02-implementing-luhn-algorithm.py

"""
Implementing the Luhn Algorithm

The Luhn algorithm is as follows:
- From the right to left, double the value of every second digit; if the product is greater than 9, 
sum the digits of the products.
- Take the sum of all the digits.
- If the sum of all the digits is a multiple of 10, then the number is valid; else it is not valid.
- Assume an example of an account number "7992739871" that will have a check digit added, 
making it of the form 7992739871x:

Example Code
Account number      7   9  9  2  7  3  9   8  7  1  x
Double every other  7  18  9  4  7  6  9  16  7  2  x
Sum 2-char digits   7   9  9  4  7  6  9   7  7  2  x
"""



card_number = '4111-1111-4555-1142'

def verify_card_number(card_number):
    sum_of_odd_digits = 0
    card_number_reversed = card_number[::-1]
    odd_digits = card_number_reversed[::2]

    for digit in odd_digits:
        sum_of_odd_digits += int(digit)

    sum_of_even_digits = 0
    even_digits = card_number_reversed[1::2]

    for digit in even_digits:
        number = int(digit) * 2
        if number >= 10:
            number = (number // 10) + (number % 10)
        sum_of_even_digits += number

    total = sum_of_odd_digits + sum_of_even_digits
    print(total)
    return total % 10 == 0


def main():
    card_translation = str.maketrans({'-': '', ' ': ''})
    translated_card_number = card_number.translate(card_translation)

    if verify_card_number(translated_card_number):
        print('VALID!')
    else:
        print('INVALID!')



if __name__ == '__main__':
    main()