Luhn algorithm

luhn.py

def luhn(cc, debug=False):
    double = True
    sum = 0
    doubled = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
    if debug:
        print('{:15}{:15}{:15}{}'.format('digit', 'sum digit', 'sum', 'double'))
    for n in (int(c) for c in reversed(cc.replace(' ', ''))):
        value = doubled[n] if double else n
        if debug:
            print('{:<15}{:<15}{:<15}{}'.format(n, value, sum, double))
        sum += doubled[n] if double else n
        double = not double

    return (9 * sum) % 10

def luhn2(cc):
    doubled = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
    nums = (int(c) for c in reversed(cc.replace(' ', '')))
    total = sum((doubled[n], n)[i % 2] for i, n in enumerate(nums))
    return (9 * total) % 10

def luhn3(cc):
    doubled = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
    nums = [int(c) for c in cc.replace(' ', '')]
    total = sum((doubled[n] for n in nums[-1::-2]), sum(nums[-2::-2]))
    return (9 * total) % 10
  
def luhn4(cc):
    sums = ((0, 2, 4, 6, 8, 1, 3, 5, 7, 9),
            (0, 1, 2, 3, 4, 5, 6, 7, 8, 9))
    nums = (int(c) for c in reversed(cc.replace(' ', '')))
    total = sum(sums[i % 2][n] for i, n in enumerate(nums))
    return (9 * total) % 10

luhn('4038 1287 8520 ')
luhn('4038 1287 8520 ', debug=True)
luhn2('4038 1287 8520 ')