import threading

from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine

from app.core.config import settings

_engine = None
_session_factory: async_sessionmaker[AsyncSession] | None = None
_lock = threading.Lock()


def _get_engine():
    global _engine
    if _engine is None:
        with _lock:
            if _engine is None:
                _engine = create_async_engine(
                    settings.database_url,
                    echo=settings.debug,
                    pool_pre_ping=True,
                    pool_recycle=300,
                )
    return _engine


def _get_session_factory():
    global _session_factory
    if _session_factory is None:
        with _lock:
            if _session_factory is None:
                _session_factory = async_sessionmaker(
                    _get_engine(), class_=AsyncSession, expire_on_commit=False
                )
    return _session_factory


async def init_db():
    """Create tables if they do not exist."""
    from app.db.models import Base  # main models

    engine = _get_engine()
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)


async def get_db():
    """Yield a DB session with proper cleanup."""
    session_factory = _get_session_factory()
    async with session_factory() as session:
        yield session