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;
293 /// Position::set_castling_right() is a helper function used to set castling
294 /// rights given the corresponding color and the rook starting square.
296 void Position::set_castling_right(Color c, Square rfrom) {
298 Square kfrom = square<KING>(c);
299 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
301 st->castlingRights |= cr;
302 castlingRightsMask[kfrom] |= cr;
303 castlingRightsMask[rfrom] |= cr;
304 castlingRookSquare[cr] = rfrom;
306 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
307 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
309 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto))
314 /// Position::set_check_info() sets king attacks to detect if a move gives check
316 void Position::set_check_info() const {
318 st->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), st->pinners[BLACK]);
319 st->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), st->pinners[WHITE]);
321 Square ksq = square<KING>(~sideToMove);
323 st->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
324 st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
325 st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
326 st->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
327 st->checkSquares[QUEEN] = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
328 st->checkSquares[KING] = 0;
332 /// Position::set_state() computes the hash keys of the position, and other
333 /// data that once computed is updated incrementally as moves are made.
334 /// The function is only used when a new position is set up, and to verify
335 /// the correctness of the StateInfo data when running in debug mode.
337 void Position::set_state() const {
339 st->key = st->materialKey = 0;
340 st->pawnKey = Zobrist::noPawns;
341 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
342 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
346 for (Bitboard b = pieces(); b; )
348 Square s = pop_lsb(b);
349 Piece pc = piece_on(s);
350 st->key ^= Zobrist::psq[pc][s];
352 if (type_of(pc) == PAWN)
353 st->pawnKey ^= Zobrist::psq[pc][s];
355 else if (type_of(pc) != KING)
356 st->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
359 if (st->epSquare != SQ_NONE)
360 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
362 if (sideToMove == BLACK)
363 st->key ^= Zobrist::side;
365 st->key ^= Zobrist::castling[st->castlingRights];
367 for (Piece pc : Pieces)
368 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
369 st->materialKey ^= Zobrist::psq[pc][cnt];
373 /// Position::set() is an overload to initialize the position object with
374 /// the given endgame code string like "KBPKN". It is mainly a helper to
375 /// get the material key out of an endgame code.
377 Position& Position::set(const string& code, Color c, StateInfo* si) {
379 assert(code[0] == 'K');
381 string sides[] = { code.substr(code.find('K', 1)), // Weak
382 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
384 assert(sides[0].length() > 0 && sides[0].length() < 8);
385 assert(sides[1].length() > 0 && sides[1].length() < 8);
387 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
389 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
390 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
392 return set(fenStr, false, si, nullptr);
396 /// Position::fen() returns a FEN representation of the position. In case of
397 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
399 string Position::fen() const {
402 std::ostringstream ss;
404 for (Rank r = RANK_8; r >= RANK_1; --r)
406 for (File f = FILE_A; f <= FILE_H; ++f)
408 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
415 ss << PieceToChar[piece_on(make_square(f, r))];
422 ss << (sideToMove == WHITE ? " w " : " b ");
424 if (can_castle(WHITE_OO))
425 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
427 if (can_castle(WHITE_OOO))
428 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
430 if (can_castle(BLACK_OO))
431 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
433 if (can_castle(BLACK_OOO))
434 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
436 if (!can_castle(ANY_CASTLING))
439 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
440 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
446 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
447 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
448 /// slider if removing that piece from the board would result in a position where
449 /// square 's' is attacked. For example, a king-attack blocking piece can be either
450 /// a pinned or a discovered check piece, according if its color is the opposite
451 /// or the same of the color of the slider.
453 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
455 Bitboard blockers = 0;
458 // Snipers are sliders that attack 's' when a piece and other snipers are removed
459 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
460 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
461 Bitboard occupancy = pieces() ^ snipers;
465 Square sniperSq = pop_lsb(snipers);
466 Bitboard b = between_bb(s, sniperSq) & occupancy;
468 if (b && !more_than_one(b))
471 if (b & pieces(color_of(piece_on(s))))
479 /// Position::attackers_to() computes a bitboard of all pieces which attack a
480 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
482 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
484 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
485 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
486 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
487 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
488 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
489 | (attacks_bb<KING>(s) & pieces(KING));
493 /// Position::legal() tests whether a pseudo-legal move is legal
495 bool Position::legal(Move m) const {
499 Color us = sideToMove;
500 Square from = from_sq(m);
501 Square to = to_sq(m);
503 assert(color_of(moved_piece(m)) == us);
504 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
506 // En passant captures are a tricky special case. Because they are rather
507 // uncommon, we do it simply by testing whether the king is attacked after
509 if (type_of(m) == EN_PASSANT)
511 Square ksq = square<KING>(us);
512 Square capsq = to - pawn_push(us);
513 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
515 assert(to == ep_square());
516 assert(moved_piece(m) == make_piece(us, PAWN));
517 assert(piece_on(capsq) == make_piece(~us, PAWN));
518 assert(piece_on(to) == NO_PIECE);
520 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
521 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
524 // Castling moves generation does not check if the castling path is clear of
525 // enemy attacks, it is delayed at a later time: now!
526 if (type_of(m) == CASTLING)
528 // After castling, the rook and king final positions are the same in
529 // Chess960 as they would be in standard chess.
530 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
531 Direction step = to > from ? WEST : EAST;
533 for (Square s = to; s != from; s += step)
534 if (attackers_to(s) & pieces(~us))
537 // In case of Chess960, verify if the Rook blocks some checks
538 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
539 return !chess960 || !(blockers_for_king(us) & to_sq(m));
542 // If the moving piece is a king, check whether the destination square is
543 // attacked by the opponent.
544 if (type_of(piece_on(from)) == KING)
545 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
547 // A non-king move is legal if and only if it is not pinned or it
548 // is moving along the ray towards or away from the king.
549 return !(blockers_for_king(us) & from)
550 || aligned(from, to, square<KING>(us));
554 /// Position::pseudo_legal() takes a random move and tests whether the move is
555 /// pseudo legal. It is used to validate moves from TT that can be corrupted
556 /// due to SMP concurrent access or hash position key aliasing.
558 bool Position::pseudo_legal(const Move m) const {
560 Color us = sideToMove;
561 Square from = from_sq(m);
562 Square to = to_sq(m);
563 Piece pc = moved_piece(m);
565 // Use a slower but simpler function for uncommon cases
566 // yet we skip the legality check of MoveList<LEGAL>().
567 if (type_of(m) != NORMAL)
568 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
569 : MoveList<NON_EVASIONS>(*this).contains(m);
571 // Is not a promotion, so promotion piece must be empty
572 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
574 // If the 'from' square is not occupied by a piece belonging to the side to
575 // move, the move is obviously not legal.
576 if (pc == NO_PIECE || color_of(pc) != us)
579 // The destination square cannot be occupied by a friendly piece
583 // Handle the special case of a pawn move
584 if (type_of(pc) == PAWN)
586 // We have already handled promotion moves, so destination
587 // cannot be on the 8th/1st rank.
588 if ((Rank8BB | Rank1BB) & to)
591 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
592 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
593 && !( (from + 2 * pawn_push(us) == to) // Not a double push
594 && (relative_rank(us, from) == RANK_2)
596 && empty(to - pawn_push(us))))
599 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
602 // Evasions generator already takes care to avoid some kind of illegal moves
603 // and legal() relies on this. We therefore have to take care that the same
604 // kind of moves are filtered out here.
607 if (type_of(pc) != KING)
609 // Double check? In this case a king move is required
610 if (more_than_one(checkers()))
613 // Our move must be a blocking interposition or a capture of the checking piece
614 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
617 // In case of king moves under check we have to remove king so as to catch
618 // invalid moves like b1a1 when opposite queen is on c1.
619 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
627 /// Position::gives_check() tests whether a pseudo-legal move gives a check
629 bool Position::gives_check(Move m) const {
632 assert(color_of(moved_piece(m)) == sideToMove);
634 Square from = from_sq(m);
635 Square to = to_sq(m);
637 // Is there a direct check?
638 if (check_squares(type_of(piece_on(from))) & to)
641 // Is there a discovered check?
642 if ( (blockers_for_king(~sideToMove) & from)
643 && !aligned(from, to, square<KING>(~sideToMove)))
652 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
654 // En passant capture with check? We have already handled the case
655 // of direct checks and ordinary discovered check, so the only case we
656 // need to handle is the unusual case of a discovered check through
657 // the captured pawn.
660 Square capsq = make_square(file_of(to), rank_of(from));
661 Bitboard b = (pieces() ^ from ^ capsq) | to;
663 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
664 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
668 // Castling is encoded as 'king captures the rook'
669 Square ksq = square<KING>(~sideToMove);
670 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
672 return (attacks_bb<ROOK>(rto) & ksq)
673 && (attacks_bb<ROOK>(rto, pieces() ^ from ^ to) & ksq);
679 /// Position::do_move() makes a move, and saves all information necessary
680 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
681 /// moves should be filtered out before this function is called.
683 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
686 assert(&newSt != st);
688 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
689 Key k = st->key ^ Zobrist::side;
691 // Copy some fields of the old state to our new StateInfo object except the
692 // ones which are going to be recalculated from scratch anyway and then switch
693 // our state pointer to point to the new (ready to be updated) state.
694 std::memcpy(&newSt, st, offsetof(StateInfo, key));
698 // Increment ply counters. In particular, rule50 will be reset to zero later on
699 // in case of a capture or a pawn move.
705 st->accumulator.computed[WHITE] = false;
706 st->accumulator.computed[BLACK] = false;
707 auto& dp = st->dirtyPiece;
710 Color us = sideToMove;
712 Square from = from_sq(m);
713 Square to = to_sq(m);
714 Piece pc = piece_on(from);
715 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
717 assert(color_of(pc) == us);
718 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
719 assert(type_of(captured) != KING);
721 if (type_of(m) == CASTLING)
723 assert(pc == make_piece(us, KING));
724 assert(captured == make_piece(us, ROOK));
727 do_castling<true>(us, from, to, rfrom, rto);
729 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
737 // If the captured piece is a pawn, update pawn hash key, otherwise
738 // update non-pawn material.
739 if (type_of(captured) == PAWN)
741 if (type_of(m) == EN_PASSANT)
743 capsq -= pawn_push(us);
745 assert(pc == make_piece(us, PAWN));
746 assert(to == st->epSquare);
747 assert(relative_rank(us, to) == RANK_6);
748 assert(piece_on(to) == NO_PIECE);
749 assert(piece_on(capsq) == make_piece(them, PAWN));
752 st->pawnKey ^= Zobrist::psq[captured][capsq];
755 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
759 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
760 dp.piece[1] = captured;
765 // Update board and piece lists
768 // Update material hash key and prefetch access to materialTable
769 k ^= Zobrist::psq[captured][capsq];
770 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
771 prefetch(thisThread->materialTable[st->materialKey]);
773 // Reset rule 50 counter
778 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
780 // Reset en passant square
781 if (st->epSquare != SQ_NONE)
783 k ^= Zobrist::enpassant[file_of(st->epSquare)];
784 st->epSquare = SQ_NONE;
787 // Update castling rights if needed
788 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
790 k ^= Zobrist::castling[st->castlingRights];
791 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
792 k ^= Zobrist::castling[st->castlingRights];
795 // Move the piece. The tricky Chess960 castling is handled earlier
796 if (type_of(m) != CASTLING)
805 move_piece(from, to);
808 // If the moving piece is a pawn do some special extra work
809 if (type_of(pc) == PAWN)
811 // Set en passant square if the moved pawn can be captured
812 if ( (int(to) ^ int(from)) == 16
813 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
815 st->epSquare = to - pawn_push(us);
816 k ^= Zobrist::enpassant[file_of(st->epSquare)];
819 else if (type_of(m) == PROMOTION)
821 Piece promotion = make_piece(us, promotion_type(m));
823 assert(relative_rank(us, to) == RANK_8);
824 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
827 put_piece(promotion, to);
831 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
833 dp.piece[dp.dirty_num] = promotion;
834 dp.from[dp.dirty_num] = SQ_NONE;
835 dp.to[dp.dirty_num] = to;
840 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
841 st->pawnKey ^= Zobrist::psq[pc][to];
842 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
843 ^ Zobrist::psq[pc][pieceCount[pc]];
846 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
849 // Update pawn hash key
850 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
852 // Reset rule 50 draw counter
857 st->capturedPiece = captured;
859 // Update the key with the final value
862 // Calculate checkers bitboard (if move gives check)
863 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
865 sideToMove = ~sideToMove;
867 // Update king attacks used for fast check detection
870 // Calculate the repetition info. It is the ply distance from the previous
871 // occurrence of the same position, negative in the 3-fold case, or zero
872 // if the position was not repeated.
874 int end = std::min(st->rule50, st->pliesFromNull);
877 StateInfo* stp = st->previous->previous;
878 for (int i = 4; i <= end; i += 2)
880 stp = stp->previous->previous;
881 if (stp->key == st->key)
883 st->repetition = stp->repetition ? -i : i;
893 /// Position::undo_move() unmakes a move. When it returns, the position should
894 /// be restored to exactly the same state as before the move was made.
896 void Position::undo_move(Move m) {
900 sideToMove = ~sideToMove;
902 Color us = sideToMove;
903 Square from = from_sq(m);
904 Square to = to_sq(m);
905 Piece pc = piece_on(to);
907 assert(empty(from) || type_of(m) == CASTLING);
908 assert(type_of(st->capturedPiece) != KING);
910 if (type_of(m) == PROMOTION)
912 assert(relative_rank(us, to) == RANK_8);
913 assert(type_of(pc) == promotion_type(m));
914 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
917 pc = make_piece(us, PAWN);
921 if (type_of(m) == CASTLING)
924 do_castling<false>(us, from, to, rfrom, rto);
928 move_piece(to, from); // Put the piece back at the source square
930 if (st->capturedPiece)
934 if (type_of(m) == EN_PASSANT)
936 capsq -= pawn_push(us);
938 assert(type_of(pc) == PAWN);
939 assert(to == st->previous->epSquare);
940 assert(relative_rank(us, to) == RANK_6);
941 assert(piece_on(capsq) == NO_PIECE);
942 assert(st->capturedPiece == make_piece(~us, PAWN));
945 put_piece(st->capturedPiece, capsq); // Restore the captured piece
949 // Finally point our state pointer back to the previous state
957 /// Position::do_castling() is a helper used to do/undo a castling move. This
958 /// is a bit tricky in Chess960 where from/to squares can overlap.
960 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
962 bool kingSide = to > from;
963 rfrom = to; // Castling is encoded as "king captures friendly rook"
964 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
965 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
967 if (Do && Eval::useNNUE)
969 auto& dp = st->dirtyPiece;
970 dp.piece[0] = make_piece(us, KING);
973 dp.piece[1] = make_piece(us, ROOK);
979 // Remove both pieces first since squares could overlap in Chess960
980 remove_piece(Do ? from : to);
981 remove_piece(Do ? rfrom : rto);
982 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
983 put_piece(make_piece(us, KING), Do ? to : from);
984 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
988 /// Position::do_null_move() is used to do a "null move": it flips
989 /// the side to move without executing any move on the board.
991 void Position::do_null_move(StateInfo& newSt) {
994 assert(&newSt != st);
996 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1001 st->dirtyPiece.dirty_num = 0;
1002 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1003 st->accumulator.computed[WHITE] = false;
1004 st->accumulator.computed[BLACK] = false;
1006 if (st->epSquare != SQ_NONE)
1008 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1009 st->epSquare = SQ_NONE;
1012 st->key ^= Zobrist::side;
1014 prefetch(TT.first_entry(key()));
1016 st->pliesFromNull = 0;
1018 sideToMove = ~sideToMove;
1024 assert(pos_is_ok());
1028 /// Position::undo_null_move() must be used to undo a "null move"
1030 void Position::undo_null_move() {
1032 assert(!checkers());
1035 sideToMove = ~sideToMove;
1039 /// Position::key_after() computes the new hash key after the given move. Needed
1040 /// for speculative prefetch. It doesn't recognize special moves like castling,
1041 /// en passant and promotions.
1043 Key Position::key_after(Move m) const {
1045 Square from = from_sq(m);
1046 Square to = to_sq(m);
1047 Piece pc = piece_on(from);
1048 Piece captured = piece_on(to);
1049 Key k = st->key ^ Zobrist::side;
1052 k ^= Zobrist::psq[captured][to];
1054 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1056 return (captured || type_of(pc) == PAWN)
1057 ? k : adjust_key50<true>(k);
1061 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1062 /// SEE value of move is greater or equal to the given threshold. We'll use an
1063 /// algorithm similar to alpha-beta pruning with a null window.
1065 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1069 // Only deal with normal moves, assume others pass a simple SEE
1070 if (type_of(m) != NORMAL)
1071 return VALUE_ZERO >= threshold;
1073 Square from = from_sq(m), to = to_sq(m);
1075 int swap = PieceValue[MG][piece_on(to)] - threshold;
1079 swap = PieceValue[MG][piece_on(from)] - swap;
1083 assert(color_of(piece_on(from)) == sideToMove);
1084 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1085 Color stm = sideToMove;
1086 Bitboard attackers = attackers_to(to, occupied);
1087 Bitboard stmAttackers, bb;
1093 attackers &= occupied;
1095 // If stm has no more attackers then give up: stm loses
1096 if (!(stmAttackers = attackers & pieces(stm)))
1099 // Don't allow pinned pieces to attack as long as there are
1100 // pinners on their original square.
1101 if (pinners(~stm) & occupied)
1103 stmAttackers &= ~blockers_for_king(stm);
1111 // Locate and remove the next least valuable attacker, and add to
1112 // the bitboard 'attackers' any X-ray attackers behind it.
1113 if ((bb = stmAttackers & pieces(PAWN)))
1115 occupied ^= least_significant_square_bb(bb);
1116 if ((swap = PawnValueMg - swap) < res)
1119 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1122 else if ((bb = stmAttackers & pieces(KNIGHT)))
1124 occupied ^= least_significant_square_bb(bb);
1125 if ((swap = KnightValueMg - swap) < res)
1129 else if ((bb = stmAttackers & pieces(BISHOP)))
1131 occupied ^= least_significant_square_bb(bb);
1132 if ((swap = BishopValueMg - swap) < res)
1135 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1138 else if ((bb = stmAttackers & pieces(ROOK)))
1140 occupied ^= least_significant_square_bb(bb);
1141 if ((swap = RookValueMg - swap) < res)
1144 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1147 else if ((bb = stmAttackers & pieces(QUEEN)))
1149 occupied ^= least_significant_square_bb(bb);
1150 if ((swap = QueenValueMg - swap) < res)
1153 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1154 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1158 // If we "capture" with the king but opponent still has attackers,
1159 // reverse the result.
1160 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1166 bool Position::see_ge(Move m, Value threshold) const {
1168 return see_ge(m, occupied, threshold);
1172 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1173 /// or by repetition. It does not detect stalemates.
1175 bool Position::is_draw(int ply) const {
1177 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1180 // Return a draw score if a position repeats once earlier but strictly
1181 // after the root, or repeats twice before or at the root.
1182 return st->repetition && st->repetition < ply;
1186 // Position::has_repeated() tests whether there has been at least one repetition
1187 // of positions since the last capture or pawn move.
1189 bool Position::has_repeated() const {
1191 StateInfo* stc = st;
1192 int end = std::min(st->rule50, st->pliesFromNull);
1195 if (stc->repetition)
1198 stc = stc->previous;
1204 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1205 /// or an earlier position has a move that directly reaches the current position.
1207 bool Position::has_game_cycle(int ply) const {
1211 int end = std::min(st->rule50, st->pliesFromNull);
1216 Key originalKey = st->key;
1217 StateInfo* stp = st->previous;
1219 for (int i = 3; i <= end; i += 2)
1221 stp = stp->previous->previous;
1223 Key moveKey = originalKey ^ stp->key;
1224 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1225 || (j = H2(moveKey), cuckoo[j] == moveKey))
1227 Move move = cuckooMove[j];
1228 Square s1 = from_sq(move);
1229 Square s2 = to_sq(move);
1231 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1236 // For nodes before or at the root, check that the move is a
1237 // repetition rather than a move to the current position.
1238 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1239 // the same location, so we have to select which square to check.
1240 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1243 // For repetitions before or at the root, require one more
1244 if (stp->repetition)
1253 /// Position::flip() flips position with the white and black sides reversed. This
1254 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1256 void Position::flip() {
1259 std::stringstream ss(fen());
1261 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1263 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1264 f.insert(0, token + (f.empty() ? " " : "/"));
1267 ss >> token; // Active color
1268 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1270 ss >> token; // Castling availability
1273 std::transform(f.begin(), f.end(), f.begin(),
1274 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1276 ss >> token; // En passant square
1277 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1279 std::getline(ss, token); // Half and full moves
1282 set(f, is_chess960(), st, this_thread());
1284 assert(pos_is_ok());
1288 /// Position::pos_is_ok() performs some consistency checks for the
1289 /// position object and raises an asserts if something wrong is detected.
1290 /// This is meant to be helpful when debugging.
1292 bool Position::pos_is_ok() const {
1294 constexpr bool Fast = true; // Quick (default) or full check?
1296 if ( (sideToMove != WHITE && sideToMove != BLACK)
1297 || piece_on(square<KING>(WHITE)) != W_KING
1298 || piece_on(square<KING>(BLACK)) != B_KING
1299 || ( ep_square() != SQ_NONE
1300 && relative_rank(sideToMove, ep_square()) != RANK_6))
1301 assert(0 && "pos_is_ok: Default");
1306 if ( pieceCount[W_KING] != 1
1307 || pieceCount[B_KING] != 1
1308 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1309 assert(0 && "pos_is_ok: Kings");
1311 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1312 || pieceCount[W_PAWN] > 8
1313 || pieceCount[B_PAWN] > 8)
1314 assert(0 && "pos_is_ok: Pawns");
1316 if ( (pieces(WHITE) & pieces(BLACK))
1317 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1318 || popcount(pieces(WHITE)) > 16
1319 || popcount(pieces(BLACK)) > 16)
1320 assert(0 && "pos_is_ok: Bitboards");
1322 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1323 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1324 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1325 assert(0 && "pos_is_ok: Bitboards");
1328 for (Piece pc : Pieces)
1329 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1330 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1331 assert(0 && "pos_is_ok: Pieces");
1333 for (Color c : { WHITE, BLACK })
1334 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1336 if (!can_castle(cr))
1339 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1340 || castlingRightsMask[castlingRookSquare[cr]] != cr
1341 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1342 assert(0 && "pos_is_ok: Castling");
1348 } // namespace Stockfish