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/>.
21 #include <cstddef> // For offsetof()
22 #include <cstring> // For std::memset, std::memcmp
25 #include <string_view>
34 #include "syzygy/tbprobe.h"
42 Key psq[PIECE_NB][SQUARE_NB];
43 Key enpassant[FILE_NB];
44 Key castling[CASTLING_RIGHT_NB];
50 constexpr std::string_view PieceToChar(" PNBRQK pnbrqk");
52 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
53 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
57 /// operator<<(Position) returns an ASCII representation of the position
59 std::ostream& operator<<(std::ostream& os, const Position& pos) {
61 os << "\n +---+---+---+---+---+---+---+---+\n";
63 for (Rank r = RANK_8; r >= RANK_1; --r)
65 for (File f = FILE_A; f <= FILE_H; ++f)
66 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
68 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
71 os << " a b c d e f g h\n"
72 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
73 << std::setfill('0') << std::setw(16) << pos.key()
74 << std::setfill(' ') << std::dec << "\nCheckers: ";
76 for (Bitboard b = pos.checkers(); b; )
77 os << UCI::square(pop_lsb(b)) << " ";
79 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
80 && !pos.can_castle(ANY_CASTLING))
83 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
86 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
87 Tablebases::ProbeState s1, s2;
88 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
89 int dtz = Tablebases::probe_dtz(p, &s2);
90 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
91 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
98 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
99 // situations. Description of the algorithm in the following paper:
100 // http://web.archive.org/web/20201107002606/https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
102 // First and second hash functions for indexing the cuckoo tables
103 inline int H1(Key h) { return h & 0x1fff; }
104 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
106 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
108 Move cuckooMove[8192];
111 /// Position::init() initializes at startup the various arrays used to compute hash keys
113 void Position::init() {
117 for (Piece pc : Pieces)
118 for (Square s = SQ_A1; s <= SQ_H8; ++s)
119 Zobrist::psq[pc][s] = rng.rand<Key>();
121 for (File f = FILE_A; f <= FILE_H; ++f)
122 Zobrist::enpassant[f] = rng.rand<Key>();
124 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
125 Zobrist::castling[cr] = rng.rand<Key>();
127 Zobrist::side = rng.rand<Key>();
129 // Prepare the cuckoo tables
130 std::memset(cuckoo, 0, sizeof(cuckoo));
131 std::memset(cuckooMove, 0, sizeof(cuckooMove));
132 [[maybe_unused]] int count = 0;
133 for (Piece pc : Pieces)
134 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
135 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
136 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
138 Move move = make_move(s1, s2);
139 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
143 std::swap(cuckoo[i], key);
144 std::swap(cuckooMove[i], move);
145 if (move == MOVE_NONE) // Arrived at empty slot?
147 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
151 assert(count == 3668);
155 /// Position::set() initializes the position object with the given FEN string.
156 /// This function is not very robust - make sure that input FENs are correct,
157 /// this is assumed to be the responsibility of the GUI.
159 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
161 A FEN string defines a particular position using only the ASCII character set.
163 A FEN string contains six fields separated by a space. The fields are:
165 1) Piece placement (from white's perspective). Each rank is described, starting
166 with rank 8 and ending with rank 1. Within each rank, the contents of each
167 square are described from file A through file H. Following the Standard
168 Algebraic Notation (SAN), each piece is identified by a single letter taken
169 from the standard English names. White pieces are designated using upper-case
170 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
171 noted using digits 1 through 8 (the number of blank squares), and "/"
174 2) Active color. "w" means white moves next, "b" means black.
176 3) Castling availability. If neither side can castle, this is "-". Otherwise,
177 this has one or more letters: "K" (White can castle kingside), "Q" (White
178 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
179 can castle queenside).
181 4) En passant target square (in algebraic notation). If there's no en passant
182 target square, this is "-". If a pawn has just made a 2-square move, this
183 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
184 if there is a pawn in position to make an en passant capture, and if there really
185 is a pawn that might have advanced two squares.
187 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
188 or capture. This is used to determine if a draw can be claimed under the
191 6) Fullmove number. The number of the full move. It starts at 1, and is
192 incremented after Black's move.
195 unsigned char col, row, token;
198 std::istringstream ss(fenStr);
200 std::memset(this, 0, sizeof(Position));
201 std::memset(si, 0, sizeof(StateInfo));
206 // 1. Piece placement
207 while ((ss >> token) && !isspace(token))
210 sq += (token - '0') * EAST; // Advance the given number of files
212 else if (token == '/')
215 else if ((idx = PieceToChar.find(token)) != string::npos) {
216 put_piece(Piece(idx), sq);
223 sideToMove = (token == 'w' ? WHITE : BLACK);
226 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
227 // Shredder-FEN that uses the letters of the columns on which the rooks began
228 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
229 // if an inner rook is associated with the castling right, the castling tag is
230 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
231 while ((ss >> token) && !isspace(token))
234 Color c = islower(token) ? BLACK : WHITE;
235 Piece rook = make_piece(c, ROOK);
237 token = char(toupper(token));
240 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
242 else if (token == 'Q')
243 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
245 else if (token >= 'A' && token <= 'H')
246 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
251 set_castling_right(c, rsq);
254 // 4. En passant square.
255 // Ignore if square is invalid or not on side to move relative rank 6.
256 bool enpassant = false;
258 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
259 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
261 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
263 // En passant square will be considered only if
264 // a) side to move have a pawn threatening epSquare
265 // b) there is an enemy pawn in front of epSquare
266 // c) there is no piece on epSquare or behind epSquare
267 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
268 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
269 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
273 st->epSquare = SQ_NONE;
275 // 5-6. Halfmove clock and fullmove number
276 ss >> std::skipws >> st->rule50 >> gamePly;
278 // Convert from fullmove starting from 1 to gamePly starting from 0,
279 // handle also common incorrect FEN with fullmove = 0.
280 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
282 chess960 = isChess960;
292 /// Position::set_castling_right() is a helper function used to set castling
293 /// rights given the corresponding color and the rook starting square.
295 void Position::set_castling_right(Color c, Square rfrom) {
297 Square kfrom = square<KING>(c);
298 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
300 st->castlingRights |= cr;
301 castlingRightsMask[kfrom] |= cr;
302 castlingRightsMask[rfrom] |= cr;
303 castlingRookSquare[cr] = rfrom;
305 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
306 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
308 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto))
313 /// Position::set_check_info() sets king attacks to detect if a move gives check
315 void Position::set_check_info() const {
317 st->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), st->pinners[BLACK]);
318 st->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), st->pinners[WHITE]);
320 Square ksq = square<KING>(~sideToMove);
322 st->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
323 st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
324 st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
325 st->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
326 st->checkSquares[QUEEN] = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
327 st->checkSquares[KING] = 0;
331 /// Position::set_state() computes the hash keys of the position, and other
332 /// data that once computed is updated incrementally as moves are made.
333 /// The function is only used when a new position is set up
335 void Position::set_state() const {
337 st->key = st->materialKey = 0;
338 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
339 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
343 for (Bitboard b = pieces(); b; )
345 Square s = pop_lsb(b);
346 Piece pc = piece_on(s);
347 st->key ^= Zobrist::psq[pc][s];
349 if (type_of(pc) != KING && type_of(pc) != PAWN)
350 st->nonPawnMaterial[color_of(pc)] += PieceValue[pc];
353 if (st->epSquare != SQ_NONE)
354 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
356 if (sideToMove == BLACK)
357 st->key ^= Zobrist::side;
359 st->key ^= Zobrist::castling[st->castlingRights];
361 for (Piece pc : Pieces)
362 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
363 st->materialKey ^= Zobrist::psq[pc][cnt];
367 /// Position::set() is an overload to initialize the position object with
368 /// the given endgame code string like "KBPKN". It is mainly a helper to
369 /// get the material key out of an endgame code.
371 Position& Position::set(const string& code, Color c, StateInfo* si) {
373 assert(code[0] == 'K');
375 string sides[] = { code.substr(code.find('K', 1)), // Weak
376 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
378 assert(sides[0].length() > 0 && sides[0].length() < 8);
379 assert(sides[1].length() > 0 && sides[1].length() < 8);
381 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
383 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
384 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
386 return set(fenStr, false, si, nullptr);
390 /// Position::fen() returns a FEN representation of the position. In case of
391 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
393 string Position::fen() const {
396 std::ostringstream ss;
398 for (Rank r = RANK_8; r >= RANK_1; --r)
400 for (File f = FILE_A; f <= FILE_H; ++f)
402 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
409 ss << PieceToChar[piece_on(make_square(f, r))];
416 ss << (sideToMove == WHITE ? " w " : " b ");
418 if (can_castle(WHITE_OO))
419 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
421 if (can_castle(WHITE_OOO))
422 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
424 if (can_castle(BLACK_OO))
425 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
427 if (can_castle(BLACK_OOO))
428 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
430 if (!can_castle(ANY_CASTLING))
433 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
434 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
440 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
441 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
442 /// slider if removing that piece from the board would result in a position where
443 /// square 's' is attacked. For example, a king-attack blocking piece can be either
444 /// a pinned or a discovered check piece, according if its color is the opposite
445 /// or the same of the color of the slider.
447 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
449 Bitboard blockers = 0;
452 // Snipers are sliders that attack 's' when a piece and other snipers are removed
453 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
454 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
455 Bitboard occupancy = pieces() ^ snipers;
459 Square sniperSq = pop_lsb(snipers);
460 Bitboard b = between_bb(s, sniperSq) & occupancy;
462 if (b && !more_than_one(b))
465 if (b & pieces(color_of(piece_on(s))))
473 /// Position::attackers_to() computes a bitboard of all pieces which attack a
474 /// given square. 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 /// Position::legal() 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)
544 || aligned(from, to, square<KING>(us));
548 /// Position::pseudo_legal() takes a random move and tests whether the move is
549 /// pseudo legal. It is used to validate moves from TT that can be corrupted
550 /// due to SMP concurrent access or hash position key aliasing.
552 bool Position::pseudo_legal(const Move m) const {
554 Color us = sideToMove;
555 Square from = from_sq(m);
556 Square to = to_sq(m);
557 Piece pc = moved_piece(m);
559 // Use a slower but simpler function for uncommon cases
560 // yet we skip the legality check of MoveList<LEGAL>().
561 if (type_of(m) != NORMAL)
562 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
563 : MoveList<NON_EVASIONS>(*this).contains(m);
565 // Is not a promotion, so promotion piece must be empty
566 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
568 // If the 'from' square is not occupied by a piece belonging to the side to
569 // move, the move is obviously not legal.
570 if (pc == NO_PIECE || color_of(pc) != us)
573 // The destination square cannot be occupied by a friendly piece
577 // Handle the special case of a pawn move
578 if (type_of(pc) == PAWN)
580 // We have already handled promotion moves, so destination
581 // cannot be on the 8th/1st rank.
582 if ((Rank8BB | Rank1BB) & to)
585 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
586 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
587 && !( (from + 2 * pawn_push(us) == to) // Not a double push
588 && (relative_rank(us, from) == RANK_2)
590 && empty(to - pawn_push(us))))
593 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
596 // Evasions generator already takes care to avoid some kind of illegal moves
597 // and legal() relies on this. We therefore have to take care that the same
598 // kind of moves are filtered out here.
601 if (type_of(pc) != KING)
603 // Double check? In this case a king move is required
604 if (more_than_one(checkers()))
607 // Our move must be a blocking interposition or a capture of the checking piece
608 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
611 // In case of king moves under check we have to remove king so as to catch
612 // invalid moves like b1a1 when opposite queen is on c1.
613 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
621 /// Position::gives_check() tests whether a pseudo-legal move gives a check
623 bool Position::gives_check(Move m) const {
626 assert(color_of(moved_piece(m)) == sideToMove);
628 Square from = from_sq(m);
629 Square to = to_sq(m);
631 // Is there a direct check?
632 if (check_squares(type_of(piece_on(from))) & to)
635 // Is there a discovered check?
636 if (blockers_for_king(~sideToMove) & from)
637 return !aligned(from, to, square<KING>(~sideToMove))
638 || type_of(m) == CASTLING;
646 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
648 // En passant capture with check? We have already handled the case
649 // of direct checks and ordinary discovered check, so the only case we
650 // need to handle is the unusual case of a discovered check through
651 // 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) & pieces(sideToMove, QUEEN, BISHOP));
662 // Castling is encoded as 'king captures the rook'
663 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
665 return check_squares(ROOK) & rto;
671 /// Position::do_move() makes a move, and saves all information necessary
672 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
673 /// moves should be filtered out before this function is called.
675 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
678 assert(&newSt != st);
680 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
681 Key k = st->key ^ Zobrist::side;
683 // Copy some fields of the old state to our new StateInfo object except the
684 // ones which are going to be recalculated from scratch anyway and then switch
685 // our state pointer to point to the new (ready to be updated) state.
686 std::memcpy(&newSt, st, offsetof(StateInfo, key));
690 // Increment ply counters. In particular, rule50 will be reset to zero later on
691 // in case of a capture or a pawn move.
697 st->accumulator.computed[WHITE] = false;
698 st->accumulator.computed[BLACK] = false;
699 auto& dp = st->dirtyPiece;
702 Color us = sideToMove;
704 Square from = from_sq(m);
705 Square to = to_sq(m);
706 Piece pc = piece_on(from);
707 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
709 assert(color_of(pc) == us);
710 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
711 assert(type_of(captured) != KING);
713 if (type_of(m) == CASTLING)
715 assert(pc == make_piece(us, KING));
716 assert(captured == make_piece(us, ROOK));
719 do_castling<true>(us, from, to, rfrom, rto);
721 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
729 // If the captured piece is a pawn, update pawn hash key, otherwise
730 // update non-pawn material.
731 if (type_of(captured) == PAWN)
733 if (type_of(m) == EN_PASSANT)
735 capsq -= pawn_push(us);
737 assert(pc == make_piece(us, PAWN));
738 assert(to == st->epSquare);
739 assert(relative_rank(us, to) == RANK_6);
740 assert(piece_on(to) == NO_PIECE);
741 assert(piece_on(capsq) == make_piece(them, PAWN));
745 st->nonPawnMaterial[them] -= PieceValue[captured];
747 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
748 dp.piece[1] = captured;
752 // Update board and piece lists
755 // Update material hash key and prefetch access to materialTable
756 k ^= Zobrist::psq[captured][capsq];
757 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
759 // Reset rule 50 counter
764 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
766 // Reset en passant square
767 if (st->epSquare != SQ_NONE)
769 k ^= Zobrist::enpassant[file_of(st->epSquare)];
770 st->epSquare = SQ_NONE;
773 // Update castling rights if needed
774 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
776 k ^= Zobrist::castling[st->castlingRights];
777 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
778 k ^= Zobrist::castling[st->castlingRights];
781 // Move the piece. The tricky Chess960 castling is handled earlier
782 if (type_of(m) != CASTLING)
788 move_piece(from, to);
791 // If the moving piece is a pawn do some special extra work
792 if (type_of(pc) == PAWN)
794 // Set en passant square if the moved pawn can be captured
795 if ( (int(to) ^ int(from)) == 16
796 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
798 st->epSquare = to - pawn_push(us);
799 k ^= Zobrist::enpassant[file_of(st->epSquare)];
802 else if (type_of(m) == PROMOTION)
804 Piece promotion = make_piece(us, promotion_type(m));
806 assert(relative_rank(us, to) == RANK_8);
807 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
810 put_piece(promotion, to);
812 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
814 dp.piece[dp.dirty_num] = promotion;
815 dp.from[dp.dirty_num] = SQ_NONE;
816 dp.to[dp.dirty_num] = to;
820 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
821 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
822 ^ Zobrist::psq[pc][pieceCount[pc]];
825 st->nonPawnMaterial[us] += PieceValue[promotion];
828 // Reset rule 50 draw counter
833 st->capturedPiece = captured;
835 // Update the key with the final value
838 // Calculate checkers bitboard (if move gives check)
839 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
841 sideToMove = ~sideToMove;
843 // Update king attacks used for fast check detection
846 // Calculate the repetition info. It is the ply distance from the previous
847 // occurrence of the same position, negative in the 3-fold case, or zero
848 // if the position was not repeated.
850 int end = std::min(st->rule50, st->pliesFromNull);
853 StateInfo* stp = st->previous->previous;
854 for (int i = 4; i <= end; i += 2)
856 stp = stp->previous->previous;
857 if (stp->key == st->key)
859 st->repetition = stp->repetition ? -i : i;
869 /// Position::undo_move() unmakes a move. When it returns, the position should
870 /// be restored to exactly the same state as before the move was made.
872 void Position::undo_move(Move m) {
876 sideToMove = ~sideToMove;
878 Color us = sideToMove;
879 Square from = from_sq(m);
880 Square to = to_sq(m);
881 Piece pc = piece_on(to);
883 assert(empty(from) || type_of(m) == CASTLING);
884 assert(type_of(st->capturedPiece) != KING);
886 if (type_of(m) == PROMOTION)
888 assert(relative_rank(us, to) == RANK_8);
889 assert(type_of(pc) == promotion_type(m));
890 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
893 pc = make_piece(us, PAWN);
897 if (type_of(m) == CASTLING)
900 do_castling<false>(us, from, to, rfrom, rto);
904 move_piece(to, from); // Put the piece back at the source square
906 if (st->capturedPiece)
910 if (type_of(m) == EN_PASSANT)
912 capsq -= pawn_push(us);
914 assert(type_of(pc) == PAWN);
915 assert(to == st->previous->epSquare);
916 assert(relative_rank(us, to) == RANK_6);
917 assert(piece_on(capsq) == NO_PIECE);
918 assert(st->capturedPiece == make_piece(~us, PAWN));
921 put_piece(st->capturedPiece, capsq); // Restore the captured piece
925 // Finally point our state pointer back to the previous state
933 /// Position::do_castling() is a helper used to do/undo a castling move. This
934 /// is a bit tricky in Chess960 where from/to squares can overlap.
936 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
938 bool kingSide = to > from;
939 rfrom = to; // Castling is encoded as "king captures friendly rook"
940 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
941 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
945 auto& dp = st->dirtyPiece;
946 dp.piece[0] = make_piece(us, KING);
949 dp.piece[1] = make_piece(us, ROOK);
955 // Remove both pieces first since squares could overlap in Chess960
956 remove_piece(Do ? from : to);
957 remove_piece(Do ? rfrom : rto);
958 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
959 put_piece(make_piece(us, KING), Do ? to : from);
960 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
964 /// Position::do_null_move() is used to do a "null move": it flips
965 /// the side to move without executing any move on the board.
967 void Position::do_null_move(StateInfo& newSt) {
970 assert(&newSt != st);
972 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
977 st->dirtyPiece.dirty_num = 0;
978 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
979 st->accumulator.computed[WHITE] = false;
980 st->accumulator.computed[BLACK] = false;
982 if (st->epSquare != SQ_NONE)
984 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
985 st->epSquare = SQ_NONE;
988 st->key ^= Zobrist::side;
990 prefetch(TT.first_entry(key()));
992 st->pliesFromNull = 0;
994 sideToMove = ~sideToMove;
1000 assert(pos_is_ok());
1004 /// Position::undo_null_move() must be used to undo a "null move"
1006 void Position::undo_null_move() {
1008 assert(!checkers());
1011 sideToMove = ~sideToMove;
1015 /// Position::key_after() computes the new hash key after the given move. Needed
1016 /// for speculative prefetch. It doesn't recognize special moves like castling,
1017 /// en passant and promotions.
1019 Key Position::key_after(Move m) const {
1021 Square from = from_sq(m);
1022 Square to = to_sq(m);
1023 Piece pc = piece_on(from);
1024 Piece captured = piece_on(to);
1025 Key k = st->key ^ Zobrist::side;
1028 k ^= Zobrist::psq[captured][to];
1030 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1032 return (captured || type_of(pc) == PAWN)
1033 ? k : adjust_key50<true>(k);
1037 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1038 /// SEE value of move is greater or equal to the given threshold. We'll use an
1039 /// algorithm similar to alpha-beta pruning with a null window.
1041 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1045 // Only deal with normal moves, assume others pass a simple SEE
1046 if (type_of(m) != NORMAL)
1047 return VALUE_ZERO >= threshold;
1049 Square from = from_sq(m), to = to_sq(m);
1051 int swap = PieceValue[piece_on(to)] - threshold;
1055 swap = PieceValue[piece_on(from)] - swap;
1059 assert(color_of(piece_on(from)) == sideToMove);
1060 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1061 Color stm = sideToMove;
1062 Bitboard attackers = attackers_to(to, occupied);
1063 Bitboard stmAttackers, bb;
1069 attackers &= occupied;
1071 // If stm has no more attackers then give up: stm loses
1072 if (!(stmAttackers = attackers & pieces(stm)))
1075 // Don't allow pinned pieces to attack as long as there are
1076 // pinners on their original square.
1077 if (pinners(~stm) & occupied)
1079 stmAttackers &= ~blockers_for_king(stm);
1087 // Locate and remove the next least valuable attacker, and add to
1088 // the bitboard 'attackers' any X-ray attackers behind it.
1089 if ((bb = stmAttackers & pieces(PAWN)))
1091 occupied ^= least_significant_square_bb(bb);
1092 if ((swap = PawnValue - swap) < res)
1095 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1098 else if ((bb = stmAttackers & pieces(KNIGHT)))
1100 occupied ^= least_significant_square_bb(bb);
1101 if ((swap = KnightValue - swap) < res)
1105 else if ((bb = stmAttackers & pieces(BISHOP)))
1107 occupied ^= least_significant_square_bb(bb);
1108 if ((swap = BishopValue - swap) < res)
1111 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1114 else if ((bb = stmAttackers & pieces(ROOK)))
1116 occupied ^= least_significant_square_bb(bb);
1117 if ((swap = RookValue - swap) < res)
1120 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1123 else if ((bb = stmAttackers & pieces(QUEEN)))
1125 occupied ^= least_significant_square_bb(bb);
1126 if ((swap = QueenValue - swap) < res)
1129 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1130 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1134 // If we "capture" with the king but opponent still has attackers,
1135 // reverse the result.
1136 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1142 bool Position::see_ge(Move m, Value threshold) const {
1144 return see_ge(m, occupied, threshold);
1148 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1149 /// or by repetition. It does not detect stalemates.
1151 bool Position::is_draw(int ply) const {
1153 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1156 // Return a draw score if a position repeats once earlier but strictly
1157 // after the root, or repeats twice before or at the root.
1158 return st->repetition && st->repetition < ply;
1162 // Position::has_repeated() tests whether there has been at least one repetition
1163 // of positions since the last capture or pawn move.
1165 bool Position::has_repeated() const {
1167 StateInfo* stc = st;
1168 int end = std::min(st->rule50, st->pliesFromNull);
1171 if (stc->repetition)
1174 stc = stc->previous;
1180 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1181 /// or an earlier position has a move that directly reaches the current position.
1183 bool Position::has_game_cycle(int ply) const {
1187 int end = std::min(st->rule50, st->pliesFromNull);
1192 Key originalKey = st->key;
1193 StateInfo* stp = st->previous;
1195 for (int i = 3; i <= end; i += 2)
1197 stp = stp->previous->previous;
1199 Key moveKey = originalKey ^ stp->key;
1200 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1201 || (j = H2(moveKey), cuckoo[j] == moveKey))
1203 Move move = cuckooMove[j];
1204 Square s1 = from_sq(move);
1205 Square s2 = to_sq(move);
1207 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1212 // For nodes before or at the root, check that the move is a
1213 // repetition rather than a move to the current position.
1214 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1215 // the same location, so we have to select which square to check.
1216 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1219 // For repetitions before or at the root, require one more
1220 if (stp->repetition)
1229 /// Position::flip() flips position with the white and black sides reversed. This
1230 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1232 void Position::flip() {
1235 std::stringstream ss(fen());
1237 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1239 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1240 f.insert(0, token + (f.empty() ? " " : "/"));
1243 ss >> token; // Active color
1244 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1246 ss >> token; // Castling availability
1249 std::transform(f.begin(), f.end(), f.begin(),
1250 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1252 ss >> token; // En passant square
1253 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1255 std::getline(ss, token); // Half and full moves
1258 set(f, is_chess960(), st, this_thread());
1260 assert(pos_is_ok());
1264 /// Position::pos_is_ok() performs some consistency checks for the
1265 /// position object and raises an asserts if something wrong is detected.
1266 /// This is meant to be helpful when debugging.
1268 bool Position::pos_is_ok() const {
1270 constexpr bool Fast = true; // Quick (default) or full check?
1272 if ( (sideToMove != WHITE && sideToMove != BLACK)
1273 || piece_on(square<KING>(WHITE)) != W_KING
1274 || piece_on(square<KING>(BLACK)) != B_KING
1275 || ( ep_square() != SQ_NONE
1276 && relative_rank(sideToMove, ep_square()) != RANK_6))
1277 assert(0 && "pos_is_ok: Default");
1282 if ( pieceCount[W_KING] != 1
1283 || pieceCount[B_KING] != 1
1284 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1285 assert(0 && "pos_is_ok: Kings");
1287 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1288 || pieceCount[W_PAWN] > 8
1289 || pieceCount[B_PAWN] > 8)
1290 assert(0 && "pos_is_ok: Pawns");
1292 if ( (pieces(WHITE) & pieces(BLACK))
1293 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1294 || popcount(pieces(WHITE)) > 16
1295 || popcount(pieces(BLACK)) > 16)
1296 assert(0 && "pos_is_ok: Bitboards");
1298 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1299 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1300 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1301 assert(0 && "pos_is_ok: Bitboards");
1304 for (Piece pc : Pieces)
1305 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1306 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1307 assert(0 && "pos_is_ok: Pieces");
1309 for (Color c : { WHITE, BLACK })
1310 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1312 if (!can_castle(cr))
1315 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1316 || castlingRightsMask[castlingRookSquare[cr]] != cr
1317 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1318 assert(0 && "pos_is_ok: Castling");
1324 } // namespace Stockfish