⚖️ 'Bad' bishop penalty (#1022)

`BadBishopPenalty` indexed by number of same color pawns
lynx-chess · Sep 17, 2024 · 9aa7aa7 · 9aa7aa7
1 parent b33f5c9
commit 9aa7aa7
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Showing 3 changed files with 5,162 additions and 5,135 deletions.
diff --git a/src/Lynx/Model/Position.cs b/src/Lynx/Model/Position.cs
@@ -617,7 +617,7 @@ internal int AdditionalPieceEvaluation(int bucket, int pieceSquareIndex, int pie
         {
             (int)Piece.P or (int)Piece.p => PawnAdditionalEvaluation(bucket, pieceSquareIndex, pieceIndex, sameSideKingSquare, oppositeSideKingSquare),
             (int)Piece.R or (int)Piece.r => RookAdditionalEvaluation(pieceSquareIndex, pieceIndex, pieceSide, enemyPawnAttacks),
-            (int)Piece.B or (int)Piece.b => BishopAdditionalEvaluation(pieceSquareIndex, pieceSide, enemyPawnAttacks),
+            (int)Piece.B or (int)Piece.b => BishopAdditionalEvaluation(pieceSquareIndex, pieceIndex, pieceSide, enemyPawnAttacks),
             (int)Piece.N or (int)Piece.n => KnightAdditionalEvaluation(pieceSquareIndex, pieceSide, enemyPawnAttacks),
             (int)Piece.Q or (int)Piece.q => QueenAdditionalEvaluation(pieceSquareIndex, pieceSide, enemyPawnAttacks),
             _ => 0
@@ -714,15 +714,29 @@ private int KnightAdditionalEvaluation(int squareIndex, int pieceSide, BitBoard
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private int BishopAdditionalEvaluation(int squareIndex, int pieceSide, BitBoard enemyPawnAttacks)
+    private int BishopAdditionalEvaluation(int squareIndex, int pieceIndex, int pieceSide, BitBoard enemyPawnAttacks)
     {
+        const int pawnToBishopOffset = (int)Piece.B - (int)Piece.P;
+
         // Mobility
         var attacksCount =
             (Attacks.BishopAttacks(squareIndex, OccupancyBitBoards[(int)Side.Both])
                 & (~(OccupancyBitBoards[pieceSide] | enemyPawnAttacks)))
             .CountBits();
 
-        return BishopMobilityBonus[attacksCount];
+        var packedBonus = BishopMobilityBonus[attacksCount];
+
+        // Bad bishop
+        var sameSidePawns = PieceBitBoards[pieceIndex - pawnToBishopOffset];
+
+        var sameColorPawns = sameSidePawns &
+            (Constants.DarkSquares[squareIndex] == 1
+                ? Constants.DarkSquaresBitBoard
+                : Constants.LightSquaresBitBoard);
+
+        packedBonus += BadBishopPenalty[sameColorPawns.CountBits()];
+
+        return packedBonus;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]