🧬 Implement SEE and order bad captures after killers but before quiet…

… moves (#554)

src/Lynx.Benchmark/LocalVariableIn_vs_NoIn.cs

@@ -90,7 +90,7 @@ public static int OldScore(this Move move, in Position position, int[,]? killerM
                 }
             }
 
-            return EvaluationConstants.CaptureMoveBaseScoreValue + EvaluationConstants.MostValueableVictimLeastValuableAttacker[sourcePiece, targetPiece];
+            return EvaluationConstants.BadCaptureMoveBaseScoreValue + EvaluationConstants.MostValueableVictimLeastValuableAttacker[sourcePiece, targetPiece];
         }
         else if (killerMoves is not null && plies is not null)
         {

src/Lynx/EvaluationConstants.cs

@@ -326,10 +326,9 @@ static EvaluationConstants()
 
     public const int TTMoveScoreValue = 2_097_152;
 
-    /// <summary>
-    /// For MVVLVA
-    /// </summary>
-    public const int CaptureMoveBaseScoreValue = 1_048_576;
+    #region Move ordering
+
+    public const int GoodCaptureMoveBaseScoreValue = 1_048_576;
 
     public const int FirstKillerMoveValue = 524_288;
 
@@ -339,13 +338,17 @@ static EvaluationConstants()
 
     public const int PromotionMoveScoreValue = 65_536;
 
+    public const int BadCaptureMoveBaseScoreValue = 32_768;
+
     //public const int MaxHistoryMoveValue => Configuration.EngineSettings.MaxHistoryMoveValue;
 
     /// <summary>
     /// Negative offset to ensure history move scores don't reach other move ordering values
     /// </summary>
     public const int BaseMoveScore = int.MinValue / 2;
 
+    #endregion
+
     /// <summary>
     /// Outside of the evaluation ranges (higher than any sensible evaluation, lower than <see cref="PositiveCheckmateDetectionLimit"/>)
     /// </summary>

src/Lynx/Model/BitBoard.cs

@@ -7,8 +7,12 @@ namespace Lynx.Model;
 
 public static class BitBoardExtensions
 {
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static bool Empty(this BitBoard board) => board == default;
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static bool NotEmpty(this BitBoard board) => board != default;
+
     public static BitBoard Initialize(params BoardSquare[] occupiedSquares)
     {
         BitBoard board = default;
@@ -101,6 +105,17 @@ public static void ToggleBits(this ref BitBoard bitboard, int squareA, int squar
         bitboard ^= (1ul << squareA | 1ul << squareB);
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static ulong LSB(this BitBoard board)
+    {
+        if (System.Runtime.Intrinsics.X86.Bmi1.IsSupported)
+        {
+            return System.Runtime.Intrinsics.X86.Bmi1.X64.ExtractLowestSetBit(board);
+        }
+
+        return board & (~board + 1);
+    }
+
     #region Static methods
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -152,6 +167,30 @@ public static int CountBits(this BitBoard board)
         return BitOperations.PopCount(board);
     }
 
+    /// <summary>
+    /// Extracts the bit that represents each square on a bitboard
+    /// </summary>
+    /// <param name="boardSquare"></param>
+    /// <returns></returns>
+    public static ulong SquareBit(int boardSquare)
+    {
+        return 1UL << boardSquare;
+    }
+
+    public static bool Contains(this BitBoard board, int boardSquare)
+    {
+        var bit = SquareBit(boardSquare);
+
+        return (board & bit) != default;
+    }
+
+    public static bool DoesNotContain(this BitBoard board, int boardSquare)
+    {
+        var bit = SquareBit(boardSquare);
+
+        return (board & bit) == default;
+    }
+
     #endregion
 
     #region Methods accepting BoardSquares

src/Lynx/Model/Position.cs

@@ -14,6 +14,12 @@ public class Position
     /// </summary>
     public BitBoard[] PieceBitBoards { get; }
 
+    public BitBoard Queens => PieceBitBoards[(int)Piece.Q] | PieceBitBoards[(int)Piece.q];
+    public BitBoard Rooks => PieceBitBoards[(int)Piece.R] | PieceBitBoards[(int)Piece.r];
+    public BitBoard Bishops => PieceBitBoards[(int)Piece.B] | PieceBitBoards[(int)Piece.b];
+    public BitBoard Knights => PieceBitBoards[(int)Piece.N] | PieceBitBoards[(int)Piece.n];
+    public BitBoard Kings => PieceBitBoards[(int)Piece.K] | PieceBitBoards[(int)Piece.k];
+
     /// <summary>
     /// Black, White, Both
     /// </summary>
@@ -583,6 +589,63 @@ public string FEN(int halfMovesWithoutCaptureOrPawnMove = 0, int fullMoveClock =
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public int CountPieces() => PieceBitBoards.Sum(b => b.CountBits());
 
+    /// <summary>
+    /// Based on Stormphrax
+    /// </summary>
+    /// <param name="square"></param>
+    /// <returns></returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public int PieceAt(int square)
+    {
+        var bit = BitBoardExtensions.SquareBit(square);
+
+        Side color;
+
+        if ((OccupancyBitBoards[(int)Side.Black] & bit) != default)
+        {
+            color = Side.Black;
+        }
+        else if ((OccupancyBitBoards[(int)Side.White] & bit) != default)
+        {
+            color = Side.White;
+        }
+        else
+        {
+            return (int)Piece.Unknown;
+        }
+
+        var offset = Utils.PieceOffset(color);
+
+        for (int pieceIndex = offset; pieceIndex < 6 + offset; ++pieceIndex)
+        {
+            if (!(PieceBitBoards[pieceIndex] & bit).Empty())
+            {
+                return pieceIndex;
+            }
+        }
+
+        System.Diagnostics.Debug.Fail($"Bit set in {Side} occupancy bitboard, but not piece found");
+
+        return (int)Piece.Unknown;
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public ulong AllAttackersTo(int square, BitBoard occupancy)
+    {
+        System.Diagnostics.Debug.Assert(square != (int)BoardSquare.noSquare);
+
+        var queens = Queens;
+        var rooks = queens | Rooks;
+        var bishops = queens | Bishops;
+
+        return (rooks & Attacks.RookAttacks(square, occupancy))
+            | (bishops & Attacks.BishopAttacks(square, occupancy))
+            | (PieceBitBoards[(int)Piece.p] & Attacks.PawnAttacks[(int)Side.White, square])
+            | (PieceBitBoards[(int)Piece.P] & Attacks.PawnAttacks[(int)Side.Black, square])
+            | (Knights & Attacks.KnightAttacks[square])
+            | (Kings & Attacks.KingAttacks[square]);
+    }
+
     /// <summary>
     /// Evaluates material and position in a NegaMax style.
     /// That is, positive scores always favour playing <see cref="Side"/>.

src/Lynx/SEE.cs

@@ -0,0 +1,169 @@
+using Lynx.Model;
+using System.Runtime.CompilerServices;
+
+namespace Lynx;
+
+/// <summary>
+/// Implementation based on Stormprhax, some comments and clarifications from Altair
+/// </summary>
+public static class SEE
+{
+    #pragma warning disable IDE0055 // Discard formatting in this region
+
+    private static readonly int[] _pieceValues =
+    [
+        100, 450, 450, 650, 1250, 0,
+        100, 450, 450, 650, 1250, 0
+    ];
+
+    #pragma warning restore IDE0055
+
+    /// <summary>
+    /// Doesn't handle non-captures, promotions and en-passants
+    /// </summary>
+    /// <param name="position"></param>
+    /// <param name="move"></param>
+    /// <param name="threshold"></param>
+    /// <returns></returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static bool IsGoodCapture(Position position, Move move, short threshold = 0)
+    {
+        System.Diagnostics.Debug.Assert(move.IsCapture(), $"{nameof(IsGoodCapture)} doesn't handle non-capture moves");
+        System.Diagnostics.Debug.Assert(move.PromotedPiece() == default, $"{nameof(IsGoodCapture)} doesn't handle promotion moves");
+        System.Diagnostics.Debug.Assert(!move.IsEnPassant(), $"{nameof(IsGoodCapture)} potentially doesn't handle en-passant moves");
+
+        var sideToMove = position.Side;
+
+        var score = Gain(position, move) - threshold;
+
+        // If taking the opponent's piece without any risk is still negative
+        if (score < 0)
+        {
+            return false;
+        }
+
+        var next = move.Piece();
+        score -= _pieceValues[next];
+
+        // If risking our piece being fully lost and the exchange value is still >= 0
+        if (score >= 0)
+        {
+            return true;
+        }
+
+        var targetSquare = move.TargetSquare();
+
+        var occupancy = position.OccupancyBitBoards[(int)Side.Both]
+            ^ BitBoardExtensions.SquareBit(move.SourceSquare())
+            ^ BitBoardExtensions.SquareBit(targetSquare);
+
+        var queens = position.Queens;
+        var bishops = queens | position.Bishops;
+        var rooks = queens | position.Rooks;
+
+        var attackers = position.AllAttackersTo(targetSquare, occupancy);
+
+        var us = Utils.OppositeSide(sideToMove);
+
+        while (true)
+        {
+            var ourAttackers = attackers & position.OccupancyBitBoards[us];
+
+            if (ourAttackers.Empty())
+            {
+                break;
+            }
+
+            var nextPiece = PopLeastValuableAttacker(position, ref occupancy, ourAttackers, us);
+
+            // After removing an attacker, there could be a sliding piece attack
+            if (nextPiece == Piece.P || nextPiece == Piece.p
+                || nextPiece == Piece.B || nextPiece == Piece.b
+                || nextPiece == Piece.Q || nextPiece == Piece.q)
+            {
+                attackers |= Attacks.BishopAttacks(targetSquare, occupancy) & bishops;
+            }
+
+            if (nextPiece == Piece.R || nextPiece == Piece.r
+                || nextPiece == Piece.Q || nextPiece == Piece.q)
+            {
+                attackers |= Attacks.RookAttacks(targetSquare, occupancy) & rooks;
+            }
+
+            // Removing used pieces from attackers
+            attackers &= occupancy;
+
+            score = -score - 1 - _pieceValues[(int)nextPiece];
+            us = Utils.OppositeSide(us);
+
+            if (score >= 0)
+            {
+                // Our only attacker is our king, but the opponent still has defenders
+                if ((nextPiece == Piece.K || nextPiece == Piece.k)
+                    && (attackers & position.OccupancyBitBoards[us]).NotEmpty())
+                {
+                    us = Utils.OppositeSide(us);
+                }
+
+                break;
+            }
+        }
+
+        return (int)sideToMove != us;
+    }
+
+    /// <summary>
+    /// Doesn't handle non-captures, promotions and en-passants
+    /// </summary>
+    /// <param name="position"></param>
+    /// <param name="move"></param>
+    /// <returns></returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static int Gain(Position position, Move move) => _pieceValues[position.PieceAt(move.TargetSquare())];
+
+    /// <summary>
+    /// Doesn't handle non-captures, promotions and en-passants
+    /// </summary>
+    /// <param name="position"></param>
+    /// <param name="move"></param>
+    /// <returns></returns>
+    [Obsolete("Since we're not handling non-captures, promotiosn and en-passants, we don't really need this")]
+    private static int CompleteGain(Position position, Move move)
+    {
+        if (move.IsCastle())
+        {
+            return 0;
+        }
+        else if (move.IsEnPassant())
+        {
+            return _pieceValues[(int)Piece.P];
+        }
+
+        var promotedPiece = move.PromotedPiece();
+
+        return promotedPiece == default
+            ? _pieceValues[position.PieceAt(move.TargetSquare())]
+            : _pieceValues[promotedPiece] - _pieceValues[(int)Piece.P];
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static Piece PopLeastValuableAttacker(Position position, ref BitBoard occupancy, BitBoard attackers, int color)
+    {
+        var start = Utils.PieceOffset(color);
+
+        for (int i = start; i < start + 6; ++i)
+        {
+            var piece = (Piece)i;
+            var board = attackers & position.PieceBitBoards[i];
+
+            if (!board.Empty())
+            {
+                occupancy ^= board.LSB();
+
+                return piece;
+            }
+        }
+
+        return Piece.Unknown;
+    }
+}