// Subject Interface
interface Subject {
  registerObserver(observer: Observer): void;
  removeObserver(observer: Observer): void;
  notifyObservers(): void;
}

// Observer Interface
interface Observer {
  update(temperature: number): void;
}

// Concrete Subject
class WeatherStation implements Subject {
  private observers: Observer[] = [];
  private temperature: number;

  setTemperature(temp: number) {
    console.log(`WeatherStation: new temperature measurement: ${temp}`);
    this.temperature = temp;
    this.notifyObservers();
  }

  registerObserver(observer: Observer): void {
    this.observers.push(observer);
  }

  removeObserver(observer: Observer): void {
    this.observers = this.observers.filter(obs => obs !== observer);
  }

  notifyObservers(): void {
    for (const observer of this.observers) {
      observer.update(this.temperature);
    }
  }
}

// Concrete Observers
class TemperatureDisplay implements Observer {
  update(temperature: number): void {
    console.log(`TemperatureDisplay: I need to update my display with the new temperature: ${temperature}`);
  }
}

class Fan implements Observer {
  update(temperature: number): void {
    if (temperature > 25) {
      console.log(`Fan: It's hot here, turning on the fan...`);
    } else {
      console.log(`Fan: It's cool, turning off the fan...`);
    }
  }
}

// Usage
const weatherStation = new WeatherStation();
const tempDisplay = new TemperatureDisplay();
const fan = new Fan();

weatherStation.registerObserver(tempDisplay);
weatherStation.registerObserver(fan);

weatherStation.setTemperature(30);
weatherStation.setTemperature(20);