"""Async DB engine, session factory, declarative base, UTC helper."""
from __future__ import annotations

from datetime import datetime, timezone
from typing import AsyncIterator

from sqlalchemy.ext.asyncio import (
    AsyncSession,
    async_sessionmaker,
    create_async_engine,
)
from sqlalchemy.orm import DeclarativeBase

from app.config import get_settings


class Base(DeclarativeBase):
    pass


_engine = None
_session_factory: async_sessionmaker[AsyncSession] | None = None


def utcnow() -> datetime:
    """All timestamps in Cassandra are tz-aware UTC by convention."""
    return datetime.now(timezone.utc)


def get_engine():
    global _engine
    if _engine is None:
        s = get_settings()
        _engine = create_async_engine(
            s.DATABASE_URL,
            pool_pre_ping=True,
            pool_recycle=3600,
            future=True,
        )
    return _engine


def get_session_factory() -> async_sessionmaker[AsyncSession]:
    global _session_factory
    if _session_factory is None:
        _session_factory = async_sessionmaker(
            get_engine(), expire_on_commit=False, class_=AsyncSession
        )
    return _session_factory


async def get_session() -> AsyncIterator[AsyncSession]:
    """FastAPI dependency yielding an async session."""
    async with get_session_factory()() as session:
        yield session