Scale eval with material

lynx-chess · Nov 13, 2024 · 5f151e5 · 5f151e5
1 parent c590b71
commit 5f151e5
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Showing 2 changed files with 25 additions and 9 deletions.
diff --git a/src/Lynx/Model/Position.cs b/src/Lynx/Model/Position.cs
@@ -560,6 +560,7 @@ public bool WasProduceByAValidMove()
         eval = (int)(eval * ((80 + (totalPawnsCount * 7)) / 128.0));
 
         eval = ScaleEvalWith50MovesDrawDistance(eval, movesWithoutCaptureOrPawnMove);
+        eval = ScaleEvalWithMaterial(eval);
 
         eval = Math.Clamp(eval, MinStaticEval, MaxStaticEval);
 
@@ -856,6 +857,21 @@ internal int KingAdditionalEvaluation(int squareIndex, int pieceSide, BitBoard e
     internal static int ScaleEvalWith50MovesDrawDistance(int eval, int movesWithoutCaptureOrPawnMove) =>
         eval * (200 - movesWithoutCaptureOrPawnMove) / 200;
 
+    /// <summary>
+    /// Formula based on Stormphrax
+    /// </summary>
+    internal  int ScaleEvalWithMaterial(int eval)
+    {
+        var material_phase =
+            (SEE.PieceValues[(int)Piece.P] * (PieceBitBoards[(int)Piece.P] & PieceBitBoards[(int)Piece.p]).CountBits())
+            + (SEE.PieceValues[(int)Piece.N] * (PieceBitBoards[(int)Piece.N] & PieceBitBoards[(int)Piece.n]).CountBits())
+            + (SEE.PieceValues[(int)Piece.B] * (PieceBitBoards[(int)Piece.B] & PieceBitBoards[(int)Piece.b]).CountBits())
+            + (SEE.PieceValues[(int)Piece.R] * (PieceBitBoards[(int)Piece.R] & PieceBitBoards[(int)Piece.r]).CountBits())
+            + (SEE.PieceValues[(int)Piece.Q] * (PieceBitBoards[(int)Piece.Q] & PieceBitBoards[(int)Piece.q]).CountBits());
+
+        return eval * (26500 + material_phase) / 32768;
+    }
+
     #endregion
 
     #region Attacks

diff --git a/src/Lynx/SEE.cs b/src/Lynx/SEE.cs
@@ -10,7 +10,7 @@ public static class SEE
 {
     #pragma warning disable IDE0055 // Discard formatting in this region
 
-    private static ReadOnlySpan<int> _pieceValues =>
+    public static ReadOnlySpan<int> PieceValues =>
     [
         100, 450, 450, 650, 1250, 0,
         100, 450, 450, 650, 1250, 0,
@@ -35,7 +35,7 @@ public static bool IsGoodCapture(Position position, Move move, int threshold = 0
 
         var sideToMove = position.Side;
 
-        var score = _pieceValues[move.CapturedPiece()] - threshold;    // Gain() - threshold
+        var score = PieceValues[move.CapturedPiece()] - threshold;    // Gain() - threshold
 
         // If taking the opponent's piece without any risk is still negative
         if (score < 0)
@@ -44,7 +44,7 @@ public static bool IsGoodCapture(Position position, Move move, int threshold = 0
         }
 
         var next = move.Piece();
-        score -= _pieceValues[next];
+        score -= PieceValues[next];
 
         // If risking our piece being fully lost and the exchange value is still >= 0
         if (score >= 0)
@@ -91,7 +91,7 @@ public static bool IsGoodCapture(Position position, Move move, int threshold = 0
             // Removing used pieces from attackers
             attackers &= occupancy;
 
-            score = -score - 1 - _pieceValues[nextPiece];
+            score = -score - 1 - PieceValues[nextPiece];
             us = Utils.OppositeSide(us);
 
             if (score >= 0)
@@ -134,7 +134,7 @@ public static bool HasPositiveScore(Position position, Move move, int threshold
             ? move.PromotedPiece()
             : move.Piece();
 
-        score -= _pieceValues[next];
+        score -= PieceValues[next];
 
         // If risking our piece being fully lost and the exchange value is still >= 0
         if (score >= 0)
@@ -181,7 +181,7 @@ public static bool HasPositiveScore(Position position, Move move, int threshold
             // Removing used pieces from attackers
             attackers &= occupancy;
 
-            score = -score - 1 - _pieceValues[nextPiece];
+            score = -score - 1 - PieceValues[nextPiece];
             us = Utils.OppositeSide(us);
 
             if (score >= 0)
@@ -209,15 +209,15 @@ private static int Gain(Move move)
         }
         else if (move.IsEnPassant())
         {
-            return _pieceValues[(int)Piece.P];
+            return PieceValues[(int)Piece.P];
         }
 
         var promotedPiece = move.PromotedPiece();
 
 #pragma warning disable S3358 // Ternary operators should not be nested
         return promotedPiece == default
-            ? _pieceValues[move.CapturedPiece()]
-            : _pieceValues[promotedPiece] - _pieceValues[(int)Piece.P] + (move.IsCapture() ? _pieceValues[move.CapturedPiece()] : 0);
+            ? PieceValues[move.CapturedPiece()]
+            : PieceValues[promotedPiece] - PieceValues[(int)Piece.P] + (move.IsCapture() ? PieceValues[move.CapturedPiece()] : 0);
 #pragma warning restore S3358 // Ternary operators should not be nested
     }