設計模式：中介者模式 (Mediator)

interface ChatMediator {
  sendMessage(message: string, sender: User): void;
  addUser(user: User): void;
}

// Colleague
class User {
  constructor(
    public name: string,
    private mediator: ChatMediator,
  ) {}

  send(message: string): void {
    console.log(`${this.name} 發送：${message}`);
    this.mediator.sendMessage(message, this);
  }

  receive(message: string, from: User): void {
    console.log(`${this.name} 收到來自 ${from.name} 的訊息：${message}`);
  }
}

// ConcreteMediator：公開聊天室
class PublicChatRoom implements ChatMediator {
  private users: User[] = [];

  addUser(user: User): void {
    this.users.push(user);
  }

  sendMessage(message: string, sender: User): void {
    this.users
      .filter(user => user !== sender)
      .forEach(user => user.receive(message, sender));
  }
}

// ConcreteMediator：私訊頻道
class DirectMessageRoom implements ChatMediator {
  private users: User[] = [];
  private messageHistory: string[] = [];

  addUser(user: User): void {
    if (this.users.length >= 2) {
      throw new Error('私訊頻道只支援兩個使用者');
    }
    this.users.push(user);
  }

  sendMessage(message: string, sender: User): void {
    this.messageHistory.push(`[${sender.name}] ${message}`);
    const recipient = this.users.find(u => u !== sender);
    recipient?.receive(message, sender);
  }

  getHistory(): string[] {
    return [...this.messageHistory];
  }
}

// 使用
const publicRoom = new PublicChatRoom();
const alice = new User('Alice', publicRoom);
const bob = new User('Bob', publicRoom);
const charlie = new User('Charlie', publicRoom);

publicRoom.addUser(alice);
publicRoom.addUser(bob);
publicRoom.addUser(charlie);

alice.send('大家好!');
// Bob 收到來自 Alice 的訊息：大家好!
// Charlie 收到來自 Alice 的訊息：大家好!