import typing

import tldextract
from flask import request, current_app


def get_allowed_root_domain() -> typing.Optional[str]:
    """Returns allowed root domain to set cookie or None."""

    root_domain = tldextract.extract(request.host).registered_domain
    if root_domain and request.host in current_app.config.get("ALLOWED_HOSTS"):
        return root_domain

    return None