import hashlib


class User:

    def __init__(self, username, password):
        '''Create a new user object. The password will be encrypted before
        storing'''
        self.username = username
        self.password = self._encrypt_pw(password)
        self.is_logged_in = False

    def _encrypt_pw(self, password):
        '''Encrypt the password with the username and return the sha digest.'''
        hash_string = self.username + password
        hash_string = hash_string.encode("utf8")
        return hashlib.sha256(hash_string).hexdigest()

    def check_password(self, password):
        '''Return True if the password is valid for this user,
        False otherwise'''
        encrypted = self._encrypt_pw(password)
        return encrypted == self.password


class Authenticator:

    def __init__(self):
        '''Construct an authenticator to manage users logging in and out.
        The User objects are stored in a simple dictionary'''
        self.users = {}

    def add_user(self, username, password):
        '''Add a User object with given username & password to the users
        dictionary after checking if given username & password are valid'''
        if username in self.users:
            raise UsernameAlreadyExists(username)
        if len(password) < 6:
            raise PasswordTooShort(username)
        self.users[username] = User(username, password)

    def login(self, username, password):
        try:
            user = self.users[username]
        except KeyError:
            raise InvalidUsername(username)
        if not user.check_password(password):
            raise InvalidPassword(username, user)
        user.is_logged_in = True
        return True

    def is_logged_in(self, username):
        if username in self.users:
            return self.users[username].is_logged_in
        return False


authenticator = Authenticator()


class Authorizor():

    def __init__(self, authenticator):
        self.authenticator = authenticator
        self.permissions = {}

    def add_permission(self, perm_name):
        '''Create a new permission that users can be added to'''
        try:
            perm_set = self.permissions[perm_name]
        except KeyError:
            self.permissions[perm_name] = set()
        else:
            raise PermissionError("Permission exists")

    def permit_user(self, perm_name, username):
        '''Grant the given permission to the given user'''
        try:
            perm_set = self.permissions[perm_name]
        except KeyError:
            raise PermissionError("Permission does not exist")
        else:
            if username not in self.authenticator.users:
                raise InvalidUsername(username)
            perm_set.add(username)

    def check_permission(self, perm_name, username):
        if not self.authenticator.is_logged_in(username):
            raise NotLoggedInError(username)
        try:
            perm_set = self.permissions[perm_name]
        except KeyError:
            raise PermissionError("Permission does not exist")
        if username not in perm_set:
            raise NotPermittedError(username)
        else:
            return True


authorizor = Authorizor(authenticator)


# Exception classes

class AuthException(Exception):

    def __init__(self, username, user = None):
        super().__init__(username, user)
        self.username = username
        self.user = user


class UsernameAlreadyExists(AuthException):
    pass


class PasswordTooShort(AuthException):
    pass


class InvalidUsername(AuthException):
    pass


class InvalidPassword(AuthException):
    pass


class NotLoggedInError(AuthException):
    pass


class NotPermittedError(AuthException):
    pass


class PermissionError(Exception):
    pass