from functools import lru_cache
from datetime import tzinfo
from typing import Sequence

import pytz # type: ignore


def user_forced() -> Sequence[str]:
    # conversion from abbreviations is always ambiguous
    # https://stackoverflow.com/questions/36067621/python-all-possible-timezone-abbreviations-for-given-timezone-name-and-vise-ve
    try:
        from my.config import time as user_config
        return user_config.tz.force_abbreviations # type: ignore[attr-defined]
    except:
        # todo log/apply policy
        return []


@lru_cache(1)
def _abbr_to_timezone_map():
    # also force UTC to always correspond to utc
    # this makes more sense than Zulu it ends up by default
    timezones = pytz.all_timezones + ['UTC'] + list(user_forced())

    res = {}
    for tzname in timezones:
        tz = pytz.timezone(tzname)
        infos = getattr(tz, '_tzinfos', []) # not sure if can rely on attr always present?
        for info in infos:
            abbr = info[-1]
            # todo could support this with a better error handling strategy?
            # otz = res.get(abbr, tz)
            # if otz != tz:
            #     raise RuntimeError(abbr, tz, otz)
            res[abbr] = tz
        # ugh. also necessary, e.g. for Zulu?? why is it not in _tzinfos?
        # note: somehow this is not the same as the tzname!
        tzn = getattr(tz, '_tzname', None)
        if tzn is not None:
            res[tzn] = tz
    return res


# todo dammit, lru_cache interferes with mypy?
@lru_cache(None)
def abbr_to_timezone(abbr: str) -> tzinfo:
    return _abbr_to_timezone_map()[abbr]


def zone_to_countrycode(zone: str) -> str:
    # todo make optional?
    return _zones_to_countrycode()[zone]


@lru_cache(1)
def _zones_to_countrycode():
    # https://stackoverflow.com/a/13020785/706389
    res = {}
    for countrycode, timezones in pytz.country_timezones.items():
        for timezone in timezones:
            res[timezone] = countrycode
    return res


# todo stuff here could be a bit more defensive? e.g. dependent on policy