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>();
128 Zobrist::noPawns = rng.rand<Key>();
130 // Prepare the cuckoo tables
131 std::memset(cuckoo, 0, sizeof(cuckoo));
132 std::memset(cuckooMove, 0, sizeof(cuckooMove));
133 [[maybe_unused]] int count = 0;
134 for (Piece pc : Pieces)
135 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
136 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
137 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
139 Move move = make_move(s1, s2);
140 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
144 std::swap(cuckoo[i], key);
145 std::swap(cuckooMove[i], move);
146 if (move == MOVE_NONE) // Arrived at empty slot?
148 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
152 assert(count == 3668);
156 /// Position::set() initializes the position object with the given FEN string.
157 /// This function is not very robust - make sure that input FENs are correct,
158 /// this is assumed to be the responsibility of the GUI.
160 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
162 A FEN string defines a particular position using only the ASCII character set.
164 A FEN string contains six fields separated by a space. The fields are:
166 1) Piece placement (from white's perspective). Each rank is described, starting
167 with rank 8 and ending with rank 1. Within each rank, the contents of each
168 square are described from file A through file H. Following the Standard
169 Algebraic Notation (SAN), each piece is identified by a single letter taken
170 from the standard English names. White pieces are designated using upper-case
171 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
172 noted using digits 1 through 8 (the number of blank squares), and "/"
175 2) Active color. "w" means white moves next, "b" means black.
177 3) Castling availability. If neither side can castle, this is "-". Otherwise,
178 this has one or more letters: "K" (White can castle kingside), "Q" (White
179 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
180 can castle queenside).
182 4) En passant target square (in algebraic notation). If there's no en passant
183 target square, this is "-". If a pawn has just made a 2-square move, this
184 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
185 if there is a pawn in position to make an en passant capture, and if there really
186 is a pawn that might have advanced two squares.
188 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
189 or capture. This is used to determine if a draw can be claimed under the
192 6) Fullmove number. The number of the full move. It starts at 1, and is
193 incremented after Black's move.
196 unsigned char col, row, token;
199 std::istringstream ss(fenStr);
201 std::memset(this, 0, sizeof(Position));
202 std::memset(si, 0, sizeof(StateInfo));
207 // 1. Piece placement
208 while ((ss >> token) && !isspace(token))
211 sq += (token - '0') * EAST; // Advance the given number of files
213 else if (token == '/')
216 else if ((idx = PieceToChar.find(token)) != string::npos) {
217 put_piece(Piece(idx), sq);
224 sideToMove = (token == 'w' ? WHITE : BLACK);
227 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
228 // Shredder-FEN that uses the letters of the columns on which the rooks began
229 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
230 // if an inner rook is associated with the castling right, the castling tag is
231 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
232 while ((ss >> token) && !isspace(token))
235 Color c = islower(token) ? BLACK : WHITE;
236 Piece rook = make_piece(c, ROOK);
238 token = char(toupper(token));
241 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
243 else if (token == 'Q')
244 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
246 else if (token >= 'A' && token <= 'H')
247 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
252 set_castling_right(c, rsq);
255 // 4. En passant square.
256 // Ignore if square is invalid or not on side to move relative rank 6.
257 bool enpassant = false;
259 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
260 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
262 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
264 // En passant square will be considered only if
265 // a) side to move have a pawn threatening epSquare
266 // b) there is an enemy pawn in front of epSquare
267 // c) there is no piece on epSquare or behind epSquare
268 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
269 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
270 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
274 st->epSquare = SQ_NONE;
276 // 5-6. Halfmove clock and fullmove number
277 ss >> std::skipws >> st->rule50 >> gamePly;
279 // Convert from fullmove starting from 1 to gamePly starting from 0,
280 // handle also common incorrect FEN with fullmove = 0.
281 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
283 chess960 = isChess960;
291 /// Position::set_castling_right() is a helper function used to set castling
292 /// rights given the corresponding color and the rook starting square.
294 void Position::set_castling_right(Color c, Square rfrom) {
296 Square kfrom = square<KING>(c);
297 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
299 st->castlingRights |= cr;
300 castlingRightsMask[kfrom] |= cr;
301 castlingRightsMask[rfrom] |= cr;
302 castlingRookSquare[cr] = rfrom;
304 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
305 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
307 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto))
312 /// Position::set_check_info() sets king attacks to detect if a move gives check
314 void Position::set_check_info() const {
316 st->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), st->pinners[BLACK]);
317 st->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), st->pinners[WHITE]);
319 Square ksq = square<KING>(~sideToMove);
321 st->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
322 st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
323 st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
324 st->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
325 st->checkSquares[QUEEN] = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
326 st->checkSquares[KING] = 0;
330 /// Position::set_state() computes the hash keys of the position, and other
331 /// data that once computed is updated incrementally as moves are made.
332 /// The function is only used when a new position is set up, and to verify
333 /// the correctness of the StateInfo data when running in debug mode.
335 void Position::set_state() const {
337 st->key = st->materialKey = 0;
338 st->pawnKey = Zobrist::noPawns;
339 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
340 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
344 for (Bitboard b = pieces(); b; )
346 Square s = pop_lsb(b);
347 Piece pc = piece_on(s);
348 st->key ^= Zobrist::psq[pc][s];
350 if (type_of(pc) == PAWN)
351 st->pawnKey ^= Zobrist::psq[pc][s];
353 else if (type_of(pc) != KING)
354 st->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
357 if (st->epSquare != SQ_NONE)
358 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
360 if (sideToMove == BLACK)
361 st->key ^= Zobrist::side;
363 st->key ^= Zobrist::castling[st->castlingRights];
365 for (Piece pc : Pieces)
366 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
367 st->materialKey ^= Zobrist::psq[pc][cnt];
371 /// Position::set() is an overload to initialize the position object with
372 /// the given endgame code string like "KBPKN". It is mainly a helper to
373 /// get the material key out of an endgame code.
375 Position& Position::set(const string& code, Color c, StateInfo* si) {
377 assert(code[0] == 'K');
379 string sides[] = { code.substr(code.find('K', 1)), // Weak
380 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
382 assert(sides[0].length() > 0 && sides[0].length() < 8);
383 assert(sides[1].length() > 0 && sides[1].length() < 8);
385 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
387 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
388 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
390 return set(fenStr, false, si, nullptr);
394 /// Position::fen() returns a FEN representation of the position. In case of
395 /// 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()) + " ")
438 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
444 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
445 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
446 /// slider if removing that piece from the board would result in a position where
447 /// square 's' is attacked. For example, a king-attack blocking piece can be either
448 /// a pinned or a discovered check piece, according if its color is the opposite
449 /// or the same of the color of the slider.
451 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
453 Bitboard blockers = 0;
456 // Snipers are sliders that attack 's' when a piece and other snipers are removed
457 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
458 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
459 Bitboard occupancy = pieces() ^ snipers;
463 Square sniperSq = pop_lsb(snipers);
464 Bitboard b = between_bb(s, sniperSq) & occupancy;
466 if (b && !more_than_one(b))
469 if (b & pieces(color_of(piece_on(s))))
477 /// Position::attackers_to() computes a bitboard of all pieces which attack a
478 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
480 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
482 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
483 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
484 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
485 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
486 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
487 | (attacks_bb<KING>(s) & pieces(KING));
491 /// Position::legal() tests whether a pseudo-legal move is legal
493 bool Position::legal(Move m) const {
497 Color us = sideToMove;
498 Square from = from_sq(m);
499 Square to = to_sq(m);
501 assert(color_of(moved_piece(m)) == us);
502 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
504 // En passant captures are a tricky special case. Because they are rather
505 // uncommon, we do it simply by testing whether the king is attacked after
507 if (type_of(m) == EN_PASSANT)
509 Square ksq = square<KING>(us);
510 Square capsq = to - pawn_push(us);
511 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
513 assert(to == ep_square());
514 assert(moved_piece(m) == make_piece(us, PAWN));
515 assert(piece_on(capsq) == make_piece(~us, PAWN));
516 assert(piece_on(to) == NO_PIECE);
518 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
519 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
522 // Castling moves generation does not check if the castling path is clear of
523 // enemy attacks, it is delayed at a later time: now!
524 if (type_of(m) == CASTLING)
526 // After castling, the rook and king final positions are the same in
527 // Chess960 as they would be in standard chess.
528 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
529 Direction step = to > from ? WEST : EAST;
531 for (Square s = to; s != from; s += step)
532 if (attackers_to(s) & pieces(~us))
535 // In case of Chess960, verify if the Rook blocks some checks
536 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
537 return !chess960 || !(blockers_for_king(us) & to_sq(m));
540 // If the moving piece is a king, check whether the destination square is
541 // attacked by the opponent.
542 if (type_of(piece_on(from)) == KING)
543 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
545 // A non-king move is legal if and only if it is not pinned or it
546 // is moving along the ray towards or away from the king.
547 return !(blockers_for_king(us) & from)
548 || aligned(from, to, square<KING>(us));
552 /// Position::pseudo_legal() takes a random move and tests whether the move is
553 /// pseudo legal. It is used to validate moves from TT that can be corrupted
554 /// due to SMP concurrent access or hash position key aliasing.
556 bool Position::pseudo_legal(const Move m) const {
558 Color us = sideToMove;
559 Square from = from_sq(m);
560 Square to = to_sq(m);
561 Piece pc = moved_piece(m);
563 // Use a slower but simpler function for uncommon cases
564 // yet we skip the legality check of MoveList<LEGAL>().
565 if (type_of(m) != NORMAL)
566 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
567 : MoveList<NON_EVASIONS>(*this).contains(m);
569 // Is not a promotion, so promotion piece must be empty
570 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
572 // If the 'from' square is not occupied by a piece belonging to the side to
573 // move, the move is obviously not legal.
574 if (pc == NO_PIECE || color_of(pc) != us)
577 // The destination square cannot be occupied by a friendly piece
581 // Handle the special case of a pawn move
582 if (type_of(pc) == PAWN)
584 // We have already handled promotion moves, so destination
585 // cannot be on the 8th/1st rank.
586 if ((Rank8BB | Rank1BB) & to)
589 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
590 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
591 && !( (from + 2 * pawn_push(us) == to) // Not a double push
592 && (relative_rank(us, from) == RANK_2)
594 && empty(to - pawn_push(us))))
597 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
600 // Evasions generator already takes care to avoid some kind of illegal moves
601 // and legal() relies on this. We therefore have to take care that the same
602 // kind of moves are filtered out here.
605 if (type_of(pc) != KING)
607 // Double check? In this case a king move is required
608 if (more_than_one(checkers()))
611 // Our move must be a blocking interposition or a capture of the checking piece
612 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
615 // In case of king moves under check we have to remove king so as to catch
616 // invalid moves like b1a1 when opposite queen is on c1.
617 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
625 /// Position::gives_check() tests whether a pseudo-legal move gives a check
627 bool Position::gives_check(Move m) const {
630 assert(color_of(moved_piece(m)) == sideToMove);
632 Square from = from_sq(m);
633 Square to = to_sq(m);
635 // Is there a direct check?
636 if (check_squares(type_of(piece_on(from))) & to)
639 // Is there a discovered check?
640 if ( (blockers_for_king(~sideToMove) & from)
641 && !aligned(from, to, square<KING>(~sideToMove)))
650 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
652 // En passant capture with check? We have already handled the case
653 // of direct checks and ordinary discovered check, so the only case we
654 // need to handle is the unusual case of a discovered check through
655 // the captured pawn.
658 Square capsq = make_square(file_of(to), rank_of(from));
659 Bitboard b = (pieces() ^ from ^ capsq) | to;
661 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
662 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
666 // Castling is encoded as 'king captures the rook'
667 Square ksq = square<KING>(~sideToMove);
668 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
670 return (attacks_bb<ROOK>(rto) & ksq)
671 && (attacks_bb<ROOK>(rto, pieces() ^ from ^ to) & ksq);
677 /// Position::do_move() makes a move, and saves all information necessary
678 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
679 /// moves should be filtered out before this function is called.
681 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
684 assert(&newSt != st);
686 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
687 Key k = st->key ^ Zobrist::side;
689 // Copy some fields of the old state to our new StateInfo object except the
690 // ones which are going to be recalculated from scratch anyway and then switch
691 // our state pointer to point to the new (ready to be updated) state.
692 std::memcpy(&newSt, st, offsetof(StateInfo, key));
696 // Increment ply counters. In particular, rule50 will be reset to zero later on
697 // in case of a capture or a pawn move.
703 st->accumulator.computed[WHITE] = false;
704 st->accumulator.computed[BLACK] = false;
705 auto& dp = st->dirtyPiece;
708 Color us = sideToMove;
710 Square from = from_sq(m);
711 Square to = to_sq(m);
712 Piece pc = piece_on(from);
713 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
715 assert(color_of(pc) == us);
716 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
717 assert(type_of(captured) != KING);
719 if (type_of(m) == CASTLING)
721 assert(pc == make_piece(us, KING));
722 assert(captured == make_piece(us, ROOK));
725 do_castling<true>(us, from, to, rfrom, rto);
727 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
735 // If the captured piece is a pawn, update pawn hash key, otherwise
736 // update non-pawn material.
737 if (type_of(captured) == PAWN)
739 if (type_of(m) == EN_PASSANT)
741 capsq -= pawn_push(us);
743 assert(pc == make_piece(us, PAWN));
744 assert(to == st->epSquare);
745 assert(relative_rank(us, to) == RANK_6);
746 assert(piece_on(to) == NO_PIECE);
747 assert(piece_on(capsq) == make_piece(them, PAWN));
750 st->pawnKey ^= Zobrist::psq[captured][capsq];
753 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
757 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
758 dp.piece[1] = captured;
763 // Update board and piece lists
766 // Update material hash key and prefetch access to materialTable
767 k ^= Zobrist::psq[captured][capsq];
768 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
769 prefetch(thisThread->materialTable[st->materialKey]);
771 // Reset rule 50 counter
776 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
778 // Reset en passant square
779 if (st->epSquare != SQ_NONE)
781 k ^= Zobrist::enpassant[file_of(st->epSquare)];
782 st->epSquare = SQ_NONE;
785 // Update castling rights if needed
786 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
788 k ^= Zobrist::castling[st->castlingRights];
789 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
790 k ^= Zobrist::castling[st->castlingRights];
793 // Move the piece. The tricky Chess960 castling is handled earlier
794 if (type_of(m) != CASTLING)
803 move_piece(from, to);
806 // If the moving piece is a pawn do some special extra work
807 if (type_of(pc) == PAWN)
809 // Set en passant square if the moved pawn can be captured
810 if ( (int(to) ^ int(from)) == 16
811 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
813 st->epSquare = to - pawn_push(us);
814 k ^= Zobrist::enpassant[file_of(st->epSquare)];
817 else if (type_of(m) == PROMOTION)
819 Piece promotion = make_piece(us, promotion_type(m));
821 assert(relative_rank(us, to) == RANK_8);
822 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
825 put_piece(promotion, to);
829 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
831 dp.piece[dp.dirty_num] = promotion;
832 dp.from[dp.dirty_num] = SQ_NONE;
833 dp.to[dp.dirty_num] = to;
838 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
839 st->pawnKey ^= Zobrist::psq[pc][to];
840 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
841 ^ Zobrist::psq[pc][pieceCount[pc]];
844 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
847 // Update pawn hash key
848 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
850 // Reset rule 50 draw counter
855 st->capturedPiece = captured;
857 // Update the key with the final value
860 // Calculate checkers bitboard (if move gives check)
861 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
863 sideToMove = ~sideToMove;
865 // Update king attacks used for fast check detection
868 // Calculate the repetition info. It is the ply distance from the previous
869 // occurrence of the same position, negative in the 3-fold case, or zero
870 // if the position was not repeated.
872 int end = std::min(st->rule50, st->pliesFromNull);
875 StateInfo* stp = st->previous->previous;
876 for (int i = 4; i <= end; i += 2)
878 stp = stp->previous->previous;
879 if (stp->key == st->key)
881 st->repetition = stp->repetition ? -i : i;
891 /// Position::undo_move() unmakes a move. When it returns, the position should
892 /// be restored to exactly the same state as before the move was made.
894 void Position::undo_move(Move m) {
898 sideToMove = ~sideToMove;
900 Color us = sideToMove;
901 Square from = from_sq(m);
902 Square to = to_sq(m);
903 Piece pc = piece_on(to);
905 assert(empty(from) || type_of(m) == CASTLING);
906 assert(type_of(st->capturedPiece) != KING);
908 if (type_of(m) == PROMOTION)
910 assert(relative_rank(us, to) == RANK_8);
911 assert(type_of(pc) == promotion_type(m));
912 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
915 pc = make_piece(us, PAWN);
919 if (type_of(m) == CASTLING)
922 do_castling<false>(us, from, to, rfrom, rto);
926 move_piece(to, from); // Put the piece back at the source square
928 if (st->capturedPiece)
932 if (type_of(m) == EN_PASSANT)
934 capsq -= pawn_push(us);
936 assert(type_of(pc) == PAWN);
937 assert(to == st->previous->epSquare);
938 assert(relative_rank(us, to) == RANK_6);
939 assert(piece_on(capsq) == NO_PIECE);
940 assert(st->capturedPiece == make_piece(~us, PAWN));
943 put_piece(st->capturedPiece, capsq); // Restore the captured piece
947 // Finally point our state pointer back to the previous state
955 /// Position::do_castling() is a helper used to do/undo a castling move. This
956 /// is a bit tricky in Chess960 where from/to squares can overlap.
958 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
960 bool kingSide = to > from;
961 rfrom = to; // Castling is encoded as "king captures friendly rook"
962 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
963 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
965 if (Do && Eval::useNNUE)
967 auto& dp = st->dirtyPiece;
968 dp.piece[0] = make_piece(us, KING);
971 dp.piece[1] = make_piece(us, ROOK);
977 // Remove both pieces first since squares could overlap in Chess960
978 remove_piece(Do ? from : to);
979 remove_piece(Do ? rfrom : rto);
980 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
981 put_piece(make_piece(us, KING), Do ? to : from);
982 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
986 /// Position::do_null_move() is used to do a "null move": it flips
987 /// the side to move without executing any move on the board.
989 void Position::do_null_move(StateInfo& newSt) {
992 assert(&newSt != st);
994 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
999 st->dirtyPiece.dirty_num = 0;
1000 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1001 st->accumulator.computed[WHITE] = false;
1002 st->accumulator.computed[BLACK] = false;
1004 if (st->epSquare != SQ_NONE)
1006 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1007 st->epSquare = SQ_NONE;
1010 st->key ^= Zobrist::side;
1012 prefetch(TT.first_entry(key()));
1014 st->pliesFromNull = 0;
1016 sideToMove = ~sideToMove;
1022 assert(pos_is_ok());
1026 /// Position::undo_null_move() must be used to undo a "null move"
1028 void Position::undo_null_move() {
1030 assert(!checkers());
1033 sideToMove = ~sideToMove;
1037 /// Position::key_after() computes the new hash key after the given move. Needed
1038 /// for speculative prefetch. It doesn't recognize special moves like castling,
1039 /// en passant and promotions.
1041 Key Position::key_after(Move m) const {
1043 Square from = from_sq(m);
1044 Square to = to_sq(m);
1045 Piece pc = piece_on(from);
1046 Piece captured = piece_on(to);
1047 Key k = st->key ^ Zobrist::side;
1050 k ^= Zobrist::psq[captured][to];
1052 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1054 return (captured || type_of(pc) == PAWN)
1055 ? k : adjust_key50<true>(k);
1059 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1060 /// SEE value of move is greater or equal to the given threshold. We'll use an
1061 /// algorithm similar to alpha-beta pruning with a null window.
1063 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1067 // Only deal with normal moves, assume others pass a simple SEE
1068 if (type_of(m) != NORMAL)
1069 return VALUE_ZERO >= threshold;
1071 Square from = from_sq(m), to = to_sq(m);
1073 int swap = PieceValue[MG][piece_on(to)] - threshold;
1077 swap = PieceValue[MG][piece_on(from)] - swap;
1081 assert(color_of(piece_on(from)) == sideToMove);
1082 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1083 Color stm = sideToMove;
1084 Bitboard attackers = attackers_to(to, occupied);
1085 Bitboard stmAttackers, bb;
1091 attackers &= occupied;
1093 // If stm has no more attackers then give up: stm loses
1094 if (!(stmAttackers = attackers & pieces(stm)))
1097 // Don't allow pinned pieces to attack as long as there are
1098 // pinners on their original square.
1099 if (pinners(~stm) & occupied)
1101 stmAttackers &= ~blockers_for_king(stm);
1109 // Locate and remove the next least valuable attacker, and add to
1110 // the bitboard 'attackers' any X-ray attackers behind it.
1111 if ((bb = stmAttackers & pieces(PAWN)))
1113 occupied ^= least_significant_square_bb(bb);
1114 if ((swap = PawnValueMg - swap) < res)
1117 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1120 else if ((bb = stmAttackers & pieces(KNIGHT)))
1122 occupied ^= least_significant_square_bb(bb);
1123 if ((swap = KnightValueMg - swap) < res)
1127 else if ((bb = stmAttackers & pieces(BISHOP)))
1129 occupied ^= least_significant_square_bb(bb);
1130 if ((swap = BishopValueMg - swap) < res)
1133 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1136 else if ((bb = stmAttackers & pieces(ROOK)))
1138 occupied ^= least_significant_square_bb(bb);
1139 if ((swap = RookValueMg - swap) < res)
1142 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1145 else if ((bb = stmAttackers & pieces(QUEEN)))
1147 occupied ^= least_significant_square_bb(bb);
1148 if ((swap = QueenValueMg - swap) < res)
1151 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1152 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1156 // If we "capture" with the king but opponent still has attackers,
1157 // reverse the result.
1158 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1164 bool Position::see_ge(Move m, Value threshold) const {
1166 return see_ge(m, occupied, threshold);
1170 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1171 /// or by repetition. It does not detect stalemates.
1173 bool Position::is_draw(int ply) const {
1175 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1178 // Return a draw score if a position repeats once earlier but strictly
1179 // after the root, or repeats twice before or at the root.
1180 return st->repetition && st->repetition < ply;
1184 // Position::has_repeated() tests whether there has been at least one repetition
1185 // of positions since the last capture or pawn move.
1187 bool Position::has_repeated() const {
1189 StateInfo* stc = st;
1190 int end = std::min(st->rule50, st->pliesFromNull);
1193 if (stc->repetition)
1196 stc = stc->previous;
1202 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1203 /// or an earlier position has a move that directly reaches the current position.
1205 bool Position::has_game_cycle(int ply) const {
1209 int end = std::min(st->rule50, st->pliesFromNull);
1214 Key originalKey = st->key;
1215 StateInfo* stp = st->previous;
1217 for (int i = 3; i <= end; i += 2)
1219 stp = stp->previous->previous;
1221 Key moveKey = originalKey ^ stp->key;
1222 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1223 || (j = H2(moveKey), cuckoo[j] == moveKey))
1225 Move move = cuckooMove[j];
1226 Square s1 = from_sq(move);
1227 Square s2 = to_sq(move);
1229 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1234 // For nodes before or at the root, check that the move is a
1235 // repetition rather than a move to the current position.
1236 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1237 // the same location, so we have to select which square to check.
1238 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1241 // For repetitions before or at the root, require one more
1242 if (stp->repetition)
1251 /// Position::flip() flips position with the white and black sides reversed. This
1252 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1254 void Position::flip() {
1257 std::stringstream ss(fen());
1259 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1261 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1262 f.insert(0, token + (f.empty() ? " " : "/"));
1265 ss >> token; // Active color
1266 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1268 ss >> token; // Castling availability
1271 std::transform(f.begin(), f.end(), f.begin(),
1272 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1274 ss >> token; // En passant square
1275 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1277 std::getline(ss, token); // Half and full moves
1280 set(f, is_chess960(), st, this_thread());
1282 assert(pos_is_ok());
1286 /// Position::pos_is_ok() performs some consistency checks for the
1287 /// position object and raises an asserts if something wrong is detected.
1288 /// This is meant to be helpful when debugging.
1290 bool Position::pos_is_ok() const {
1292 constexpr bool Fast = true; // Quick (default) or full check?
1294 if ( (sideToMove != WHITE && sideToMove != BLACK)
1295 || piece_on(square<KING>(WHITE)) != W_KING
1296 || piece_on(square<KING>(BLACK)) != B_KING
1297 || ( ep_square() != SQ_NONE
1298 && relative_rank(sideToMove, ep_square()) != RANK_6))
1299 assert(0 && "pos_is_ok: Default");
1304 if ( pieceCount[W_KING] != 1
1305 || pieceCount[B_KING] != 1
1306 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1307 assert(0 && "pos_is_ok: Kings");
1309 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1310 || pieceCount[W_PAWN] > 8
1311 || pieceCount[B_PAWN] > 8)
1312 assert(0 && "pos_is_ok: Pawns");
1314 if ( (pieces(WHITE) & pieces(BLACK))
1315 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1316 || popcount(pieces(WHITE)) > 16
1317 || popcount(pieces(BLACK)) > 16)
1318 assert(0 && "pos_is_ok: Bitboards");
1320 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1321 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1322 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1323 assert(0 && "pos_is_ok: Bitboards");
1326 for (Piece pc : Pieces)
1327 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1328 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1329 assert(0 && "pos_is_ok: Pieces");
1331 for (Color c : { WHITE, BLACK })
1332 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1334 if (!can_castle(cr))
1337 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1338 || castlingRightsMask[castlingRookSquare[cr]] != cr
1339 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1340 assert(0 && "pos_is_ok: Castling");
1346 } // namespace Stockfish