🔍 Add third killer move (#517)

src/Lynx/EvaluationConstants.cs

@@ -329,7 +329,9 @@ static EvaluationConstants()
 
     public const int SecondKillerMoveValue = 262_144;
 
-    public const int PromotionMoveScoreValue = 131_072;
+    public const int ThirdKillerMoveValue = 131_072;
+
+    public const int PromotionMoveScoreValue = 65_536;
 
     //public const int MaxHistoryMoveValue => Configuration.EngineSettings.MaxHistoryMoveValue;
 

src/Lynx/Search/Helpers.cs

@@ -129,6 +129,11 @@ internal int ScoreMove(Move move, int depth, bool useKillerAndPositionMoves, Mov
                 return EvaluationConstants.SecondKillerMoveValue;
             }
 
+            if (_killerMoves[2, depth] == move)
+            {
+                return EvaluationConstants.ThirdKillerMoveValue;
+            }
+
             // History move or 0 if not found
             return EvaluationConstants.BaseMoveScore + _historyMoves[move.Piece(), move.TargetSquare()];
         }

src/Lynx/Search/IDDFS.cs

@@ -10,7 +10,7 @@ public sealed partial class Engine
 {
     private readonly Stopwatch _stopWatch = new();
     private readonly Move[] _pVTable = new Move[Configuration.EngineSettings.MaxDepth * (Configuration.EngineSettings.MaxDepth + 1) / 2];
-    private readonly int[,] _killerMoves = new int[2, Configuration.EngineSettings.MaxDepth];
+    private readonly int[,] _killerMoves = new int[3, Configuration.EngineSettings.MaxDepth];
     private readonly int[,] _historyMoves = new int[12, 64];
     private readonly int[] _maxDepthReached = new int[Constants.AbsoluteMaxDepth];
     private TranspositionTable _tt = Array.Empty<TranspositionTableElement>();
@@ -21,7 +21,7 @@ public sealed partial class Engine
     private bool _isScoringPV;
 
     private SearchResult? _previousSearchResult;
-    private readonly int[,] _previousKillerMoves = new int[2, Configuration.EngineSettings.MaxDepth];
+    private readonly int[,] _previousKillerMoves = new int[3, Configuration.EngineSettings.MaxDepth];
 
     private readonly Move _defaultMove = default;
 
@@ -265,6 +265,7 @@ private int CheckPonderHit(ref SearchResult? lastSearchResult, int depth)
             {
                 _killerMoves[0, d] = _previousKillerMoves[0, d + 2];
                 _killerMoves[1, d] = _previousKillerMoves[1, d + 2];
+                _killerMoves[2, d] = _previousKillerMoves[2, d + 2];
             }
 
             // Re-search from depth 1

src/Lynx/Search/NegaMax.cs

@@ -314,6 +314,7 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool parentWasNullM
                 // 🔍 Killer moves
                 if (!move.IsCapture() && move.PromotedPiece() == default && move != _killerMoves[0, ply])
                 {
+                    _killerMoves[2, ply] = _killerMoves[1, ply];
                     _killerMoves[1, ply] = _killerMoves[0, ply];
                     _killerMoves[0, ply] = move;
                 }