🐛 Clamp static evaluation within +- checkmate limits (#510)

* Clamp static eval This avoids engine misbehavior for positions such as: `QQQQQQQQ/QQQQQQQQ/QQQQQQQQ/QQQQQQQQ/QQQQQQQQ/QQQQQQQQ/QPPPPPPP/K6k b - - 0 1` from https://lichess.org/49dBqSDK
lynx-chess · Nov 20, 2023 · bcacef3 · bcacef3
1 parent f4afe2c
commit bcacef3
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diff --git a/src/Lynx/EvaluationConstants.cs b/src/Lynx/EvaluationConstants.cs
@@ -312,6 +312,10 @@ static EvaluationConstants()
  /// </summary>
  public const int NegativeCheckmateDetectionLimit = -27_000; // -CheckMateBaseEvaluation + (Constants.AbsoluteMaxDepth + 45) * DepthCheckmateFactor;
 
+ public const int MinEval = NegativeCheckmateDetectionLimit + 1;
+
+ public const int MaxEval = PositiveCheckmateDetectionLimit - 1;
+
  public const int PVMoveScoreValue = 4_194_304;
 
  public const int TTMoveScoreValue = 2_097_152;

diff --git a/src/Lynx/Model/Position.cs b/src/Lynx/Model/Position.cs
@@ -693,6 +693,8 @@ public string FEN(int halfMovesWithoutCaptureOrPawnMove = 0, int fullMoveClock =
 
  var eval = ((middleGameScore * gamePhase) + (endGameScore * endGamePhase)) / maxPhase;
 
+ eval = Math.Clamp(eval, EvaluationConstants.MinEval, EvaluationConstants.MaxEval);
+
  var sideEval = Side == Side.White
  ? eval
  : -eval;