import base64
import ipaddress
import struct

from .abstract import Session
from .memory import MemorySession
from ..crypto import AuthKey

_STRUCT_PREFORMAT = '>B{}sH256s'

CURRENT_VERSION = '1'


class _AsyncString(str):
    """
    Ugly hack to enable awaiting strings.
    """
    def __await__(self):
        yield
        return self


class StringSession(MemorySession):
    """
    This session file can be easily saved and loaded as a string. According
    to the initial design, it contains only the data that is necessary for
    successful connection and authentication, so takeout ID is not stored.

    It is thought to be used where you don't want to create any on-disk
    files but would still like to be able to save and load existing sessions
    by other means.

    You can use custom `encode` and `decode` functions, if present:

    * `encode` definition must be ``def encode(value: bytes) -> str:``.
    * `decode` definition must be ``def decode(value: str) -> bytes:``.
    """
    def __init__(self, string: str = None):
        super().__init__()
        if string:
            if string[0] != CURRENT_VERSION:
                raise ValueError('Not a valid string')

            string = string[1:]
            ip_len = 4 if len(string) == 352 else 16
            self._dc_id, ip, self._port, key = struct.unpack(
                _STRUCT_PREFORMAT.format(ip_len), StringSession.decode(string))

            self._server_address = ipaddress.ip_address(ip).compressed
            if any(key):
                self._auth_key = AuthKey(key)

    @staticmethod
    def encode(x: bytes) -> str:
        return base64.urlsafe_b64encode(x).decode('ascii')

    @staticmethod
    def decode(x: str) -> bytes:
        return base64.urlsafe_b64decode(x)

    def save(self: Session):
        # save should be async but that would break code which relies on sync StringSession.
        # Return a type which behaves like a string for all intents and purposes, but can be awaited.
        # The await is necessary for the library to save all sessions in the same way.
        if not self.auth_key:
            return _AsyncString('')

        ip = ipaddress.ip_address(self.server_address).packed
        return _AsyncString(CURRENT_VERSION + StringSession.encode(struct.pack(
            _STRUCT_PREFORMAT.format(len(ip)),
            self.dc_id,
            ip,
            self.port,
            self.auth_key.key
        )))
