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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) 2008-2012 Marco Costalba, Joona Kiiski, 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 "bitboard.h"
#include "types.h"
/// The checkInfo struct is initialized at c'tor time and keeps info used
/// to detect if a move gives check.
class Position;
class Thread;
struct CheckInfo {
explicit CheckInfo(const Position&);
Bitboard dcCandidates;
Bitboard pinned;
Bitboard checkSq[8];
Square ksq;
/// The StateInfo struct stores information we need 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), an StateInfo object
/// must be passed as a parameter.
struct StateInfo {
Key pawnKey, materialKey;
Value npMaterial[2];
int castleRights, rule50, pliesFromNull;
Score psqScore;
Square epSquare;
Key key;
Bitboard checkersBB;
PieceType capturedType;
StateInfo* previous;
struct ReducedStateInfo {
Key pawnKey, materialKey;
Value npMaterial[2];
int castleRights, rule50, pliesFromNull;
Score psqScore;
Square epSquare;
/// The position data structure. A position consists of the following data:
/// * For each piece type, a bitboard representing the squares occupied
/// by pieces of that type.
/// * For each color, a bitboard representing the squares occupied by
/// pieces of that color.
/// * A bitboard of all occupied squares.
/// * A bitboard of all checking pieces.
/// * A 64-entry array of pieces, indexed by the squares of the board.
/// * The current side to move.
/// * Information about the castling rights for both sides.
/// * The initial files of the kings and both pairs of rooks. This is
/// used to implement the Chess960 castling rules.
/// * The en passant square (which is SQ_NONE if no en passant capture is
/// possible).
/// * The squares of the kings for both sides.
/// * Hash keys for the position itself, the current pawn structure, and
/// the current material situation.
/// * Hash keys for all previous positions in the game for detecting
/// repetition draws.
/// * A counter for detecting 50 move rule draws.
class Position {
Position() {}
Position(const Position& p, Thread* t) { *this = p; thisThread = t; }
Position(const std::string& f, bool c960, Thread* t) { from_fen(f, c960, t); }
Position& operator=(const Position&);
// Text input/output
void from_fen(const std::string& fen, bool isChess960, Thread* th);
const std::string to_fen() const;
void print(Move m = MOVE_NONE) 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 king_square(Color c) const;
Square ep_square() const;
bool is_empty(Square s) const;
const Square* piece_list(Color c, PieceType pt) const;
int piece_count(Color c, PieceType pt) const;
// Castling
int can_castle(CastleRight f) const;
int can_castle(Color c) const;
bool castle_impeded(Color c, CastlingSide s) const;
Square castle_rook_square(Color c, CastlingSide s) const;
// Checking
bool in_check() const;
Bitboard checkers() const;
Bitboard discovered_check_candidates() const;
Bitboard pinned_pieces() const;
// Attacks to/from a given square
Bitboard attackers_to(Square s) const;
Bitboard attackers_to(Square s, Bitboard occ) const;
Bitboard attacks_from(Piece p, Square s) const;
static Bitboard attacks_from(Piece p, Square s, Bitboard occ);
template<PieceType> Bitboard attacks_from(Square s) const;
template<PieceType> Bitboard attacks_from(Square s, Color c) const;
// Properties of moves
bool move_gives_check(Move m, const CheckInfo& ci) const;
bool move_attacks_square(Move m, Square s) const;
bool move_is_legal(const Move m) const;
bool pl_move_is_legal(Move m, Bitboard pinned) const;
bool is_pseudo_legal(const Move m) const;
bool is_capture(Move m) const;
bool is_capture_or_promotion(Move m) const;
bool is_passed_pawn_push(Move m) const;
Piece piece_moved(Move m) const;
PieceType captured_piece_type() const;
// Piece specific
bool pawn_is_passed(Color c, Square s) const;
bool pawn_on_7th(Color c) const;
bool opposite_bishops() const;
bool bishop_pair(Color c) const;
// Doing and undoing moves
void do_move(Move m, StateInfo& st);
void do_move(Move m, StateInfo& st, const CheckInfo& ci, bool moveIsCheck);
void undo_move(Move m);
template<bool Do> void do_null_move(StateInfo& st);
// Static exchange evaluation
int see(Move m) const;
int see_sign(Move m) const;
// Accessing hash keys
Key key() const;
Key exclusion_key() const;
Key pawn_key() const;
Key material_key() const;
// Incremental piece-square evaluation
Score psq_score() const;
Score psq_delta(Piece p, Square from, Square to) const;
Value non_pawn_material(Color c) const;
// Other properties of the position
Color side_to_move() const;
int startpos_ply_counter() const;
bool is_chess960() const;
Thread* this_thread() const;
int64_t nodes_searched() const;
void set_nodes_searched(int64_t n);
template<bool SkipRepetition> bool is_draw() const;
// Position consistency check, for debugging
bool pos_is_ok(int* failedStep = NULL) const;
void flip();
// Initialization helpers (used while setting up a position)
void clear();
void put_piece(Piece p, Square s);
void set_castle_right(Color c, Square rfrom);
// Helper template functions
template<bool Do> void do_castle_move(Move m);
template<bool FindPinned> Bitboard hidden_checkers() const;
// Computing hash keys from scratch (for initialization and debugging)
Key compute_key() const;
Key compute_pawn_key() const;
Key compute_material_key() const;
// Computing incremental evaluation scores and material counts
Score compute_psq_score() const;
Value compute_non_pawn_material(Color c) const;
// Board and pieces
Piece board[64]; // [square]
Bitboard byTypeBB[8]; // [pieceType]
Bitboard byColorBB[2]; // [color]
int pieceCount[2][8]; // [color][pieceType]
Square pieceList[2][8][16]; // [color][pieceType][index]
int index[64]; // [square]
// Other info
int castleRightsMask[64]; // [square]
Square castleRookSquare[2][2]; // [color][side]
Bitboard castlePath[2][2]; // [color][side]
StateInfo startState;
int64_t nodes;
int startPosPly;
Color sideToMove;
Thread* thisThread;
StateInfo* st;
int chess960;
inline int64_t Position::nodes_searched() const {
return nodes;
inline void Position::set_nodes_searched(int64_t n) {
nodes = n;
inline Piece Position::piece_on(Square s) const {
return board[s];
inline Piece Position::piece_moved(Move m) const {
return board[from_sq(m)];
inline bool Position::is_empty(Square s) const {
return board[s] == NO_PIECE;
inline Color Position::side_to_move() const {
return sideToMove;
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]);
inline int Position::piece_count(Color c, PieceType pt) const {
return pieceCount[c][pt];
inline const Square* Position::piece_list(Color c, PieceType pt) const {
return pieceList[c][pt];
inline Square Position::ep_square() const {
return st->epSquare;
inline Square Position::king_square(Color c) const {
return pieceList[c][KING][0];
inline int Position::can_castle(CastleRight f) const {
return st->castleRights & f;
inline int Position::can_castle(Color c) const {
return st->castleRights & ((WHITE_OO | WHITE_OOO) << (2 * c));
inline bool Position::castle_impeded(Color c, CastlingSide s) const {
return byTypeBB[ALL_PIECES] & castlePath[c][s];
inline Square Position::castle_rook_square(Color c, CastlingSide s) const {
return castleRookSquare[c][s];
template<PieceType Pt>
inline Bitboard Position::attacks_from(Square s) const {
return Pt == BISHOP || Pt == ROOK ? attacks_bb<Pt>(s, 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 p, Square s) const {
return attacks_from(p, 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 bool Position::in_check() const {
return st->checkersBB != 0;
inline Bitboard Position::discovered_check_candidates() const {
return hidden_checkers<false>();
inline Bitboard Position::pinned_pieces() const {
return hidden_checkers<true>();
inline bool Position::pawn_is_passed(Color c, Square s) const {
return !(pieces(~c, PAWN) & passed_pawn_mask(c, s));
inline Key Position::key() const {
return st->key;
inline Key Position::exclusion_key() const {
return st->key ^ Zobrist::exclusion;
inline Key Position::pawn_key() const {
return st->pawnKey;
inline Key Position::material_key() const {
return st->materialKey;
inline Score Position::psq_delta(Piece p, Square from, Square to) const {
return pieceSquareTable[p][to] - pieceSquareTable[p][from];
inline Score Position::psq_score() const {
return st->psqScore;
inline Value Position::non_pawn_material(Color c) const {
return st->npMaterial[c];
inline bool Position::is_passed_pawn_push(Move m) const {
return type_of(piece_moved(m)) == PAWN
&& pawn_is_passed(sideToMove, to_sq(m));
inline int Position::startpos_ply_counter() const {
return startPosPly + st->pliesFromNull; // HACK
inline bool Position::opposite_bishops() const {
return pieceCount[WHITE][BISHOP] == 1
&& pieceCount[BLACK][BISHOP] == 1
&& opposite_colors(pieceList[WHITE][BISHOP][0], pieceList[BLACK][BISHOP][0]);
inline bool Position::bishop_pair(Color c) const {
return pieceCount[c][BISHOP] >= 2
&& opposite_colors(pieceList[c][BISHOP][0], pieceList[c][BISHOP][1]);
inline bool Position::pawn_on_7th(Color c) const {
return pieces(c, PAWN) & rank_bb(relative_rank(c, RANK_7));
inline bool Position::is_chess960() const {
return chess960;
inline bool Position::is_capture_or_promotion(Move m) const {
return type_of(m) ? type_of(m) != CASTLE : !is_empty(to_sq(m));
inline bool Position::is_capture(Move m) const {
// Note that castle is coded as "king captures the rook"
return (!is_empty(to_sq(m)) && type_of(m) != CASTLE) || type_of(m) == ENPASSANT;
inline PieceType Position::captured_piece_type() const {
return st->capturedType;
inline Thread* Position::this_thread() const {
return thisThread;
#endif // !defined(POSITION_H_INCLUDED)
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