luhn.py

# USE THIS SCRIPT TO VALIDATE THE NUMBER OF A CREDIT CARD USING THE LUHN ALGORITHM

# The Luhn algorithm is as follows:

# 1. 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.

# 2. Take the sum of all the digits.

# 3. If the sum of all the digits is a multiple of 10,
# then the number is valid; else it is not valid.


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
    
    return total % 10 == 0

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

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

main()