luhn_tester.py

luhn_tester.py

#! /usr/bin/python3
import random

def luhn_residue(digits):
    return sum(sum(divmod(int(d)*(1 + i%2), 10))
                 for i, d in enumerate(digits[::-1])) % 10

def get_valid_luhn(N, preamble = ""):
    part = preamble+''.join(str(random.randrange(0,9)) for _ in range(N-1-len(preamble)))
    res = luhn_residue('{}{}'.format(part, 0))
    return '{}{}'.format(part, -res%10)

if __name__ == "__main__":
    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument("digits", type=int)
    parser.add_argument("preamble", default="", nargs="?")
    args = parser.parse_args()
    if len(args.preamble) == args.digits:
        input_luhn = args.preamble
        given_digit = input_luhn[-1]
        input_luhn = input_luhn[0:-1] + '0'
        check_digit = str(-luhn_residue(input_luhn)%10)
        assert check_digit == given_digit
        print('valid')
    else:
        output = get_valid_luhn(args.digits, args.preamble)
        assert len(output) == args.digits
        print(output)