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

Asked 2009-10-10T14:29:35.113
12

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.

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.

delphi delphi-2010
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1 Answer

1

Maybe adding Uppercasing or lowercasing aSubstr and aString parameters before doing the search can make Deltics purpose case insensitive. I think he left you to do this before calling RPos. but maybe an optional parameter can do the job.

this is how Deltic's purpose should look:

function RPos(const aSubStr, aString : String; const aStartPos: Integer;
              const aCaseInSensitive:boolean=true): Integer; overload;
var
  i, _startPos: Integer;
  pStr: PChar;
  pSub: PChar;
  _subStr, _string: string;
begin

 if aCaseInSensitive then
 begin
  _subStr := lowercase( aSubstr );
  _string := lowercase( aString );
 end
 else 
 begin
  _subStr := aSubstr:
  _string := aString;
 end;

 pSub := Pointer(_subStr);

 if aStartPos = -1 then
    _startPos :=  Length(_string) - Length(_subStr) + 1
 else
    _startPos := aStartPos;

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

 result := 0;
end;


function RPos(const aSubStr, aString : String; 
              const aCaseInSensitive:boolean=true): Integer; overload;
begin
  result := RPos(aSubStr, aString, Length(aString) - Length(aSubStr) + 1,
                 aCaseInSensitive);
end;
answered 2011-08-04T01:49:49.033

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