Question

I have to check if I have duplicate paths in a FileListBox (FileListBox has the role of some kind of job list or play list).
Using Delphi’s SameText, CompareStr, CompareText, takes 6 seconds. So I came with my own compare function which is (just) a bit faster but not fast enough. Any ideas how to improve it?

function SameFile(CONST Path1, Path2: string): Boolean;
VAR i: Integer;
begin
 Result:= Length(Path1)= Length(Path2);                                         { if they have different lenghts then obviously are not the same file }
 if Result then
  for i:= Length(Path1) downto 1 DO                                             { start from the end because it is more likely to find the difference there }
   if Path1[i]<> Path2[i] then
    begin
     Result:= FALSE;
     Break;
    end;
end;

I use it like this:

 for x:= JList.Count-1 downto 1 DO
  begin
   sMaster:= JList.Items[x];
   for y:= x-1 downto 0 DO
    if SameFile(sMaster, JList.Items[y]) then
     begin
      JList.Items.Delete (x); { REMOVE DUPLICATES }
      Break;
     end;
  end;

Note: The chance of having duplicates is small so Delete is not called often. Also the list cannot be sorted because the items are added by user and sometimes the order may be important.

Update:
The thing is that I lose the asvantage of my code because it is Pascal.
It would be nice if the comparison loop ( Path1[i]<> Path2[i] ) would be optimized to use Borland’s ASM code.

---

Delphi 7, Win XP 32 bit, Tests were done with 577 items in the list. Deleting the items from list IS NOT A PROBLEM because it happens rarely.

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CONCLUSION

As Svein Bringsli pointed, my code is slow not because of the comparing algorithm but because of TListBox. The BEST solution was provided by Marcelo Cantos. Thanks a lot Marcelo.
I accepted Svein’s answer because it answers directly my question “how to make my comparison function faster” with “there is no point to make it faster”.
For the moment I implemented the dirty and quick to implement solution: when I have under 200 files, I use my slow code to check the duplicates. If there are more than 200 files I use dwrbudr’s solution (which is damn fast) considering that if the user has so many files, the order is irrelevant anyway (human brain cannot track so many items).

I want to thank you all for ideas and especially Svein for revealing the truth: (Borland’s) visual controls are damn slow!

Answer 1

Using your code as a starting point, I modified it to take a copy of the list before searching for duplicates. The time went from 5,5 seconds to about 0,5 seconds.

vSL := TStringList.Create;
try
  vSL.Assign(jList.Items);
  vSL.Sorted := true;
  for x:= vSL.Count-1 downto 1 DO
  begin
   sMaster:= vSL[x];
   for y:= x-1 downto 0 DO
    if SameFile(sMaster, vSL[y]) then
     begin
      vSL.Delete (x); { REMOVE DUPLICATES }
      jList.Items.Delete (x);
      Break;
     end;
  end;
finally
  vSL.Free;
end;

Obviously, this is not a good way to do it, but it demonstrates that TFileListBox is in itself quite slow. I don’t believe you can gain much by optimizing your compare-function.

To demonstrate this, I replaced your SameFile function with the following, but kept the rest of your code:

function SameFile(CONST Path1, Path2: string): Boolean;
VAR i: Integer;
begin
  Result := false; //Pretty darn fast code!!!
end;

The time went from 5,6 seconds to 5,5 seconds. I don’t think there’s much more to gain there 🙂