"""Main file."""


def find_next_valid_upc(upc: int, skip: int = 1) -> int:
    """Find next valid UPC based on an existing UPC value.

    Args:
        upc (int): A current UPC value to help generate the next valid UPC.
        skip_power (int): How far to advance the next found UPC.

    Returns:
        int: A valid UPC.
    """

    if skip < 1:
        raise ValueError(f'{skip} must be >= 1')

    upc_found = False

    while not upc_found:
        upc = upc+(10 * skip)
        checksum = calculate_upc_checksum(upc)
        new_upc = str(upc)[0:-1] + str(checksum)
        upc_found = len(new_upc) == 12

    return int(new_upc)


def validate_upc(upc: int) -> bool:
    """Validates a upc.

    Args:
        upc (int): A UPC to validate.

    Returns:
        bool: Whether or not the UPC is valid.
    """
    # UCP must be 12 characters long
    if len(str(upc)) != 12:
        raise ValueError('{upc} does not contain 12 characters.')

    # Checksum is last digit
    checksum = str(upc)[-1]

    if calculate_upc_checksum(upc) == int(checksum):
        return True

    return False


def calculate_upc_checksum(upc: int) -> bool:
    """Calculates the checksum on a upc.

    Args:
        upc (int): A UPC to check.

    Returns:
        int: The checksum of the upc.
    """
    if len(str(upc)) == 12:
        upc = str(upc)[0:-1]

    if len(str(upc)) != 11:
        raise ValueError(f'UPC value of length {len(str(upc))} is invalid.')

    even_items = []
    odd_items = []

    # Loop through UPC without checksum and store values
    for pos, num in enumerate(str(upc)):
        if (pos+1) % 2 == 0:
            even_items.append(int(num))
        else:
            odd_items.append(int(num))

    # Create the check for 6 odd values
    odd_check = sum(odd_items)*3

    # Create the check for 5 even values
    even_check = sum(even_items)

    # Sum checks round up to nearest ten
    sum_of_checks = even_check + odd_check

    diff_from_ten = 10 - (sum_of_checks % 10)

    return diff_from_ten


def report_upc_valid(upc: int) -> bool:
    """Report if a UPC is valid.

    Args:
        upc (int): A UPC to check.
    """
    is_valid = validate_upc(upc)

    report_valid = 'is' if is_valid else 'is not'

    print(f'UPC {upc} {report_valid} valid.')

    return is_valid


def main():
    """Main method."""
    upc = 193483736817

    is_valid = report_upc_valid(upc)

    if not is_valid:
        print('Finding valid UPC...')
        new_upc = find_next_valid_upc(upc)

        print(f'Next valid UPC: \'{new_upc}\'')

        report_upc_valid(new_upc)


if __name__ == '__main__':
    main()