Author: Giannis Sampaziotis

A routine to validate and identify Visa/MasterCard/Amex credit cards. Might come 
handy if you want to perform arbitrary card checking. Original code by Daniel J. 
Karnes (1990)

Answer:
1   
2   {-------------------------------------------------
3     Credit card validator
4   
5     Returns:
6   
7      0  : Card is invalid or unknown
8      1  : Card is a valid AmEx
9      2  : Card is a valid Visa
10     3  : Card is a valid MasterCard
11  
12  -------------------------------------------------}
13  
14  function Vc(c: string): integer;
15  var
16    card: string[21];
17    Vcard: array[0..21] of byte absolute card;
18    Xcard: integer;
19    Cstr: string[21];
20    y, x: integer;
21  begin
22    Cstr := '';
23    fillchar(Vcard, 22, #0);
24    card := c;
25    for x := 1 to 20 do
26      if (Vcard[x] in [48..57]) then
27        Cstr := Cstr + chr(Vcard[x]);
28    card := '';
29    card := Cstr;
30    Xcard := 0;
31    if not odd(length(card)) then
32      for x := (length(card) - 1) downto 1 do
33      begin
34        if odd(x) then
35          y := ((Vcard[x] - 48) * 2)
36        else
37          y := (Vcard[x] - 48);
38        if (y >= 10) then
39          y := ((y - 10) + 1);
40        Xcard := (Xcard + y)
41      end
42    else
43      for x := (length(card) - 1) downto 1 do
44      begin
45        if odd(x) then
46          y := (Vcard[x] - 48)
47        else
48          y := ((Vcard[x] - 48) * 2);
49        if (y >= 10) then
50          y := ((y - 10) + 1);
51        Xcard := (Xcard + y)
52      end;
53    x := (10 - (Xcard mod 10));
54    if (x = 10) then
55      x := 0;
56    if (x = (Vcard[length(card)] - 48)) then
57      Vc := ord(Cstr[1]) - ord('2')
58    else
59      Vc := 0
60  end;