Add last-lap exercise

Author: ErikSchierboom

exercises/practice/split-second-stopwatch/SplitSecondStopwatch.cs

@@ -2,31 +2,67 @@ public enum StopwatchState
 {
     Ready,
     Running,
-    Paused,
-    Stopped
+    Paused
 }
 
 public class SplitSecondStopwatch(TimeProvider time)
 {
+    private readonly List<TimeSpan> _previousLapSegments = new();
+    private DateTimeOffset? _currentSegmentStart;
+    
     public StopwatchState State { get; private set; }
 
+    public List<TimeSpan> PreviousLaps { get; } = new();
+    
+    public TimeSpan CurrentLap => PreviousLapSegments + CurrentSegment;
+
+    private TimeSpan PreviousLapSegments => _previousLapSegments.Aggregate(TimeSpan.Zero, (total, segment) => total + segment);
+
+    private TimeSpan CurrentSegment => _currentSegmentStart is {} start ? time.GetUtcNow() - start : TimeSpan.Zero;
+
     public void Start()
     {
-        throw new NotImplementedException();
+        if (State == StopwatchState.Paused)
+            _currentSegmentStart = null;
+        else if (State == StopwatchState.Running)
+            throw new InvalidOperationException();
+        
+        _currentSegmentStart ??= time.GetUtcNow();
+        
+        State = StopwatchState.Running;
     }
 
     public void Stop()
     {
-        throw new NotImplementedException();
+        if (State == StopwatchState.Ready)
+            return;
+        
+        PreviousLaps.Add(CurrentLap);
+        _currentSegmentStart = null;
+        State = StopwatchState.Ready;
     }
 
     public void Split()
     {
-        throw new NotImplementedException();
+        AddSegment();
     }
 
     public void Pause()
     {
-        throw new NotImplementedException();
+        State = StopwatchState.Paused;
+        AddSegment();
+    }
+
+    public void Reset()
+    {
+        Stop();
+        _previousLapSegments.Clear();
+        Start();
+    }
+
+    private void AddSegment()
+    {
+        _previousLapSegments.Add(CurrentSegment);
+        _currentSegmentStart = null;
     }
 }