"""数据库引擎、Session 与 Declarative Base。

引擎采用懒初始化：首次访问 `get_engine()` 时才创建连接池，
避免 import 时的副作用，并支持 `dispose()` 后重新加载配置。
"""

from __future__ import annotations

from collections.abc import Generator

from sqlalchemy import create_engine
from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker

from .config import get_settings

_engine: create_engine | None = None
_session_local: sessionmaker | None = None


def get_engine() -> create_engine:
    """返回全局 Engine 实例；首次调用时懒创建。"""
    global _engine, _session_local
    if _engine is None:
        s = get_settings()
        _engine = create_engine(
            s.db_url(),
            pool_pre_ping=True,
            pool_size=s.database.pool_size,
            pool_recycle=s.database.pool_recycle,
            future=True,
        )
        _session_local = sessionmaker(
            bind=_engine, autoflush=False, autocommit=False, future=True,
        )
    return _engine


def dispose_engine() -> None:
    """关闭连接池并清除缓存，下次访问时重新创建（配合 reload_settings 使用）。"""
    global _engine, _session_local
    if _engine is not None:
        _engine.dispose()
        _engine = None
        _session_local = None


def get_session_local() -> sessionmaker:
    """返回全局 SessionLocal；确保 Engine 已初始化。"""
    get_engine()
    return _session_local  # type: ignore[return-value]


class Base(DeclarativeBase):
    pass


def get_db() -> Generator[Session, None, None]:
    """FastAPI 依赖：为每个请求产出一个 Session。"""
    db = get_session_local()()
    try:
        yield db
    finally:
        db.close()


def init_db_schema() -> None:
    """按需建表（幂等）。先导入 models 以注册映射。"""
    from . import models  # noqa: F401
    get_engine()
    Base.metadata.create_all(bind=_engine)