// 不自动运行（Next.js 路由处理器，npm run dev 后 POST /api/chat）
// 服务端调用大模型并以流的形式返回；模型密钥只在服务端，浏览器拿不到。
export const runtime = "nodejs";

async function* fakeLLM(question: string, signal: AbortSignal): AsyncGenerator<string> {
  // 真实项目：调用模型 SDK 的流式接口，并把 signal 传进去
  const words = `关于“${question}”：签收 7 天内可以无理由退货。`.split("");
  for (const w of words) {
    if (signal.aborted) return; // 用户停止或断开：不再继续生成（节省费用）
    await new Promise((r) => setTimeout(r, 30));
    yield w;
  }
}

export async function POST(request: Request): Promise<Response> {
  const { question } = (await request.json()) as { question?: unknown };
  if (typeof question !== "string" || question.length === 0 || question.length > 500) {
    return Response.json({ error: "question 必须是 1~500 字的字符串" }, { status: 400 });
  }
  const encoder = new TextEncoder();
  const stream = new ReadableStream<Uint8Array>({
    async start(controller) {
      try {
        for await (const token of fakeLLM(question, request.signal)) {
          controller.enqueue(encoder.encode(token));
        }
      } finally {
        controller.close();
      }
    },
  });
  return new Response(stream, {
    headers: { "Content-Type": "text/plain; charset=utf-8", "Cache-Control": "no-cache", "X-Accel-Buffering": "no" },
  });
}