Question

I was trying to speed up a certain routine in an application, and my profiler, AQTime, identified one method in particular as a bottleneck. The method has been with us for years, and is part of a “misc”-unit:

function cwLeftPad(aString:string; aCharCount:integer; aChar:char): string;
var
  i,vLength:integer;
begin
  Result := aString;
  vLength := Length(aString);
  for I := (vLength + 1) to aCharCount do    
    Result := aChar + Result;
end;

In the part of the program that I’m optimizing at the moment the method was called ~35k times, and it took a stunning 56% of the execution time!

It’s easy to see that it’s a horrible way to left-pad a string, so I replaced it with

function cwLeftPad(const aString:string; aCharCount:integer; aChar:char): string; 
begin
  Result := StringOfChar(aChar, aCharCount-length(aString))+aString;
end;

which gave a significant boost. Total running time went from 10,2 sec to 5,4 sec. Awesome! But, cwLeftPad still accounts for about 13% of the total running time. Is there an easy way to optimize this method further?

Answer 1

Your new function involves three strings, the input, the result from StringOfChar, and the function result. One of them gets destroyed when your function returns. You could do it in two, with nothing getting destroyed or re-allocated.

1. Allocate a string of the total required length.
2. Fill the first portion of it with your padding character.
3. Fill the rest of it with the input string.

Here’s an example:

function cwLeftPad(const aString: AnsiString; aCharCount: Integer; aChar: AnsiChar): AnsiString;
var
  PadCount: Integer;
begin
  PadCount := ACharCount - Length(AString);
  if PadCount > 0 then begin
    SetLength(Result, ACharCount);
    FillChar(Result[1], PadCount, AChar);
    Move(AString[1], Result[PadCount + 1], Length(AString));
  end else
    Result := AString;
end;

I don’t know whether Delphi 2009 and later provide a double-byte Char-based equivalent of FillChar, and if they do, I don’t know what it’s called, so I have changed the signature of the function to explicitly use AnsiString. If you need WideString or UnicodeString, you’ll have to find the FillChar replacement that handles two-byte characters. (FillChar has a confusing name as of Delphi 2009 since it doesn’t handle full-sized Char values.)

Another thing to consider is whether you really need to call that function so often in the first place. The fastest code is the code that never runs.