Easy way to add stable sorting to TList and TStringList

Question

I use TList/TObjectList and TStringList (with associated objects) for a multitude of tasks, either as-is, or as basis for more complex structures. While the sort functionali

Answer 1

From similar question How Can I Replace StringList.Sort with a Stable Sort in Delphi?, linked here in comment by lkessler I need to copy to here really easy trick as mentioned in question.

You can easily make quick sort behave stable just by adding initial order numbers into data to sort and adding last comparation condition in CustomSort compare function to compare this initial order numbers.

Easy, fast and smart. Costs only one extra integer (or byte, or use some reserved storage like TComponent.Tag if You sort TComponents) on each sortable item and one initialization loop over them.

TObjectToSort = class
  ...
  Index: Integer;
end;

function MyStableSortComparer(List: TStringList; Index1, Index2: Integer): Integer;
var
  o1, o2: TObjectToSort; 
begin
  o1 := TObjectToSort(List.Objects[Index1]);
  o2 := TObjectToSort(List.Objects[Index2]);
  ...
  if Result = 0 then
    Result := o1.Index - o2.Index;
end;


for i := 0 to MyStrtingList.Count - 1 do
  TObjectToSort(MyStrtingList.Objects[i]).Index := i;
MyStrtingList.CustomSort(MyStableSortComparer);