added method calculate_luhn_checksum

smash/web/algorithm/luhn_algorithm.py

+def digits_of(n):
+    return [int(d) for d in str(n)]
+
+
 class LuhnAlgorithm(object):
+
     def __init__(self):
         pass
 
     @staticmethod
     def luhn_checksum(card_number):
-        def digits_of(n):
-            return [int(d) for d in str(n)]
-
         digits = digits_of(card_number)
         odd_digits = digits[-1::-2]
         even_digits = digits[-2::-2]
@@ -19,3 +21,7 @@ class LuhnAlgorithm(object):
     @staticmethod
     def is_luhn_valid(card_number):
         return LuhnAlgorithm.luhn_checksum(card_number) == 0
+
+    @staticmethod
+    def calculate_luhn_checksum(card_number):
+        return (10 - LuhnAlgorithm.luhn_checksum(card_number + '0')) % 10