Add last-lap exercise

Author: ErikSchierboom

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

@@ -1,32 +1,57 @@
 public enum StopwatchState
 {
-    Ready,
-    Running,
-    Paused,
-    Stopped
+    Stopped,
+    Started
 }
 
 public class SplitSecondStopwatch(TimeProvider time)
 {
-    public StopwatchState State { get; private set; }
+    private readonly List<TimeSpan> _splits = new();
+    private DateTimeOffset? _splitStart;
+    
+    private TimeSpan PreviousSplits => _splits.Aggregate(TimeSpan.Zero, (total, split) => total + split);
+    private TimeSpan CurrentSplit => _splitStart is {} start ? time.GetUtcNow() - start : TimeSpan.Zero;
+
+    public StopwatchState State { get; private set; } = StopwatchState.Stopped;
+    public List<TimeSpan> Laps { get; } = [];
+    public TimeSpan Split => CurrentSplit + PreviousSplits;
+    public TimeSpan Total => Split + Laps.Aggregate(TimeSpan.Zero, (total, split) => total + split);
 
     public void Start()
     {
-        throw new NotImplementedException();
+        if (State != StopwatchState.Stopped)
+            throw new InvalidOperationException("Can't start a stopwatch that is not stopped.");
+        
+        _splitStart = time.GetUtcNow();
+        State = StopwatchState.Started;
     }
 
     public void Stop()
     {
-        throw new NotImplementedException();
-    }
+        if (State != StopwatchState.Started)
+            throw new InvalidOperationException("Can't stop a stopwatch that is not started.");
 
-    public void Split()
+        _splits.Add(CurrentSplit);
+        _splitStart = null;
+        State = StopwatchState.Stopped;
+    }
+    
+    public void Reset()
     {
-        throw new NotImplementedException();
+        if (State != StopwatchState.Stopped)
+            throw new InvalidOperationException("Can't reset a stopwatch that is not stopped.");
+        
+        _splits.Clear();
+        _splitStart = null;
+        State = StopwatchState.Stopped;
     }
-
-    public void Pause()
+    
+    public void Lap()
     {
-        throw new NotImplementedException();
+        if (State != StopwatchState.Started)
+            throw new InvalidOperationException("Can't lap a stopwatch that is not started stopped.");
+        
+        _splits.Add(CurrentSplit);
+        _splitStart = time.GetUtcNow();
     }
 }