Credit Card Validator

program CreditCardValidator; // COPY INTO LAZARUS TO RUN

uses sysutils;

var
  cardNum: string;
  total, digit, currentDigit: integer;

begin
  while true do
    begin
      writeln('Enter a 16-digit credit card number: ');
      readln(cardNum);
      total := 0;
      for digit := 1 to 15 do
        begin
          if digit mod 2 = 1 then
            begin
              currentDigit := strtoint(cardnum[digit]) * 2;
                while length(inttostr(currentDigit)) <> 1 do
                  currentDigit := strtoint(inttostr(currentDigit)[1]) + strtoint(inttostr(currentDigit)[2]);
              total += currentDigit;
            end
          else
            total += strtoint(cardNum[digit]);
        end;
      if strtoint(cardNum[16]) = 10 - (total mod 10) then
        writeln('Credit card valid!')
      else
        writeln('Invalid credit card!');
    end;
end.