Verilen bir kredi karti numarasi'nin gecerli olup olmadiginin kontrolü

{-------------------------------------------------
Credit card validator

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

-------------------------------------------------}

function Vc(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
    Vc := ord(Cstr[1])-ord('2')
else
    Vc := 0
end;