const express = require('express');
const fs = require('fs');
const path = require('path');
const crypto = require('crypto');

const app = express();
const PORT = process.env.PORT || 3003;

// ── 中间件 ────────────────────────────────────────────
app.use(express.json());
app.use(express.static(path.join(__dirname, 'public')));

// 统一设置 JSON 响应的 charset
app.use((_req, res, next) => {
  res.setHeader('Content-Type', 'application/json; charset=utf-8');
  next();
});

// ── 数据层 ────────────────────────────────────────────
const DATA_DIR = path.join(__dirname, 'data');
const DATA_FILE = path.join(DATA_DIR, 'todos.json');

function ensureDataDir() {
  if (!fs.existsSync(DATA_DIR)) {
    fs.mkdirSync(DATA_DIR, { recursive: true });
  }
}

function readTodos() {
  ensureDataDir();
  try {
    if (!fs.existsSync(DATA_FILE)) return [];
    const raw = fs.readFileSync(DATA_FILE, 'utf-8');
    return JSON.parse(raw);
  } catch {
    // 文件损坏时备份并重建
    if (fs.existsSync(DATA_FILE)) {
      const backup = DATA_FILE + '.bak.' + Date.now();
      fs.copyFileSync(DATA_FILE, backup);
      console.warn(`数据文件损坏，已备份至 ${backup}`);
    }
    return [];
  }
}

let todos = readTodos();
let writeTimer = null;

function writeTodos() {
  clearTimeout(writeTimer);
  writeTimer = setTimeout(() => {
    ensureDataDir();
    fs.writeFileSync(DATA_FILE, JSON.stringify(todos, null, 2), 'utf-8');
  }, 300); // 300ms 防抖
}

function generateId() {
  return crypto.randomUUID();
}

// ── 辅助函数 ──────────────────────────────────────────
/** 递归收集所有子孙节点 ID */
function collectDescendantIds(parentId) {
  const ids = [];
  const children = todos.filter(t => t.parentId === parentId);
  for (const child of children) {
    ids.push(child.id);
    ids.push(...collectDescendantIds(child.id));
  }
  return ids;
}

/** 获取某个任务的最大层级深度（用于限制三级） */
function getDepth(itemId) {
  let depth = 0;
  let current = todos.find(t => t.id === itemId);
  while (current && current.parentId) {
    depth++;
    current = todos.find(t => t.id === current.parentId);
  }
  return depth;
}

// ── API 路由 ──────────────────────────────────────────
// 获取所有任务（排除已删除的）
app.get('/api/todos', (_req, res) => {
  res.json(todos.filter(t => t.status !== 'deleted'));
});

// 创建任务
app.post('/api/todos', (req, res) => {
  const { title, parentId } = req.body;

  if (!title || !title.trim()) {
    return res.status(400).json({ error: '标题不能为空' });
  }

  // 检查层级限制：最多三级（主/子/孙）
  if (parentId) {
    const parent = todos.find(t => t.id === parentId);
    if (!parent) {
      return res.status(404).json({ error: '父任务不存在' });
    }
    const parentDepth = getDepth(parentId);
    if (parentDepth >= 2) {
      return res.status(400).json({ error: '最多支持三级任务（主/子/孙）' });
    }
  }

  const now = new Date().toISOString();
  const todo = {
    id: generateId(),
    title: title.trim(),
    status: 'active',
    parentId: parentId || null,
    createdAt: now,
    updatedAt: now,
  };

  todos.push(todo);
  writeTodos();
  res.status(201).json(todo);
});

// 更新任务
app.patch('/api/todos/:id', (req, res) => {
  const { id } = req.params;
  const { title, status } = req.body;

  const index = todos.findIndex(t => t.id === id);
  if (index === -1) {
    return res.status(404).json({ error: '任务不存在' });
  }

  const todo = todos[index];
  const now = new Date().toISOString();

  if (title !== undefined) {
    if (!title.trim()) {
      return res.status(400).json({ error: '标题不能为空' });
    }
    todo.title = title.trim();
  }

  if (status !== undefined) {
    if (!['active', 'completed', 'abandoned', 'deleted'].includes(status)) {
      return res.status(400).json({ error: '无效的状态值' });
    }
    todo.status = status;
    todo.updatedAt = now;

    // 级联完成：当标记为 completed 时，所有子孙也变为 completed
    if (status === 'completed') {
      const descendantIds = collectDescendantIds(id);
      for (const descId of descendantIds) {
        const desc = todos.find(t => t.id === descId);
        if (desc) {
          desc.status = 'completed';
          desc.updatedAt = now;
        }
      }
    }
  }

  todo.updatedAt = now;
  writeTodos();
  res.json(todo);
});

// 删除任务（软删除：标记为 deleted，级联子孙）
app.delete('/api/todos/:id', (req, res) => {
  const { id } = req.params;

  const todo = todos.find(t => t.id === id);
  if (!todo) {
    return res.status(404).json({ error: '任务不存在' });
  }

  const idsToDelete = new Set([id, ...collectDescendantIds(id)]);
  const now = new Date().toISOString();
  let count = 0;

  for (const t of todos) {
    if (idsToDelete.has(t.id)) {
      t.status = 'deleted';
      t.updatedAt = now;
      count++;
    }
  }

  writeTodos();
  res.json({ deleted: count });
});

// ── 设置接口 ──────────────────────────────────────────
const SETTINGS_FILE = path.join(DATA_DIR, 'settings.json');

function readSettings() {
  ensureDataDir();
  try {
    if (!fs.existsSync(SETTINGS_FILE)) return { autoSaveEnabled: false, autoSaveSec: 10 };
    return JSON.parse(fs.readFileSync(SETTINGS_FILE, 'utf-8'));
  } catch {
    return { autoSaveEnabled: false, autoSaveSec: 10 };
  }
}

function writeSettings(s) {
  ensureDataDir();
  fs.writeFileSync(SETTINGS_FILE, JSON.stringify(s, null, 2), 'utf-8');
}

app.get('/api/settings', (_req, res) => {
  res.json(readSettings());
});

app.put('/api/settings', (req, res) => {
  const s = readSettings();
  if (req.body.autoSaveEnabled !== undefined) s.autoSaveEnabled = req.body.autoSaveEnabled;
  if (req.body.autoSaveSec !== undefined) s.autoSaveSec = req.body.autoSaveSec;
  writeSettings(s);
  res.json(s);
});

// 回退到 index.html（SPA）
app.get('*', (_req, res) => {
  res.sendFile(path.join(__dirname, 'public', 'index.html'));
});

// ── 启动服务 ──────────────────────────────────────────
function startServer(port) {
  const server = app.listen(port, () => {
    console.log(`✅ Todo List 服务已启动: http://localhost:${port}`);
    console.log(`📁 数据文件: ${DATA_FILE}`);
  });

  server.on('error', (err) => {
    // 生产环境（PM2 管理端口）不自动跳，直接抛错
    if (err.code === 'EADDRINUSE' && process.env.NODE_ENV !== 'production') {
      console.warn(`⚠️  端口 ${port} 被占用，尝试 ${port + 1}…`);
      startServer(port + 1);
    } else {
      throw err;
    }
  });
}

startServer(PORT);