Added draw by fifty move rule

src/Lunarr/Board.cs

@@ -698,9 +698,7 @@ public uint FourBitCastlingRights()
     public void MakeNullMove()
     {
         PlyCount++;
-
-        // A null move is irreversible.
-        FiftyMovePlyCount = 0;
+        FiftyMovePlyCount++;
 
         CapturedPieceType = None;
 
@@ -753,7 +751,7 @@ public void UnmakeNullMove()
         Opponent ^= 1;
 
         PlyCount--;
-        FiftyMovePlyCount = previousPosition.FiftyMovePlyCount;
+        FiftyMovePlyCount--;
 
         TT.CalculateCurrentEntryIndex(this);
 

src/Lunarr/Engine.cs

@@ -1447,6 +1447,17 @@ public static bool IsMateLossScore(int score)
     public static int MatedIn(int ply) => -Checkmate + ply;
 
 
+    /// <summary>
+    /// After 50 moves (100 ply) if there were no captures or pawn moves the game is a draw, unless the last move delivered checkmate.
+    /// </summary>
+    /// <remarks>
+    /// Since verifying a checkmate would involve generating all legal moves (which is expensive), 
+    /// <see cref="Board.IsInCheck"/> is used instead. This means some draws will not be seen immediately
+    /// </remarks>
+    public static bool IsDrawByFiftyMoveRule() => 
+        t_board.FiftyMovePlyCount > 99 &&
+        (t_board.FiftyMovePlyCount != 100 || !t_board.IsInCheck(t_board.Friendly));
+
     /// <summary>If a position is reached three times, it's a draw.</summary>
     /// <remarks>The current implementation returns true even if the position was only reached twice.</remarks>
     public static bool IsDrawByRepetition() => t_board.PositionKeyHistory.Count(other => other == t_board.ZobristKey) >= 2;