"""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）并校验既有表列与模型一致（fail-fast on schema drift）。

    先导入 models 注册映射；create_all 用 IF NOT EXISTS 仅补缺失的表；
    随后对每张已存在的表检查模型声明的列是否齐全，缺列即抛 RuntimeError，
    避免运行期才以晦涩的 OperationalError 暴露 schema 漂移。
    """
    from sqlalchemy import inspect

    from . import models  # noqa: F401
    engine = get_engine()
    Base.metadata.create_all(bind=engine)

    inspector = inspect(engine)
    missing: list[str] = []
    for table, mapper in Base.registry.mappers.items():
        if not inspector.has_table(table):
            continue
        db_cols = {c["name"] for c in inspector.get_columns(table)}
        for model_col in mapper.columns.keys():
            if model_col not in db_cols:
                missing.append(f"{table}.{model_col}")
    if missing:
        raise RuntimeError(
            "数据库 schema 与模型不一致，缺少列: " + ", ".join(missing)
            + "。请执行 sql/schema.sql 或迁移脚本更新表结构。"
        )