Convert Hexadecimal to Integer

FUNCTION HexToInt(S: STRING): LongInt;
CONST
   DecDigits: SET OF '0'..'9' = ['0'..'9'];
   HexVals: ARRAY[0..$F] OF Integer = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
      $A, $B, $C, $D, $E, $F);
   UpCaseHexLetters: SET OF 'A'..'F' = ['A'..'F'];
   LowCaseHexLetters: SET OF 'a'..'f' = ['a'..'f'];
   varv: LongInt;
   i: integer; LookUpIndex: integer;
BEGIN
   IF length(S) <= 8 THEN
   BEGIN
      v := 0;
      FOR i := 1 TO length(S) DO
      BEGIN
{$R-}
         v := v SHL 4;
{$R+}
         IF S[i] IN DecDigits THEN
         BEGIN
            LookUpIndex := Ord(S[i]) - Ord('0'); END
         ELSE
         BEGIN
            IF S[i] IN UpCaseHexLetters THEN
            BEGIN
               LookUpIndex := Ord(S[i]) - Ord('A') + $A;
            END
            ELSE
            BEGIN
               IF S[i] IN LowCaseHexLetters THEN
               BEGIN
                  LookUpIndex := Ord(S[i]) - Ord('a') + $A;
               END ELSE BEGIN LookUpIndex := 0;
               END;
            END;
         END;
         v := v OR HexVals[LookUpIndex];
      END;
      result := v;
   END
   ELSE
   BEGIN
      result := 0;
   END;
END;