Any build-in Delphi function like PosEx that finds a sub-string starting from the back of the string?

Question

Is there any Delphi D2010 function like PosEx that finds a sub-string inside a string starting from the end of the string?

I'm removing all the calls to the FastStrings library and one of the functions I was using was FastPosBack:

function FastPosBack(const aSourceString, aFindString : AnsiString; const aSourceLen, aFindLen, StartPos : Integer) : Integer;

I found LastDelimiter but it's not quite the same thing since it only finds the last delimiter and I can't specify a start position.

Thanks!

Update: Following DR comment, I've created this function:

function FastPosBack(const aSourceString, aFindString : String; const aSourceLen, aFindLen, StartPos : Integer) : Integer;
var
  RevSourceString, RevFindString: string;
begin
  RevSourceString := AnsiReverseString(aSourceString);
  RevFindString := AnsiReverseString(aFindString);

  Result := Length(aSourceString) - PosEx(RevFindString, RevSourceString, StartPos) + 1;
end;

Is there any more effective way of doing this? On a 1000000 loop cycle, Pos takes 47ms while FastPosBack takes 234ms to complete.

Answer 1

Try this/these:

function RPos(const aSubStr, aString : String; const aStartPos: Integer): Integer; overload;
var
  i: Integer;
  pStr: PChar;
  pSub: PChar;
begin
  pSub := Pointer(aSubStr);

  for i := aStartPos downto 1 do
  begin
    pStr := @(aString[i]);
    if (pStr^ = pSub^) then
    begin
      if CompareMem(pSub, pStr, Length(aSubStr)) then
      begin
        result := i;
        EXIT;
      end;
    end;
  end;

  result := 0;
end;


function RPos(const aSubStr, aString : String): Integer; overload;
begin
  result := RPos(aSubStr, aString, Length(aString) - Length(aSubStr) + 1);
end;

The overload provides a way to call RPos using the most efficient startpos for searching from the very end of the string without having to calculate that yourself. For efficiency no checking is performed on startpos when explicitly specified.

In my SmokeTest performance testing suite this comes out about 20% faster than your FastPosBack (which incidentally contains an "off by one" error as well as requiring some parameters which it doesn't actually use).

Answer 2

You can use Pos in combination with ReverseString (from StrUtils)