"""Timezone helpers for web UI."""
from contextvars import ContextVar
from datetime import datetime
from typing import Optional
from zoneinfo import ZoneInfo, ZoneInfoNotFoundError

_current_timezone: ContextVar[str] = ContextVar("current_timezone", default="UTC")


def set_current_timezone(tz: str) -> None:
    _current_timezone.set(tz)


def get_current_timezone() -> str:
    return _current_timezone.get()


def get_common_timezones() -> list[str]:
    """Return a curated list of common IANA timezones for the UI."""
    return [
        "UTC",
        "Europe/Moscow",
        "Europe/Samara",
        "Asia/Krasnoyarsk",
        "Asia/Novosibirsk",
        "Asia/Irkutsk",
        "Asia/Yakutsk",
        "Asia/Vladivostok",
        "Asia/Kamchatka",
        "Asia/Almaty",
        "Asia/Tashkent",
        "Asia/Tbilisi",
    ]


def _ensure_aware(dt: datetime, default_tz: str = "UTC") -> datetime:
    if dt.tzinfo is None:
        return dt.replace(tzinfo=ZoneInfo(default_tz))
    return dt


def localtime(
    value: Optional[datetime],
    tz: Optional[str] = None,
    fmt: str = "%Y-%m-%d %H:%M",
) -> str:
    """Convert a UTC datetime to the configured timezone and format it."""
    if value is None:
        return "-"
    tz_name = tz or get_current_timezone()
    try:
        zone = ZoneInfo(tz_name)
    except ZoneInfoNotFoundError:
        zone = ZoneInfo("UTC")
    dt = _ensure_aware(value)
    return dt.astimezone(zone).strftime(fmt)


def localtime_iso(value: Optional[datetime], tz: Optional[str] = None) -> str:
    """Convert a UTC datetime to the configured timezone and return ISO string."""
    if value is None:
        return ""
    tz_name = tz or get_current_timezone()
    try:
        zone = ZoneInfo(tz_name)
    except ZoneInfoNotFoundError:
        zone = ZoneInfo("UTC")
    dt = _ensure_aware(value)
    return dt.astimezone(zone).isoformat()