Как проверить правильность номера средитной карточки

{-------------------------------------------------
  Returns:

   0  : Card is invalid or unknown
   1  : Card is a valid AmEx
   2  : Card is a valid Visa
   3  : Card is a valid MasterCard

  Ergebnis:

   0  : Unbekannte Karte
   1  : AmEx Karte
   2  : Visa Karte
   3  : MasterCard Karte
  -------------------------------------------------}

function CheckCC(c: string): Integer;
var
  card: string[21];
  Vcard: array[0..21] of Byte absolute card;
  Xcard: Integer;
  Cstr: string[21];
  y, x: Integer;
begin
  Cstr := '';
  FillChar(Vcard, 22, #0);
  card := c;
  for x := 1 to 20 do
    if (Vcard[x] in [48..57]) then
      Cstr := Cstr + chr(Vcard[x]);
  card := '';
  card := Cstr;
  Xcard := 0;
  if not odd(Length(card)) then
    for x := (Length(card) - 1) downto 1 do
    begin
      if odd(x) then
        y := ((Vcard[x] - 48) * 2)
      else
        y := (Vcard[x] - 48);
      if (y >= 10) then
        y := ((y - 10) + 1);
      Xcard := (Xcard + y)
    end
  else
    for x := (Length(card) - 1) downto 1 do
    begin
      if odd(x) then
        y := (Vcard[x] - 48)
      else
        y := ((Vcard[x] - 48) * 2);
      if (y >= 10) then
        y := ((y - 10) + 1);
      Xcard := (Xcard + y)
    end;
  x := (10 - (Xcard mod 10));
  if (x = 10) then
    x := 0;
  if (x = (Vcard[Length(card)] - 48)) then
    Result := Ord(Cstr[1]) - Ord('2')
  else
    Result := 0
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  case CheckCC(Edit1.Text) of
    0: Label1.Caption := 'Card is invalid or unknown';
    1: Label1.Caption := 'Card is a valid AmEx';
    2: Label1.Caption := 'Card is a valid Visa';
    3: Label1.Caption := 'Card is a valid MasterCard';
  end;
end;