ValidaCartao

function TFmPrinc.validaCartaoCredito(cn: String): ShortInt;
var
  i, j, Sum : Integer;
begin
  if (Length(cn) > 20) OR (Length(cn) < 15) then
    Result := -1
  else
    try 
      Sum := 0;

      for i := Length(cn) - 1 downto 1 do
      begin
        j := Length(cn) - i;

        if(j mod 2 = 1) then
          inc(sum, StrToInt(cn[i]))
        else
          begin
            J := StrToInt(cn[i]) * 2;

            if(J > 9) then
              inc(sum, (StrToInt(IntToStr(J)[1]) + StrToInt(IntToStr(J)[2])) * 2)
            else
              inc(sum, J);
          end;
      end;

      if((10 - (Sum mod 10)) = StrToInt(cn[Length(cn)])) then
        Result := 1
      else
        Result := -1;
    except
      Result := -1;
    end;
end;