import inspect
import re
from typing import Callable


class InvalidUsernameException(Exception):
    pass


class InvalidForwardingAddressException(Exception):
    pass


valid_username_re = re.compile(r'^[a-z][\w-]+$')

# Only allow usernames and email addresses to be set in ~/.forward
valid_forwarding_address_re = re.compile(r'^[\w-]+|[\w.+-]+@[\w-]+(\.[\w-]+)*$')


def is_valid_username(username: str) -> bool:
    """Returns True if the username has a valid format."""
    return valid_username_re.match(username) is not None


def check_valid_username(f: Callable) -> Callable:
    """
    A decorator which raises an Exception if the username passed
    to f is invalid.
    f must accept `username` as a regular argument.
    """
    argspec = inspect.getfullargspec(f)
    try:
        arg_idx = argspec.args.index('username')
    except ValueError:
        return f

    def wrapper(*args, **kwargs):
        username = None
        if arg_idx < len(args):
            username = args[arg_idx]
        elif 'username' in kwargs:
            username = kwargs['username']
        if username is not None and not is_valid_username(username):
            raise InvalidUsernameException(username)
        return f(*args, **kwargs)

    return wrapper


def is_valid_forwarding_address(address: str) -> bool:
    """Returns True if the address is a valid forwarding address."""
    return valid_forwarding_address_re.match(address) is not None


def is_valid_shell(shell: str) -> bool:
    """Returns True if this shell is in /etc/shells, otherwise False."""
    return shell in [
        line.strip() for line in open('/etc/shells')
        if line != '' and not line.isspace()
    ]


def is_valid_term(term: str) -> bool:
    return len(term) == 5 and \
        term[0] in ['s', 'f', 'w'] and \
        term[1:5].isdigit()