#  This is a self-loading procedure that can tell whether a MasterCard
#  or Visa credit card number is plausible by constructing a special
#  checksum of the digits.   The checksum used for credit cards is
#  designed to catch transposed or incorrect digits.  For fun, try it
#  on the cards in your wallet!

#  Copyright (c) 1990-2012 by Hamilton Laboratories.  All rights reserved.

proc mcvisa(card)
   local i, j, sum
   @ sum = 0
   # reverse the digits and discard any spaces.
   @ card = reverse(card)
   if (card =~ "* *") @ card = $card:gs/ //
   if (card =~ "*[^0-9]*") return "bad"
   for i = 1 to strlen(card) do
      @ j = substr(card, i, 1)
      if (i % 2 == 0) then
         @ j *= 2
         if (j >= 10) @ j -= 9;
      end
      @ sum += j
   end
   return sum % 10 == 0 ? "good" : "bad"
end

mcvisa $argv