{$F+,D+,L+}

unit Vericard;

interface

  function Vc(c : string) : char;

implementation

  function Vc(c : string) : char;
  var
    card : string[21];
    Vcard : array[0..21] of byte absolute card;
    Xcard : integer;
    Cstr : string[21];
    y, x : integer;
  begin
    x := 0;
    Cstr := '                ';
    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 := Cstr[1]
    else
      Vc := #0
  end;

END.

{ .....................DRIVER EXAMple........  }

{$A-,B+,D-,E-,F-,I+,L-,N-,O-,R+,S+,V+}
{$M 2048,0,4096}

program ValiCard;

  { Test routine for the Mod 10 Check Digit CC validator... }

uses
  dos,
  crt,
  VeriCard;

var
  card : string[22];
  k : char;

  procedure Squawk(Noise : byte);
  begin
    case Noise of
      1 : begin
            Sound(400);
            Delay(200);
            Sound(200);
            Delay(200);
            Nosound
          end;
      2 : begin
            Sound(392);
            Delay(55);
            Nosound;
            Delay(30);
            Sound(523);
            Delay(55);
            Nosound;
            Delay(30);
            Sound(659);
            Delay(55);
            Nosound;
            Delay(30);
            Sound(784);
            Delay(277);
            Nosound;
            Delay(30);
            Sound(659);
            Delay(55);
            Nosound;
            Delay(30);
            Sound(784);
            Delay(1200);
            Nosound
          end
    end                                { case }
  end;

BEGIN
  k := #0;
  clrscr;
  fillchar(card, 22, #0);
  writeln('VC: Integer Modulo-10 Visa/Mastercard/Amex Check-Digit');
  writeln('    verification routine. (c) 1990 Daniel J. Karnes');
  writeln;
  write('    Please enter a Credit Card number: ');
  readln(card);
  writeln;
  writeln;
  if (length(card) > 12) then
    k := Vc(card);
  if (k in ['3', '4', '5']) then
    Squawk(2)
  else
    Squawk(1);
  case k of
    #0 : writeln('    Could NOT verify this number with any card type.')
    '3' : writeln('    Card was verified as a valid Amex Card Number.');
    '4' : writeln('    Card was verified as a valid VISA Card Number.');
    '5' : writeln('    Card was verified as a valid Mastercard Number.')
  end
END.

...................
Hope that helps. I've only tried it on one card number BUT it did work
for the one and the info was received from someone in the business.