Implementation of Luhn Algorithm

Psuedo Code

GET user's credit card number

SET total to 0

LOOP backwards from the last digit to the first one at a time

# IF the position of the current digit is even THEN
    # DOUBLE the value of the current digit
    # IF the doubled value is more than 9 THEN
        # SUM the digits of the doubled value
    # ENDIF
   # SUM the calculated value and the total
# ELSE
    # SUM the current digit and the total
# ENDIF

END loop

IF total % 10 == 0 THEN

# SHOW Number is valid

ELSE

# SHOW number is invalid

ENDIF