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⚖️ Trapped rook IV - only when can't castle to that side #2358

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⚖️ Trapped rook IV - only when can't castle to that side #2358

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4309107
Add TrappedRookPenalty
eduherminio Jan 9, 2026
0f0db2e
Increase mobility and rank requirements
eduherminio Jan 10, 2026
6df78dc
Differenciate being able to castle vs not
eduherminio Jan 11, 2026
730ee35
Merge with main
eduherminio Jan 11, 2026
a676b0c
Retune with only buried values
eduherminio Jan 11, 2026
e936ad6
Exclude non-DFRC posiitions
eduherminio Jan 12, 2026
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75 changes: 67 additions & 8 deletions src/Lynx/Evaluation.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -167,7 +167,7 @@ public partial class Position
{
bitboard = bitboard.WithoutLS1B(out var pieceSquareIndex);

packedScore += AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, whiteBucket, blackBucket, pieceIndex, (int)Side.White, blackPawnAttacks, blackKing);
packedScore += AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, whiteBucket, blackBucket, pieceIndex, (int)Side.White, blackPawnAttacks, whiteKing, blackKing);
}
}

Expand All @@ -184,7 +184,7 @@ public partial class Position
{
bitboard = bitboard.WithoutLS1B(out var pieceSquareIndex);

packedScore -= AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, blackBucket, whiteBucket, pieceIndex, (int)Side.Black, whitePawnAttacks, whiteKing);
packedScore -= AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, blackBucket, whiteBucket, pieceIndex, (int)Side.Black, whitePawnAttacks, blackKing, whiteKing);
}
}
}
Expand Down Expand Up @@ -295,7 +295,7 @@ public partial class Position

IncrementalPhaseAccumulator += GamePhaseByPiece[pieceIndex];

packedScore += AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, whiteBucket, blackBucket, pieceIndex, (int)Side.White, blackPawnAttacks, blackKing);
packedScore += AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, whiteBucket, blackBucket, pieceIndex, (int)Side.White, blackPawnAttacks, whiteKing, blackKing);
}
}

Expand All @@ -316,7 +316,7 @@ public partial class Position

IncrementalPhaseAccumulator += GamePhaseByPiece[pieceIndex];

packedScore -= AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, blackBucket, whiteBucket, pieceIndex, (int)Side.Black, whitePawnAttacks, whiteKing);
packedScore -= AdditionalPieceEvaluation(ref evaluationContext, pieceSquareIndex, blackBucket, whiteBucket, pieceIndex, (int)Side.Black, whitePawnAttacks, blackKing, whiteKing);
}
}

Expand Down Expand Up @@ -573,11 +573,11 @@ public static int EvaluateFinalPosition(int ply, bool isInCheck)
/// Doesn't include <see cref="Piece.P"/>, <see cref="Piece.p"/>, <see cref="Piece.K"/> and <see cref="Piece.k"/> evaluation
/// </summary>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
internal int AdditionalPieceEvaluation(ref EvaluationContext evaluationContext, int pieceSquareIndex, int bucket, int oppositeSideBucket, int pieceIndex, int pieceSide, BitBoard enemyPawnAttacks, int oppositeSideKingSquare)
internal int AdditionalPieceEvaluation(ref EvaluationContext evaluationContext, int pieceSquareIndex, int bucket, int oppositeSideBucket, int pieceIndex, int pieceSide, BitBoard enemyPawnAttacks, int sameSideKingSquare, int oppositeSideKingSquare)
{
return pieceIndex switch
{
(int)Piece.R or (int)Piece.r => RookAdditionalEvaluation(ref evaluationContext, pieceSquareIndex, bucket, oppositeSideBucket, pieceIndex, pieceSide, enemyPawnAttacks, oppositeSideKingSquare),
(int)Piece.R or (int)Piece.r => RookAdditionalEvaluation(ref evaluationContext, pieceSquareIndex, bucket, oppositeSideBucket, pieceIndex, pieceSide, enemyPawnAttacks, sameSideKingSquare, oppositeSideKingSquare),
(int)Piece.B or (int)Piece.b => BishopAdditionalEvaluation(ref evaluationContext, pieceSquareIndex, pieceIndex, pieceSide, enemyPawnAttacks, oppositeSideKingSquare),
(int)Piece.N or (int)Piece.n => KnightAdditionalEvaluation(ref evaluationContext, pieceSquareIndex, pieceIndex, pieceSide, enemyPawnAttacks, oppositeSideKingSquare),
(int)Piece.Q or (int)Piece.q => QueenAdditionalEvaluation(ref evaluationContext, pieceSquareIndex, pieceIndex, pieceSide, enemyPawnAttacks, oppositeSideKingSquare),
Expand Down Expand Up @@ -794,7 +794,7 @@ private int BishopAdditionalEvaluation(ref EvaluationContext evaluationContext,
}

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private int RookAdditionalEvaluation(ref EvaluationContext evaluationContext, int squareIndex, int bucket, int oppositeSideBucket, int pieceIndex, int pieceSide, BitBoard enemyPawnAttacks, int oppositeSideKingSquare)
private int RookAdditionalEvaluation(ref EvaluationContext evaluationContext, int squareIndex, int bucket, int oppositeSideBucket, int pieceIndex, int pieceSide, BitBoard enemyPawnAttacks, int sameSideKingSquare, int oppositeSideKingSquare)
{
const int pawnToRookOffset = (int)Piece.R - (int)Piece.P;
var sameSidePawns = _pieceBitBoards[pieceIndex - pawnToRookOffset];
Expand Down Expand Up @@ -832,6 +832,65 @@ private int RookAdditionalEvaluation(ref EvaluationContext evaluationContext, in
packedBonus += SemiOpenFileRookBonus[bucket][file];
packedBonus += SemiOpenFileRookEnemyBonus[oppositeSideBucket][file];
}
// Trapped rook
else if (Configuration.EngineSettings.IsChess960
&& mobility <= 3)
{
var rank = Constants.Rank[squareIndex];

if (pieceIndex == (int)Piece.r)
{
rank = 7 - rank;
}

if (rank <= 1)
{
const int EFile = 4;

var rookFile = Constants.File[squareIndex];
var kingFile = Constants.File[sameSideKingSquare];

if (kingFile != rookFile)
{
// Queenside rook
if (kingFile < rookFile && kingFile >= EFile)
{
if (pieceSide == (int)Side.White)
{
if ((_castle & (int)CastlingRights.WQ) == default)
{
packedBonus += BuriedRookPenalty;
}
}
else
{
if ((_castle & (int)CastlingRights.BQ) == default)
{
packedBonus += BuriedRookPenalty;
}
}
}
// Kingside rook
else if (kingFile <= EFile)
{
if (pieceSide == (int)Side.White)
{
if ((_castle & (int)CastlingRights.WK) == default)
{
packedBonus += BuriedRookPenalty;
}
}
else
{
if ((_castle & (int)CastlingRights.BK) == default)
{
packedBonus += BuriedRookPenalty;
}
}
}
}
}
}

// Connected rooks
if ((attacks & _pieceBitBoards[pieceIndex]).CountBits() >= 1)
Expand Down Expand Up @@ -1008,7 +1067,7 @@ private int Threats(EvaluationContext evaluationContext, Side side, int opposite
// since ~attacksBySide[(int)side] is always false given we're evaluation threats
var defendedSquares = attacks[oppositeSidePawnIndex]
| (attacksBySide[oppositeSide] & ~attacksBySide[(int)side]);

for (int i = (int)Piece.N; i <= (int)Piece.K; ++i)
{
var ourPiecesIndex = 6 + i - oppositeSideOffset;
Expand Down
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