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 // Returns an ASCII representation of the position
65 std::ostream& operator<<(std::ostream& os, const Position& pos) {
67 os << "\n +---+---+---+---+---+---+---+---+\n";
69 for (Rank r = RANK_8; r >= RANK_1; --r)
71 for (File f = FILE_A; f <= FILE_H; ++f)
72 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
74 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
77 os << " a b c d e f g h\n"
78 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase << std::setfill('0')
79 << std::setw(16) << pos.key() << std::setfill(' ') << std::dec << "\nCheckers: ";
81 for (Bitboard b = pos.checkers(); b;)
82 os << UCI::square(pop_lsb(b)) << " ";
84 if (int(Tablebases::MaxCardinality) >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
87 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
90 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
91 Tablebases::ProbeState s1, s2;
92 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
93 int dtz = Tablebases::probe_dtz(p, &s2);
94 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
95 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
102 // Implements Marcel van Kervinck's cuckoo algorithm to detect repetition of positions
103 // for 3-fold repetition draws. The algorithm uses two hash tables with Zobrist hashes
104 // to allow fast detection of recurring positions. For details see:
105 // http://web.archive.org/web/20201107002606/https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
107 // First and second hash functions for indexing the cuckoo tables
108 inline int H1(Key h) { return h & 0x1fff; }
109 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
111 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
113 Move cuckooMove[8192];
116 // Initializes at startup the various arrays used to compute hash keys
117 void Position::init() {
121 for (Piece pc : Pieces)
122 for (Square s = SQ_A1; s <= SQ_H8; ++s)
123 Zobrist::psq[pc][s] = rng.rand<Key>();
125 for (File f = FILE_A; f <= FILE_H; ++f)
126 Zobrist::enpassant[f] = rng.rand<Key>();
128 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
129 Zobrist::castling[cr] = rng.rand<Key>();
131 Zobrist::side = rng.rand<Key>();
132 Zobrist::noPawns = rng.rand<Key>();
134 // Prepare the cuckoo tables
135 std::memset(cuckoo, 0, sizeof(cuckoo));
136 std::memset(cuckooMove, 0, sizeof(cuckooMove));
137 [[maybe_unused]] int count = 0;
138 for (Piece pc : Pieces)
139 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
140 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
141 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
143 Move move = make_move(s1, s2);
144 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
148 std::swap(cuckoo[i], key);
149 std::swap(cuckooMove[i], move);
150 if (move == MOVE_NONE) // Arrived at empty slot?
152 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
156 assert(count == 3668);
160 // Initializes the position object with the given FEN string.
161 // This function is not very robust - make sure that input FENs are correct,
162 // this is assumed to be the responsibility of the GUI.
163 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
165 A FEN string defines a particular position using only the ASCII character set.
167 A FEN string contains six fields separated by a space. The fields are:
169 1) Piece placement (from white's perspective). Each rank is described, starting
170 with rank 8 and ending with rank 1. Within each rank, the contents of each
171 square are described from file A through file H. Following the Standard
172 Algebraic Notation (SAN), each piece is identified by a single letter taken
173 from the standard English names. White pieces are designated using upper-case
174 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
175 noted using digits 1 through 8 (the number of blank squares), and "/"
178 2) Active color. "w" means white moves next, "b" means black.
180 3) Castling availability. If neither side can castle, this is "-". Otherwise,
181 this has one or more letters: "K" (White can castle kingside), "Q" (White
182 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
183 can castle queenside).
185 4) En passant target square (in algebraic notation). If there's no en passant
186 target square, this is "-". If a pawn has just made a 2-square move, this
187 is the position "behind" the pawn. Following X-FEN standard, this is recorded
188 only if there is a pawn in position to make an en passant capture, and if
189 there really is a pawn that might have advanced two squares.
191 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
192 or capture. This is used to determine if a draw can be claimed under the
195 6) Fullmove number. The number of the full move. It starts at 1, and is
196 incremented after Black's move.
199 unsigned char col, row, token;
202 std::istringstream ss(fenStr);
204 std::memset(this, 0, sizeof(Position));
205 std::memset(si, 0, sizeof(StateInfo));
210 // 1. Piece placement
211 while ((ss >> token) && !isspace(token))
214 sq += (token - '0') * EAST; // Advance the given number of files
216 else if (token == '/')
219 else if ((idx = PieceToChar.find(token)) != string::npos)
221 put_piece(Piece(idx), sq);
228 sideToMove = (token == 'w' ? WHITE : BLACK);
231 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
232 // Shredder-FEN that uses the letters of the columns on which the rooks began
233 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
234 // if an inner rook is associated with the castling right, the castling tag is
235 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
236 while ((ss >> token) && !isspace(token))
239 Color c = islower(token) ? BLACK : WHITE;
240 Piece rook = make_piece(c, ROOK);
242 token = char(toupper(token));
245 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq)
248 else if (token == 'Q')
249 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq)
252 else if (token >= 'A' && token <= 'H')
253 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
258 set_castling_right(c, rsq);
261 // 4. En passant square.
262 // Ignore if square is invalid or not on side to move relative rank 6.
263 bool enpassant = false;
265 if (((ss >> col) && (col >= 'a' && col <= 'h'))
266 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
268 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
270 // En passant square will be considered only if
271 // a) side to move have a pawn threatening epSquare
272 // b) there is an enemy pawn in front of epSquare
273 // c) there is no piece on epSquare or behind epSquare
274 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
275 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
276 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
280 st->epSquare = SQ_NONE;
282 // 5-6. Halfmove clock and fullmove number
283 ss >> std::skipws >> st->rule50 >> gamePly;
285 // Convert from fullmove starting from 1 to gamePly starting from 0,
286 // handle also common incorrect FEN with fullmove = 0.
287 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
289 chess960 = isChess960;
299 // Helper function used to set castling
300 // rights given the corresponding color and the rook starting square.
301 void Position::set_castling_right(Color c, Square rfrom) {
303 Square kfrom = square<KING>(c);
304 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE : QUEEN_SIDE);
306 st->castlingRights |= cr;
307 castlingRightsMask[kfrom] |= cr;
308 castlingRightsMask[rfrom] |= cr;
309 castlingRookSquare[cr] = rfrom;
311 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
312 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
314 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto)) & ~(kfrom | rfrom);
318 // Sets king attacks to detect if a move gives check
319 void Position::set_check_info() const {
321 update_slider_blockers(WHITE);
322 update_slider_blockers(BLACK);
324 Square ksq = square<KING>(~sideToMove);
326 st->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
327 st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
328 st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
329 st->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
330 st->checkSquares[QUEEN] = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
331 st->checkSquares[KING] = 0;
335 // Computes the hash keys of the position, and other
336 // data that once computed is updated incrementally as moves are made.
337 // The function is only used when a new position is set up
338 void Position::set_state() const {
340 st->key = st->materialKey = 0;
341 st->pawnKey = Zobrist::noPawns;
342 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
343 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
347 for (Bitboard b = pieces(); b;)
349 Square s = pop_lsb(b);
350 Piece pc = piece_on(s);
351 st->key ^= Zobrist::psq[pc][s];
353 if (type_of(pc) == PAWN)
354 st->pawnKey ^= Zobrist::psq[pc][s];
356 else if (type_of(pc) != KING)
357 st->nonPawnMaterial[color_of(pc)] += PieceValue[pc];
360 if (st->epSquare != SQ_NONE)
361 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
363 if (sideToMove == BLACK)
364 st->key ^= Zobrist::side;
366 st->key ^= Zobrist::castling[st->castlingRights];
368 for (Piece pc : Pieces)
369 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
370 st->materialKey ^= Zobrist::psq[pc][cnt];
374 // Overload to initialize the position object with the given endgame code string
375 // like "KBPKN". It's mainly a helper to get the material key out of an endgame code.
376 Position& Position::set(const string& code, Color c, StateInfo* si) {
378 assert(code[0] == 'K');
380 string sides[] = {code.substr(code.find('K', 1)), // Weak
381 code.substr(0, std::min(code.find('v'), code.find('K', 1)))}; // Strong
383 assert(sides[0].length() > 0 && sides[0].length() < 8);
384 assert(sides[1].length() > 0 && sides[1].length() < 8);
386 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
388 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/" + sides[1]
389 + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
391 return set(fenStr, false, si, nullptr);
395 // Returns a FEN representation of the position. In case of
396 // Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
397 string Position::fen() const {
400 std::ostringstream ss;
402 for (Rank r = RANK_8; r >= RANK_1; --r)
404 for (File f = FILE_A; f <= FILE_H; ++f)
406 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
413 ss << PieceToChar[piece_on(make_square(f, r))];
420 ss << (sideToMove == WHITE ? " w " : " b ");
422 if (can_castle(WHITE_OO))
423 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO))) : 'K');
425 if (can_castle(WHITE_OOO))
426 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
428 if (can_castle(BLACK_OO))
429 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO))) : 'k');
431 if (can_castle(BLACK_OOO))
432 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
434 if (!can_castle(ANY_CASTLING))
437 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ") << st->rule50
438 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
443 // Calculates st->blockersForKing[c] and st->pinners[~c],
444 // which store respectively the pieces preventing king of color c from being in check
445 // and the slider pieces of color ~c pinning pieces of color c to the king.
446 void Position::update_slider_blockers(Color c) const {
448 Square ksq = square<KING>(c);
450 st->blockersForKing[c] = 0;
453 // Snipers are sliders that attack 's' when a piece and other snipers are removed
454 Bitboard snipers = ((attacks_bb<ROOK>(ksq) & pieces(QUEEN, ROOK))
455 | (attacks_bb<BISHOP>(ksq) & pieces(QUEEN, BISHOP)))
457 Bitboard occupancy = pieces() ^ snipers;
461 Square sniperSq = pop_lsb(snipers);
462 Bitboard b = between_bb(ksq, sniperSq) & occupancy;
464 if (b && !more_than_one(b))
466 st->blockersForKing[c] |= b;
468 st->pinners[~c] |= sniperSq;
474 // Computes a bitboard of all pieces which attack a given square.
475 // Slider attacks use the occupied bitboard to indicate occupancy.
476 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
478 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
479 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
480 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
481 | (attacks_bb<ROOK>(s, occupied) & pieces(ROOK, QUEEN))
482 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
483 | (attacks_bb<KING>(s) & pieces(KING));
487 // Tests whether a pseudo-legal move is legal
488 bool Position::legal(Move m) const {
492 Color us = sideToMove;
493 Square from = from_sq(m);
494 Square to = to_sq(m);
496 assert(color_of(moved_piece(m)) == us);
497 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
499 // En passant captures are a tricky special case. Because they are rather
500 // uncommon, we do it simply by testing whether the king is attacked after
502 if (type_of(m) == EN_PASSANT)
504 Square ksq = square<KING>(us);
505 Square capsq = to - pawn_push(us);
506 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
508 assert(to == ep_square());
509 assert(moved_piece(m) == make_piece(us, PAWN));
510 assert(piece_on(capsq) == make_piece(~us, PAWN));
511 assert(piece_on(to) == NO_PIECE);
513 return !(attacks_bb<ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
514 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
517 // Castling moves generation does not check if the castling path is clear of
518 // enemy attacks, it is delayed at a later time: now!
519 if (type_of(m) == CASTLING)
521 // After castling, the rook and king final positions are the same in
522 // Chess960 as they would be in standard chess.
523 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
524 Direction step = to > from ? WEST : EAST;
526 for (Square s = to; s != from; s += step)
527 if (attackers_to(s) & pieces(~us))
530 // In case of Chess960, verify if the Rook blocks some checks.
531 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
532 return !chess960 || !(blockers_for_king(us) & to_sq(m));
535 // If the moving piece is a king, check whether the destination square is
536 // attacked by the opponent.
537 if (type_of(piece_on(from)) == KING)
538 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
540 // A non-king move is legal if and only if it is not pinned or it
541 // is moving along the ray towards or away from the king.
542 return !(blockers_for_king(us) & from) || aligned(from, to, square<KING>(us));
546 // Takes a random move and tests whether the move is
547 // pseudo-legal. It is used to validate moves from TT that can be corrupted
548 // due to SMP concurrent access or hash position key aliasing.
549 bool Position::pseudo_legal(const Move m) const {
551 Color us = sideToMove;
552 Square from = from_sq(m);
553 Square to = to_sq(m);
554 Piece pc = moved_piece(m);
556 // Use a slower but simpler function for uncommon cases
557 // yet we skip the legality check of MoveList<LEGAL>().
558 if (type_of(m) != NORMAL)
559 return checkers() ? MoveList<EVASIONS>(*this).contains(m)
560 : MoveList<NON_EVASIONS>(*this).contains(m);
562 // Is not a promotion, so the promotion piece must be empty
563 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
565 // If the 'from' square is not occupied by a piece belonging to the side to
566 // move, the move is obviously not legal.
567 if (pc == NO_PIECE || color_of(pc) != us)
570 // The destination square cannot be occupied by a friendly piece
574 // Handle the special case of a pawn move
575 if (type_of(pc) == PAWN)
577 // We have already handled promotion moves, so destination cannot be on the 8th/1st rank
578 if ((Rank8BB | Rank1BB) & to)
581 if (!(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
582 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
583 && !((from + 2 * pawn_push(us) == to) // Not a double push
584 && (relative_rank(us, from) == RANK_2) && empty(to) && empty(to - pawn_push(us))))
587 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
590 // Evasions generator already takes care to avoid some kind of illegal moves
591 // and legal() relies on this. We therefore have to take care that the same
592 // kind of moves are filtered out here.
595 if (type_of(pc) != KING)
597 // Double check? In this case, a king move is required
598 if (more_than_one(checkers()))
601 // Our move must be a blocking interposition or a capture of the checking piece
602 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
605 // In case of king moves under check we have to remove the king so as to catch
606 // invalid moves like b1a1 when opposite queen is on c1.
607 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
615 // Tests whether a pseudo-legal move gives a check
616 bool Position::gives_check(Move m) const {
619 assert(color_of(moved_piece(m)) == sideToMove);
621 Square from = from_sq(m);
622 Square to = to_sq(m);
624 // Is there a direct check?
625 if (check_squares(type_of(piece_on(from))) & to)
628 // Is there a discovered check?
629 if (blockers_for_king(~sideToMove) & from)
630 return !aligned(from, to, square<KING>(~sideToMove)) || type_of(m) == CASTLING;
638 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
640 // En passant capture with check? We have already handled the case of direct
641 // checks and ordinary discovered check, so the only case we need to handle
642 // is the unusual case of a discovered check through the captured pawn.
644 Square capsq = make_square(file_of(to), rank_of(from));
645 Bitboard b = (pieces() ^ from ^ capsq) | to;
647 return (attacks_bb<ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
648 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b)
649 & pieces(sideToMove, QUEEN, BISHOP));
653 // Castling is encoded as 'king captures the rook'
654 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
656 return check_squares(ROOK) & rto;
662 // Makes a move, and saves all information necessary
663 // to a StateInfo object. The move is assumed to be legal. Pseudo-legal
664 // moves should be filtered out before this function is called.
665 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
668 assert(&newSt != st);
670 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
671 Key k = st->key ^ Zobrist::side;
673 // Copy some fields of the old state to our new StateInfo object except the
674 // ones which are going to be recalculated from scratch anyway and then switch
675 // our state pointer to point to the new (ready to be updated) state.
676 std::memcpy(&newSt, st, offsetof(StateInfo, key));
680 // Increment ply counters. In particular, rule50 will be reset to zero later on
681 // in case of a capture or a pawn move.
687 st->accumulator.computed[WHITE] = false;
688 st->accumulator.computed[BLACK] = false;
689 auto& dp = st->dirtyPiece;
692 Color us = sideToMove;
694 Square from = from_sq(m);
695 Square to = to_sq(m);
696 Piece pc = piece_on(from);
697 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
699 assert(color_of(pc) == us);
700 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
701 assert(type_of(captured) != KING);
703 if (type_of(m) == CASTLING)
705 assert(pc == make_piece(us, KING));
706 assert(captured == make_piece(us, ROOK));
709 do_castling<true>(us, from, to, rfrom, rto);
711 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
719 // If the captured piece is a pawn, update pawn hash key, otherwise
720 // update non-pawn material.
721 if (type_of(captured) == PAWN)
723 if (type_of(m) == EN_PASSANT)
725 capsq -= pawn_push(us);
727 assert(pc == make_piece(us, PAWN));
728 assert(to == st->epSquare);
729 assert(relative_rank(us, to) == RANK_6);
730 assert(piece_on(to) == NO_PIECE);
731 assert(piece_on(capsq) == make_piece(them, PAWN));
734 st->pawnKey ^= Zobrist::psq[captured][capsq];
737 st->nonPawnMaterial[them] -= PieceValue[captured];
739 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
740 dp.piece[1] = captured;
744 // Update board and piece lists
747 // Update material hash key and prefetch access to materialTable
748 k ^= Zobrist::psq[captured][capsq];
749 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
751 // Reset rule 50 counter
756 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
758 // Reset en passant square
759 if (st->epSquare != SQ_NONE)
761 k ^= Zobrist::enpassant[file_of(st->epSquare)];
762 st->epSquare = SQ_NONE;
765 // Update castling rights if needed
766 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
768 k ^= Zobrist::castling[st->castlingRights];
769 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
770 k ^= Zobrist::castling[st->castlingRights];
773 // Move the piece. The tricky Chess960 castling is handled earlier
774 if (type_of(m) != CASTLING)
780 move_piece(from, to);
783 // If the moving piece is a pawn do some special extra work
784 if (type_of(pc) == PAWN)
786 // Set en passant square if the moved pawn can be captured
787 if ((int(to) ^ int(from)) == 16
788 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
790 st->epSquare = to - pawn_push(us);
791 k ^= Zobrist::enpassant[file_of(st->epSquare)];
794 else if (type_of(m) == PROMOTION)
796 Piece promotion = make_piece(us, promotion_type(m));
798 assert(relative_rank(us, to) == RANK_8);
799 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
802 put_piece(promotion, to);
804 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
806 dp.piece[dp.dirty_num] = promotion;
807 dp.from[dp.dirty_num] = SQ_NONE;
808 dp.to[dp.dirty_num] = to;
812 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
813 st->pawnKey ^= Zobrist::psq[pc][to];
815 Zobrist::psq[promotion][pieceCount[promotion] - 1] ^ Zobrist::psq[pc][pieceCount[pc]];
818 st->nonPawnMaterial[us] += PieceValue[promotion];
821 // Update pawn hash key
822 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
824 // Reset rule 50 draw counter
829 st->capturedPiece = captured;
831 // Update the key with the final value
834 // Calculate checkers bitboard (if move gives check)
835 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
837 sideToMove = ~sideToMove;
839 // Update king attacks used for fast check detection
842 // Calculate the repetition info. It is the ply distance from the previous
843 // occurrence of the same position, negative in the 3-fold case, or zero
844 // if the position was not repeated.
846 int end = std::min(st->rule50, st->pliesFromNull);
849 StateInfo* stp = st->previous->previous;
850 for (int i = 4; i <= end; i += 2)
852 stp = stp->previous->previous;
853 if (stp->key == st->key)
855 st->repetition = stp->repetition ? -i : i;
865 // Unmakes a move. When it returns, the position should
866 // be restored to exactly the same state as before the move was made.
867 void Position::undo_move(Move m) {
871 sideToMove = ~sideToMove;
873 Color us = sideToMove;
874 Square from = from_sq(m);
875 Square to = to_sq(m);
876 Piece pc = piece_on(to);
878 assert(empty(from) || type_of(m) == CASTLING);
879 assert(type_of(st->capturedPiece) != KING);
881 if (type_of(m) == PROMOTION)
883 assert(relative_rank(us, to) == RANK_8);
884 assert(type_of(pc) == promotion_type(m));
885 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
888 pc = make_piece(us, PAWN);
892 if (type_of(m) == CASTLING)
895 do_castling<false>(us, from, to, rfrom, rto);
899 move_piece(to, from); // Put the piece back at the source square
901 if (st->capturedPiece)
905 if (type_of(m) == EN_PASSANT)
907 capsq -= pawn_push(us);
909 assert(type_of(pc) == PAWN);
910 assert(to == st->previous->epSquare);
911 assert(relative_rank(us, to) == RANK_6);
912 assert(piece_on(capsq) == NO_PIECE);
913 assert(st->capturedPiece == make_piece(~us, PAWN));
916 put_piece(st->capturedPiece, capsq); // Restore the captured piece
920 // Finally point our state pointer back to the previous state
928 // Helper used to do/undo a castling move. This is a bit
929 // tricky in Chess960 where from/to squares can overlap.
931 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
933 bool kingSide = to > from;
934 rfrom = to; // Castling is encoded as "king captures friendly rook"
935 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
936 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
940 auto& dp = st->dirtyPiece;
941 dp.piece[0] = make_piece(us, KING);
944 dp.piece[1] = make_piece(us, ROOK);
950 // Remove both pieces first since squares could overlap in Chess960
951 remove_piece(Do ? from : to);
952 remove_piece(Do ? rfrom : rto);
953 board[Do ? from : to] = board[Do ? rfrom : rto] =
954 NO_PIECE; // remove_piece does not do this for us
955 put_piece(make_piece(us, KING), Do ? to : from);
956 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
960 // Used to do a "null move": it flips
961 // the side to move without executing any move on the board.
962 void Position::do_null_move(StateInfo& newSt) {
965 assert(&newSt != st);
967 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
972 st->dirtyPiece.dirty_num = 0;
973 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
974 st->accumulator.computed[WHITE] = false;
975 st->accumulator.computed[BLACK] = false;
977 if (st->epSquare != SQ_NONE)
979 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
980 st->epSquare = SQ_NONE;
983 st->key ^= Zobrist::side;
985 prefetch(TT.first_entry(key()));
987 st->pliesFromNull = 0;
989 sideToMove = ~sideToMove;
999 // Must be used to undo a "null move"
1000 void Position::undo_null_move() {
1002 assert(!checkers());
1005 sideToMove = ~sideToMove;
1009 // Computes the new hash key after the given move. Needed
1010 // for speculative prefetch. It doesn't recognize special moves like castling,
1011 // en passant and promotions.
1012 Key Position::key_after(Move m) const {
1014 Square from = from_sq(m);
1015 Square to = to_sq(m);
1016 Piece pc = piece_on(from);
1017 Piece captured = piece_on(to);
1018 Key k = st->key ^ Zobrist::side;
1021 k ^= Zobrist::psq[captured][to];
1023 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1025 return (captured || type_of(pc) == PAWN) ? k : adjust_key50<true>(k);
1029 // Tests if the SEE (Static Exchange Evaluation)
1030 // value of move is greater or equal to the given threshold. We'll use an
1031 // algorithm similar to alpha-beta pruning with a null window.
1032 bool Position::see_ge(Move m, Value threshold) const {
1036 // Only deal with normal moves, assume others pass a simple SEE
1037 if (type_of(m) != NORMAL)
1038 return VALUE_ZERO >= threshold;
1040 Square from = from_sq(m), to = to_sq(m);
1042 int swap = PieceValue[piece_on(to)] - threshold;
1046 swap = PieceValue[piece_on(from)] - swap;
1050 assert(color_of(piece_on(from)) == sideToMove);
1051 Bitboard occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1052 Color stm = sideToMove;
1053 Bitboard attackers = attackers_to(to, occupied);
1054 Bitboard stmAttackers, bb;
1060 attackers &= occupied;
1062 // If stm has no more attackers then give up: stm loses
1063 if (!(stmAttackers = attackers & pieces(stm)))
1066 // Don't allow pinned pieces to attack as long as there are
1067 // pinners on their original square.
1068 if (pinners(~stm) & occupied)
1070 stmAttackers &= ~blockers_for_king(stm);
1078 // Locate and remove the next least valuable attacker, and add to
1079 // the bitboard 'attackers' any X-ray attackers behind it.
1080 if ((bb = stmAttackers & pieces(PAWN)))
1082 if ((swap = PawnValue - swap) < res)
1084 occupied ^= least_significant_square_bb(bb);
1086 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1089 else if ((bb = stmAttackers & pieces(KNIGHT)))
1091 if ((swap = KnightValue - swap) < res)
1093 occupied ^= least_significant_square_bb(bb);
1096 else if ((bb = stmAttackers & pieces(BISHOP)))
1098 if ((swap = BishopValue - swap) < res)
1100 occupied ^= least_significant_square_bb(bb);
1102 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1105 else if ((bb = stmAttackers & pieces(ROOK)))
1107 if ((swap = RookValue - swap) < res)
1109 occupied ^= least_significant_square_bb(bb);
1111 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1114 else if ((bb = stmAttackers & pieces(QUEEN)))
1116 if ((swap = QueenValue - swap) < res)
1118 occupied ^= least_significant_square_bb(bb);
1120 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1121 | (attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN));
1125 // If we "capture" with the king but the opponent still has attackers,
1126 // reverse the result.
1127 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1133 // Tests whether the position is drawn by 50-move rule
1134 // or by repetition. It does not detect stalemates.
1135 bool Position::is_draw(int ply) const {
1137 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1140 // Return a draw score if a position repeats once earlier but strictly
1141 // after the root, or repeats twice before or at the root.
1142 return st->repetition && st->repetition < ply;
1146 // Tests whether there has been at least one repetition
1147 // of positions since the last capture or pawn move.
1148 bool Position::has_repeated() const {
1150 StateInfo* stc = st;
1151 int end = std::min(st->rule50, st->pliesFromNull);
1154 if (stc->repetition)
1157 stc = stc->previous;
1163 // Tests if the position has a move which draws by repetition,
1164 // or an earlier position has a move that directly reaches the current position.
1165 bool Position::has_game_cycle(int ply) const {
1169 int end = std::min(st->rule50, st->pliesFromNull);
1174 Key originalKey = st->key;
1175 StateInfo* stp = st->previous;
1177 for (int i = 3; i <= end; i += 2)
1179 stp = stp->previous->previous;
1181 Key moveKey = originalKey ^ stp->key;
1182 if ((j = H1(moveKey), cuckoo[j] == moveKey) || (j = H2(moveKey), cuckoo[j] == moveKey))
1184 Move move = cuckooMove[j];
1185 Square s1 = from_sq(move);
1186 Square s2 = to_sq(move);
1188 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1193 // For nodes before or at the root, check that the move is a
1194 // repetition rather than a move to the current position.
1195 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1196 // the same location, so we have to select which square to check.
1197 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1200 // For repetitions before or at the root, require one more
1201 if (stp->repetition)
1210 // Flips position with the white and black sides reversed. This
1211 // is only useful for debugging e.g. for finding evaluation symmetry bugs.
1212 void Position::flip() {
1215 std::stringstream ss(fen());
1217 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1219 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1220 f.insert(0, token + (f.empty() ? " " : "/"));
1223 ss >> token; // Active color
1224 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1226 ss >> token; // Castling availability
1229 std::transform(f.begin(), f.end(), f.begin(),
1230 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1232 ss >> token; // En passant square
1233 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1235 std::getline(ss, token); // Half and full moves
1238 set(f, is_chess960(), st, this_thread());
1240 assert(pos_is_ok());
1244 // Performs some consistency checks for the position object
1245 // and raise an assert if something wrong is detected.
1246 // This is meant to be helpful when debugging.
1247 bool Position::pos_is_ok() const {
1249 constexpr bool Fast = true; // Quick (default) or full check?
1251 if ((sideToMove != WHITE && sideToMove != BLACK) || piece_on(square<KING>(WHITE)) != W_KING
1252 || piece_on(square<KING>(BLACK)) != B_KING
1253 || (ep_square() != SQ_NONE && relative_rank(sideToMove, ep_square()) != RANK_6))
1254 assert(0 && "pos_is_ok: Default");
1259 if (pieceCount[W_KING] != 1 || pieceCount[B_KING] != 1
1260 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1261 assert(0 && "pos_is_ok: Kings");
1263 if ((pieces(PAWN) & (Rank1BB | Rank8BB)) || pieceCount[W_PAWN] > 8 || pieceCount[B_PAWN] > 8)
1264 assert(0 && "pos_is_ok: Pawns");
1266 if ((pieces(WHITE) & pieces(BLACK)) || (pieces(WHITE) | pieces(BLACK)) != pieces()
1267 || popcount(pieces(WHITE)) > 16 || popcount(pieces(BLACK)) > 16)
1268 assert(0 && "pos_is_ok: Bitboards");
1270 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1271 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1272 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1273 assert(0 && "pos_is_ok: Bitboards");
1276 for (Piece pc : Pieces)
1277 if (pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1278 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1279 assert(0 && "pos_is_ok: Pieces");
1281 for (Color c : {WHITE, BLACK})
1282 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1284 if (!can_castle(cr))
1287 if (piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1288 || castlingRightsMask[castlingRookSquare[cr]] != cr
1289 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1290 assert(0 && "pos_is_ok: Castling");
1296 } // namespace Stockfish