import json


class UserManager:
    """管理用户数据的增删查改，并支持 JSON 文件保存和加载。"""

    def __init__(self):
        self.users = []      # 存放所有用户字典
        self.next_id = 1     # 下一个要分配的 id

    def add_user(self, name, age):
        """添加用户，自动分配 id，返回该用户字典。"""
        user = {"id": self.next_id, "name": name, "age": age}
        self.users.append(user)
        self.next_id += 1
        return user

    def get_user(self, user_id):
        """按 id 查询用户，不存在返回 None。"""
        for user in self.users:
            if user["id"] == user_id:
                return user
        return None

    def update_age(self, user_id, age):
        """修改用户年龄，成功返回 True，不存在返回 False。"""
        for user in self.users:
            if user["id"] == user_id:
                user["age"] = age
                return True
        return False

    def remove_user(self, user_id):
        """删除用户，成功返回 True，不存在返回 False。"""
        for user in self.users:
            if user["id"] == user_id:
                self.users.remove(user)
                return True
        return False

    def list_users(self):
        """列出所有用户（按添加顺序）。"""
        return self.users

    def save_to_json(self, filepath):
        """把所有用户保存为 JSON 文件。"""
        with open(filepath, "w", encoding="utf-8") as f:
            json.dump(self.users, f, ensure_ascii=False, indent=2)

    def load_from_json(self, filepath):
        """从 JSON 文件加载用户，覆盖当前数据。"""
        with open(filepath, "r", encoding="utf-8") as f:
            self.users = json.load(f)
        # 后续添加的 id 接续已加载的最大 id
        if self.users:
            self.next_id = max(user["id"] for user in self.users) + 1
        else:
            self.next_id = 1