prevent millisecond drift in wartime

fix #130

Author: dealloc

src/Helldivers-2-Core/Mapping/MappingContext.cs

@@ -66,8 +66,15 @@ internal MappingContext(WarId warId,
         InvariantWarStatus = warStatuses.FirstOrDefault().Value
                              ?? throw new InvalidOperationException("No warstatus available");
 
+        
         var gameTime = DateTime.UnixEpoch.AddSeconds(warInfo.StartDate + InvariantWarStatus.Time);
-        GameTimeDeviation = DateTime.UtcNow.Subtract(gameTime);
+        GameTimeDeviation = TruncateToSeconds(DateTime.UtcNow).Subtract(gameTime);
         RelativeGameStart = DateTime.UnixEpoch.Add(GameTimeDeviation).AddSeconds(warInfo.StartDate);
     }
+
+    /// <summary>
+    /// ArrowHead doesn't send timestamps more accurate than seconds, so we truncate our relative time to seconds.
+    /// This prevents timestamps for the same value from being different (due to milli/micro second differences).
+    /// </summary>
+    private static DateTime TruncateToSeconds(DateTime dateTime) => dateTime.AddTicks(-(dateTime.Ticks % TimeSpan.TicksPerSecond));
 }