// Simple Todo App
// 数据保存在 localStorage 中

(function () {
  "use strict";

  const STORAGE_KEY = "simple-todo-app.tasks";

  // DOM 元素
  const form = document.getElementById("todo-form");
  const input = document.getElementById("todo-input");
  const list = document.getElementById("todo-list");
  const countEl = document.getElementById("todo-count");
  const emptyState = document.getElementById("empty-state");
  const clearCompletedBtn = document.getElementById("clear-completed");

  /**
   * 从 localStorage 读取任务列表
   * @returns {Array<{id: string, text: string, completed: boolean}>}
   */
  function loadTasks() {
    try {
      const raw = localStorage.getItem(STORAGE_KEY);
      if (!raw) return [];
      const parsed = JSON.parse(raw);
      return Array.isArray(parsed) ? parsed : [];
    } catch (err) {
      console.error("读取任务失败:", err);
      return [];
    }
  }

  /**
   * 保存任务列表到 localStorage
   * @param {Array} tasks
   */
  function saveTasks(tasks) {
    try {
      localStorage.setItem(STORAGE_KEY, JSON.stringify(tasks));
    } catch (err) {
      console.error("保存任务失败:", err);
    }
  }

  // 当前任务状态
  let tasks = loadTasks();

  /**
   * 生成唯一 ID
   */
  function generateId() {
    return Date.now().toString(36) + Math.random().toString(36).slice(2, 8);
  }

  /**
   * 渲染任务列表
   */
  function render() {
    list.innerHTML = "";

    tasks.forEach(function (task) {
      const li = document.createElement("li");
      li.className = "todo-item" + (task.completed ? " completed" : "");
      li.dataset.id = task.id;

      // 复选框
      const checkbox = document.createElement("input");
      checkbox.type = "checkbox";
      checkbox.checked = task.completed;
      checkbox.addEventListener("change", function () {
        toggleTask(task.id);
      });

      // 文本
      const span = document.createElement("span");
      span.className = "todo-text";
      span.textContent = task.text;

      // 删除按钮
      const delBtn = document.createElement("button");
      delBtn.className = "delete-btn";
      delBtn.type = "button";
      delBtn.setAttribute("aria-label", "删除任务");
      delBtn.textContent = "✕";
      delBtn.addEventListener("click", function () {
        deleteTask(task.id);
      });

      li.appendChild(checkbox);
      li.appendChild(span);
      li.appendChild(delBtn);
      list.appendChild(li);
    });

    // 空状态
    if (tasks.length === 0) {
      emptyState.classList.remove("hidden");
    } else {
      emptyState.classList.add("hidden");
    }

    // 统计
    const remaining = tasks.filter(function (t) {
      return !t.completed;
    }).length;
    countEl.textContent = "共 " + tasks.length + " 项 · 未完成 " + remaining + " 项";
  }

  /**
   * 添加任务
   */
  function addTask(text) {
    const trimmed = text.trim();
    if (!trimmed) return;

    tasks.push({
      id: generateId(),
      text: trimmed,
      completed: false
    });

    saveTasks(tasks);
    render();
  }

  /**
   * 切换任务完成状态
   */
  function toggleTask(id) {
    tasks = tasks.map(function (t) {
      return t.id === id ? Object.assign({}, t, { completed: !t.completed }) : t;
    });
    saveTasks(tasks);
    render();
  }

  /**
   * 删除任务
   */
  function deleteTask(id) {
    tasks = tasks.filter(function (t) {
      return t.id !== id;
    });
    saveTasks(tasks);
    render();
  }

  /**
   * 清除已完成任务
   */
  function clearCompleted() {
    tasks = tasks.filter(function (t) {
      return !t.completed;
    });
    saveTasks(tasks);
    render();
  }

  // 事件绑定
  form.addEventListener("submit", function (e) {
    e.preventDefault();
    addTask(input.value);
    input.value = "";
    input.focus();
  });

  clearCompletedBtn.addEventListener("click", clearCompleted);

  // 初始渲染
  render();

  // 自动聚焦输入框
  input.focus();
})();