/**
 * Mock LLM Server — E2E 冒烟测试的本地 OpenAI 兼容 Provider（v0.8.1 P2-5）
 *
 * 实现 DeepSeek 适配器实际消费的最小协议面：
 * - POST {baseURL}/chat/completions（stream=true）：按 SSE 推送一段固定文本 +
 *   finish_reason=stop + usage + [DONE]；
 * - POST /chat/completions（stream=false）：非流式 JSON（压缩摘要等内部调用兜底）。
 *
 * 端口随机分配（127.0.0.1），测试结束关闭 —— 不触外网、不落真实会话数据。
 */

import { createServer, type Server } from 'http';
import type { AddressInfo } from 'net';

/** 流式回复的固定文本（断言锚点） */
export const MOCK_REPLY = 'Hello from mock LLM. E2E smoke reply.';

export interface MockLLMHandle {
  url: string;
  close: () => Promise<void>;
  /** 收到的 chat/completions 请求体列表（断言用） */
  readonly requests: Array<Record<string, unknown>>;
}

export function startMockLLM(): Promise<MockLLMHandle> {
  const requests: Array<Record<string, unknown>> = [];
  const server: Server = createServer((req, res) => {
    if (!req.url?.includes('/chat/completions')) {
      res.writeHead(404).end();
      return;
    }
    let body = '';
    req.on('data', (chunk) => {
      body += chunk;
    });
    req.on('end', () => {
      let parsed: Record<string, unknown> = {};
      try {
        parsed = JSON.parse(body) as Record<string, unknown>;
      } catch {
        /* 忽略解析失败 */
      }
      requests.push(parsed);

      const isStream = parsed.stream === true;
      if (!isStream) {
        res.writeHead(200, { 'Content-Type': 'application/json' });
        res.end(
          JSON.stringify({
            id: 'mock-1',
            model: parsed.model ?? 'mock',
            choices: [
              {
                index: 0,
                message: { role: 'assistant', content: MOCK_REPLY },
                finish_reason: 'stop',
              },
            ],
            usage: { prompt_tokens: 10, completion_tokens: 8, total_tokens: 18 },
          }),
        );
        return;
      }

      res.writeHead(200, {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        Connection: 'keep-alive',
      });
      const frame = (payload: Record<string, unknown>): void => {
        res.write(`data: ${JSON.stringify(payload)}\n\n`);
      };
      // 首 chunk：正文增量
      frame({
        id: 'mock-1',
        model: parsed.model ?? 'mock',
        choices: [{ index: 0, delta: { role: 'assistant', content: MOCK_REPLY } }],
      });
      // 末帧：finish_reason + usage
      frame({
        id: 'mock-1',
        model: parsed.model ?? 'mock',
        choices: [{ index: 0, delta: {}, finish_reason: 'stop' }],
        usage: { prompt_tokens: 10, completion_tokens: 8, total_tokens: 18 },
      });
      res.write('data: [DONE]\n\n');
      res.end();
    });
  });

  return new Promise((resolve) => {
    server.listen(0, '127.0.0.1', () => {
      const addr = server.address() as AddressInfo;
      resolve({
        url: `http://127.0.0.1:${addr.port}`,
        close: () => new Promise<void>((r) => server.close(() => r())),
        requests,
      });
    });
  });
}