What is an efficient way of deleting a large block of items from the start of a TList in Delphi

Question

Delete (0) from a TList is expensive because all the subsequent items need to be moved down. If I need to delete a large number of items from the start of an even larger list what's the fastest way?

Answer 1

Deleting a large range of elements from the beginning of a TList is expensive. Although the class name flatters to deceive, a TList is in fact an array. In TList there is no facility to delete a range–each item must be deleted individually and then the rest of the list is moved down. For a large range that's going to provoke an awful lot of reallocations and full list moves.

If you had a more modern Delphi you could use the generic list class TList<T> and avail yourself of the DeleteRange method. The documentation includes this vital note:

This is an O(ACount) operation.

In Delphi 2006 you can write something with equivalent performance characteristics like this:

procedure DeleteRange(List: TList; AIndex, ACount: Integer);
var
  i: Integer;
  NewCount: Integer;
begin
  NewCount := List.Count-ACount;
  Assert(AIndex>=0);
  Assert(ACount>=0);
  Assert(NewCount>=0);
  for i := AIndex to NewCount-1 do
    List[i] := List[i+ACount]
  List.Count := NewCount;
end;