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Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Stockfish is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <>.
#include <cassert>
#include <deque>
#include <memory> // For std::unique_ptr
#include <string>
#include "bitboard.h"
#include "types.h"
/// StateInfo struct stores information needed to restore a Position object to
/// its previous state when we retract a move. Whenever a move is made on the
/// board (by calling Position::do_move), a StateInfo object must be passed.
struct StateInfo {
// Copied when making a move
Key pawnKey;
Key materialKey;
Value nonPawnMaterial[COLOR_NB];
int castlingRights;
int rule50;
int pliesFromNull;
Score psq;
Square epSquare;
// Not copied when making a move (will be recomputed anyhow)
Key key;
Bitboard checkersBB;
Piece capturedPiece;
StateInfo* previous;
Bitboard blockersForKing[COLOR_NB];
Bitboard pinnersForKing[COLOR_NB];
Bitboard checkSquares[PIECE_TYPE_NB];
// In a std::deque references to elements are unaffected upon resizing
typedef std::unique_ptr<std::deque<StateInfo>> StateListPtr;
/// Position class stores information regarding the board representation as
/// pieces, side to move, hash keys, castling info, etc. Important methods are
/// do_move() and undo_move(), used by the search to update node info when
/// traversing the search tree.
class Thread;
class Position {
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();
// 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;
extern std::ostream& operator<<(std::ostream& os, const Position& pos);
inline Color Position::side_to_move() const {
return sideToMove;
inline bool Position::empty(Square s) const {
return board[s] == NO_PIECE;
inline Piece Position::piece_on(Square s) const {
return board[s];
inline Piece Position::moved_piece(Move m) const {
return board[from_sq(m)];
inline Bitboard Position::pieces() const {
return byTypeBB[ALL_PIECES];
inline Bitboard Position::pieces(PieceType pt) const {
return byTypeBB[pt];
inline Bitboard Position::pieces(PieceType pt1, PieceType pt2) const {
return byTypeBB[pt1] | byTypeBB[pt2];
inline Bitboard Position::pieces(Color c) const {
return byColorBB[c];
inline Bitboard Position::pieces(Color c, PieceType pt) const {
return byColorBB[c] & byTypeBB[pt];
inline Bitboard Position::pieces(Color c, PieceType pt1, PieceType pt2) const {
return byColorBB[c] & (byTypeBB[pt1] | byTypeBB[pt2]);
template<PieceType Pt> inline int Position::count(Color c) const {
return pieceCount[make_piece(c, Pt)];
template<PieceType Pt> inline const Square* Position::squares(Color c) const {
return pieceList[make_piece(c, Pt)];
template<PieceType Pt> inline Square Position::square(Color c) const {
assert(pieceCount[make_piece(c, Pt)] == 1);
return pieceList[make_piece(c, Pt)][0];
inline Square Position::ep_square() const {
return st->epSquare;
inline int Position::can_castle(CastlingRight cr) const {
return st->castlingRights & cr;
inline int Position::can_castle(Color c) const {
return st->castlingRights & ((WHITE_OO | WHITE_OOO) << (2 * c));
inline bool Position::castling_impeded(CastlingRight cr) const {
return byTypeBB[ALL_PIECES] & castlingPath[cr];
inline Square Position::castling_rook_square(CastlingRight cr) const {
return castlingRookSquare[cr];
template<PieceType Pt>
inline Bitboard Position::attacks_from(Square s) const {
return Pt == BISHOP || Pt == ROOK ? attacks_bb<Pt>(s, byTypeBB[ALL_PIECES])
: Pt == QUEEN ? attacks_from<ROOK>(s) | attacks_from<BISHOP>(s)
: StepAttacksBB[Pt][s];
inline Bitboard Position::attacks_from<PAWN>(Square s, Color c) const {
return StepAttacksBB[make_piece(c, PAWN)][s];
inline Bitboard Position::attacks_from(Piece pc, Square s) const {
return attacks_bb(pc, s, byTypeBB[ALL_PIECES]);
inline Bitboard Position::attackers_to(Square s) const {
return attackers_to(s, byTypeBB[ALL_PIECES]);
inline Bitboard Position::checkers() const {
return st->checkersBB;
inline Bitboard Position::discovered_check_candidates() const {
return st->blockersForKing[~sideToMove] & pieces(sideToMove);
inline Bitboard Position::pinned_pieces(Color c) const {
return st->blockersForKing[c] & pieces(c);
inline Bitboard Position::check_squares(PieceType pt) const {
return st->checkSquares[pt];
inline bool Position::pawn_passed(Color c, Square s) const {
return !(pieces(~c, PAWN) & passed_pawn_mask(c, s));
inline bool Position::advanced_pawn_push(Move m) const {
return type_of(moved_piece(m)) == PAWN
&& relative_rank(sideToMove, from_sq(m)) > RANK_4;
inline Key Position::key() const {
return st->key;
inline Key Position::pawn_key() const {
return st->pawnKey;
inline Key Position::material_key() const {
return st->materialKey;
inline Score Position::psq_score() const {
return st->psq;
inline Value Position::non_pawn_material(Color c) const {
return st->nonPawnMaterial[c];
inline int Position::game_ply() const {
return gamePly;
inline int Position::rule50_count() const {
return st->rule50;
inline uint64_t Position::nodes_searched() const {
return nodes;
inline bool Position::opposite_bishops() const {
return pieceCount[W_BISHOP] == 1
&& pieceCount[B_BISHOP] == 1
&& opposite_colors(square<BISHOP>(WHITE), square<BISHOP>(BLACK));
inline bool Position::is_chess960() const {
return chess960;
inline bool Position::capture_or_promotion(Move m) const {
return type_of(m) != NORMAL ? type_of(m) != CASTLING : !empty(to_sq(m));
inline bool Position::capture(Move m) const {
// Castling is encoded as "king captures rook"
return (!empty(to_sq(m)) && type_of(m) != CASTLING) || type_of(m) == ENPASSANT;
inline Piece Position::captured_piece() const {
return st->capturedPiece;
inline Thread* Position::this_thread() const {
return thisThread;
inline void Position::put_piece(Piece pc, Square s) {
board[s] = pc;
byTypeBB[ALL_PIECES] |= s;
byTypeBB[type_of(pc)] |= s;
byColorBB[color_of(pc)] |= s;
index[s] = pieceCount[pc]++;
pieceList[pc][index[s]] = s;
pieceCount[make_piece(color_of(pc), ALL_PIECES)]++;
inline void Position::remove_piece(Piece pc, Square s) {
// WARNING: This is not a reversible operation. If we remove a piece in
// do_move() and then replace it in undo_move() we will put it at the end of
// the list and not in its original place, it means index[] and pieceList[]
// are not invariant to a do_move() + undo_move() sequence.
byTypeBB[ALL_PIECES] ^= s;
byTypeBB[type_of(pc)] ^= s;
byColorBB[color_of(pc)] ^= s;
/* board[s] = NO_PIECE; Not needed, overwritten by the capturing one */
Square lastSquare = pieceList[pc][--pieceCount[pc]];
index[lastSquare] = index[s];
pieceList[pc][index[lastSquare]] = lastSquare;
pieceList[pc][pieceCount[pc]] = SQ_NONE;
pieceCount[make_piece(color_of(pc), ALL_PIECES)]--;
inline void Position::move_piece(Piece pc, Square from, Square to) {
// index[from] is not updated and becomes stale. This works as long as index[]
// is accessed just by known occupied squares.
Bitboard from_to_bb = SquareBB[from] ^ SquareBB[to];
byTypeBB[ALL_PIECES] ^= from_to_bb;
byTypeBB[type_of(pc)] ^= from_to_bb;
byColorBB[color_of(pc)] ^= from_to_bb;
board[from] = NO_PIECE;
board[to] = pc;
index[to] = index[from];
pieceList[pc][index[to]] = to;
#endif // #ifndef POSITION_H_INCLUDED