2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2014 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
37 static const string PieceToChar(" PNBRQK pnbrqk");
41 Value PieceValue[PHASE_NB][PIECE_NB] = {
42 { VALUE_ZERO, PawnValueMg, KnightValueMg, BishopValueMg, RookValueMg, QueenValueMg },
43 { VALUE_ZERO, PawnValueEg, KnightValueEg, BishopValueEg, RookValueEg, QueenValueEg } };
45 static Score psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
49 Key psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
50 Key enpassant[FILE_NB];
51 Key castling[CASTLING_RIGHT_NB];
56 Key Position::exclusion_key() const { return st->key ^ Zobrist::exclusion;}
60 // min_attacker() is a helper function used by see() to locate the least
61 // valuable attacker for the side to move, remove the attacker we just found
62 // from the bitboards and scan for new X-ray attacks behind it.
64 template<int Pt> FORCE_INLINE
65 PieceType min_attacker(const Bitboard* bb, const Square& to, const Bitboard& stmAttackers,
66 Bitboard& occupied, Bitboard& attackers) {
68 Bitboard b = stmAttackers & bb[Pt];
70 return min_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
72 occupied ^= b & ~(b - 1);
74 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
75 attackers |= attacks_bb<BISHOP>(to, occupied) & (bb[BISHOP] | bb[QUEEN]);
77 if (Pt == ROOK || Pt == QUEEN)
78 attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
80 attackers &= occupied; // After X-ray that may add already processed pieces
84 template<> FORCE_INLINE
85 PieceType min_attacker<KING>(const Bitboard*, const Square&, const Bitboard&, Bitboard&, Bitboard&) {
86 return KING; // No need to update bitboards: it is the last cycle
94 CheckInfo::CheckInfo(const Position& pos) {
96 Color them = ~pos.side_to_move();
97 ksq = pos.king_square(them);
99 pinned = pos.pinned_pieces(pos.side_to_move());
100 dcCandidates = pos.discovered_check_candidates();
102 checkSq[PAWN] = pos.attacks_from<PAWN>(ksq, them);
103 checkSq[KNIGHT] = pos.attacks_from<KNIGHT>(ksq);
104 checkSq[BISHOP] = pos.attacks_from<BISHOP>(ksq);
105 checkSq[ROOK] = pos.attacks_from<ROOK>(ksq);
106 checkSq[QUEEN] = checkSq[BISHOP] | checkSq[ROOK];
111 /// Position::init() initializes at startup the various arrays used to compute
112 /// hash keys and the piece square tables. The latter is a two-step operation:
113 /// Firstly, the white halves of the tables are copied from PSQT[] tables.
114 /// Secondly, the black halves of the tables are initialized by flipping and
115 /// changing the sign of the white scores.
117 void Position::init() {
121 for (Color c = WHITE; c <= BLACK; ++c)
122 for (PieceType pt = PAWN; pt <= KING; ++pt)
123 for (Square s = SQ_A1; s <= SQ_H8; ++s)
124 Zobrist::psq[c][pt][s] = rk.rand<Key>();
126 for (File f = FILE_A; f <= FILE_H; ++f)
127 Zobrist::enpassant[f] = rk.rand<Key>();
129 for (int cf = NO_CASTLING; cf <= ANY_CASTLING; ++cf)
134 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
135 Zobrist::castling[cf] ^= k ? k : rk.rand<Key>();
139 Zobrist::side = rk.rand<Key>();
140 Zobrist::exclusion = rk.rand<Key>();
142 for (PieceType pt = PAWN; pt <= KING; ++pt)
144 PieceValue[MG][make_piece(BLACK, pt)] = PieceValue[MG][pt];
145 PieceValue[EG][make_piece(BLACK, pt)] = PieceValue[EG][pt];
147 Score v = make_score(PieceValue[MG][pt], PieceValue[EG][pt]);
149 for (Square s = SQ_A1; s <= SQ_H8; ++s)
151 psq[WHITE][pt][ s] = (v + PSQT[pt][s]);
152 psq[BLACK][pt][~s] = -(v + PSQT[pt][s]);
158 /// Position::operator=() creates a copy of 'pos'. We want the new born Position
159 /// object to not depend on any external data so we detach state pointer from
162 Position& Position::operator=(const Position& pos) {
164 std::memcpy(this, &pos, sizeof(Position));
175 /// Position::clear() erases the position object to a pristine state, with an
176 /// empty board, white to move, and no castling rights.
178 void Position::clear() {
180 std::memset(this, 0, sizeof(Position));
181 startState.epSquare = SQ_NONE;
184 for (int i = 0; i < PIECE_TYPE_NB; ++i)
185 for (int j = 0; j < 16; ++j)
186 pieceList[WHITE][i][j] = pieceList[BLACK][i][j] = SQ_NONE;
190 /// Position::set() initializes the position object with the given FEN string.
191 /// This function is not very robust - make sure that input FENs are correct,
192 /// this is assumed to be the responsibility of the GUI.
194 void Position::set(const string& fenStr, bool isChess960, Thread* th) {
196 A FEN string defines a particular position using only the ASCII character set.
198 A FEN string contains six fields separated by a space. The fields are:
200 1) Piece placement (from white's perspective). Each rank is described, starting
201 with rank 8 and ending with rank 1. Within each rank, the contents of each
202 square are described from file A through file H. Following the Standard
203 Algebraic Notation (SAN), each piece is identified by a single letter taken
204 from the standard English names. White pieces are designated using upper-case
205 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
206 noted using digits 1 through 8 (the number of blank squares), and "/"
209 2) Active color. "w" means white moves next, "b" means black.
211 3) Castling availability. If neither side can castle, this is "-". Otherwise,
212 this has one or more letters: "K" (White can castle kingside), "Q" (White
213 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
214 can castle queenside).
216 4) En passant target square (in algebraic notation). If there's no en passant
217 target square, this is "-". If a pawn has just made a 2-square move, this
218 is the position "behind" the pawn. This is recorded regardless of whether
219 there is a pawn in position to make an en passant capture.
221 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
222 or capture. This is used to determine if a draw can be claimed under the
225 6) Fullmove number. The number of the full move. It starts at 1, and is
226 incremented after Black's move.
229 unsigned char col, row, token;
232 std::istringstream ss(fenStr);
237 // 1. Piece placement
238 while ((ss >> token) && !isspace(token))
241 sq += Square(token - '0'); // Advance the given number of files
243 else if (token == '/')
246 else if ((idx = PieceToChar.find(token)) != string::npos)
248 put_piece(sq, color_of(Piece(idx)), type_of(Piece(idx)));
255 sideToMove = (token == 'w' ? WHITE : BLACK);
258 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
259 // Shredder-FEN that uses the letters of the columns on which the rooks began
260 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
261 // if an inner rook is associated with the castling right, the castling tag is
262 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
263 while ((ss >> token) && !isspace(token))
266 Color c = islower(token) ? BLACK : WHITE;
268 token = char(toupper(token));
271 for (rsq = relative_square(c, SQ_H1); type_of(piece_on(rsq)) != ROOK; --rsq) {}
273 else if (token == 'Q')
274 for (rsq = relative_square(c, SQ_A1); type_of(piece_on(rsq)) != ROOK; ++rsq) {}
276 else if (token >= 'A' && token <= 'H')
277 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
282 set_castling_right(c, rsq);
285 // 4. En passant square. Ignore if no pawn capture is possible
286 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
287 && ((ss >> row) && (row == '3' || row == '6')))
289 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
291 if (!(attackers_to(st->epSquare) & pieces(sideToMove, PAWN)))
292 st->epSquare = SQ_NONE;
295 // 5-6. Halfmove clock and fullmove number
296 ss >> std::skipws >> st->rule50 >> gamePly;
298 // Convert from fullmove starting from 1 to ply starting from 0,
299 // handle also common incorrect FEN with fullmove = 0.
300 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
302 chess960 = isChess960;
310 /// Position::set_castling_right() is a helper function used to set castling
311 /// rights given the corresponding color and the rook starting square.
313 void Position::set_castling_right(Color c, Square rfrom) {
315 Square kfrom = king_square(c);
316 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
317 CastlingRight cr = (c | cs);
319 st->castlingRights |= cr;
320 castlingRightsMask[kfrom] |= cr;
321 castlingRightsMask[rfrom] |= cr;
322 castlingRookSquare[cr] = rfrom;
324 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
325 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
327 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
328 if (s != kfrom && s != rfrom)
329 castlingPath[cr] |= s;
331 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
332 if (s != kfrom && s != rfrom)
333 castlingPath[cr] |= s;
337 /// Position::set_state() computes the hash keys of the position, and other
338 /// data that once computed is updated incrementally as moves are made.
339 /// The function is only used when a new position is set up, and to verify
340 /// the correctness of the StateInfo data when running in debug mode.
342 void Position::set_state(StateInfo* si) const {
344 si->key = si->pawnKey = si->materialKey = 0;
345 si->npMaterial[WHITE] = si->npMaterial[BLACK] = VALUE_ZERO;
346 si->psq = SCORE_ZERO;
348 si->checkersBB = attackers_to(king_square(sideToMove)) & pieces(~sideToMove);
350 for (Bitboard b = pieces(); b; )
352 Square s = pop_lsb(&b);
353 Piece pc = piece_on(s);
354 si->key ^= Zobrist::psq[color_of(pc)][type_of(pc)][s];
355 si->psq += psq[color_of(pc)][type_of(pc)][s];
358 if (ep_square() != SQ_NONE)
359 si->key ^= Zobrist::enpassant[file_of(ep_square())];
361 if (sideToMove == BLACK)
362 si->key ^= Zobrist::side;
364 si->key ^= Zobrist::castling[st->castlingRights];
366 for (Bitboard b = pieces(PAWN); b; )
368 Square s = pop_lsb(&b);
369 si->pawnKey ^= Zobrist::psq[color_of(piece_on(s))][PAWN][s];
372 for (Color c = WHITE; c <= BLACK; ++c)
373 for (PieceType pt = PAWN; pt <= KING; ++pt)
374 for (int cnt = 0; cnt < pieceCount[c][pt]; ++cnt)
375 si->materialKey ^= Zobrist::psq[c][pt][cnt];
377 for (Color c = WHITE; c <= BLACK; ++c)
378 for (PieceType pt = KNIGHT; pt <= QUEEN; ++pt)
379 si->npMaterial[c] += pieceCount[c][pt] * PieceValue[MG][pt];
383 /// Position::fen() returns a FEN representation of the position. In case of
384 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
386 const string Position::fen() const {
389 std::ostringstream ss;
391 for (Rank r = RANK_8; r >= RANK_1; --r)
393 for (File f = FILE_A; f <= FILE_H; ++f)
395 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
402 ss << PieceToChar[piece_on(make_square(f, r))];
409 ss << (sideToMove == WHITE ? " w " : " b ");
411 if (can_castle(WHITE_OO))
412 ss << (chess960 ? to_char(file_of(castling_rook_square(WHITE | KING_SIDE)), false) : 'K');
414 if (can_castle(WHITE_OOO))
415 ss << (chess960 ? to_char(file_of(castling_rook_square(WHITE | QUEEN_SIDE)), false) : 'Q');
417 if (can_castle(BLACK_OO))
418 ss << (chess960 ? to_char(file_of(castling_rook_square(BLACK | KING_SIDE)), true) : 'k');
420 if (can_castle(BLACK_OOO))
421 ss << (chess960 ? to_char(file_of(castling_rook_square(BLACK | QUEEN_SIDE)), true) : 'q');
423 if (!can_castle(WHITE) && !can_castle(BLACK))
426 ss << (ep_square() == SQ_NONE ? " - " : " " + to_string(ep_square()) + " ")
427 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
433 /// Position::pretty() returns an ASCII representation of the position to be
434 /// printed to the standard output together with the move's san notation.
436 const string Position::pretty(Move m) const {
438 std::ostringstream ss;
441 ss << "\nMove: " << (sideToMove == BLACK ? ".." : "")
442 << move_to_san(*const_cast<Position*>(this), m);
444 ss << "\n +---+---+---+---+---+---+---+---+\n";
446 for (Rank r = RANK_8; r >= RANK_1; --r)
448 for (File f = FILE_A; f <= FILE_H; ++f)
449 ss << " | " << PieceToChar[piece_on(make_square(f, r))];
451 ss << " |\n +---+---+---+---+---+---+---+---+\n";
454 ss << "\nFen: " << fen() << "\nKey: " << std::hex << std::uppercase
455 << std::setfill('0') << std::setw(16) << st->key << "\nCheckers: ";
457 for (Bitboard b = checkers(); b; )
458 ss << to_string(pop_lsb(&b)) << " ";
460 ss << "\nLegal moves: ";
461 for (MoveList<LEGAL> it(*this); *it; ++it)
462 ss << move_to_san(*const_cast<Position*>(this), *it) << " ";
468 /// Position::game_phase() calculates the game phase of the position
470 Phase Position::game_phase() const {
472 Value npm = st->npMaterial[WHITE] + st->npMaterial[BLACK];
474 return npm >= MidgameLimit ? PHASE_MIDGAME
475 : npm <= EndgameLimit ? PHASE_ENDGAME
476 : Phase(((npm - EndgameLimit) * 128) / (MidgameLimit - EndgameLimit));
480 /// Position::check_blockers() returns a bitboard of all the pieces with color
481 /// 'c' that are blocking check on the king with color 'kingColor'. A piece
482 /// blocks a check if removing that piece from the board would result in a
483 /// position where the king is in check. A check blocking piece can be either a
484 /// pinned or a discovered check piece, according if its color 'c' is the same
485 /// or the opposite of 'kingColor'.
487 Bitboard Position::check_blockers(Color c, Color kingColor) const {
489 Bitboard b, pinners, result = 0;
490 Square ksq = king_square(kingColor);
492 // Pinners are sliders that give check when a pinned piece is removed
493 pinners = ( (pieces( ROOK, QUEEN) & PseudoAttacks[ROOK ][ksq])
494 | (pieces(BISHOP, QUEEN) & PseudoAttacks[BISHOP][ksq])) & pieces(~kingColor);
498 b = between_bb(ksq, pop_lsb(&pinners)) & pieces();
500 if (!more_than_one(b))
501 result |= b & pieces(c);
507 /// Position::attackers_to() computes a bitboard of all pieces which attack a
508 /// given square. Slider attacks use the occ bitboard to indicate occupancy.
510 Bitboard Position::attackers_to(Square s, Bitboard occ) const {
512 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
513 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
514 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
515 | (attacks_bb<ROOK>(s, occ) & pieces(ROOK, QUEEN))
516 | (attacks_bb<BISHOP>(s, occ) & pieces(BISHOP, QUEEN))
517 | (attacks_from<KING>(s) & pieces(KING));
521 /// Position::legal() tests whether a pseudo-legal move is legal
523 bool Position::legal(Move m, Bitboard pinned) const {
526 assert(pinned == pinned_pieces(sideToMove));
528 Color us = sideToMove;
529 Square from = from_sq(m);
531 assert(color_of(moved_piece(m)) == us);
532 assert(piece_on(king_square(us)) == make_piece(us, KING));
534 // En passant captures are a tricky special case. Because they are rather
535 // uncommon, we do it simply by testing whether the king is attacked after
537 if (type_of(m) == ENPASSANT)
539 Square ksq = king_square(us);
540 Square to = to_sq(m);
541 Square capsq = to - pawn_push(us);
542 Bitboard occ = (pieces() ^ from ^ capsq) | to;
544 assert(to == ep_square());
545 assert(moved_piece(m) == make_piece(us, PAWN));
546 assert(piece_on(capsq) == make_piece(~us, PAWN));
547 assert(piece_on(to) == NO_PIECE);
549 return !(attacks_bb< ROOK>(ksq, occ) & pieces(~us, QUEEN, ROOK))
550 && !(attacks_bb<BISHOP>(ksq, occ) & pieces(~us, QUEEN, BISHOP));
553 // If the moving piece is a king, check whether the destination
554 // square is attacked by the opponent. Castling moves are checked
555 // for legality during move generation.
556 if (type_of(piece_on(from)) == KING)
557 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
559 // A non-king move is legal if and only if it is not pinned or it
560 // is moving along the ray towards or away from the king.
563 || aligned(from, to_sq(m), king_square(us));
567 /// Position::pseudo_legal() takes a random move and tests whether the move is
568 /// pseudo legal. It is used to validate moves from TT that can be corrupted
569 /// due to SMP concurrent access or hash position key aliasing.
571 bool Position::pseudo_legal(const Move m) const {
573 Color us = sideToMove;
574 Square from = from_sq(m);
575 Square to = to_sq(m);
576 Piece pc = moved_piece(m);
578 // Use a slower but simpler function for uncommon cases
579 if (type_of(m) != NORMAL)
580 return MoveList<LEGAL>(*this).contains(m);
582 // Is not a promotion, so promotion piece must be empty
583 if (promotion_type(m) - 2 != NO_PIECE_TYPE)
586 // If the 'from' square is not occupied by a piece belonging to the side to
587 // move, the move is obviously not legal.
588 if (pc == NO_PIECE || color_of(pc) != us)
591 // The destination square cannot be occupied by a friendly piece
595 // Handle the special case of a pawn move
596 if (type_of(pc) == PAWN)
598 // We have already handled promotion moves, so destination
599 // cannot be on the 8th/1st rank.
600 if (rank_of(to) == relative_rank(us, RANK_8))
603 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
605 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
607 && !( (from + 2 * pawn_push(us) == to) // Not a double push
608 && (rank_of(from) == relative_rank(us, RANK_2))
610 && empty(to - pawn_push(us))))
613 else if (!(attacks_from(pc, from) & to))
616 // Evasions generator already takes care to avoid some kind of illegal moves
617 // and legal() relies on this. We therefore have to take care that the same
618 // kind of moves are filtered out here.
621 if (type_of(pc) != KING)
623 // Double check? In this case a king move is required
624 if (more_than_one(checkers()))
627 // Our move must be a blocking evasion or a capture of the checking piece
628 if (!((between_bb(lsb(checkers()), king_square(us)) | checkers()) & to))
631 // In case of king moves under check we have to remove king so as to catch
632 // invalid moves like b1a1 when opposite queen is on c1.
633 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
641 /// Position::gives_check() tests whether a pseudo-legal move gives a check
643 bool Position::gives_check(Move m, const CheckInfo& ci) const {
646 assert(ci.dcCandidates == discovered_check_candidates());
647 assert(color_of(moved_piece(m)) == sideToMove);
649 Square from = from_sq(m);
650 Square to = to_sq(m);
651 PieceType pt = type_of(piece_on(from));
653 // Is there a direct check?
654 if (ci.checkSq[pt] & to)
657 // Is there a discovered check?
658 if ( unlikely(ci.dcCandidates)
659 && (ci.dcCandidates & from)
660 && !aligned(from, to, ci.ksq))
669 return attacks_bb(Piece(promotion_type(m)), to, pieces() ^ from) & ci.ksq;
671 // En passant capture with check? We have already handled the case
672 // of direct checks and ordinary discovered check, so the only case we
673 // need to handle is the unusual case of a discovered check through
674 // the captured pawn.
677 Square capsq = make_square(file_of(to), rank_of(from));
678 Bitboard b = (pieces() ^ from ^ capsq) | to;
680 return (attacks_bb< ROOK>(ci.ksq, b) & pieces(sideToMove, QUEEN, ROOK))
681 | (attacks_bb<BISHOP>(ci.ksq, b) & pieces(sideToMove, QUEEN, BISHOP));
686 Square rfrom = to; // Castling is encoded as 'King captures the rook'
687 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
688 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
690 return (PseudoAttacks[ROOK][rto] & ci.ksq)
691 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & ci.ksq);
700 /// Position::do_move() makes a move, and saves all information necessary
701 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
702 /// moves should be filtered out before this function is called.
704 void Position::do_move(Move m, StateInfo& newSt) {
707 do_move(m, newSt, ci, gives_check(m, ci));
710 void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool moveIsCheck) {
713 assert(&newSt != st);
718 // Copy some fields of the old state to our new StateInfo object except the
719 // ones which are going to be recalculated from scratch anyway and then switch
720 // our state pointer to point to the new (ready to be updated) state.
721 std::memcpy(&newSt, st, StateCopySize64 * sizeof(uint64_t));
726 // Update side to move
729 // Increment ply counters. In particular, rule50 will be reset to zero later on
730 // in case of a capture or a pawn move.
735 Color us = sideToMove;
737 Square from = from_sq(m);
738 Square to = to_sq(m);
739 Piece pc = piece_on(from);
740 PieceType pt = type_of(pc);
741 PieceType captured = type_of(m) == ENPASSANT ? PAWN : type_of(piece_on(to));
743 assert(color_of(pc) == us);
744 assert(piece_on(to) == NO_PIECE || color_of(piece_on(to)) == them || type_of(m) == CASTLING);
745 assert(captured != KING);
747 if (type_of(m) == CASTLING)
749 assert(pc == make_piece(us, KING));
752 do_castling<true>(from, to, rfrom, rto);
754 captured = NO_PIECE_TYPE;
755 st->psq += psq[us][ROOK][rto] - psq[us][ROOK][rfrom];
756 k ^= Zobrist::psq[us][ROOK][rfrom] ^ Zobrist::psq[us][ROOK][rto];
763 // If the captured piece is a pawn, update pawn hash key, otherwise
764 // update non-pawn material.
765 if (captured == PAWN)
767 if (type_of(m) == ENPASSANT)
769 capsq += pawn_push(them);
772 assert(to == st->epSquare);
773 assert(relative_rank(us, to) == RANK_6);
774 assert(piece_on(to) == NO_PIECE);
775 assert(piece_on(capsq) == make_piece(them, PAWN));
777 board[capsq] = NO_PIECE;
780 st->pawnKey ^= Zobrist::psq[them][PAWN][capsq];
783 st->npMaterial[them] -= PieceValue[MG][captured];
785 // Update board and piece lists
786 remove_piece(capsq, them, captured);
788 // Update material hash key and prefetch access to materialTable
789 k ^= Zobrist::psq[them][captured][capsq];
790 st->materialKey ^= Zobrist::psq[them][captured][pieceCount[them][captured]];
791 prefetch((char*)thisThread->materialTable[st->materialKey]);
793 // Update incremental scores
794 st->psq -= psq[them][captured][capsq];
796 // Reset rule 50 counter
801 k ^= Zobrist::psq[us][pt][from] ^ Zobrist::psq[us][pt][to];
803 // Reset en passant square
804 if (st->epSquare != SQ_NONE)
806 k ^= Zobrist::enpassant[file_of(st->epSquare)];
807 st->epSquare = SQ_NONE;
810 // Update castling rights if needed
811 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
813 int cr = castlingRightsMask[from] | castlingRightsMask[to];
814 k ^= Zobrist::castling[st->castlingRights & cr];
815 st->castlingRights &= ~cr;
818 // Prefetch TT access as soon as we know the new hash key
819 prefetch((char*)TT.first_entry(k));
821 // Move the piece. The tricky Chess960 castling is handled earlier
822 if (type_of(m) != CASTLING)
823 move_piece(from, to, us, pt);
825 // If the moving piece is a pawn do some special extra work
828 // Set en-passant square if the moved pawn can be captured
829 if ( (int(to) ^ int(from)) == 16
830 && (attacks_from<PAWN>(from + pawn_push(us), us) & pieces(them, PAWN)))
832 st->epSquare = Square((from + to) / 2);
833 k ^= Zobrist::enpassant[file_of(st->epSquare)];
836 else if (type_of(m) == PROMOTION)
838 PieceType promotion = promotion_type(m);
840 assert(relative_rank(us, to) == RANK_8);
841 assert(promotion >= KNIGHT && promotion <= QUEEN);
843 remove_piece(to, us, PAWN);
844 put_piece(to, us, promotion);
847 k ^= Zobrist::psq[us][PAWN][to] ^ Zobrist::psq[us][promotion][to];
848 st->pawnKey ^= Zobrist::psq[us][PAWN][to];
849 st->materialKey ^= Zobrist::psq[us][promotion][pieceCount[us][promotion]-1]
850 ^ Zobrist::psq[us][PAWN][pieceCount[us][PAWN]];
852 // Update incremental score
853 st->psq += psq[us][promotion][to] - psq[us][PAWN][to];
856 st->npMaterial[us] += PieceValue[MG][promotion];
859 // Update pawn hash key and prefetch access to pawnsTable
860 st->pawnKey ^= Zobrist::psq[us][PAWN][from] ^ Zobrist::psq[us][PAWN][to];
861 prefetch((char*)thisThread->pawnsTable[st->pawnKey]);
863 // Reset rule 50 draw counter
867 // Update incremental scores
868 st->psq += psq[us][pt][to] - psq[us][pt][from];
871 st->capturedType = captured;
873 // Update the key with the final value
876 // Update checkers bitboard: piece must be already moved due to attacks_from()
881 if (type_of(m) != NORMAL)
882 st->checkersBB = attackers_to(king_square(them)) & pieces(us);
886 if (ci.checkSq[pt] & to)
887 st->checkersBB |= to;
890 if (unlikely(ci.dcCandidates) && (ci.dcCandidates & from))
893 st->checkersBB |= attacks_from<ROOK>(king_square(them)) & pieces(us, QUEEN, ROOK);
896 st->checkersBB |= attacks_from<BISHOP>(king_square(them)) & pieces(us, QUEEN, BISHOP);
901 sideToMove = ~sideToMove;
907 /// Position::undo_move() unmakes a move. When it returns, the position should
908 /// be restored to exactly the same state as before the move was made.
910 void Position::undo_move(Move m) {
914 sideToMove = ~sideToMove;
916 Color us = sideToMove;
917 Square from = from_sq(m);
918 Square to = to_sq(m);
919 PieceType pt = type_of(piece_on(to));
921 assert(empty(from) || type_of(m) == CASTLING);
922 assert(st->capturedType != KING);
924 if (type_of(m) == PROMOTION)
926 assert(pt == promotion_type(m));
927 assert(relative_rank(us, to) == RANK_8);
928 assert(promotion_type(m) >= KNIGHT && promotion_type(m) <= QUEEN);
930 remove_piece(to, us, promotion_type(m));
931 put_piece(to, us, PAWN);
935 if (type_of(m) == CASTLING)
938 do_castling<false>(from, to, rfrom, rto);
942 move_piece(to, from, us, pt); // Put the piece back at the source square
944 if (st->capturedType)
948 if (type_of(m) == ENPASSANT)
950 capsq -= pawn_push(us);
953 assert(to == st->previous->epSquare);
954 assert(relative_rank(us, to) == RANK_6);
955 assert(piece_on(capsq) == NO_PIECE);
958 put_piece(capsq, ~us, st->capturedType); // Restore the captured piece
962 // Finally point our state pointer back to the previous state
970 /// Position::do_castling() is a helper used to do/undo a castling move. This
971 /// is a bit tricky, especially in Chess960.
973 void Position::do_castling(Square from, Square& to, Square& rfrom, Square& rto) {
975 bool kingSide = to > from;
976 rfrom = to; // Castling is encoded as "king captures friendly rook"
977 rto = relative_square(sideToMove, kingSide ? SQ_F1 : SQ_D1);
978 to = relative_square(sideToMove, kingSide ? SQ_G1 : SQ_C1);
980 // Remove both pieces first since squares could overlap in Chess960
981 remove_piece(Do ? from : to, sideToMove, KING);
982 remove_piece(Do ? rfrom : rto, sideToMove, ROOK);
983 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
984 put_piece(Do ? to : from, sideToMove, KING);
985 put_piece(Do ? rto : rfrom, sideToMove, ROOK);
989 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
990 /// the side to move without executing any move on the board.
992 void Position::do_null_move(StateInfo& newSt) {
996 std::memcpy(&newSt, st, sizeof(StateInfo)); // Fully copy here
1001 if (st->epSquare != SQ_NONE)
1003 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1004 st->epSquare = SQ_NONE;
1007 st->key ^= Zobrist::side;
1008 prefetch((char*)TT.first_entry(st->key));
1011 st->pliesFromNull = 0;
1013 sideToMove = ~sideToMove;
1015 assert(pos_is_ok());
1018 void Position::undo_null_move() {
1020 assert(!checkers());
1023 sideToMove = ~sideToMove;
1027 /// Position::see() is a static exchange evaluator: It tries to estimate the
1028 /// material gain or loss resulting from a move.
1030 Value Position::see_sign(Move m) const {
1034 // Early return if SEE cannot be negative because captured piece value
1035 // is not less then capturing one. Note that king moves always return
1036 // here because king midgame value is set to 0.
1037 if (PieceValue[MG][moved_piece(m)] <= PieceValue[MG][piece_on(to_sq(m))])
1038 return VALUE_KNOWN_WIN;
1043 Value Position::see(Move m) const {
1046 Bitboard occupied, attackers, stmAttackers;
1056 swapList[0] = PieceValue[MG][piece_on(to)];
1057 stm = color_of(piece_on(from));
1058 occupied = pieces() ^ from;
1060 // Castling moves are implemented as king capturing the rook so cannot be
1061 // handled correctly. Simply return 0 that is always the correct value
1062 // unless in the rare case the rook ends up under attack.
1063 if (type_of(m) == CASTLING)
1066 if (type_of(m) == ENPASSANT)
1068 occupied ^= to - pawn_push(stm); // Remove the captured pawn
1069 swapList[0] = PieceValue[MG][PAWN];
1072 // Find all attackers to the destination square, with the moving piece
1073 // removed, but possibly an X-ray attacker added behind it.
1074 attackers = attackers_to(to, occupied) & occupied;
1076 // If the opponent has no attackers we are finished
1078 stmAttackers = attackers & pieces(stm);
1082 // The destination square is defended, which makes things rather more
1083 // difficult to compute. We proceed by building up a "swap list" containing
1084 // the material gain or loss at each stop in a sequence of captures to the
1085 // destination square, where the sides alternately capture, and always
1086 // capture with the least valuable piece. After each capture, we look for
1087 // new X-ray attacks from behind the capturing piece.
1088 captured = type_of(piece_on(from));
1091 assert(slIndex < 32);
1093 // Add the new entry to the swap list
1094 swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][captured];
1096 // Locate and remove the next least valuable attacker
1097 captured = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1099 // Stop before processing a king capture
1100 if (captured == KING)
1102 if (stmAttackers == attackers)
1109 stmAttackers = attackers & pieces(stm);
1112 } while (stmAttackers);
1114 // Having built the swap list, we negamax through it to find the best
1115 // achievable score from the point of view of the side to move.
1117 swapList[slIndex - 1] = std::min(-swapList[slIndex], swapList[slIndex - 1]);
1123 /// Position::is_draw() tests whether the position is drawn by material, 50 moves
1124 /// rule or repetition. It does not detect stalemates.
1126 bool Position::is_draw() const {
1129 && (non_pawn_material(WHITE) + non_pawn_material(BLACK) <= BishopValueMg))
1132 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1135 StateInfo* stp = st;
1136 for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
1138 stp = stp->previous->previous;
1140 if (stp->key == st->key)
1141 return true; // Draw at first repetition
1148 /// Position::flip() flips position with the white and black sides reversed. This
1149 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1151 static char toggle_case(char c) {
1152 return char(islower(c) ? toupper(c) : tolower(c));
1155 void Position::flip() {
1158 std::stringstream ss(fen());
1160 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1162 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1163 f.insert(0, token + (f.empty() ? " " : "/"));
1166 ss >> token; // Active color
1167 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1169 ss >> token; // Castling availability
1172 std::transform(f.begin(), f.end(), f.begin(), toggle_case);
1174 ss >> token; // En passant square
1175 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1177 std::getline(ss, token); // Half and full moves
1180 set(f, is_chess960(), this_thread());
1182 assert(pos_is_ok());
1186 /// Position::pos_is_ok() performs some consistency checks for the position object.
1187 /// This is meant to be helpful when debugging.
1189 bool Position::pos_is_ok(int* step) const {
1191 // Which parts of the position should be verified?
1192 const bool all = false;
1194 const bool testBitboards = all || false;
1195 const bool testState = all || false;
1196 const bool testKingCount = all || false;
1197 const bool testKingCapture = all || false;
1198 const bool testPieceCounts = all || false;
1199 const bool testPieceList = all || false;
1200 const bool testCastlingSquares = all || false;
1205 if ( (sideToMove != WHITE && sideToMove != BLACK)
1206 || piece_on(king_square(WHITE)) != W_KING
1207 || piece_on(king_square(BLACK)) != B_KING
1208 || ( ep_square() != SQ_NONE
1209 && relative_rank(sideToMove, ep_square()) != RANK_6))
1212 if (step && ++*step, testBitboards)
1214 // The intersection of the white and black pieces must be empty
1215 if (pieces(WHITE) & pieces(BLACK))
1218 // The union of the white and black pieces must be equal to all
1220 if ((pieces(WHITE) | pieces(BLACK)) != pieces())
1223 // Separate piece type bitboards must have empty intersections
1224 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1225 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1226 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1230 if (step && ++*step, testState)
1234 if ( st->key != si.key
1235 || st->pawnKey != si.pawnKey
1236 || st->materialKey != si.materialKey
1237 || st->npMaterial[WHITE] != si.npMaterial[WHITE]
1238 || st->npMaterial[BLACK] != si.npMaterial[BLACK]
1239 || st->psq != si.psq
1240 || st->checkersBB != si.checkersBB)
1244 if (step && ++*step, testKingCount)
1245 if ( std::count(board, board + SQUARE_NB, W_KING) != 1
1246 || std::count(board, board + SQUARE_NB, B_KING) != 1)
1249 if (step && ++*step, testKingCapture)
1250 if (attackers_to(king_square(~sideToMove)) & pieces(sideToMove))
1253 if (step && ++*step, testPieceCounts)
1254 for (Color c = WHITE; c <= BLACK; ++c)
1255 for (PieceType pt = PAWN; pt <= KING; ++pt)
1256 if (pieceCount[c][pt] != popcount<Full>(pieces(c, pt)))
1259 if (step && ++*step, testPieceList)
1260 for (Color c = WHITE; c <= BLACK; ++c)
1261 for (PieceType pt = PAWN; pt <= KING; ++pt)
1262 for (int i = 0; i < pieceCount[c][pt]; ++i)
1263 if ( board[pieceList[c][pt][i]] != make_piece(c, pt)
1264 || index[pieceList[c][pt][i]] != i)
1267 if (step && ++*step, testCastlingSquares)
1268 for (Color c = WHITE; c <= BLACK; ++c)
1269 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1271 if (!can_castle(c | s))
1274 if ( (castlingRightsMask[king_square(c)] & (c | s)) != (c | s)
1275 || piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1276 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s))