// 不自动运行：需要先 npm install（express 5）。Node 23.6+ 可以直接运行 .ts（类型擦除）
// Express 5 BFF 示例：请求 id、输入校验、统一错误处理、async 路由（Express 5 会自动捕获 Promise 拒绝）、SSE 流式转发
import express, { type NextFunction, type Request, type Response } from "express";
import { randomUUID } from "node:crypto";

export class HttpError extends Error {
  status: number;
  code: string;
  constructor(status: number, code: string, message: string) {
    super(message);
    this.status = status;
    this.code = code;
  }
}

export type Deps = {
  // 上游 AI 服务：返回逐段文本的异步迭代器；真实项目里是对 Python 服务的 fetch 流式调用
  askLLM: (question: string, signal: AbortSignal) => AsyncIterable<string>;
  findOrder: (id: string) => Promise<{ id: string; status: string } | null>;
};

export function createApp(deps: Deps) {
  const app = express();
  app.use(express.json({ limit: "100kb" }));                       // 限制请求体大小

  app.use((req, res, next) => {                                      // 请求 id：贯穿日志和上游调用，便于排查
    const id = req.get("x-request-id") ?? randomUUID();
    res.locals.requestId = id;
    res.set("x-request-id", id);
    next();
  });

  app.get("/healthz", (_req, res) => {
    res.json({ ok: true });
  });

  // Express 5：async 处理函数抛出的错误会自动交给错误处理中间件（Express 4 需要手动 next(err)）
  app.get("/orders/:id", async (req, res) => {
    const id = req.params.id;
    if (!/^[A-Z]\d{3,}$/.test(id)) throw new HttpError(400, "BAD_ORDER_ID", "订单号格式不正确");
    const order = await deps.findOrder(id);
    if (!order) throw new HttpError(404, "ORDER_NOT_FOUND", "订单不存在");
    res.json(order);
  });

  app.post("/chat", async (req, res) => {
    const question: unknown = req.body?.question;
    if (typeof question !== "string" || question.length === 0 || question.length > 500) {
      throw new HttpError(400, "BAD_QUESTION", "question 必须是 1~500 字的字符串");
    }
    const controller = new AbortController();
    res.on("close", () => controller.abort());                       // 客户端断开：通知上游停止生成
    res.set({ "Content-Type": "text/event-stream", "Cache-Control": "no-cache", "X-Accel-Buffering": "no" });
    res.flushHeaders();
    try {
      for await (const token of deps.askLLM(question, controller.signal)) {
        if (controller.signal.aborted) break;
        res.write(`data: ${JSON.stringify({ token })}\n\n`);
      }
      res.write("event: done\ndata: {}\n\n");
    } catch (e) {
      // 响应头已经发出，不能再改状态码：用一个 error 事件告诉前端
      res.write(`event: error\ndata: ${JSON.stringify({ message: "生成失败，请重试" })}\n\n`);
      console.error(res.locals.requestId, e);
    } finally {
      res.end();
    }
  });

  app.use((_req, _res, next) => next(new HttpError(404, "NOT_FOUND", "接口不存在")));

  // 统一错误处理：4 个参数。不把内部错误细节返回给客户端
  app.use((err: unknown, _req: Request, res: Response, _next: NextFunction) => {
    if (err instanceof HttpError) {
      res.status(err.status).json({ code: err.code, message: err.message, requestId: res.locals.requestId });
      return;
    }
    const status = (err as { status?: number }).status;
    if (status === 400 || status === 413) {                          // express.json 的解析错误 / 请求体过大
      res.status(status).json({ code: "BAD_REQUEST", message: "请求体无效", requestId: res.locals.requestId });
      return;
    }
    console.error(res.locals.requestId, err);
    res.status(500).json({ code: "INTERNAL", message: "服务器内部错误", requestId: res.locals.requestId });
  });

  return app;
}