from __future__ import annotations

from datetime import UTC, datetime, timedelta
from zoneinfo import ZoneInfo, ZoneInfoNotFoundError

DEFAULT_TIME_ZONE = "UTC"
_TIMESTAMP_KEYS = ("observed_at", "opened_at", "resolved_at")


def validate_time_zone(value: str) -> str:
    name = value.strip() or DEFAULT_TIME_ZONE
    try:
        ZoneInfo(name)
    except ZoneInfoNotFoundError as exc:
        raise ValueError(f"invalid timezone: {name}") from exc
    return name


def require_utc_datetime(value: datetime) -> datetime:
    if value.tzinfo is None or value.utcoffset() is None:
        raise ValueError("timestamp must include UTC timezone")
    if value.utcoffset() != timedelta(0):
        raise ValueError("timestamp must be UTC")
    return value.astimezone(UTC)


def to_utc(value: datetime) -> datetime:
    if value.tzinfo is None or value.utcoffset() is None:
        return value.replace(tzinfo=UTC)
    return value.astimezone(UTC)


def format_display_datetime(value: str | datetime | None, time_zone: str) -> str:
    if not value:
        return ""
    if isinstance(value, datetime):
        dt = value
    else:
        dt = datetime.fromisoformat(value.replace("Z", "+00:00"))
    return (
        to_utc(dt)
        .astimezone(ZoneInfo(validate_time_zone(time_zone)))
        .strftime("%Y-%m-%d %H:%M:%S %Z")
    )


def payload_with_display_times(payload: dict, time_zone: str) -> dict:
    out = dict(payload)
    out["time_zone"] = validate_time_zone(time_zone)
    for key in _TIMESTAMP_KEYS:
        if payload.get(key):
            out[f"{key}_display"] = format_display_datetime(payload[key], out["time_zone"])
    return out