Enforce HLC drift bounds on backward clock + harden vector clock parsing

src/Distributed/VectorClock.cs

@@ -384,7 +384,7 @@ public override string ToString()
     /// </summary>
     public static VectorClock Parse(string s)
     {
-        if (string.IsNullOrEmpty(s))
+        if (string.IsNullOrWhiteSpace(s))
             return new VectorClock();
 
         var entries = s.Split(',', StringSplitOptions.RemoveEmptyEntries);
@@ -394,7 +394,7 @@ public static VectorClock Parse(string s)
         var pairs = new List<(ushort nodeId, ulong counter)>();
         foreach (var entry in entries)
         {
-            var parts = entry.Split(':');
+            var parts = entry.Split(':', 2, StringSplitOptions.None);
             if (parts.Length != 2)
                 throw new FormatException($"Invalid vector clock entry: {entry}");
 
@@ -428,7 +428,7 @@ private static VectorClock CreateCanonical(List<(ushort nodeId, ulong counter)>
             counters.Add(pair.counter);
         }
 
-        return new VectorClock(nodeIds.ToArray(), counters.ToArray());
+        return new VectorClock([.. nodeIds], [.. counters]);
     }
 
     /// <summary>
@@ -437,7 +437,7 @@ private static VectorClock CreateCanonical(List<(ushort nodeId, ulong counter)>
     public static bool TryParse(string? s, out VectorClock result)
     {
         result = default;
-        if (string.IsNullOrEmpty(s))
+        if (string.IsNullOrWhiteSpace(s))
         {
             result = new VectorClock();
             return true;

src/Distributed/VectorClockCoordinator.cs

@@ -26,7 +26,7 @@ namespace Clockworks.Distributed;
 public sealed class VectorClockCoordinator
 {
     private readonly ushort _nodeId;
-    private readonly object _lock = new();
+    private readonly Lock _lock = new();
     private VectorClock _current;
 
     /// <summary>

src/HlcGuidFactory.cs

@@ -159,19 +159,18 @@ public void NewGuids(Span<Guid> destination)
                     // Counter overflow - must advance logical time
                     _logicalTimeMs++;
                     _counter = 0;
-
-                    // Check drift bounds
-                    var drift = _logicalTimeMs - physicalTimeMs;
-                    if (drift > _options.MaxDriftMs)
-                    {
-                        if (_options.ThrowOnExcessiveDrift)
-                        {
-                            throw new HlcDriftException(drift, _options.MaxDriftMs);
-                        }
-                        // Otherwise, we accept the drift (for high-throughput scenarios)
-                    }
                 }
             }
+
+            var drift = _logicalTimeMs - physicalTimeMs;
+            if (drift > _options.MaxDriftMs)
+            {
+                if (_options.ThrowOnExcessiveDrift)
+                {
+                    throw new HlcDriftException(drift, _options.MaxDriftMs);
+                }
+                // Otherwise, we accept the drift (for high-throughput scenarios)
+            }
 
             timestamp = new HlcTimestamp(_logicalTimeMs, _counter, _nodeId);
 

tests/HlcGuidFactoryDriftTests.cs

@@ -0,0 +1,43 @@
+using Xunit;
+
+namespace Clockworks.Tests;
+
+public sealed class HlcGuidFactoryDriftTests
+{
+    [Fact]
+    public void NewGuidWithHlc_WhenClockMovesBackwardBeyondMaxDrift_ThrowsImmediately_WhenStrict()
+    {
+        var tp = new SimulatedTimeProvider();
+
+        using var factory = new HlcGuidFactory(
+            tp,
+            nodeId: 1,
+            options: new HlcOptions { MaxDriftMs = 0, ThrowOnExcessiveDrift = true });
+
+        _ = factory.NewGuidWithHlc();
+
+        var now = tp.GetUtcNow();
+        tp.SetUtcNow(now - TimeSpan.FromMilliseconds(1));
+
+        Assert.Throws<HlcDriftException>(() => factory.NewGuidWithHlc());
+    }
+
+    [Fact]
+    public void NewGuidWithHlc_WhenClockMovesBackwardBeyondMaxDrift_DoesNotThrow_WhenNotStrict()
+    {
+        var tp = new SimulatedTimeProvider();
+
+        using var factory = new HlcGuidFactory(
+            tp,
+            nodeId: 1,
+            options: new HlcOptions { MaxDriftMs = 0, ThrowOnExcessiveDrift = false });
+
+        _ = factory.NewGuidWithHlc();
+
+        var now = tp.GetUtcNow();
+        tp.SetUtcNow(now - TimeSpan.FromMilliseconds(1));
+
+        var ex = Record.Exception(() => factory.NewGuidWithHlc());
+        Assert.Null(ex);
+    }
+}