2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2023 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
27 #include <initializer_list>
31 #include <string_view>
37 #include "nnue/nnue_common.h"
38 #include "syzygy/tbprobe.h"
49 Key psq[PIECE_NB][SQUARE_NB];
50 Key enpassant[FILE_NB];
51 Key castling[CASTLING_RIGHT_NB];
57 constexpr std::string_view PieceToChar(" PNBRQK pnbrqk");
59 constexpr Piece Pieces[] = {W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
60 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING};
64 // operator<<(Position) returns an ASCII representation of the position
66 std::ostream& operator<<(std::ostream& os, const Position& pos) {
68 os << "\n +---+---+---+---+---+---+---+---+\n";
70 for (Rank r = RANK_8; r >= RANK_1; --r)
72 for (File f = FILE_A; f <= FILE_H; ++f)
73 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
75 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
78 os << " a b c d e f g h\n"
79 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase << std::setfill('0')
80 << std::setw(16) << pos.key() << std::setfill(' ') << std::dec << "\nCheckers: ";
82 for (Bitboard b = pos.checkers(); b;)
83 os << UCI::square(pop_lsb(b)) << " ";
85 if (int(Tablebases::MaxCardinality) >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
88 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
91 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
92 Tablebases::ProbeState s1, s2;
93 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
94 int dtz = Tablebases::probe_dtz(p, &s2);
95 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
96 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
103 // Implements Marcel van Kervinck's cuckoo algorithm to detect repetition of positions
104 // for 3-fold repetition draws. The algorithm uses two hash tables with Zobrist hashes
105 // to allow fast detection of recurring positions. For details see:
106 // http://web.archive.org/web/20201107002606/https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
108 // First and second hash functions for indexing the cuckoo tables
109 inline int H1(Key h) { return h & 0x1fff; }
110 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
112 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
114 Move cuckooMove[8192];
117 // Position::init() initializes at startup the various arrays used to compute hash keys
119 void Position::init() {
123 for (Piece pc : Pieces)
124 for (Square s = SQ_A1; s <= SQ_H8; ++s)
125 Zobrist::psq[pc][s] = rng.rand<Key>();
127 for (File f = FILE_A; f <= FILE_H; ++f)
128 Zobrist::enpassant[f] = rng.rand<Key>();
130 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
131 Zobrist::castling[cr] = rng.rand<Key>();
133 Zobrist::side = rng.rand<Key>();
135 // Prepare the cuckoo tables
136 std::memset(cuckoo, 0, sizeof(cuckoo));
137 std::memset(cuckooMove, 0, sizeof(cuckooMove));
138 [[maybe_unused]] int count = 0;
139 for (Piece pc : Pieces)
140 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
141 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
142 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
144 Move move = make_move(s1, s2);
145 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
149 std::swap(cuckoo[i], key);
150 std::swap(cuckooMove[i], move);
151 if (move == MOVE_NONE) // Arrived at empty slot?
153 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
157 assert(count == 3668);
161 // Position::set() initializes the position object with the given FEN string.
162 // This function is not very robust - make sure that input FENs are correct,
163 // this is assumed to be the responsibility of the GUI.
165 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
167 A FEN string defines a particular position using only the ASCII character set.
169 A FEN string contains six fields separated by a space. The fields are:
171 1) Piece placement (from white's perspective). Each rank is described, starting
172 with rank 8 and ending with rank 1. Within each rank, the contents of each
173 square are described from file A through file H. Following the Standard
174 Algebraic Notation (SAN), each piece is identified by a single letter taken
175 from the standard English names. White pieces are designated using upper-case
176 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
177 noted using digits 1 through 8 (the number of blank squares), and "/"
180 2) Active color. "w" means white moves next, "b" means black.
182 3) Castling availability. If neither side can castle, this is "-". Otherwise,
183 this has one or more letters: "K" (White can castle kingside), "Q" (White
184 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
185 can castle queenside).
187 4) En passant target square (in algebraic notation). If there's no en passant
188 target square, this is "-". If a pawn has just made a 2-square move, this
189 is the position "behind" the pawn. Following X-FEN standard, this is recorded
190 only if there is a pawn in position to make an en passant capture, and if
191 there really is a pawn that might have advanced two squares.
193 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
194 or capture. This is used to determine if a draw can be claimed under the
197 6) Fullmove number. The number of the full move. It starts at 1, and is
198 incremented after Black's move.
201 unsigned char col, row, token;
204 std::istringstream ss(fenStr);
206 std::memset(this, 0, sizeof(Position));
207 std::memset(si, 0, sizeof(StateInfo));
212 // 1. Piece placement
213 while ((ss >> token) && !isspace(token))
216 sq += (token - '0') * EAST; // Advance the given number of files
218 else if (token == '/')
221 else if ((idx = PieceToChar.find(token)) != string::npos)
223 put_piece(Piece(idx), sq);
230 sideToMove = (token == 'w' ? WHITE : BLACK);
233 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
234 // Shredder-FEN that uses the letters of the columns on which the rooks began
235 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
236 // if an inner rook is associated with the castling right, the castling tag is
237 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
238 while ((ss >> token) && !isspace(token))
241 Color c = islower(token) ? BLACK : WHITE;
242 Piece rook = make_piece(c, ROOK);
244 token = char(toupper(token));
247 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq)
250 else if (token == 'Q')
251 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq)
254 else if (token >= 'A' && token <= 'H')
255 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
260 set_castling_right(c, rsq);
263 // 4. En passant square.
264 // Ignore if square is invalid or not on side to move relative rank 6.
265 bool enpassant = false;
267 if (((ss >> col) && (col >= 'a' && col <= 'h'))
268 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
270 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
272 // En passant square will be considered only if
273 // a) side to move have a pawn threatening epSquare
274 // b) there is an enemy pawn in front of epSquare
275 // c) there is no piece on epSquare or behind epSquare
276 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
277 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
278 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
282 st->epSquare = SQ_NONE;
284 // 5-6. Halfmove clock and fullmove number
285 ss >> std::skipws >> st->rule50 >> gamePly;
287 // Convert from fullmove starting from 1 to gamePly starting from 0,
288 // handle also common incorrect FEN with fullmove = 0.
289 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
291 chess960 = isChess960;
301 // Position::set_castling_right() is a helper function used to set castling
302 // rights given the corresponding color and the rook starting square.
304 void Position::set_castling_right(Color c, Square rfrom) {
306 Square kfrom = square<KING>(c);
307 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE : QUEEN_SIDE);
309 st->castlingRights |= cr;
310 castlingRightsMask[kfrom] |= cr;
311 castlingRightsMask[rfrom] |= cr;
312 castlingRookSquare[cr] = rfrom;
314 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
315 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
317 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto)) & ~(kfrom | rfrom);
321 // Position::set_check_info() sets king attacks to detect if a move gives check
323 void Position::set_check_info() const {
325 update_slider_blockers(WHITE);
326 update_slider_blockers(BLACK);
328 Square ksq = square<KING>(~sideToMove);
330 st->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
331 st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
332 st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
333 st->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
334 st->checkSquares[QUEEN] = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
335 st->checkSquares[KING] = 0;
339 // Position::set_state() computes the hash keys of the position, and other
340 // data that once computed is updated incrementally as moves are made.
341 // The function is only used when a new position is set up
343 void Position::set_state() const {
345 st->key = st->materialKey = 0;
346 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
347 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
351 for (Bitboard b = pieces(); b;)
353 Square s = pop_lsb(b);
354 Piece pc = piece_on(s);
355 st->key ^= Zobrist::psq[pc][s];
357 if (type_of(pc) != KING && type_of(pc) != PAWN)
358 st->nonPawnMaterial[color_of(pc)] += PieceValue[pc];
361 if (st->epSquare != SQ_NONE)
362 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
364 if (sideToMove == BLACK)
365 st->key ^= Zobrist::side;
367 st->key ^= Zobrist::castling[st->castlingRights];
369 for (Piece pc : Pieces)
370 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
371 st->materialKey ^= Zobrist::psq[pc][cnt];
375 // Position::set() is an overload to initialize the position object with
376 // the given endgame code string like "KBPKN". It is mainly a helper to
377 // get the material key out of an endgame code.
379 Position& Position::set(const string& code, Color c, StateInfo* si) {
381 assert(code[0] == 'K');
383 string sides[] = {code.substr(code.find('K', 1)), // Weak
384 code.substr(0, std::min(code.find('v'), code.find('K', 1)))}; // Strong
386 assert(sides[0].length() > 0 && sides[0].length() < 8);
387 assert(sides[1].length() > 0 && sides[1].length() < 8);
389 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
391 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/" + sides[1]
392 + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
394 return set(fenStr, false, si, nullptr);
398 // Position::fen() returns a FEN representation of the position. In case of
399 // Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
401 string Position::fen() const {
404 std::ostringstream ss;
406 for (Rank r = RANK_8; r >= RANK_1; --r)
408 for (File f = FILE_A; f <= FILE_H; ++f)
410 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
417 ss << PieceToChar[piece_on(make_square(f, r))];
424 ss << (sideToMove == WHITE ? " w " : " b ");
426 if (can_castle(WHITE_OO))
427 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO))) : 'K');
429 if (can_castle(WHITE_OOO))
430 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
432 if (can_castle(BLACK_OO))
433 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO))) : 'k');
435 if (can_castle(BLACK_OOO))
436 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
438 if (!can_castle(ANY_CASTLING))
441 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ") << st->rule50
442 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
447 // update_slider_blockers() calculates st->blockersForKing[c] and st->pinners[~c],
448 // which store respectively the pieces preventing king of color c from being in check
449 // and the slider pieces of color ~c pinning pieces of color c to the king.
450 void Position::update_slider_blockers(Color c) const {
452 Square ksq = square<KING>(c);
454 st->blockersForKing[c] = 0;
457 // Snipers are sliders that attack 's' when a piece and other snipers are removed
458 Bitboard snipers = ((attacks_bb<ROOK>(ksq) & pieces(QUEEN, ROOK))
459 | (attacks_bb<BISHOP>(ksq) & pieces(QUEEN, BISHOP)))
461 Bitboard occupancy = pieces() ^ snipers;
465 Square sniperSq = pop_lsb(snipers);
466 Bitboard b = between_bb(ksq, sniperSq) & occupancy;
468 if (b && !more_than_one(b))
470 st->blockersForKing[c] |= b;
472 st->pinners[~c] |= sniperSq;
478 // Position::attackers_to() computes a bitboard of all pieces which attack a
479 // given square. Slider attacks use the occupied bitboard to indicate occupancy.
481 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
483 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
484 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
485 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
486 | (attacks_bb<ROOK>(s, occupied) & pieces(ROOK, QUEEN))
487 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
488 | (attacks_bb<KING>(s) & pieces(KING));
492 // Position::legal() tests whether a pseudo-legal move is legal
494 bool Position::legal(Move m) const {
498 Color us = sideToMove;
499 Square from = from_sq(m);
500 Square to = to_sq(m);
502 assert(color_of(moved_piece(m)) == us);
503 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
505 // En passant captures are a tricky special case. Because they are rather
506 // uncommon, we do it simply by testing whether the king is attacked after
508 if (type_of(m) == EN_PASSANT)
510 Square ksq = square<KING>(us);
511 Square capsq = to - pawn_push(us);
512 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
514 assert(to == ep_square());
515 assert(moved_piece(m) == make_piece(us, PAWN));
516 assert(piece_on(capsq) == make_piece(~us, PAWN));
517 assert(piece_on(to) == NO_PIECE);
519 return !(attacks_bb<ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
520 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
523 // Castling moves generation does not check if the castling path is clear of
524 // enemy attacks, it is delayed at a later time: now!
525 if (type_of(m) == CASTLING)
527 // After castling, the rook and king final positions are the same in
528 // Chess960 as they would be in standard chess.
529 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
530 Direction step = to > from ? WEST : EAST;
532 for (Square s = to; s != from; s += step)
533 if (attackers_to(s) & pieces(~us))
536 // In case of Chess960, verify if the Rook blocks some checks.
537 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
538 return !chess960 || !(blockers_for_king(us) & to_sq(m));
541 // If the moving piece is a king, check whether the destination square is
542 // attacked by the opponent.
543 if (type_of(piece_on(from)) == KING)
544 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
546 // A non-king move is legal if and only if it is not pinned or it
547 // is moving along the ray towards or away from the king.
548 return !(blockers_for_king(us) & from) || aligned(from, to, square<KING>(us));
552 // Position::pseudo_legal() takes a random move and tests whether the move is
553 // pseudo-legal. It is used to validate moves from TT that can be corrupted
554 // due to SMP concurrent access or hash position key aliasing.
556 bool Position::pseudo_legal(const Move m) const {
558 Color us = sideToMove;
559 Square from = from_sq(m);
560 Square to = to_sq(m);
561 Piece pc = moved_piece(m);
563 // Use a slower but simpler function for uncommon cases
564 // yet we skip the legality check of MoveList<LEGAL>().
565 if (type_of(m) != NORMAL)
566 return checkers() ? MoveList<EVASIONS>(*this).contains(m)
567 : MoveList<NON_EVASIONS>(*this).contains(m);
569 // Is not a promotion, so the promotion piece must be empty
570 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
572 // If the 'from' square is not occupied by a piece belonging to the side to
573 // move, the move is obviously not legal.
574 if (pc == NO_PIECE || color_of(pc) != us)
577 // The destination square cannot be occupied by a friendly piece
581 // Handle the special case of a pawn move
582 if (type_of(pc) == PAWN)
584 // We have already handled promotion moves, so destination
585 // cannot be on the 8th/1st rank.
586 if ((Rank8BB | Rank1BB) & to)
589 if (!(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
590 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
591 && !((from + 2 * pawn_push(us) == to) // Not a double push
592 && (relative_rank(us, from) == RANK_2) && empty(to) && empty(to - pawn_push(us))))
595 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
598 // Evasions generator already takes care to avoid some kind of illegal moves
599 // and legal() relies on this. We therefore have to take care that the same
600 // kind of moves are filtered out here.
603 if (type_of(pc) != KING)
605 // Double check? In this case, a king move is required
606 if (more_than_one(checkers()))
609 // Our move must be a blocking interposition or a capture of the checking piece
610 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
613 // In case of king moves under check we have to remove the king so as to catch
614 // invalid moves like b1a1 when opposite queen is on c1.
615 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
623 // Position::gives_check() tests whether a pseudo-legal move gives a check
625 bool Position::gives_check(Move m) const {
628 assert(color_of(moved_piece(m)) == sideToMove);
630 Square from = from_sq(m);
631 Square to = to_sq(m);
633 // Is there a direct check?
634 if (check_squares(type_of(piece_on(from))) & to)
637 // Is there a discovered check?
638 if (blockers_for_king(~sideToMove) & from)
639 return !aligned(from, to, square<KING>(~sideToMove)) || type_of(m) == CASTLING;
647 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
649 // En passant capture with check? We have already handled the case
650 // of direct checks and ordinary discovered check, so the only case we
651 // need to handle is the unusual case of a discovered check through
652 // the captured pawn.
654 Square capsq = make_square(file_of(to), rank_of(from));
655 Bitboard b = (pieces() ^ from ^ capsq) | to;
657 return (attacks_bb<ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
658 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b)
659 & pieces(sideToMove, QUEEN, BISHOP));
663 // Castling is encoded as 'king captures the rook'
664 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
666 return check_squares(ROOK) & rto;
672 // Position::do_move() makes a move, and saves all information necessary
673 // to a StateInfo object. The move is assumed to be legal. Pseudo-legal
674 // moves should be filtered out before this function is called.
676 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
679 assert(&newSt != st);
681 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
682 Key k = st->key ^ Zobrist::side;
684 // Copy some fields of the old state to our new StateInfo object except the
685 // ones which are going to be recalculated from scratch anyway and then switch
686 // our state pointer to point to the new (ready to be updated) state.
687 std::memcpy(&newSt, st, offsetof(StateInfo, key));
691 // Increment ply counters. In particular, rule50 will be reset to zero later on
692 // in case of a capture or a pawn move.
698 st->accumulator.computed[WHITE] = false;
699 st->accumulator.computed[BLACK] = false;
700 auto& dp = st->dirtyPiece;
703 Color us = sideToMove;
705 Square from = from_sq(m);
706 Square to = to_sq(m);
707 Piece pc = piece_on(from);
708 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
710 assert(color_of(pc) == us);
711 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
712 assert(type_of(captured) != KING);
714 if (type_of(m) == CASTLING)
716 assert(pc == make_piece(us, KING));
717 assert(captured == make_piece(us, ROOK));
720 do_castling<true>(us, from, to, rfrom, rto);
722 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
730 // If the captured piece is a pawn, update pawn hash key, otherwise
731 // update non-pawn material.
732 if (type_of(captured) == PAWN)
734 if (type_of(m) == EN_PASSANT)
736 capsq -= pawn_push(us);
738 assert(pc == make_piece(us, PAWN));
739 assert(to == st->epSquare);
740 assert(relative_rank(us, to) == RANK_6);
741 assert(piece_on(to) == NO_PIECE);
742 assert(piece_on(capsq) == make_piece(them, PAWN));
746 st->nonPawnMaterial[them] -= PieceValue[captured];
748 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
749 dp.piece[1] = captured;
753 // Update board and piece lists
756 // Update material hash key and prefetch access to materialTable
757 k ^= Zobrist::psq[captured][capsq];
758 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
760 // Reset rule 50 counter
765 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
767 // Reset en passant square
768 if (st->epSquare != SQ_NONE)
770 k ^= Zobrist::enpassant[file_of(st->epSquare)];
771 st->epSquare = SQ_NONE;
774 // Update castling rights if needed
775 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
777 k ^= Zobrist::castling[st->castlingRights];
778 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
779 k ^= Zobrist::castling[st->castlingRights];
782 // Move the piece. The tricky Chess960 castling is handled earlier
783 if (type_of(m) != CASTLING)
789 move_piece(from, to);
792 // If the moving piece is a pawn do some special extra work
793 if (type_of(pc) == PAWN)
795 // Set en passant square if the moved pawn can be captured
796 if ((int(to) ^ int(from)) == 16
797 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
799 st->epSquare = to - pawn_push(us);
800 k ^= Zobrist::enpassant[file_of(st->epSquare)];
803 else if (type_of(m) == PROMOTION)
805 Piece promotion = make_piece(us, promotion_type(m));
807 assert(relative_rank(us, to) == RANK_8);
808 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
811 put_piece(promotion, to);
813 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
815 dp.piece[dp.dirty_num] = promotion;
816 dp.from[dp.dirty_num] = SQ_NONE;
817 dp.to[dp.dirty_num] = to;
821 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
823 Zobrist::psq[promotion][pieceCount[promotion] - 1] ^ Zobrist::psq[pc][pieceCount[pc]];
826 st->nonPawnMaterial[us] += PieceValue[promotion];
829 // Reset rule 50 draw counter
834 st->capturedPiece = captured;
836 // Update the key with the final value
839 // Calculate checkers bitboard (if move gives check)
840 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
842 sideToMove = ~sideToMove;
844 // Update king attacks used for fast check detection
847 // Calculate the repetition info. It is the ply distance from the previous
848 // occurrence of the same position, negative in the 3-fold case, or zero
849 // if the position was not repeated.
851 int end = std::min(st->rule50, st->pliesFromNull);
854 StateInfo* stp = st->previous->previous;
855 for (int i = 4; i <= end; i += 2)
857 stp = stp->previous->previous;
858 if (stp->key == st->key)
860 st->repetition = stp->repetition ? -i : i;
870 // Position::undo_move() unmakes a move. When it returns, the position should
871 // be restored to exactly the same state as before the move was made.
873 void Position::undo_move(Move m) {
877 sideToMove = ~sideToMove;
879 Color us = sideToMove;
880 Square from = from_sq(m);
881 Square to = to_sq(m);
882 Piece pc = piece_on(to);
884 assert(empty(from) || type_of(m) == CASTLING);
885 assert(type_of(st->capturedPiece) != KING);
887 if (type_of(m) == PROMOTION)
889 assert(relative_rank(us, to) == RANK_8);
890 assert(type_of(pc) == promotion_type(m));
891 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
894 pc = make_piece(us, PAWN);
898 if (type_of(m) == CASTLING)
901 do_castling<false>(us, from, to, rfrom, rto);
905 move_piece(to, from); // Put the piece back at the source square
907 if (st->capturedPiece)
911 if (type_of(m) == EN_PASSANT)
913 capsq -= pawn_push(us);
915 assert(type_of(pc) == PAWN);
916 assert(to == st->previous->epSquare);
917 assert(relative_rank(us, to) == RANK_6);
918 assert(piece_on(capsq) == NO_PIECE);
919 assert(st->capturedPiece == make_piece(~us, PAWN));
922 put_piece(st->capturedPiece, capsq); // Restore the captured piece
926 // Finally point our state pointer back to the previous state
934 // Position::do_castling() is a helper used to do/undo a castling move. This
935 // is a bit tricky in Chess960 where from/to squares can overlap.
937 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
939 bool kingSide = to > from;
940 rfrom = to; // Castling is encoded as "king captures friendly rook"
941 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
942 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
946 auto& dp = st->dirtyPiece;
947 dp.piece[0] = make_piece(us, KING);
950 dp.piece[1] = make_piece(us, ROOK);
956 // Remove both pieces first since squares could overlap in Chess960
957 remove_piece(Do ? from : to);
958 remove_piece(Do ? rfrom : rto);
959 board[Do ? from : to] = board[Do ? rfrom : rto] =
960 NO_PIECE; // remove_piece does not do this for us
961 put_piece(make_piece(us, KING), Do ? to : from);
962 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
966 // Position::do_null_move() is used to do a "null move": it flips
967 // the side to move without executing any move on the board.
969 void Position::do_null_move(StateInfo& newSt) {
972 assert(&newSt != st);
974 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
979 st->dirtyPiece.dirty_num = 0;
980 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
981 st->accumulator.computed[WHITE] = false;
982 st->accumulator.computed[BLACK] = false;
984 if (st->epSquare != SQ_NONE)
986 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
987 st->epSquare = SQ_NONE;
990 st->key ^= Zobrist::side;
992 prefetch(TT.first_entry(key()));
994 st->pliesFromNull = 0;
996 sideToMove = ~sideToMove;
1002 assert(pos_is_ok());
1006 // Position::undo_null_move() must be used to undo a "null move"
1008 void Position::undo_null_move() {
1010 assert(!checkers());
1013 sideToMove = ~sideToMove;
1017 // Position::key_after() computes the new hash key after the given move. Needed
1018 // for speculative prefetch. It doesn't recognize special moves like castling,
1019 // en passant and promotions.
1021 Key Position::key_after(Move m) const {
1023 Square from = from_sq(m);
1024 Square to = to_sq(m);
1025 Piece pc = piece_on(from);
1026 Piece captured = piece_on(to);
1027 Key k = st->key ^ Zobrist::side;
1030 k ^= Zobrist::psq[captured][to];
1032 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1034 return (captured || type_of(pc) == PAWN) ? k : adjust_key50<true>(k);
1038 // Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1039 // SEE value of move is greater or equal to the given threshold. We'll use an
1040 // algorithm similar to alpha-beta pruning with a null window.
1042 bool Position::see_ge(Move m, Value threshold) const {
1046 // Only deal with normal moves, assume others pass a simple SEE
1047 if (type_of(m) != NORMAL)
1048 return VALUE_ZERO >= threshold;
1050 Square from = from_sq(m), to = to_sq(m);
1052 int swap = PieceValue[piece_on(to)] - threshold;
1056 swap = PieceValue[piece_on(from)] - swap;
1060 assert(color_of(piece_on(from)) == sideToMove);
1061 Bitboard occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1062 Color stm = sideToMove;
1063 Bitboard attackers = attackers_to(to, occupied);
1064 Bitboard stmAttackers, bb;
1070 attackers &= occupied;
1072 // If stm has no more attackers then give up: stm loses
1073 if (!(stmAttackers = attackers & pieces(stm)))
1076 // Don't allow pinned pieces to attack as long as there are
1077 // pinners on their original square.
1078 if (pinners(~stm) & occupied)
1080 stmAttackers &= ~blockers_for_king(stm);
1088 // Locate and remove the next least valuable attacker, and add to
1089 // the bitboard 'attackers' any X-ray attackers behind it.
1090 if ((bb = stmAttackers & pieces(PAWN)))
1092 if ((swap = PawnValue - swap) < res)
1094 occupied ^= least_significant_square_bb(bb);
1096 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1099 else if ((bb = stmAttackers & pieces(KNIGHT)))
1101 if ((swap = KnightValue - swap) < res)
1103 occupied ^= least_significant_square_bb(bb);
1106 else if ((bb = stmAttackers & pieces(BISHOP)))
1108 if ((swap = BishopValue - swap) < res)
1110 occupied ^= least_significant_square_bb(bb);
1112 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1115 else if ((bb = stmAttackers & pieces(ROOK)))
1117 if ((swap = RookValue - swap) < res)
1119 occupied ^= least_significant_square_bb(bb);
1121 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1124 else if ((bb = stmAttackers & pieces(QUEEN)))
1126 if ((swap = QueenValue - swap) < res)
1128 occupied ^= least_significant_square_bb(bb);
1130 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1131 | (attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN));
1135 // If we "capture" with the king but the opponent still has attackers,
1136 // reverse the result.
1137 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1143 // Position::is_draw() tests whether the position is drawn by 50-move rule
1144 // or by repetition. It does not detect stalemates.
1146 bool Position::is_draw(int ply) const {
1148 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1151 // Return a draw score if a position repeats once earlier but strictly
1152 // after the root, or repeats twice before or at the root.
1153 return st->repetition && st->repetition < ply;
1157 // Position::has_repeated() tests whether there has been at least one repetition
1158 // of positions since the last capture or pawn move.
1160 bool Position::has_repeated() const {
1162 StateInfo* stc = st;
1163 int end = std::min(st->rule50, st->pliesFromNull);
1166 if (stc->repetition)
1169 stc = stc->previous;
1175 // Position::has_game_cycle() tests if the position has a move which draws by repetition,
1176 // or an earlier position has a move that directly reaches the current position.
1178 bool Position::has_game_cycle(int ply) const {
1182 int end = std::min(st->rule50, st->pliesFromNull);
1187 Key originalKey = st->key;
1188 StateInfo* stp = st->previous;
1190 for (int i = 3; i <= end; i += 2)
1192 stp = stp->previous->previous;
1194 Key moveKey = originalKey ^ stp->key;
1195 if ((j = H1(moveKey), cuckoo[j] == moveKey) || (j = H2(moveKey), cuckoo[j] == moveKey))
1197 Move move = cuckooMove[j];
1198 Square s1 = from_sq(move);
1199 Square s2 = to_sq(move);
1201 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1206 // For nodes before or at the root, check that the move is a
1207 // repetition rather than a move to the current position.
1208 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1209 // the same location, so we have to select which square to check.
1210 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1213 // For repetitions before or at the root, require one more
1214 if (stp->repetition)
1223 // Position::flip() flips position with the white and black sides reversed. This
1224 // is only useful for debugging e.g. for finding evaluation symmetry bugs.
1226 void Position::flip() {
1229 std::stringstream ss(fen());
1231 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1233 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1234 f.insert(0, token + (f.empty() ? " " : "/"));
1237 ss >> token; // Active color
1238 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1240 ss >> token; // Castling availability
1243 std::transform(f.begin(), f.end(), f.begin(),
1244 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1246 ss >> token; // En passant square
1247 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1249 std::getline(ss, token); // Half and full moves
1252 set(f, is_chess960(), st, this_thread());
1254 assert(pos_is_ok());
1258 // Position::pos_is_ok() performs some consistency checks for the
1259 // position object and raise an assert if something wrong is detected.
1260 // This is meant to be helpful when debugging.
1262 bool Position::pos_is_ok() const {
1264 constexpr bool Fast = true; // Quick (default) or full check?
1266 if ((sideToMove != WHITE && sideToMove != BLACK) || piece_on(square<KING>(WHITE)) != W_KING
1267 || piece_on(square<KING>(BLACK)) != B_KING
1268 || (ep_square() != SQ_NONE && relative_rank(sideToMove, ep_square()) != RANK_6))
1269 assert(0 && "pos_is_ok: Default");
1274 if (pieceCount[W_KING] != 1 || pieceCount[B_KING] != 1
1275 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1276 assert(0 && "pos_is_ok: Kings");
1278 if ((pieces(PAWN) & (Rank1BB | Rank8BB)) || pieceCount[W_PAWN] > 8 || pieceCount[B_PAWN] > 8)
1279 assert(0 && "pos_is_ok: Pawns");
1281 if ((pieces(WHITE) & pieces(BLACK)) || (pieces(WHITE) | pieces(BLACK)) != pieces()
1282 || popcount(pieces(WHITE)) > 16 || popcount(pieces(BLACK)) > 16)
1283 assert(0 && "pos_is_ok: Bitboards");
1285 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1286 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1287 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1288 assert(0 && "pos_is_ok: Bitboards");
1291 for (Piece pc : Pieces)
1292 if (pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1293 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1294 assert(0 && "pos_is_ok: Pieces");
1296 for (Color c : {WHITE, BLACK})
1297 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1299 if (!can_castle(cr))
1302 if (piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1303 || castlingRightsMask[castlingRookSquare[cr]] != cr
1304 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1305 assert(0 && "pos_is_ok: Castling");
1311 } // namespace Stockfish