# Calculate and verify checksums of hexadecimal values

import sys

def checksum(inputMessage, operation):
    """Take in a hex value, and an operation to perform. If the
    operation is 'c', create a checksum and return that inputMessage with
    the checksum appended. If the operation is 'v', validate the checksum.
    Return 0 if it's invalid, and 1 if it's valid.
    """
    sum = 0
    digits = [int(i,16) for i in inputMessage]

    for i in range(0, len(digits)):
        sum = (sum + digits[i]) % 16

    if operation == 'c':
        sum = (sum ^ 15) + 1
        return inputMessage + str(sum)

    if operation == 'v':
        if sum == 0:
            return 1
        else:
            return 0


def main():
    """Pass the hex value to the program as a commandline argument.
    Example: python3 checksum.py BF1942
    """

    hexValue = sys.argv[1]
    messageWithChecksum = checksum(hexValue, 'c')

    print("Generate checksum")
    print("Message with checksum appended: " + messageWithChecksum)
    print("Validate checksum: " +
          "Valid" if checksum(messageWithChecksum, 'v') else "Failed")


if __name__ == '__main__':
    main()