Stockfish

Representation of The Game State The Position Class The Position class is the core data structure in Stockfish that represents the complete state of a chess game at any given moment. It stores the board, pieces, game state, and provides methods to query and manipulate the position. class Position { public: static void init(); Position() = default; Position(const Position&) = delete; Position& operator=(const Position&) = delete; // FEN string input/output Position& set(const std::string& fenStr, bool isChess960, StateInfo* si, Thread* th); const std::string fen() const; // Position representation Bitboard pieces() const; Bitboard pieces(PieceType pt) const; Bitboard pieces(PieceType pt1, PieceType pt2) const; Bitboard pieces(Color c) const; Bitboard pieces(Color c, PieceType pt) const; Bitboard pieces(Color c, PieceType pt1, PieceType pt2) const; Piece piece_on(Square s) const; Square ep_square() const; bool empty(Square s) const; template<PieceType Pt> int count(Color c) const; template<PieceType Pt> const Square* squares(Color c) const; template<PieceType Pt> Square square(Color c) const; // Castling int can_castle(Color c) const; int can_castle(CastlingRight cr) const; bool castling_impeded(CastlingRight cr) const; Square castling_rook_square(CastlingRight cr) const; // Checking Bitboard checkers() const; Bitboard discovered_check_candidates() const; Bitboard pinned_pieces(Color c) const; Bitboard check_squares(PieceType pt) const; // Attacks to/from a given square Bitboard attackers_to(Square s) const; Bitboard attackers_to(Square s, Bitboard occupied) const; Bitboard attacks_from(Piece pc, Square s) const; template<PieceType> Bitboard attacks_from(Square s) const; template<PieceType> Bitboard attacks_from(Square s, Color c) const; Bitboard slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const; // Properties of moves bool legal(Move m) const; bool pseudo_legal(const Move m) const; bool capture(Move m) const; bool capture_or_promotion(Move m) const; bool gives_check(Move m) const; bool advanced_pawn_push(Move m) const; Piece moved_piece(Move m) const; Piece captured_piece() const; // Piece specific bool pawn_passed(Color c, Square s) const; bool opposite_bishops() const; // Doing and undoing moves void do_move(Move m, StateInfo& st, bool givesCheck); void undo_move(Move m); void do_null_move(StateInfo& st); void undo_null_move(); // Static Exchange Evaluation bool see_ge(Move m, Value value) const; // Accessing hash keys Key key() const; Key key_after(Move m) const; Key material_key() const; Key pawn_key() const; // Other properties of the position Color side_to_move() const; Phase game_phase() const; int game_ply() const; bool is_chess960() const; Thread* this_thread() const; uint64_t nodes_searched() const; bool is_draw() const; int rule50_count() const; Score psq_score() const; Value non_pawn_material(Color c) const; // Position consistency check, for debugging bool pos_is_ok(int* failedStep = nullptr) const; void flip(); private: // Initialization helpers (used while setting up a position) void set_castling_right(Color c, Square rfrom); void set_state(StateInfo* si) const; void set_check_info(StateInfo* si) const; // Other helpers void put_piece(Piece pc, Square s); void remove_piece(Piece pc, Square s); void move_piece(Piece pc, Square from, Square to); template<bool Do> void do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto); // Data members Piece board[SQUARE_NB]; Bitboard byTypeBB[PIECE_TYPE_NB]; Bitboard byColorBB[COLOR_NB]; int pieceCount[PIECE_NB]; Square pieceList[PIECE_NB][16]; int index[SQUARE_NB]; int castlingRightsMask[SQUARE_NB]; Square castlingRookSquare[CASTLING_RIGHT_NB]; Bitboard castlingPath[CASTLING_RIGHT_NB]; uint64_t nodes; int gamePly; Color sideToMove; Thread* thisThread; StateInfo* st; bool chess960; }; Class Design Decisions 1. Non-Copyable Position(const Position&) = delete; Position& operator=(const Position&) = delete; Cannot be copied (copy constructor and assignment deleted) Why? Positions are heavy objects with complex state Must be moved or passed by reference/pointer Prevents accidental expensive copies 2. Default Constructor Position() = default; Creates uninitialized position Must call set() to initialize with FEN string Core Data Members 1. Board Representation - MailBox Piece board[SQUARE_NB]; // SQUARE_NB = 64 Mailbox representation: Direct lookup “what piece is on square X?” board[e4] → returns W_KNIGHT or NO_PIECE Fast for: “piece_on(Square s)” 2. Bitboard Representation Technically we need 12 bitboards to represent the all the pieces on chessboard. ...

C++ Used in Stockfish Stockfish is written in a style of C++ that prioritizes performance, predictability, and compile-time resolution over traditional object-oriented design. Rather than heavy use of classes, inheritance, or virtual functions, the engine relies on enums, inline functions, templates, bitwise operations, and plain data structures. This makes the code extremely fast, cache-friendly, and suitable for deep search loops executed billions of times. Enums as Core Types Enums form the backbone of Stockfish’s type system. Instead of using classes for concepts like pieces, squares, colors, or moves, Stockfish represents them as enums with carefully chosen integer values. ...