Luhn Algorithm.

luhn.py

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+# 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()