrderByPropertyEntry> dataToSort, OrderByPropertyCompar
                 }
 
                 // If we're doing a unique sort and the entry is not unique, discard the duplicate entry
-                if (Unique && !uniqueSet.Add(dataToSort[dataIndex]))
+                if (Unique && !uniqueSet.Add(dataToSort[dataIndex + discardedDuplicates]))
                 {
                     discardedDuplicates++;
-                    if (dataIndex != dataToSort.Count - discardedDuplicates)
-                    {
-                        // When discarding duplicates, replace them with an item at the end of the list and
-                        // adjust our counter so that we check the item we just swapped in next
-                        dataToSort[dataIndex] = dataToSort[dataToSort.Count - discardedDuplicates];
-                        dataIndex--;
-                    }
-
+                    dataIndex--;
                     continue;
                 }
 
+                // Shift next non-duplicate entry into place
+                if (discardedDuplicates > 0)
+                {
+                    dataToSort[dataIndex] = dataToSort[dataIndex + discardedDuplicates];
+                }
+
                 // Add the current item to the heap and bubble it up into the correct position
                 int childIndex = dataIndex;
                 while (childIndex > 0)

test/powershell/Modules/Microsoft.PowerShell.Utility/Sort-Object.Tests.ps1

@@ -296,6 +296,18 @@ Describe 'Sort-Object Stable Unit Tests' -Tags 'CI' {
 			$results[1]  | Should -Be 1
 			$results[2]  | Should -Be 3
 		}
+		
+		It "Sort-Object with Unique Stable should return proper records"{
+			$Record1=[PSCustomObject]@{Key="A";Record="A1"}
+			$Record2=[PSCustomObject]@{Key="A";Record="A2"}
+			$Record3=[PSCustomObject]@{Key="B";Record="B1"}
+			$Record4=[PSCustomObject]@{Key="B";Record="B2"}
+			$Records = @($Record1,$Record2,$Record3,$Record4)
+			$results = $Records | Sort-Object -Stable -Unique -Property Key
+	
+			$results[0] | Should -Be $Records[0]
+			$results[1] | Should -Be $Records[2]
+		}
 	}
 }