"""Time helpers: the DB holds naive UTC, presentation converts to display_timezone (#42)."""
from datetime import datetime, timezone
from zoneinfo import ZoneInfo

from src.backend.config import settings


def utcnow() -> datetime:
    """Naive UTC now (the whole DB moves to naive UTC, #42)."""
    return datetime.now(timezone.utc).replace(tzinfo=None)


def as_utc_naive(dt: datetime | None) -> datetime | None:
    """Aware -> UTC naive; naive is taken as UTC already; None stays None."""
    if dt is None or dt.tzinfo is None:
        return dt
    return dt.astimezone(timezone.utc).replace(tzinfo=None)


def to_utc_aware(dt: datetime) -> datetime:
    """Naive (UTC) or aware datetime -> aware UTC."""
    if dt.tzinfo is None:
        return dt.replace(tzinfo=timezone.utc)
    return dt.astimezone(timezone.utc)


def iso_utc(dt: datetime | None) -> str:
    """ISO 8601 with explicit +00:00; "" for None (CSV cells)."""
    return to_utc_aware(dt).isoformat() if dt else ""


def to_local(dt: datetime) -> datetime:
    """Naive UTC (or aware) datetime -> aware datetime in settings.display_timezone."""
    return to_utc_aware(dt).astimezone(ZoneInfo(settings.display_timezone))