feat: null move pruning

pkg/board.go

@@ -371,6 +371,17 @@ func (board *Board) IsPieceAtSquareWhite(square byte) bool {
 	return (board.WhitePawns|board.WhiteKnights|board.WhiteBishops|board.WhiteRooks|board.WhiteQueens|board.WhiteKing)&mask != 0
 }
 
+// IsInCheck returns true if the side to move is in check.
+func (board *Board) IsInCheck(whiteToMove bool) bool {
+	var kingSq byte
+	if whiteToMove {
+		kingSq = byte(bits.TrailingZeros64(board.WhiteKing))
+	} else {
+		kingSq = byte(bits.TrailingZeros64(board.BlackKing))
+	}
+	return board.IsSquareAttacked(kingSq, whiteToMove)
+}
+
 // SquareToRank returns the rank (0-7) of a square index (0 = rank 0, 7 = rank 7)
 func (board *Board) SquareToRank(square byte) byte {
 	return square / 8

pkg/move.go

@@ -260,6 +260,11 @@ func (board *Board) Move(move Move) MoveState {
 				board.Castling.BlackQueenSide = false
 			}
 		}
+		/** Defensive: king capture is illegal.
+		if move.Captured == WhiteKing || move.Captured == BlackKing {
+			panic(fmt.Sprintf("Illegal move: %s captures the king", move.ToMove()))
+		}
+		*/
 	}
 
 	board.WhiteToMove = !board.WhiteToMove

pkg/search.go

@@ -10,6 +10,8 @@ const (
 	SearchMaxDepth      = 16        // Maximum depth to search
 	MaxEvaluationScore  = 1_000_000 // Maximum score for wining
 	MaxEvaluationTimeMs = 3_000     // Maximum time for a search at the root level
+	NullMoveReduction   = 2         // Reduction for null move pruning (R)
+	NullMoveMinDepth    = 3         // Minimum depth for null mov
 )
 
 type SearchOptions struct {
@@ -187,6 +189,15 @@ func (board *Board) ParallelRootSearch(depth int, tt *TranspositionTable, moves
 }
 
 func (board *Board) AlphaBetaSearch(depth int, maximizing bool, alpha int, beta int, tt *TranspositionTable, stats *SearchResult, ctx *SearchContext, ply int) int {
+	/*
+		defer func() {
+			if r := recover(); r != nil {
+				stack := debug.Stack()
+				fmt.Printf("PANIC in AlphaBetaSearch: %v\nStacktrace:\n%s\nContext: depth=%d maximizing=%v alpha=%d beta=%d ply=%d board=%v\n", r, stack, depth, maximizing, alpha, beta, ply, board)
+				panic(r)
+			}
+		}()
+	*/
 	// Check for cancellation at every node
 	select {
 	case <-ctx.Done:
@@ -218,6 +229,29 @@ func (board *Board) AlphaBetaSearch(depth int, maximizing bool, alpha int, beta
 		return board.MateOrStalemateScore(maximizing)
 	}
 
+	// --- Null Move Pruning ---
+	if depth >= NullMoveMinDepth && ply > 0 && !board.IsInCheck(board.WhiteToMove) && len(board.GenerateLegalMoves()) > 0 {
+		nullBoard := board.Clone()
+		nullBoard.WhiteToMove = !nullBoard.WhiteToMove // Switch side to move (null move)
+		nullEval := 0
+		newDepth := depth - NullMoveReduction - 1
+		if newDepth < 1 {
+			newDepth = 1
+		}
+		if maximizing {
+			nullEval = nullBoard.AlphaBetaSearch(newDepth, false, alpha, beta, tt, stats, ctx, ply+1)
+			if nullEval >= beta {
+				return beta // Fail-hard beta cutoff
+			}
+		} else {
+			nullEval = nullBoard.AlphaBetaSearch(newDepth, true, alpha, beta, tt, stats, ctx, ply+1)
+			if nullEval <= alpha {
+				return alpha // Fail-hard alpha cutoff
+			}
+		}
+	}
+
+	// --- Alpha-Beta Pruning ---
 	var result int
 	var bestMove Move
 	if maximizing {