2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2023 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
27 #include <initializer_list>
31 #include <string_view>
37 #include "nnue/nnue_common.h"
38 #include "syzygy/tbprobe.h"
49 Key psq[PIECE_NB][SQUARE_NB];
50 Key enpassant[FILE_NB];
51 Key castling[CASTLING_RIGHT_NB];
57 constexpr std::string_view PieceToChar(" PNBRQK pnbrqk");
59 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
60 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
64 /// operator<<(Position) returns an ASCII representation of the position
66 std::ostream& operator<<(std::ostream& os, const Position& pos) {
68 os << "\n +---+---+---+---+---+---+---+---+\n";
70 for (Rank r = RANK_8; r >= RANK_1; --r)
72 for (File f = FILE_A; f <= FILE_H; ++f)
73 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
75 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
78 os << " a b c d e f g h\n"
79 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
80 << std::setfill('0') << std::setw(16) << pos.key()
81 << std::setfill(' ') << std::dec << "\nCheckers: ";
83 for (Bitboard b = pos.checkers(); b; )
84 os << UCI::square(pop_lsb(b)) << " ";
86 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
87 && !pos.can_castle(ANY_CASTLING))
90 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
93 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
94 Tablebases::ProbeState s1, s2;
95 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
96 int dtz = Tablebases::probe_dtz(p, &s2);
97 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
98 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
105 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
106 // situations. Description of the algorithm in the following paper:
107 // http://web.archive.org/web/20201107002606/https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
109 // First and second hash functions for indexing the cuckoo tables
110 inline int H1(Key h) { return h & 0x1fff; }
111 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
113 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
115 Move cuckooMove[8192];
118 /// Position::init() initializes at startup the various arrays used to compute hash keys
120 void Position::init() {
124 for (Piece pc : Pieces)
125 for (Square s = SQ_A1; s <= SQ_H8; ++s)
126 Zobrist::psq[pc][s] = rng.rand<Key>();
128 for (File f = FILE_A; f <= FILE_H; ++f)
129 Zobrist::enpassant[f] = rng.rand<Key>();
131 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
132 Zobrist::castling[cr] = rng.rand<Key>();
134 Zobrist::side = rng.rand<Key>();
136 // Prepare the cuckoo tables
137 std::memset(cuckoo, 0, sizeof(cuckoo));
138 std::memset(cuckooMove, 0, sizeof(cuckooMove));
139 [[maybe_unused]] int count = 0;
140 for (Piece pc : Pieces)
141 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
142 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
143 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
145 Move move = make_move(s1, s2);
146 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
150 std::swap(cuckoo[i], key);
151 std::swap(cuckooMove[i], move);
152 if (move == MOVE_NONE) // Arrived at empty slot?
154 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
158 assert(count == 3668);
162 /// Position::set() initializes the position object with the given FEN string.
163 /// This function is not very robust - make sure that input FENs are correct,
164 /// this is assumed to be the responsibility of the GUI.
166 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
168 A FEN string defines a particular position using only the ASCII character set.
170 A FEN string contains six fields separated by a space. The fields are:
172 1) Piece placement (from white's perspective). Each rank is described, starting
173 with rank 8 and ending with rank 1. Within each rank, the contents of each
174 square are described from file A through file H. Following the Standard
175 Algebraic Notation (SAN), each piece is identified by a single letter taken
176 from the standard English names. White pieces are designated using upper-case
177 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
178 noted using digits 1 through 8 (the number of blank squares), and "/"
181 2) Active color. "w" means white moves next, "b" means black.
183 3) Castling availability. If neither side can castle, this is "-". Otherwise,
184 this has one or more letters: "K" (White can castle kingside), "Q" (White
185 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
186 can castle queenside).
188 4) En passant target square (in algebraic notation). If there's no en passant
189 target square, this is "-". If a pawn has just made a 2-square move, this
190 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
191 if there is a pawn in position to make an en passant capture, and if there really
192 is a pawn that might have advanced two squares.
194 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
195 or capture. This is used to determine if a draw can be claimed under the
198 6) Fullmove number. The number of the full move. It starts at 1, and is
199 incremented after Black's move.
202 unsigned char col, row, token;
205 std::istringstream ss(fenStr);
207 std::memset(this, 0, sizeof(Position));
208 std::memset(si, 0, sizeof(StateInfo));
213 // 1. Piece placement
214 while ((ss >> token) && !isspace(token))
217 sq += (token - '0') * EAST; // Advance the given number of files
219 else if (token == '/')
222 else if ((idx = PieceToChar.find(token)) != string::npos) {
223 put_piece(Piece(idx), sq);
230 sideToMove = (token == 'w' ? WHITE : BLACK);
233 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
234 // Shredder-FEN that uses the letters of the columns on which the rooks began
235 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
236 // if an inner rook is associated with the castling right, the castling tag is
237 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
238 while ((ss >> token) && !isspace(token))
241 Color c = islower(token) ? BLACK : WHITE;
242 Piece rook = make_piece(c, ROOK);
244 token = char(toupper(token));
247 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
249 else if (token == 'Q')
250 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 /// Position::set_castling_right() is a helper function used to set castling
300 /// rights given the corresponding color and the rook starting square.
302 void Position::set_castling_right(Color c, Square rfrom) {
304 Square kfrom = square<KING>(c);
305 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
307 st->castlingRights |= cr;
308 castlingRightsMask[kfrom] |= cr;
309 castlingRightsMask[rfrom] |= cr;
310 castlingRookSquare[cr] = rfrom;
312 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
313 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
315 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto))
320 /// Position::set_check_info() sets king attacks to detect if a move gives check
322 void Position::set_check_info() const {
324 update_slider_blockers(WHITE);
325 update_slider_blockers(BLACK);
327 Square ksq = square<KING>(~sideToMove);
329 st->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
330 st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
331 st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
332 st->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
333 st->checkSquares[QUEEN] = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
334 st->checkSquares[KING] = 0;
338 /// Position::set_state() computes the hash keys of the position, and other
339 /// data that once computed is updated incrementally as moves are made.
340 /// The function is only used when a new position is set up
342 void Position::set_state() const {
344 st->key = st->materialKey = 0;
345 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
346 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
350 for (Bitboard b = pieces(); b; )
352 Square s = pop_lsb(b);
353 Piece pc = piece_on(s);
354 st->key ^= Zobrist::psq[pc][s];
356 if (type_of(pc) != KING && type_of(pc) != PAWN)
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 /// Position::set() is an 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.
378 Position& Position::set(const string& code, Color c, StateInfo* si) {
380 assert(code[0] == 'K');
382 string sides[] = { code.substr(code.find('K', 1)), // Weak
383 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
385 assert(sides[0].length() > 0 && sides[0].length() < 8);
386 assert(sides[1].length() > 0 && sides[1].length() < 8);
388 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
390 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
391 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
393 return set(fenStr, false, si, nullptr);
397 /// Position::fen() returns a FEN representation of the position. In case of
398 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
400 string Position::fen() const {
403 std::ostringstream ss;
405 for (Rank r = RANK_8; r >= RANK_1; --r)
407 for (File f = FILE_A; f <= FILE_H; ++f)
409 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
416 ss << PieceToChar[piece_on(make_square(f, r))];
423 ss << (sideToMove == WHITE ? " w " : " b ");
425 if (can_castle(WHITE_OO))
426 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
428 if (can_castle(WHITE_OOO))
429 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
431 if (can_castle(BLACK_OO))
432 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
434 if (can_castle(BLACK_OOO))
435 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
437 if (!can_castle(ANY_CASTLING))
440 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
441 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
446 /// update_slider_blockers() calculates st->blockersForKing[c] and st->pinners[~c],
447 /// which store respectively the pieces preventing king of color c from being in check
448 /// and the slider pieces of color ~c pinning pieces of color c to the king.
449 void Position::update_slider_blockers(Color c) const {
451 Square ksq = square<KING>(c);
453 st->blockersForKing[c] = 0;
456 // Snipers are sliders that attack 's' when a piece and other snipers are removed
457 Bitboard snipers = ( (attacks_bb< ROOK>(ksq) & pieces(QUEEN, ROOK))
458 | (attacks_bb<BISHOP>(ksq) & pieces(QUEEN, BISHOP))) & pieces(~c);
459 Bitboard occupancy = pieces() ^ snipers;
463 Square sniperSq = pop_lsb(snipers);
464 Bitboard b = between_bb(ksq, sniperSq) & occupancy;
466 if (b && !more_than_one(b))
468 st->blockersForKing[c] |= b;
470 st->pinners[~c] |= sniperSq;
476 /// Position::attackers_to() computes a bitboard of all pieces which attack a
477 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
479 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
481 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
482 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
483 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
484 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
485 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
486 | (attacks_bb<KING>(s) & pieces(KING));
490 /// Position::legal() tests whether a pseudo-legal move is legal
492 bool Position::legal(Move m) const {
496 Color us = sideToMove;
497 Square from = from_sq(m);
498 Square to = to_sq(m);
500 assert(color_of(moved_piece(m)) == us);
501 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
503 // En passant captures are a tricky special case. Because they are rather
504 // uncommon, we do it simply by testing whether the king is attacked after
506 if (type_of(m) == EN_PASSANT)
508 Square ksq = square<KING>(us);
509 Square capsq = to - pawn_push(us);
510 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
512 assert(to == ep_square());
513 assert(moved_piece(m) == make_piece(us, PAWN));
514 assert(piece_on(capsq) == make_piece(~us, PAWN));
515 assert(piece_on(to) == NO_PIECE);
517 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
518 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
521 // Castling moves generation does not check if the castling path is clear of
522 // enemy attacks, it is delayed at a later time: now!
523 if (type_of(m) == CASTLING)
525 // After castling, the rook and king final positions are the same in
526 // Chess960 as they would be in standard chess.
527 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
528 Direction step = to > from ? WEST : EAST;
530 for (Square s = to; s != from; s += step)
531 if (attackers_to(s) & pieces(~us))
534 // In case of Chess960, verify if the Rook blocks some checks
535 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
536 return !chess960 || !(blockers_for_king(us) & to_sq(m));
539 // If the moving piece is a king, check whether the destination square is
540 // attacked by the opponent.
541 if (type_of(piece_on(from)) == KING)
542 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
544 // A non-king move is legal if and only if it is not pinned or it
545 // is moving along the ray towards or away from the king.
546 return !(blockers_for_king(us) & from)
547 || aligned(from, to, square<KING>(us));
551 /// Position::pseudo_legal() takes a random move and tests whether the move is
552 /// pseudo legal. It is used to validate moves from TT that can be corrupted
553 /// due to SMP concurrent access or hash position key aliasing.
555 bool Position::pseudo_legal(const Move m) const {
557 Color us = sideToMove;
558 Square from = from_sq(m);
559 Square to = to_sq(m);
560 Piece pc = moved_piece(m);
562 // Use a slower but simpler function for uncommon cases
563 // yet we skip the legality check of MoveList<LEGAL>().
564 if (type_of(m) != NORMAL)
565 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
566 : MoveList<NON_EVASIONS>(*this).contains(m);
568 // Is not a promotion, so promotion piece must be empty
569 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
571 // If the 'from' square is not occupied by a piece belonging to the side to
572 // move, the move is obviously not legal.
573 if (pc == NO_PIECE || color_of(pc) != us)
576 // The destination square cannot be occupied by a friendly piece
580 // Handle the special case of a pawn move
581 if (type_of(pc) == PAWN)
583 // We have already handled promotion moves, so destination
584 // cannot be on the 8th/1st rank.
585 if ((Rank8BB | Rank1BB) & to)
588 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
589 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
590 && !( (from + 2 * pawn_push(us) == to) // Not a double push
591 && (relative_rank(us, from) == RANK_2)
593 && empty(to - pawn_push(us))))
596 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
599 // Evasions generator already takes care to avoid some kind of illegal moves
600 // and legal() relies on this. We therefore have to take care that the same
601 // kind of moves are filtered out here.
604 if (type_of(pc) != KING)
606 // Double check? In this case a king move is required
607 if (more_than_one(checkers()))
610 // Our move must be a blocking interposition or a capture of the checking piece
611 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
614 // In case of king moves under check we have to remove king so as to catch
615 // invalid moves like b1a1 when opposite queen is on c1.
616 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
624 /// Position::gives_check() tests whether a pseudo-legal move gives a check
626 bool Position::gives_check(Move m) const {
629 assert(color_of(moved_piece(m)) == sideToMove);
631 Square from = from_sq(m);
632 Square to = to_sq(m);
634 // Is there a direct check?
635 if (check_squares(type_of(piece_on(from))) & to)
638 // Is there a discovered check?
639 if (blockers_for_king(~sideToMove) & from)
640 return !aligned(from, to, square<KING>(~sideToMove))
641 || type_of(m) == CASTLING;
649 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
651 // En passant capture with check? We have already handled the case
652 // of direct checks and ordinary discovered check, so the only case we
653 // need to handle is the unusual case of a discovered check through
654 // the captured pawn.
657 Square capsq = make_square(file_of(to), rank_of(from));
658 Bitboard b = (pieces() ^ from ^ capsq) | to;
660 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
661 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
665 // Castling is encoded as 'king captures the rook'
666 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
668 return check_squares(ROOK) & rto;
674 /// Position::do_move() makes a move, and saves all information necessary
675 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
676 /// moves should be filtered out before this function is called.
678 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
681 assert(&newSt != st);
683 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
684 Key k = st->key ^ Zobrist::side;
686 // Copy some fields of the old state to our new StateInfo object except the
687 // ones which are going to be recalculated from scratch anyway and then switch
688 // our state pointer to point to the new (ready to be updated) state.
689 std::memcpy(&newSt, st, offsetof(StateInfo, key));
693 // Increment ply counters. In particular, rule50 will be reset to zero later on
694 // in case of a capture or a pawn move.
700 st->accumulator.computed[WHITE] = false;
701 st->accumulator.computed[BLACK] = false;
702 auto& dp = st->dirtyPiece;
705 Color us = sideToMove;
707 Square from = from_sq(m);
708 Square to = to_sq(m);
709 Piece pc = piece_on(from);
710 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
712 assert(color_of(pc) == us);
713 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
714 assert(type_of(captured) != KING);
716 if (type_of(m) == CASTLING)
718 assert(pc == make_piece(us, KING));
719 assert(captured == make_piece(us, ROOK));
722 do_castling<true>(us, from, to, rfrom, rto);
724 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
732 // If the captured piece is a pawn, update pawn hash key, otherwise
733 // update non-pawn material.
734 if (type_of(captured) == PAWN)
736 if (type_of(m) == EN_PASSANT)
738 capsq -= pawn_push(us);
740 assert(pc == make_piece(us, PAWN));
741 assert(to == st->epSquare);
742 assert(relative_rank(us, to) == RANK_6);
743 assert(piece_on(to) == NO_PIECE);
744 assert(piece_on(capsq) == make_piece(them, PAWN));
748 st->nonPawnMaterial[them] -= PieceValue[captured];
750 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
751 dp.piece[1] = captured;
755 // Update board and piece lists
758 // Update material hash key and prefetch access to materialTable
759 k ^= Zobrist::psq[captured][capsq];
760 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
762 // Reset rule 50 counter
767 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
769 // Reset en passant square
770 if (st->epSquare != SQ_NONE)
772 k ^= Zobrist::enpassant[file_of(st->epSquare)];
773 st->epSquare = SQ_NONE;
776 // Update castling rights if needed
777 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
779 k ^= Zobrist::castling[st->castlingRights];
780 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
781 k ^= Zobrist::castling[st->castlingRights];
784 // Move the piece. The tricky Chess960 castling is handled earlier
785 if (type_of(m) != CASTLING)
791 move_piece(from, to);
794 // If the moving piece is a pawn do some special extra work
795 if (type_of(pc) == PAWN)
797 // Set en passant square if the moved pawn can be captured
798 if ( (int(to) ^ int(from)) == 16
799 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
801 st->epSquare = to - pawn_push(us);
802 k ^= Zobrist::enpassant[file_of(st->epSquare)];
805 else if (type_of(m) == PROMOTION)
807 Piece promotion = make_piece(us, promotion_type(m));
809 assert(relative_rank(us, to) == RANK_8);
810 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
813 put_piece(promotion, to);
815 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
817 dp.piece[dp.dirty_num] = promotion;
818 dp.from[dp.dirty_num] = SQ_NONE;
819 dp.to[dp.dirty_num] = to;
823 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
824 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
825 ^ Zobrist::psq[pc][pieceCount[pc]];
828 st->nonPawnMaterial[us] += PieceValue[promotion];
831 // Reset rule 50 draw counter
836 st->capturedPiece = captured;
838 // Update the key with the final value
841 // Calculate checkers bitboard (if move gives check)
842 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
844 sideToMove = ~sideToMove;
846 // Update king attacks used for fast check detection
849 // Calculate the repetition info. It is the ply distance from the previous
850 // occurrence of the same position, negative in the 3-fold case, or zero
851 // if the position was not repeated.
853 int end = std::min(st->rule50, st->pliesFromNull);
856 StateInfo* stp = st->previous->previous;
857 for (int i = 4; i <= end; i += 2)
859 stp = stp->previous->previous;
860 if (stp->key == st->key)
862 st->repetition = stp->repetition ? -i : i;
872 /// Position::undo_move() unmakes a move. When it returns, the position should
873 /// be restored to exactly the same state as before the move was made.
875 void Position::undo_move(Move m) {
879 sideToMove = ~sideToMove;
881 Color us = sideToMove;
882 Square from = from_sq(m);
883 Square to = to_sq(m);
884 Piece pc = piece_on(to);
886 assert(empty(from) || type_of(m) == CASTLING);
887 assert(type_of(st->capturedPiece) != KING);
889 if (type_of(m) == PROMOTION)
891 assert(relative_rank(us, to) == RANK_8);
892 assert(type_of(pc) == promotion_type(m));
893 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
896 pc = make_piece(us, PAWN);
900 if (type_of(m) == CASTLING)
903 do_castling<false>(us, from, to, rfrom, rto);
907 move_piece(to, from); // Put the piece back at the source square
909 if (st->capturedPiece)
913 if (type_of(m) == EN_PASSANT)
915 capsq -= pawn_push(us);
917 assert(type_of(pc) == PAWN);
918 assert(to == st->previous->epSquare);
919 assert(relative_rank(us, to) == RANK_6);
920 assert(piece_on(capsq) == NO_PIECE);
921 assert(st->capturedPiece == make_piece(~us, PAWN));
924 put_piece(st->capturedPiece, capsq); // Restore the captured piece
928 // Finally point our state pointer back to the previous state
936 /// Position::do_castling() is a helper used to do/undo a castling move. This
937 /// is a bit tricky in Chess960 where from/to squares can overlap.
939 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
941 bool kingSide = to > from;
942 rfrom = to; // Castling is encoded as "king captures friendly rook"
943 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
944 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
948 auto& dp = st->dirtyPiece;
949 dp.piece[0] = make_piece(us, KING);
952 dp.piece[1] = make_piece(us, ROOK);
958 // Remove both pieces first since squares could overlap in Chess960
959 remove_piece(Do ? from : to);
960 remove_piece(Do ? rfrom : rto);
961 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
962 put_piece(make_piece(us, KING), Do ? to : from);
963 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
967 /// Position::do_null_move() is used to do a "null move": it flips
968 /// the side to move without executing any move on the board.
970 void Position::do_null_move(StateInfo& newSt) {
973 assert(&newSt != st);
975 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
980 st->dirtyPiece.dirty_num = 0;
981 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
982 st->accumulator.computed[WHITE] = false;
983 st->accumulator.computed[BLACK] = false;
985 if (st->epSquare != SQ_NONE)
987 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
988 st->epSquare = SQ_NONE;
991 st->key ^= Zobrist::side;
993 prefetch(TT.first_entry(key()));
995 st->pliesFromNull = 0;
997 sideToMove = ~sideToMove;
1003 assert(pos_is_ok());
1007 /// Position::undo_null_move() must be used to undo a "null move"
1009 void Position::undo_null_move() {
1011 assert(!checkers());
1014 sideToMove = ~sideToMove;
1018 /// Position::key_after() computes the new hash key after the given move. Needed
1019 /// for speculative prefetch. It doesn't recognize special moves like castling,
1020 /// en passant and promotions.
1022 Key Position::key_after(Move m) const {
1024 Square from = from_sq(m);
1025 Square to = to_sq(m);
1026 Piece pc = piece_on(from);
1027 Piece captured = piece_on(to);
1028 Key k = st->key ^ Zobrist::side;
1031 k ^= Zobrist::psq[captured][to];
1033 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1035 return (captured || type_of(pc) == PAWN)
1036 ? k : adjust_key50<true>(k);
1040 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1041 /// SEE value of move is greater or equal to the given threshold. We'll use an
1042 /// algorithm similar to alpha-beta pruning with a null window.
1044 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1048 // Only deal with normal moves, assume others pass a simple SEE
1049 if (type_of(m) != NORMAL)
1050 return VALUE_ZERO >= threshold;
1052 Square from = from_sq(m), to = to_sq(m);
1054 int swap = PieceValue[piece_on(to)] - threshold;
1058 swap = PieceValue[piece_on(from)] - swap;
1062 assert(color_of(piece_on(from)) == sideToMove);
1063 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1064 Color stm = sideToMove;
1065 Bitboard attackers = attackers_to(to, occupied);
1066 Bitboard stmAttackers, bb;
1072 attackers &= occupied;
1074 // If stm has no more attackers then give up: stm loses
1075 if (!(stmAttackers = attackers & pieces(stm)))
1078 // Don't allow pinned pieces to attack as long as there are
1079 // pinners on their original square.
1080 if (pinners(~stm) & occupied)
1082 stmAttackers &= ~blockers_for_king(stm);
1090 // Locate and remove the next least valuable attacker, and add to
1091 // the bitboard 'attackers' any X-ray attackers behind it.
1092 if ((bb = stmAttackers & pieces(PAWN)))
1094 occupied ^= least_significant_square_bb(bb);
1095 if ((swap = PawnValue - swap) < res)
1098 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1101 else if ((bb = stmAttackers & pieces(KNIGHT)))
1103 occupied ^= least_significant_square_bb(bb);
1104 if ((swap = KnightValue - swap) < res)
1108 else if ((bb = stmAttackers & pieces(BISHOP)))
1110 occupied ^= least_significant_square_bb(bb);
1111 if ((swap = BishopValue - swap) < res)
1114 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1117 else if ((bb = stmAttackers & pieces(ROOK)))
1119 occupied ^= least_significant_square_bb(bb);
1120 if ((swap = RookValue - swap) < res)
1123 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1126 else if ((bb = stmAttackers & pieces(QUEEN)))
1128 occupied ^= least_significant_square_bb(bb);
1129 if ((swap = QueenValue - swap) < res)
1132 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1133 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1137 // If we "capture" with the king but opponent still has attackers,
1138 // reverse the result.
1139 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1145 bool Position::see_ge(Move m, Value threshold) const {
1147 return see_ge(m, occupied, threshold);
1151 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1152 /// or by repetition. It does not detect stalemates.
1154 bool Position::is_draw(int ply) const {
1156 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1159 // Return a draw score if a position repeats once earlier but strictly
1160 // after the root, or repeats twice before or at the root.
1161 return st->repetition && st->repetition < ply;
1165 // Position::has_repeated() tests whether there has been at least one repetition
1166 // of positions since the last capture or pawn move.
1168 bool Position::has_repeated() const {
1170 StateInfo* stc = st;
1171 int end = std::min(st->rule50, st->pliesFromNull);
1174 if (stc->repetition)
1177 stc = stc->previous;
1183 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1184 /// or an earlier position has a move that directly reaches the current position.
1186 bool Position::has_game_cycle(int ply) const {
1190 int end = std::min(st->rule50, st->pliesFromNull);
1195 Key originalKey = st->key;
1196 StateInfo* stp = st->previous;
1198 for (int i = 3; i <= end; i += 2)
1200 stp = stp->previous->previous;
1202 Key moveKey = originalKey ^ stp->key;
1203 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1204 || (j = H2(moveKey), cuckoo[j] == moveKey))
1206 Move move = cuckooMove[j];
1207 Square s1 = from_sq(move);
1208 Square s2 = to_sq(move);
1210 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1215 // For nodes before or at the root, check that the move is a
1216 // repetition rather than a move to the current position.
1217 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1218 // the same location, so we have to select which square to check.
1219 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1222 // For repetitions before or at the root, require one more
1223 if (stp->repetition)
1232 /// Position::flip() flips position with the white and black sides reversed. This
1233 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1235 void Position::flip() {
1238 std::stringstream ss(fen());
1240 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1242 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1243 f.insert(0, token + (f.empty() ? " " : "/"));
1246 ss >> token; // Active color
1247 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1249 ss >> token; // Castling availability
1252 std::transform(f.begin(), f.end(), f.begin(),
1253 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1255 ss >> token; // En passant square
1256 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1258 std::getline(ss, token); // Half and full moves
1261 set(f, is_chess960(), st, this_thread());
1263 assert(pos_is_ok());
1267 /// Position::pos_is_ok() performs some consistency checks for the
1268 /// position object and raises an asserts if something wrong is detected.
1269 /// This is meant to be helpful when debugging.
1271 bool Position::pos_is_ok() const {
1273 constexpr bool Fast = true; // Quick (default) or full check?
1275 if ( (sideToMove != WHITE && sideToMove != BLACK)
1276 || piece_on(square<KING>(WHITE)) != W_KING
1277 || piece_on(square<KING>(BLACK)) != B_KING
1278 || ( ep_square() != SQ_NONE
1279 && relative_rank(sideToMove, ep_square()) != RANK_6))
1280 assert(0 && "pos_is_ok: Default");
1285 if ( pieceCount[W_KING] != 1
1286 || pieceCount[B_KING] != 1
1287 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1288 assert(0 && "pos_is_ok: Kings");
1290 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1291 || pieceCount[W_PAWN] > 8
1292 || pieceCount[B_PAWN] > 8)
1293 assert(0 && "pos_is_ok: Pawns");
1295 if ( (pieces(WHITE) & pieces(BLACK))
1296 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1297 || popcount(pieces(WHITE)) > 16
1298 || popcount(pieces(BLACK)) > 16)
1299 assert(0 && "pos_is_ok: Bitboards");
1301 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1302 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1303 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1304 assert(0 && "pos_is_ok: Bitboards");
1307 for (Piece pc : Pieces)
1308 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1309 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1310 assert(0 && "pos_is_ok: Pieces");
1312 for (Color c : { WHITE, BLACK })
1313 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1315 if (!can_castle(cr))
1318 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1319 || castlingRightsMask[castlingRookSquare[cr]] != cr
1320 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1321 assert(0 && "pos_is_ok: Castling");
1327 } // namespace Stockfish