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
375 // the given endgame code string like "KBPKN". It is mainly a helper to
376 // get the material key out of an endgame code.
377 Position& Position::set(const string& code, Color c, StateInfo* si) {
379 assert(code[0] == 'K');
381 string sides[] = {code.substr(code.find('K', 1)), // Weak
382 code.substr(0, std::min(code.find('v'), code.find('K', 1)))}; // Strong
384 assert(sides[0].length() > 0 && sides[0].length() < 8);
385 assert(sides[1].length() > 0 && sides[1].length() < 8);
387 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
389 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/" + sides[1]
390 + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
392 return set(fenStr, false, si, nullptr);
396 // Returns a FEN representation of the position. In case of
397 // Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
398 string Position::fen() const {
401 std::ostringstream ss;
403 for (Rank r = RANK_8; r >= RANK_1; --r)
405 for (File f = FILE_A; f <= FILE_H; ++f)
407 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
414 ss << PieceToChar[piece_on(make_square(f, r))];
421 ss << (sideToMove == WHITE ? " w " : " b ");
423 if (can_castle(WHITE_OO))
424 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO))) : 'K');
426 if (can_castle(WHITE_OOO))
427 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
429 if (can_castle(BLACK_OO))
430 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO))) : 'k');
432 if (can_castle(BLACK_OOO))
433 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
435 if (!can_castle(ANY_CASTLING))
438 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ") << st->rule50
439 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
444 // Calculates st->blockersForKing[c] and st->pinners[~c],
445 // which store respectively the pieces preventing king of color c from being in check
446 // and the slider pieces of color ~c pinning pieces of color c to the king.
447 void Position::update_slider_blockers(Color c) const {
449 Square ksq = square<KING>(c);
451 st->blockersForKing[c] = 0;
454 // Snipers are sliders that attack 's' when a piece and other snipers are removed
455 Bitboard snipers = ((attacks_bb<ROOK>(ksq) & pieces(QUEEN, ROOK))
456 | (attacks_bb<BISHOP>(ksq) & pieces(QUEEN, BISHOP)))
458 Bitboard occupancy = pieces() ^ snipers;
462 Square sniperSq = pop_lsb(snipers);
463 Bitboard b = between_bb(ksq, sniperSq) & occupancy;
465 if (b && !more_than_one(b))
467 st->blockersForKing[c] |= b;
469 st->pinners[~c] |= sniperSq;
475 // Computes a bitboard of all pieces which attack a
476 // given square. Slider attacks use the occupied bitboard to indicate occupancy.
477 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
479 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
480 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
481 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
482 | (attacks_bb<ROOK>(s, occupied) & pieces(ROOK, QUEEN))
483 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
484 | (attacks_bb<KING>(s) & pieces(KING));
488 // Tests whether a pseudo-legal move is legal
489 bool Position::legal(Move m) const {
493 Color us = sideToMove;
494 Square from = from_sq(m);
495 Square to = to_sq(m);
497 assert(color_of(moved_piece(m)) == us);
498 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
500 // En passant captures are a tricky special case. Because they are rather
501 // uncommon, we do it simply by testing whether the king is attacked after
503 if (type_of(m) == EN_PASSANT)
505 Square ksq = square<KING>(us);
506 Square capsq = to - pawn_push(us);
507 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
509 assert(to == ep_square());
510 assert(moved_piece(m) == make_piece(us, PAWN));
511 assert(piece_on(capsq) == make_piece(~us, PAWN));
512 assert(piece_on(to) == NO_PIECE);
514 return !(attacks_bb<ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
515 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
518 // Castling moves generation does not check if the castling path is clear of
519 // enemy attacks, it is delayed at a later time: now!
520 if (type_of(m) == CASTLING)
522 // After castling, the rook and king final positions are the same in
523 // Chess960 as they would be in standard chess.
524 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
525 Direction step = to > from ? WEST : EAST;
527 for (Square s = to; s != from; s += step)
528 if (attackers_to(s) & pieces(~us))
531 // In case of Chess960, verify if the Rook blocks some checks.
532 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
533 return !chess960 || !(blockers_for_king(us) & to_sq(m));
536 // If the moving piece is a king, check whether the destination square is
537 // attacked by the opponent.
538 if (type_of(piece_on(from)) == KING)
539 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
541 // A non-king move is legal if and only if it is not pinned or it
542 // is moving along the ray towards or away from the king.
543 return !(blockers_for_king(us) & from) || aligned(from, to, square<KING>(us));
547 // Takes a random move and tests whether the move is
548 // pseudo-legal. It is used to validate moves from TT that can be corrupted
549 // due to SMP concurrent access or hash position key aliasing.
550 bool Position::pseudo_legal(const Move m) const {
552 Color us = sideToMove;
553 Square from = from_sq(m);
554 Square to = to_sq(m);
555 Piece pc = moved_piece(m);
557 // Use a slower but simpler function for uncommon cases
558 // yet we skip the legality check of MoveList<LEGAL>().
559 if (type_of(m) != NORMAL)
560 return checkers() ? MoveList<EVASIONS>(*this).contains(m)
561 : MoveList<NON_EVASIONS>(*this).contains(m);
563 // Is not a promotion, so the promotion piece must be empty
564 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
566 // If the 'from' square is not occupied by a piece belonging to the side to
567 // move, the move is obviously not legal.
568 if (pc == NO_PIECE || color_of(pc) != us)
571 // The destination square cannot be occupied by a friendly piece
575 // Handle the special case of a pawn move
576 if (type_of(pc) == PAWN)
578 // We have already handled promotion moves, so destination
579 // cannot be on the 8th/1st rank.
580 if ((Rank8BB | Rank1BB) & to)
583 if (!(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
584 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
585 && !((from + 2 * pawn_push(us) == to) // Not a double push
586 && (relative_rank(us, from) == RANK_2) && empty(to) && empty(to - pawn_push(us))))
589 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
592 // Evasions generator already takes care to avoid some kind of illegal moves
593 // and legal() relies on this. We therefore have to take care that the same
594 // kind of moves are filtered out here.
597 if (type_of(pc) != KING)
599 // Double check? In this case, a king move is required
600 if (more_than_one(checkers()))
603 // Our move must be a blocking interposition or a capture of the checking piece
604 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
607 // In case of king moves under check we have to remove the king so as to catch
608 // invalid moves like b1a1 when opposite queen is on c1.
609 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
617 // Tests whether a pseudo-legal move gives a check
618 bool Position::gives_check(Move m) const {
621 assert(color_of(moved_piece(m)) == sideToMove);
623 Square from = from_sq(m);
624 Square to = to_sq(m);
626 // Is there a direct check?
627 if (check_squares(type_of(piece_on(from))) & to)
630 // Is there a discovered check?
631 if (blockers_for_king(~sideToMove) & from)
632 return !aligned(from, to, square<KING>(~sideToMove)) || type_of(m) == CASTLING;
640 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
642 // En passant capture with check? We have already handled the case
643 // of direct checks and ordinary discovered check, so the only case we
644 // need to handle is the unusual case of a discovered check through
645 // the captured pawn.
647 Square capsq = make_square(file_of(to), rank_of(from));
648 Bitboard b = (pieces() ^ from ^ capsq) | to;
650 return (attacks_bb<ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
651 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b)
652 & pieces(sideToMove, QUEEN, BISHOP));
656 // Castling is encoded as 'king captures the rook'
657 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
659 return check_squares(ROOK) & rto;
665 // Makes a move, and saves all information necessary
666 // to a StateInfo object. The move is assumed to be legal. Pseudo-legal
667 // moves should be filtered out before this function is called.
668 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
671 assert(&newSt != st);
673 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
674 Key k = st->key ^ Zobrist::side;
676 // Copy some fields of the old state to our new StateInfo object except the
677 // ones which are going to be recalculated from scratch anyway and then switch
678 // our state pointer to point to the new (ready to be updated) state.
679 std::memcpy(&newSt, st, offsetof(StateInfo, key));
683 // Increment ply counters. In particular, rule50 will be reset to zero later on
684 // in case of a capture or a pawn move.
690 st->accumulator.computed[WHITE] = false;
691 st->accumulator.computed[BLACK] = false;
692 auto& dp = st->dirtyPiece;
695 Color us = sideToMove;
697 Square from = from_sq(m);
698 Square to = to_sq(m);
699 Piece pc = piece_on(from);
700 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
702 assert(color_of(pc) == us);
703 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
704 assert(type_of(captured) != KING);
706 if (type_of(m) == CASTLING)
708 assert(pc == make_piece(us, KING));
709 assert(captured == make_piece(us, ROOK));
712 do_castling<true>(us, from, to, rfrom, rto);
714 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
722 // If the captured piece is a pawn, update pawn hash key, otherwise
723 // update non-pawn material.
724 if (type_of(captured) == PAWN)
726 if (type_of(m) == EN_PASSANT)
728 capsq -= pawn_push(us);
730 assert(pc == make_piece(us, PAWN));
731 assert(to == st->epSquare);
732 assert(relative_rank(us, to) == RANK_6);
733 assert(piece_on(to) == NO_PIECE);
734 assert(piece_on(capsq) == make_piece(them, PAWN));
737 st->pawnKey ^= Zobrist::psq[captured][capsq];
740 st->nonPawnMaterial[them] -= PieceValue[captured];
742 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
743 dp.piece[1] = captured;
747 // Update board and piece lists
750 // Update material hash key and prefetch access to materialTable
751 k ^= Zobrist::psq[captured][capsq];
752 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
754 // Reset rule 50 counter
759 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
761 // Reset en passant square
762 if (st->epSquare != SQ_NONE)
764 k ^= Zobrist::enpassant[file_of(st->epSquare)];
765 st->epSquare = SQ_NONE;
768 // Update castling rights if needed
769 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
771 k ^= Zobrist::castling[st->castlingRights];
772 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
773 k ^= Zobrist::castling[st->castlingRights];
776 // Move the piece. The tricky Chess960 castling is handled earlier
777 if (type_of(m) != CASTLING)
783 move_piece(from, to);
786 // If the moving piece is a pawn do some special extra work
787 if (type_of(pc) == PAWN)
789 // Set en passant square if the moved pawn can be captured
790 if ((int(to) ^ int(from)) == 16
791 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
793 st->epSquare = to - pawn_push(us);
794 k ^= Zobrist::enpassant[file_of(st->epSquare)];
797 else if (type_of(m) == PROMOTION)
799 Piece promotion = make_piece(us, promotion_type(m));
801 assert(relative_rank(us, to) == RANK_8);
802 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
805 put_piece(promotion, to);
807 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
809 dp.piece[dp.dirty_num] = promotion;
810 dp.from[dp.dirty_num] = SQ_NONE;
811 dp.to[dp.dirty_num] = to;
815 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
816 st->pawnKey ^= Zobrist::psq[pc][to];
818 Zobrist::psq[promotion][pieceCount[promotion] - 1] ^ Zobrist::psq[pc][pieceCount[pc]];
821 st->nonPawnMaterial[us] += PieceValue[promotion];
824 // Update pawn hash key
825 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
827 // Reset rule 50 draw counter
832 st->capturedPiece = captured;
834 // Update the key with the final value
837 // Calculate checkers bitboard (if move gives check)
838 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
840 sideToMove = ~sideToMove;
842 // Update king attacks used for fast check detection
845 // Calculate the repetition info. It is the ply distance from the previous
846 // occurrence of the same position, negative in the 3-fold case, or zero
847 // if the position was not repeated.
849 int end = std::min(st->rule50, st->pliesFromNull);
852 StateInfo* stp = st->previous->previous;
853 for (int i = 4; i <= end; i += 2)
855 stp = stp->previous->previous;
856 if (stp->key == st->key)
858 st->repetition = stp->repetition ? -i : i;
868 // Unmakes a move. When it returns, the position should
869 // be restored to exactly the same state as before the move was made.
870 void Position::undo_move(Move m) {
874 sideToMove = ~sideToMove;
876 Color us = sideToMove;
877 Square from = from_sq(m);
878 Square to = to_sq(m);
879 Piece pc = piece_on(to);
881 assert(empty(from) || type_of(m) == CASTLING);
882 assert(type_of(st->capturedPiece) != KING);
884 if (type_of(m) == PROMOTION)
886 assert(relative_rank(us, to) == RANK_8);
887 assert(type_of(pc) == promotion_type(m));
888 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
891 pc = make_piece(us, PAWN);
895 if (type_of(m) == CASTLING)
898 do_castling<false>(us, from, to, rfrom, rto);
902 move_piece(to, from); // Put the piece back at the source square
904 if (st->capturedPiece)
908 if (type_of(m) == EN_PASSANT)
910 capsq -= pawn_push(us);
912 assert(type_of(pc) == PAWN);
913 assert(to == st->previous->epSquare);
914 assert(relative_rank(us, to) == RANK_6);
915 assert(piece_on(capsq) == NO_PIECE);
916 assert(st->capturedPiece == make_piece(~us, PAWN));
919 put_piece(st->capturedPiece, capsq); // Restore the captured piece
923 // Finally point our state pointer back to the previous state
931 // Helper used to do/undo a castling move. This
932 // is a bit tricky in Chess960 where from/to squares can overlap.
934 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
936 bool kingSide = to > from;
937 rfrom = to; // Castling is encoded as "king captures friendly rook"
938 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
939 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
943 auto& dp = st->dirtyPiece;
944 dp.piece[0] = make_piece(us, KING);
947 dp.piece[1] = make_piece(us, ROOK);
953 // Remove both pieces first since squares could overlap in Chess960
954 remove_piece(Do ? from : to);
955 remove_piece(Do ? rfrom : rto);
956 board[Do ? from : to] = board[Do ? rfrom : rto] =
957 NO_PIECE; // remove_piece does not do this for us
958 put_piece(make_piece(us, KING), Do ? to : from);
959 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
963 // Used to do a "null move": it flips
964 // the side to move without executing any move on the board.
965 void Position::do_null_move(StateInfo& newSt) {
968 assert(&newSt != st);
970 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
975 st->dirtyPiece.dirty_num = 0;
976 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
977 st->accumulator.computed[WHITE] = false;
978 st->accumulator.computed[BLACK] = false;
980 if (st->epSquare != SQ_NONE)
982 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
983 st->epSquare = SQ_NONE;
986 st->key ^= Zobrist::side;
988 prefetch(TT.first_entry(key()));
990 st->pliesFromNull = 0;
992 sideToMove = ~sideToMove;
1002 // Must be used to undo a "null move"
1003 void Position::undo_null_move() {
1005 assert(!checkers());
1008 sideToMove = ~sideToMove;
1012 // Computes the new hash key after the given move. Needed
1013 // for speculative prefetch. It doesn't recognize special moves like castling,
1014 // en passant and promotions.
1015 Key Position::key_after(Move m) const {
1017 Square from = from_sq(m);
1018 Square to = to_sq(m);
1019 Piece pc = piece_on(from);
1020 Piece captured = piece_on(to);
1021 Key k = st->key ^ Zobrist::side;
1024 k ^= Zobrist::psq[captured][to];
1026 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1028 return (captured || type_of(pc) == PAWN) ? k : adjust_key50<true>(k);
1032 // Tests if the SEE (Static Exchange Evaluation)
1033 // value of move is greater or equal to the given threshold. We'll use an
1034 // algorithm similar to alpha-beta pruning with a null window.
1035 bool Position::see_ge(Move m, Value threshold) const {
1039 // Only deal with normal moves, assume others pass a simple SEE
1040 if (type_of(m) != NORMAL)
1041 return VALUE_ZERO >= threshold;
1043 Square from = from_sq(m), to = to_sq(m);
1045 int swap = PieceValue[piece_on(to)] - threshold;
1049 swap = PieceValue[piece_on(from)] - swap;
1053 assert(color_of(piece_on(from)) == sideToMove);
1054 Bitboard occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1055 Color stm = sideToMove;
1056 Bitboard attackers = attackers_to(to, occupied);
1057 Bitboard stmAttackers, bb;
1063 attackers &= occupied;
1065 // If stm has no more attackers then give up: stm loses
1066 if (!(stmAttackers = attackers & pieces(stm)))
1069 // Don't allow pinned pieces to attack as long as there are
1070 // pinners on their original square.
1071 if (pinners(~stm) & occupied)
1073 stmAttackers &= ~blockers_for_king(stm);
1081 // Locate and remove the next least valuable attacker, and add to
1082 // the bitboard 'attackers' any X-ray attackers behind it.
1083 if ((bb = stmAttackers & pieces(PAWN)))
1085 if ((swap = PawnValue - swap) < res)
1087 occupied ^= least_significant_square_bb(bb);
1089 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1092 else if ((bb = stmAttackers & pieces(KNIGHT)))
1094 if ((swap = KnightValue - swap) < res)
1096 occupied ^= least_significant_square_bb(bb);
1099 else if ((bb = stmAttackers & pieces(BISHOP)))
1101 if ((swap = BishopValue - swap) < res)
1103 occupied ^= least_significant_square_bb(bb);
1105 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1108 else if ((bb = stmAttackers & pieces(ROOK)))
1110 if ((swap = RookValue - swap) < res)
1112 occupied ^= least_significant_square_bb(bb);
1114 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1117 else if ((bb = stmAttackers & pieces(QUEEN)))
1119 if ((swap = QueenValue - swap) < res)
1121 occupied ^= least_significant_square_bb(bb);
1123 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1124 | (attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN));
1128 // If we "capture" with the king but the opponent still has attackers,
1129 // reverse the result.
1130 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1136 // Tests whether the position is drawn by 50-move rule
1137 // or by repetition. It does not detect stalemates.
1138 bool Position::is_draw(int ply) const {
1140 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1143 // Return a draw score if a position repeats once earlier but strictly
1144 // after the root, or repeats twice before or at the root.
1145 return st->repetition && st->repetition < ply;
1149 // Tests whether there has been at least one repetition
1150 // of positions since the last capture or pawn move.
1151 bool Position::has_repeated() const {
1153 StateInfo* stc = st;
1154 int end = std::min(st->rule50, st->pliesFromNull);
1157 if (stc->repetition)
1160 stc = stc->previous;
1166 // Tests if the position has a move which draws by repetition,
1167 // or an earlier position has a move that directly reaches the current position.
1168 bool Position::has_game_cycle(int ply) const {
1172 int end = std::min(st->rule50, st->pliesFromNull);
1177 Key originalKey = st->key;
1178 StateInfo* stp = st->previous;
1180 for (int i = 3; i <= end; i += 2)
1182 stp = stp->previous->previous;
1184 Key moveKey = originalKey ^ stp->key;
1185 if ((j = H1(moveKey), cuckoo[j] == moveKey) || (j = H2(moveKey), cuckoo[j] == moveKey))
1187 Move move = cuckooMove[j];
1188 Square s1 = from_sq(move);
1189 Square s2 = to_sq(move);
1191 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1196 // For nodes before or at the root, check that the move is a
1197 // repetition rather than a move to the current position.
1198 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1199 // the same location, so we have to select which square to check.
1200 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1203 // For repetitions before or at the root, require one more
1204 if (stp->repetition)
1213 // Flips position with the white and black sides reversed. This
1214 // is only useful for debugging e.g. for finding evaluation symmetry bugs.
1215 void Position::flip() {
1218 std::stringstream ss(fen());
1220 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1222 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1223 f.insert(0, token + (f.empty() ? " " : "/"));
1226 ss >> token; // Active color
1227 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1229 ss >> token; // Castling availability
1232 std::transform(f.begin(), f.end(), f.begin(),
1233 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1235 ss >> token; // En passant square
1236 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1238 std::getline(ss, token); // Half and full moves
1241 set(f, is_chess960(), st, this_thread());
1243 assert(pos_is_ok());
1247 // Performs some consistency checks for the
1248 // position object and raise an assert if something wrong is detected.
1249 // This is meant to be helpful when debugging.
1250 bool Position::pos_is_ok() const {
1252 constexpr bool Fast = true; // Quick (default) or full check?
1254 if ((sideToMove != WHITE && sideToMove != BLACK) || piece_on(square<KING>(WHITE)) != W_KING
1255 || piece_on(square<KING>(BLACK)) != B_KING
1256 || (ep_square() != SQ_NONE && relative_rank(sideToMove, ep_square()) != RANK_6))
1257 assert(0 && "pos_is_ok: Default");
1262 if (pieceCount[W_KING] != 1 || pieceCount[B_KING] != 1
1263 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1264 assert(0 && "pos_is_ok: Kings");
1266 if ((pieces(PAWN) & (Rank1BB | Rank8BB)) || pieceCount[W_PAWN] > 8 || pieceCount[B_PAWN] > 8)
1267 assert(0 && "pos_is_ok: Pawns");
1269 if ((pieces(WHITE) & pieces(BLACK)) || (pieces(WHITE) | pieces(BLACK)) != pieces()
1270 || popcount(pieces(WHITE)) > 16 || popcount(pieces(BLACK)) > 16)
1271 assert(0 && "pos_is_ok: Bitboards");
1273 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1274 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1275 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1276 assert(0 && "pos_is_ok: Bitboards");
1279 for (Piece pc : Pieces)
1280 if (pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1281 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1282 assert(0 && "pos_is_ok: Pieces");
1284 for (Color c : {WHITE, BLACK})
1285 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1287 if (!can_castle(cr))
1290 if (piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1291 || castlingRightsMask[castlingRookSquare[cr]] != cr
1292 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1293 assert(0 && "pos_is_ok: Castling");
1299 } // namespace Stockfish