/**
 * 数据预取Hook
 * 用于关键数据的预加载，提升用户体验
 * 
 * 特性：
 * - 应用启动时预取关键数据
 * - 页面导航时预取相关数据
 * - 智能预取策略（基于用户行为）
 * - 并发控制和优先级管理
 */

import { useCallback, useEffect, useRef } from 'react';
import { postManager } from '../stores/postManager';
import { groupManager } from '../stores/groupManager';
import { userManager } from '../stores/userManager';
import { messageManager } from '../stores/messageManager';

// ==================== 预取配置 ====================

/** 预取任务优先级 */
enum Priority {
  HIGH = 0,
  MEDIUM = 1,
  LOW = 2,
}

/** 预取任务接口 */
interface PrefetchTask {
  key: string;
  executor: () => Promise<any>;
  priority: Priority;
}

/** 预取服务类 */
class PrefetchService {
  /** 预取任务队列 */
  private queue: PrefetchTask[] = [];

  /** 是否正在处理队列 */
  private isProcessing = false;

  /** 最大并发数 */
  private readonly MAX_CONCURRENCY = 3;

  /** 当前活跃的任务数 */
  private activeCount = 0;

  /** 是否启用调试日志 */
  private readonly DEBUG = __DEV__ || false;

  /**
   * 添加预取任务到队列
   * @param task 预取任务
   */
  schedule(task: PrefetchTask): void {
    // 检查是否已有相同key的任务
    const existingIndex = this.queue.findIndex(t => t.key === task.key);
    if (existingIndex >= 0) {
      // 如果新任务优先级更高，替换旧任务
      if (task.priority < this.queue[existingIndex].priority) {
        this.queue[existingIndex] = task;
      }
      return;
    }

    // 按优先级插入队列
    let insertIndex = this.queue.findIndex(t => t.priority > task.priority);
    if (insertIndex === -1) {
      insertIndex = this.queue.length;
    }
    this.queue.splice(insertIndex, 0, task);

    this.processQueue();
  }

  /**
   * 处理预取队列
   */
  private async processQueue(): Promise<void> {
    if (this.isProcessing || this.queue.length === 0) return;

    this.isProcessing = true;

    while (this.queue.length > 0 && this.activeCount < this.MAX_CONCURRENCY) {
      const task = this.queue.shift();
      if (!task) break;

      this.activeCount++;

      // 异步执行任务
      this.executeTask(task).finally(() => {
        this.activeCount--;
        this.processQueue();
      });
    }

    this.isProcessing = false;
  }

  /**
   * 执行单个预取任务
   */
  private async executeTask(task: PrefetchTask): Promise<void> {
    try {
      await task.executor();
    } catch (error) {
      if (this.DEBUG) {
        console.warn(`[Prefetch] 任务失败: ${task.key}`, error);
      }
    }
  }

  /**
   * 清空预取队列
   */
  clearQueue(): void {
    this.queue = [];
  }

  /**
   * 获取队列长度
   */
  getQueueLength(): number {
    return this.queue.length;
  }
}

/** 全局预取服务实例 */
const prefetchService = new PrefetchService();

// ==================== 预取函数 ====================

/**
 * 预取帖子数据
 * @param types 帖子类型数组
 */
function prefetchPosts(types: string[] = ['recommend', 'hot']): void {
  types.forEach((type, index) => {
    prefetchService.schedule({
      key: `posts:${type}:1`,
      executor: () => postManager.getPosts(type, 1, 20),
      priority: index === 0 ? Priority.HIGH : Priority.MEDIUM,
    });
  });
}

/**
 * 预取会话数据
 */
function prefetchConversations(): void {
  prefetchService.schedule({
    key: 'conversations:list',
    executor: async () => {
      await messageManager.initialize();
      await messageManager.fetchConversations(true);
      return messageManager.getConversations();
    },
    priority: Priority.HIGH,
  });

  // 同时预取未读数
  prefetchService.schedule({
    key: 'conversations:unread',
    executor: async () => {
      await messageManager.initialize();
      await messageManager.fetchUnreadCount();
      return messageManager.getUnreadCount();
    },
    priority: Priority.HIGH,
  });
}

/**
 * 预取用户信息
 */
function prefetchUserInfo(): void {
  prefetchService.schedule({
    key: 'users:me',
    executor: () => userManager.getCurrentUser(),
    priority: Priority.HIGH,
  });
}

/**
 * 预取会话详情
 * @param conversationIds 会话ID数组
 */
function prefetchConversationDetails(conversationIds: string[]): void {
  conversationIds.forEach(id => {
    prefetchService.schedule({
      key: `conversations:detail:${id}`,
      executor: async () => {
        await messageManager.initialize();
        return messageManager.fetchConversationDetail(id);
      },
      priority: Priority.LOW,
    });
  });
}

/**
 * 预取群组成员
 * @param groupIds 群组ID数组
 */
function prefetchGroupMembers(groupIds: string[]): void {
  groupIds.forEach(id => {
    prefetchService.schedule({
      key: `groups:members:${id}`,
      executor: async () => {
        const response = await groupManager.getMembers(id, 1, 50);
        return response.list || [];
      },
      priority: Priority.LOW,
    });
  });
}

/**
 * 应用启动时预取
 * 预取最关键的数据
 */
function prefetchOnAppLaunch(): void {
  // 高优先级：用户信息
  prefetchUserInfo();

  // 高优先级：会话列表和未读数
  prefetchConversations();

  // 中优先级：帖子列表
  prefetchPosts(['recommend']);
}

/**
 * 进入消息页面时预取
 */
function prefetchMessageScreen(): void {
  // 获取当前会话列表，预取前几个会话的详情
  const conversations = messageManager.getConversations();
  const topConversationIds = conversations.slice(0, 5).map((c: any) => c.id);

  // 预取会话详情
  prefetchConversationDetails(topConversationIds);

  // 预取群组成员（如果是群聊）
  const groupIds = conversations
    .filter((c: any) => c.type === 'group' && c.group?.id)
    .slice(0, 3)
    .map((c: any) => String(c.group!.id));

  prefetchGroupMembers(groupIds);
}

/**
 * 进入首页时预取
 */
function prefetchHomeScreen(): void {
  prefetchPosts(['recommend', 'hot', 'latest']);
}

// ==================== React Hook ====================

/**
 * 数据预取Hook
 * 
 * @example
 * ```tsx
 * const { prefetchOnAppLaunch, prefetchMessageScreen } = usePrefetch();
 * 
 * useEffect(() => {
 *   prefetchOnAppLaunch();
 * }, []);
 * ```
 */
export function usePrefetch() {
  /** 是否已完成初始预取 */
  const hasInitialPrefetched = useRef(false);

  /** 预取帖子 */
  const prefetchPostsData = useCallback((types?: string[]) => {
    prefetchPosts(types);
  }, []);

  /** 预取会话 */
  const prefetchConversationsData = useCallback(() => {
    prefetchConversations();
  }, []);

  /** 预取用户信息 */
  const prefetchUserData = useCallback(() => {
    prefetchUserInfo();
  }, []);

  /** 应用启动预取 */
  const doPrefetchOnAppLaunch = useCallback(() => {
    if (hasInitialPrefetched.current) return;
    hasInitialPrefetched.current = true;
    prefetchOnAppLaunch();
  }, []);

  /** 消息页面预取 */
  const doPrefetchMessageScreen = useCallback(() => {
    prefetchMessageScreen();
  }, []);

  /** 首页预取 */
  const doPrefetchHomeScreen = useCallback(() => {
    prefetchHomeScreen();
  }, []);

  /** 清空预取队列 */
  const clearPrefetchQueue = useCallback(() => {
    prefetchService.clearQueue();
  }, []);

  /** 获取队列长度 */
  const getQueueLength = useCallback(() => {
    return prefetchService.getQueueLength();
  }, []);

  return {
    // 基础预取方法
    prefetchPosts: prefetchPostsData,
    prefetchConversations: prefetchConversationsData,
    prefetchUserInfo: prefetchUserData,

    // 场景预取方法
    prefetchOnAppLaunch: doPrefetchOnAppLaunch,
    prefetchMessageScreen: doPrefetchMessageScreen,
    prefetchHomeScreen: doPrefetchHomeScreen,

    // 工具方法
    clearPrefetchQueue,
    getQueueLength,
  };
}

// ==================== 导出 ====================

export {
  prefetchPosts,
  prefetchConversations,
  prefetchUserInfo,
  prefetchOnAppLaunch,
  prefetchMessageScreen,
  prefetchHomeScreen,
  prefetchService,
};

export default usePrefetch;