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 st->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), st->pinners[BLACK]);
325 st->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), st->pinners[WHITE]);
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;
447 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
448 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
449 /// slider if removing that piece from the board would result in a position where
450 /// square 's' is attacked. For example, a king-attack blocking piece can be either
451 /// a pinned or a discovered check piece, according if its color is the opposite
452 /// or the same of the color of the slider.
454 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
456 Bitboard blockers = 0;
459 // Snipers are sliders that attack 's' when a piece and other snipers are removed
460 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
461 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
462 Bitboard occupancy = pieces() ^ snipers;
466 Square sniperSq = pop_lsb(snipers);
467 Bitboard b = between_bb(s, sniperSq) & occupancy;
469 if (b && !more_than_one(b))
472 if (b & pieces(color_of(piece_on(s))))
480 /// Position::attackers_to() computes a bitboard of all pieces which attack a
481 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
483 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
485 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
486 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
487 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
488 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
489 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
490 | (attacks_bb<KING>(s) & pieces(KING));
494 /// Position::legal() tests whether a pseudo-legal move is legal
496 bool Position::legal(Move m) const {
500 Color us = sideToMove;
501 Square from = from_sq(m);
502 Square to = to_sq(m);
504 assert(color_of(moved_piece(m)) == us);
505 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
507 // En passant captures are a tricky special case. Because they are rather
508 // uncommon, we do it simply by testing whether the king is attacked after
510 if (type_of(m) == EN_PASSANT)
512 Square ksq = square<KING>(us);
513 Square capsq = to - pawn_push(us);
514 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
516 assert(to == ep_square());
517 assert(moved_piece(m) == make_piece(us, PAWN));
518 assert(piece_on(capsq) == make_piece(~us, PAWN));
519 assert(piece_on(to) == NO_PIECE);
521 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
522 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
525 // Castling moves generation does not check if the castling path is clear of
526 // enemy attacks, it is delayed at a later time: now!
527 if (type_of(m) == CASTLING)
529 // After castling, the rook and king final positions are the same in
530 // Chess960 as they would be in standard chess.
531 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
532 Direction step = to > from ? WEST : EAST;
534 for (Square s = to; s != from; s += step)
535 if (attackers_to(s) & pieces(~us))
538 // In case of Chess960, verify if the Rook blocks some checks
539 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
540 return !chess960 || !(blockers_for_king(us) & to_sq(m));
543 // If the moving piece is a king, check whether the destination square is
544 // attacked by the opponent.
545 if (type_of(piece_on(from)) == KING)
546 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
548 // A non-king move is legal if and only if it is not pinned or it
549 // is moving along the ray towards or away from the king.
550 return !(blockers_for_king(us) & from)
551 || aligned(from, to, square<KING>(us));
555 /// Position::pseudo_legal() takes a random move and tests whether the move is
556 /// pseudo legal. It is used to validate moves from TT that can be corrupted
557 /// due to SMP concurrent access or hash position key aliasing.
559 bool Position::pseudo_legal(const Move m) const {
561 Color us = sideToMove;
562 Square from = from_sq(m);
563 Square to = to_sq(m);
564 Piece pc = moved_piece(m);
566 // Use a slower but simpler function for uncommon cases
567 // yet we skip the legality check of MoveList<LEGAL>().
568 if (type_of(m) != NORMAL)
569 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
570 : MoveList<NON_EVASIONS>(*this).contains(m);
572 // Is not a promotion, so promotion piece must be empty
573 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
575 // If the 'from' square is not occupied by a piece belonging to the side to
576 // move, the move is obviously not legal.
577 if (pc == NO_PIECE || color_of(pc) != us)
580 // The destination square cannot be occupied by a friendly piece
584 // Handle the special case of a pawn move
585 if (type_of(pc) == PAWN)
587 // We have already handled promotion moves, so destination
588 // cannot be on the 8th/1st rank.
589 if ((Rank8BB | Rank1BB) & to)
592 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
593 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
594 && !( (from + 2 * pawn_push(us) == to) // Not a double push
595 && (relative_rank(us, from) == RANK_2)
597 && empty(to - pawn_push(us))))
600 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
603 // Evasions generator already takes care to avoid some kind of illegal moves
604 // and legal() relies on this. We therefore have to take care that the same
605 // kind of moves are filtered out here.
608 if (type_of(pc) != KING)
610 // Double check? In this case a king move is required
611 if (more_than_one(checkers()))
614 // Our move must be a blocking interposition or a capture of the checking piece
615 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
618 // In case of king moves under check we have to remove king so as to catch
619 // invalid moves like b1a1 when opposite queen is on c1.
620 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
628 /// Position::gives_check() tests whether a pseudo-legal move gives a check
630 bool Position::gives_check(Move m) const {
633 assert(color_of(moved_piece(m)) == sideToMove);
635 Square from = from_sq(m);
636 Square to = to_sq(m);
638 // Is there a direct check?
639 if (check_squares(type_of(piece_on(from))) & to)
642 // Is there a discovered check?
643 if (blockers_for_king(~sideToMove) & from)
644 return !aligned(from, to, square<KING>(~sideToMove))
645 || type_of(m) == CASTLING;
653 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
655 // En passant capture with check? We have already handled the case
656 // of direct checks and ordinary discovered check, so the only case we
657 // need to handle is the unusual case of a discovered check through
658 // the captured pawn.
661 Square capsq = make_square(file_of(to), rank_of(from));
662 Bitboard b = (pieces() ^ from ^ capsq) | to;
664 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
665 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
669 // Castling is encoded as 'king captures the rook'
670 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
672 return check_squares(ROOK) & rto;
678 /// Position::do_move() makes a move, and saves all information necessary
679 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
680 /// moves should be filtered out before this function is called.
682 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
685 assert(&newSt != st);
687 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
688 Key k = st->key ^ Zobrist::side;
690 // Copy some fields of the old state to our new StateInfo object except the
691 // ones which are going to be recalculated from scratch anyway and then switch
692 // our state pointer to point to the new (ready to be updated) state.
693 std::memcpy(&newSt, st, offsetof(StateInfo, key));
697 // Increment ply counters. In particular, rule50 will be reset to zero later on
698 // in case of a capture or a pawn move.
704 st->accumulator.computed[WHITE] = false;
705 st->accumulator.computed[BLACK] = false;
706 auto& dp = st->dirtyPiece;
709 Color us = sideToMove;
711 Square from = from_sq(m);
712 Square to = to_sq(m);
713 Piece pc = piece_on(from);
714 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
716 assert(color_of(pc) == us);
717 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
718 assert(type_of(captured) != KING);
720 if (type_of(m) == CASTLING)
722 assert(pc == make_piece(us, KING));
723 assert(captured == make_piece(us, ROOK));
726 do_castling<true>(us, from, to, rfrom, rto);
728 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
736 // If the captured piece is a pawn, update pawn hash key, otherwise
737 // update non-pawn material.
738 if (type_of(captured) == PAWN)
740 if (type_of(m) == EN_PASSANT)
742 capsq -= pawn_push(us);
744 assert(pc == make_piece(us, PAWN));
745 assert(to == st->epSquare);
746 assert(relative_rank(us, to) == RANK_6);
747 assert(piece_on(to) == NO_PIECE);
748 assert(piece_on(capsq) == make_piece(them, PAWN));
752 st->nonPawnMaterial[them] -= PieceValue[captured];
754 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
755 dp.piece[1] = captured;
759 // Update board and piece lists
762 // Update material hash key and prefetch access to materialTable
763 k ^= Zobrist::psq[captured][capsq];
764 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
766 // Reset rule 50 counter
771 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
773 // Reset en passant square
774 if (st->epSquare != SQ_NONE)
776 k ^= Zobrist::enpassant[file_of(st->epSquare)];
777 st->epSquare = SQ_NONE;
780 // Update castling rights if needed
781 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
783 k ^= Zobrist::castling[st->castlingRights];
784 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
785 k ^= Zobrist::castling[st->castlingRights];
788 // Move the piece. The tricky Chess960 castling is handled earlier
789 if (type_of(m) != CASTLING)
795 move_piece(from, to);
798 // If the moving piece is a pawn do some special extra work
799 if (type_of(pc) == PAWN)
801 // Set en passant square if the moved pawn can be captured
802 if ( (int(to) ^ int(from)) == 16
803 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
805 st->epSquare = to - pawn_push(us);
806 k ^= Zobrist::enpassant[file_of(st->epSquare)];
809 else if (type_of(m) == PROMOTION)
811 Piece promotion = make_piece(us, promotion_type(m));
813 assert(relative_rank(us, to) == RANK_8);
814 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
817 put_piece(promotion, to);
819 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
821 dp.piece[dp.dirty_num] = promotion;
822 dp.from[dp.dirty_num] = SQ_NONE;
823 dp.to[dp.dirty_num] = to;
827 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
828 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
829 ^ Zobrist::psq[pc][pieceCount[pc]];
832 st->nonPawnMaterial[us] += PieceValue[promotion];
835 // Reset rule 50 draw counter
840 st->capturedPiece = captured;
842 // Update the key with the final value
845 // Calculate checkers bitboard (if move gives check)
846 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
848 sideToMove = ~sideToMove;
850 // Update king attacks used for fast check detection
853 // Calculate the repetition info. It is the ply distance from the previous
854 // occurrence of the same position, negative in the 3-fold case, or zero
855 // if the position was not repeated.
857 int end = std::min(st->rule50, st->pliesFromNull);
860 StateInfo* stp = st->previous->previous;
861 for (int i = 4; i <= end; i += 2)
863 stp = stp->previous->previous;
864 if (stp->key == st->key)
866 st->repetition = stp->repetition ? -i : i;
876 /// Position::undo_move() unmakes a move. When it returns, the position should
877 /// be restored to exactly the same state as before the move was made.
879 void Position::undo_move(Move m) {
883 sideToMove = ~sideToMove;
885 Color us = sideToMove;
886 Square from = from_sq(m);
887 Square to = to_sq(m);
888 Piece pc = piece_on(to);
890 assert(empty(from) || type_of(m) == CASTLING);
891 assert(type_of(st->capturedPiece) != KING);
893 if (type_of(m) == PROMOTION)
895 assert(relative_rank(us, to) == RANK_8);
896 assert(type_of(pc) == promotion_type(m));
897 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
900 pc = make_piece(us, PAWN);
904 if (type_of(m) == CASTLING)
907 do_castling<false>(us, from, to, rfrom, rto);
911 move_piece(to, from); // Put the piece back at the source square
913 if (st->capturedPiece)
917 if (type_of(m) == EN_PASSANT)
919 capsq -= pawn_push(us);
921 assert(type_of(pc) == PAWN);
922 assert(to == st->previous->epSquare);
923 assert(relative_rank(us, to) == RANK_6);
924 assert(piece_on(capsq) == NO_PIECE);
925 assert(st->capturedPiece == make_piece(~us, PAWN));
928 put_piece(st->capturedPiece, capsq); // Restore the captured piece
932 // Finally point our state pointer back to the previous state
940 /// Position::do_castling() is a helper used to do/undo a castling move. This
941 /// is a bit tricky in Chess960 where from/to squares can overlap.
943 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
945 bool kingSide = to > from;
946 rfrom = to; // Castling is encoded as "king captures friendly rook"
947 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
948 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
952 auto& dp = st->dirtyPiece;
953 dp.piece[0] = make_piece(us, KING);
956 dp.piece[1] = make_piece(us, ROOK);
962 // Remove both pieces first since squares could overlap in Chess960
963 remove_piece(Do ? from : to);
964 remove_piece(Do ? rfrom : rto);
965 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
966 put_piece(make_piece(us, KING), Do ? to : from);
967 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
971 /// Position::do_null_move() is used to do a "null move": it flips
972 /// the side to move without executing any move on the board.
974 void Position::do_null_move(StateInfo& newSt) {
977 assert(&newSt != st);
979 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
984 st->dirtyPiece.dirty_num = 0;
985 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
986 st->accumulator.computed[WHITE] = false;
987 st->accumulator.computed[BLACK] = false;
989 if (st->epSquare != SQ_NONE)
991 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
992 st->epSquare = SQ_NONE;
995 st->key ^= Zobrist::side;
997 prefetch(TT.first_entry(key()));
999 st->pliesFromNull = 0;
1001 sideToMove = ~sideToMove;
1007 assert(pos_is_ok());
1011 /// Position::undo_null_move() must be used to undo a "null move"
1013 void Position::undo_null_move() {
1015 assert(!checkers());
1018 sideToMove = ~sideToMove;
1022 /// Position::key_after() computes the new hash key after the given move. Needed
1023 /// for speculative prefetch. It doesn't recognize special moves like castling,
1024 /// en passant and promotions.
1026 Key Position::key_after(Move m) const {
1028 Square from = from_sq(m);
1029 Square to = to_sq(m);
1030 Piece pc = piece_on(from);
1031 Piece captured = piece_on(to);
1032 Key k = st->key ^ Zobrist::side;
1035 k ^= Zobrist::psq[captured][to];
1037 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1039 return (captured || type_of(pc) == PAWN)
1040 ? k : adjust_key50<true>(k);
1044 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1045 /// SEE value of move is greater or equal to the given threshold. We'll use an
1046 /// algorithm similar to alpha-beta pruning with a null window.
1048 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1052 // Only deal with normal moves, assume others pass a simple SEE
1053 if (type_of(m) != NORMAL)
1054 return VALUE_ZERO >= threshold;
1056 Square from = from_sq(m), to = to_sq(m);
1058 int swap = PieceValue[piece_on(to)] - threshold;
1062 swap = PieceValue[piece_on(from)] - swap;
1066 assert(color_of(piece_on(from)) == sideToMove);
1067 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1068 Color stm = sideToMove;
1069 Bitboard attackers = attackers_to(to, occupied);
1070 Bitboard stmAttackers, bb;
1076 attackers &= occupied;
1078 // If stm has no more attackers then give up: stm loses
1079 if (!(stmAttackers = attackers & pieces(stm)))
1082 // Don't allow pinned pieces to attack as long as there are
1083 // pinners on their original square.
1084 if (pinners(~stm) & occupied)
1086 stmAttackers &= ~blockers_for_king(stm);
1094 // Locate and remove the next least valuable attacker, and add to
1095 // the bitboard 'attackers' any X-ray attackers behind it.
1096 if ((bb = stmAttackers & pieces(PAWN)))
1098 occupied ^= least_significant_square_bb(bb);
1099 if ((swap = PawnValue - swap) < res)
1102 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1105 else if ((bb = stmAttackers & pieces(KNIGHT)))
1107 occupied ^= least_significant_square_bb(bb);
1108 if ((swap = KnightValue - swap) < res)
1112 else if ((bb = stmAttackers & pieces(BISHOP)))
1114 occupied ^= least_significant_square_bb(bb);
1115 if ((swap = BishopValue - swap) < res)
1118 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1121 else if ((bb = stmAttackers & pieces(ROOK)))
1123 occupied ^= least_significant_square_bb(bb);
1124 if ((swap = RookValue - swap) < res)
1127 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1130 else if ((bb = stmAttackers & pieces(QUEEN)))
1132 occupied ^= least_significant_square_bb(bb);
1133 if ((swap = QueenValue - swap) < res)
1136 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1137 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1141 // If we "capture" with the king but opponent still has attackers,
1142 // reverse the result.
1143 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1149 bool Position::see_ge(Move m, Value threshold) const {
1151 return see_ge(m, occupied, threshold);
1155 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1156 /// or by repetition. It does not detect stalemates.
1158 bool Position::is_draw(int ply) const {
1160 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1163 // Return a draw score if a position repeats once earlier but strictly
1164 // after the root, or repeats twice before or at the root.
1165 return st->repetition && st->repetition < ply;
1169 // Position::has_repeated() tests whether there has been at least one repetition
1170 // of positions since the last capture or pawn move.
1172 bool Position::has_repeated() const {
1174 StateInfo* stc = st;
1175 int end = std::min(st->rule50, st->pliesFromNull);
1178 if (stc->repetition)
1181 stc = stc->previous;
1187 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1188 /// or an earlier position has a move that directly reaches the current position.
1190 bool Position::has_game_cycle(int ply) const {
1194 int end = std::min(st->rule50, st->pliesFromNull);
1199 Key originalKey = st->key;
1200 StateInfo* stp = st->previous;
1202 for (int i = 3; i <= end; i += 2)
1204 stp = stp->previous->previous;
1206 Key moveKey = originalKey ^ stp->key;
1207 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1208 || (j = H2(moveKey), cuckoo[j] == moveKey))
1210 Move move = cuckooMove[j];
1211 Square s1 = from_sq(move);
1212 Square s2 = to_sq(move);
1214 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1219 // For nodes before or at the root, check that the move is a
1220 // repetition rather than a move to the current position.
1221 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1222 // the same location, so we have to select which square to check.
1223 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1226 // For repetitions before or at the root, require one more
1227 if (stp->repetition)
1236 /// Position::flip() flips position with the white and black sides reversed. This
1237 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1239 void Position::flip() {
1242 std::stringstream ss(fen());
1244 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1246 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1247 f.insert(0, token + (f.empty() ? " " : "/"));
1250 ss >> token; // Active color
1251 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1253 ss >> token; // Castling availability
1256 std::transform(f.begin(), f.end(), f.begin(),
1257 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1259 ss >> token; // En passant square
1260 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1262 std::getline(ss, token); // Half and full moves
1265 set(f, is_chess960(), st, this_thread());
1267 assert(pos_is_ok());
1271 /// Position::pos_is_ok() performs some consistency checks for the
1272 /// position object and raises an asserts if something wrong is detected.
1273 /// This is meant to be helpful when debugging.
1275 bool Position::pos_is_ok() const {
1277 constexpr bool Fast = true; // Quick (default) or full check?
1279 if ( (sideToMove != WHITE && sideToMove != BLACK)
1280 || piece_on(square<KING>(WHITE)) != W_KING
1281 || piece_on(square<KING>(BLACK)) != B_KING
1282 || ( ep_square() != SQ_NONE
1283 && relative_rank(sideToMove, ep_square()) != RANK_6))
1284 assert(0 && "pos_is_ok: Default");
1289 if ( pieceCount[W_KING] != 1
1290 || pieceCount[B_KING] != 1
1291 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1292 assert(0 && "pos_is_ok: Kings");
1294 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1295 || pieceCount[W_PAWN] > 8
1296 || pieceCount[B_PAWN] > 8)
1297 assert(0 && "pos_is_ok: Pawns");
1299 if ( (pieces(WHITE) & pieces(BLACK))
1300 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1301 || popcount(pieces(WHITE)) > 16
1302 || popcount(pieces(BLACK)) > 16)
1303 assert(0 && "pos_is_ok: Bitboards");
1305 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1306 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1307 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1308 assert(0 && "pos_is_ok: Bitboards");
1311 for (Piece pc : Pieces)
1312 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1313 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1314 assert(0 && "pos_is_ok: Pieces");
1316 for (Color c : { WHITE, BLACK })
1317 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1319 if (!can_castle(cr))
1322 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1323 || castlingRightsMask[castlingRookSquare[cr]] != cr
1324 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1325 assert(0 && "pos_is_ok: Castling");
1331 } // namespace Stockfish