Ch2Weather: add TemperatureStatsDisplay

Ch2Weather/Weather.cs

@@ -92,4 +92,37 @@ public class HeatIndexDisplay : IWeatherObserver, IWeatherDisplay {
         var heatIndex = ComputeHeatIndex(_weatherRecord.Temperature, _weatherRecord.Humidity);
         Console.WriteLine($"Heat Index is {heatIndex}");
     }
+}
+
+public class TemperatureStatsDisplay : IWeatherObserver, IWeatherDisplay {
+    private IWeatherSubject _subject;
+    private double _n = 0;
+    private double _avg = 0;
+    private double _min = 0;
+    private double _max = 0;
+
+    public TemperatureStatsDisplay(IWeatherSubject subject) {
+        _subject = subject;
+        subject.Subscribe(this);
+    }
+    
+    public void Update(WeatherRecord wr) {
+        var temp = wr.Temperature;
+        if (_n == 0) {
+            _n = 1;
+            _avg = temp;
+            _min = temp;
+            _max = temp;
+        } else {
+            _min = double.Min(_min, temp);
+            _max = double.Max(_max, temp);
+            _avg = (_avg * _n + temp) / (_n + 1);
+            _n += 1;
+        }
+        Display();
+    }
+
+    public void Display() {
+        Console.WriteLine($"Avg/Max/Min Temperature: {_avg}/{_max}/{_min}");
+    }
 }