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
336 void Position::set_state() const {
338 st->key = st->materialKey = 0;
339 st->pawnKey = Zobrist::noPawns;
340 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
341 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
345 for (Bitboard b = pieces(); b; )
347 Square s = pop_lsb(b);
348 Piece pc = piece_on(s);
349 st->key ^= Zobrist::psq[pc][s];
351 if (type_of(pc) == PAWN)
352 st->pawnKey ^= Zobrist::psq[pc][s];
354 else if (type_of(pc) != KING)
355 st->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
358 if (st->epSquare != SQ_NONE)
359 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
361 if (sideToMove == BLACK)
362 st->key ^= Zobrist::side;
364 st->key ^= Zobrist::castling[st->castlingRights];
366 for (Piece pc : Pieces)
367 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
368 st->materialKey ^= Zobrist::psq[pc][cnt];
372 /// Position::set() is an overload to initialize the position object with
373 /// the given endgame code string like "KBPKN". It is mainly a helper to
374 /// get the material key out of an endgame code.
376 Position& Position::set(const string& code, Color c, StateInfo* si) {
378 assert(code[0] == 'K');
380 string sides[] = { code.substr(code.find('K', 1)), // Weak
381 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
383 assert(sides[0].length() > 0 && sides[0].length() < 8);
384 assert(sides[1].length() > 0 && sides[1].length() < 8);
386 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
388 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
389 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
391 return set(fenStr, false, si, nullptr);
395 /// Position::fen() returns a FEN representation of the position. In case of
396 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
398 string Position::fen() const {
401 std::ostringstream ss;
403 for (Rank r = RANK_8; r >= RANK_1; --r)
405 for (File f = FILE_A; f <= FILE_H; ++f)
407 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
414 ss << PieceToChar[piece_on(make_square(f, r))];
421 ss << (sideToMove == WHITE ? " w " : " b ");
423 if (can_castle(WHITE_OO))
424 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
426 if (can_castle(WHITE_OOO))
427 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
429 if (can_castle(BLACK_OO))
430 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
432 if (can_castle(BLACK_OOO))
433 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
435 if (!can_castle(ANY_CASTLING))
438 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
439 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
445 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
446 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
447 /// slider if removing that piece from the board would result in a position where
448 /// square 's' is attacked. For example, a king-attack blocking piece can be either
449 /// a pinned or a discovered check piece, according if its color is the opposite
450 /// or the same of the color of the slider.
452 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
454 Bitboard blockers = 0;
457 // Snipers are sliders that attack 's' when a piece and other snipers are removed
458 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
459 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
460 Bitboard occupancy = pieces() ^ snipers;
464 Square sniperSq = pop_lsb(snipers);
465 Bitboard b = between_bb(s, sniperSq) & occupancy;
467 if (b && !more_than_one(b))
470 if (b & pieces(color_of(piece_on(s))))
478 /// Position::attackers_to() computes a bitboard of all pieces which attack a
479 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
481 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
483 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
484 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
485 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
486 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
487 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
488 | (attacks_bb<KING>(s) & pieces(KING));
492 /// Position::legal() tests whether a pseudo-legal move is legal
494 bool Position::legal(Move m) const {
498 Color us = sideToMove;
499 Square from = from_sq(m);
500 Square to = to_sq(m);
502 assert(color_of(moved_piece(m)) == us);
503 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
505 // En passant captures are a tricky special case. Because they are rather
506 // uncommon, we do it simply by testing whether the king is attacked after
508 if (type_of(m) == EN_PASSANT)
510 Square ksq = square<KING>(us);
511 Square capsq = to - pawn_push(us);
512 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
514 assert(to == ep_square());
515 assert(moved_piece(m) == make_piece(us, PAWN));
516 assert(piece_on(capsq) == make_piece(~us, PAWN));
517 assert(piece_on(to) == NO_PIECE);
519 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
520 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
523 // Castling moves generation does not check if the castling path is clear of
524 // enemy attacks, it is delayed at a later time: now!
525 if (type_of(m) == CASTLING)
527 // After castling, the rook and king final positions are the same in
528 // Chess960 as they would be in standard chess.
529 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
530 Direction step = to > from ? WEST : EAST;
532 for (Square s = to; s != from; s += step)
533 if (attackers_to(s) & pieces(~us))
536 // In case of Chess960, verify if the Rook blocks some checks
537 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
538 return !chess960 || !(blockers_for_king(us) & to_sq(m));
541 // If the moving piece is a king, check whether the destination square is
542 // attacked by the opponent.
543 if (type_of(piece_on(from)) == KING)
544 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
546 // A non-king move is legal if and only if it is not pinned or it
547 // is moving along the ray towards or away from the king.
548 return !(blockers_for_king(us) & from)
549 || aligned(from, to, square<KING>(us));
553 /// Position::pseudo_legal() takes a random move and tests whether the move is
554 /// pseudo legal. It is used to validate moves from TT that can be corrupted
555 /// due to SMP concurrent access or hash position key aliasing.
557 bool Position::pseudo_legal(const Move m) const {
559 Color us = sideToMove;
560 Square from = from_sq(m);
561 Square to = to_sq(m);
562 Piece pc = moved_piece(m);
564 // Use a slower but simpler function for uncommon cases
565 // yet we skip the legality check of MoveList<LEGAL>().
566 if (type_of(m) != NORMAL)
567 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
568 : MoveList<NON_EVASIONS>(*this).contains(m);
570 // Is not a promotion, so promotion piece must be empty
571 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
573 // If the 'from' square is not occupied by a piece belonging to the side to
574 // move, the move is obviously not legal.
575 if (pc == NO_PIECE || color_of(pc) != us)
578 // The destination square cannot be occupied by a friendly piece
582 // Handle the special case of a pawn move
583 if (type_of(pc) == PAWN)
585 // We have already handled promotion moves, so destination
586 // cannot be on the 8th/1st rank.
587 if ((Rank8BB | Rank1BB) & to)
590 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
591 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
592 && !( (from + 2 * pawn_push(us) == to) // Not a double push
593 && (relative_rank(us, from) == RANK_2)
595 && empty(to - pawn_push(us))))
598 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
601 // Evasions generator already takes care to avoid some kind of illegal moves
602 // and legal() relies on this. We therefore have to take care that the same
603 // kind of moves are filtered out here.
606 if (type_of(pc) != KING)
608 // Double check? In this case a king move is required
609 if (more_than_one(checkers()))
612 // Our move must be a blocking interposition or a capture of the checking piece
613 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
616 // In case of king moves under check we have to remove king so as to catch
617 // invalid moves like b1a1 when opposite queen is on c1.
618 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
626 /// Position::gives_check() tests whether a pseudo-legal move gives a check
628 bool Position::gives_check(Move m) const {
631 assert(color_of(moved_piece(m)) == sideToMove);
633 Square from = from_sq(m);
634 Square to = to_sq(m);
636 // Is there a direct check?
637 if (check_squares(type_of(piece_on(from))) & to)
640 // Is there a discovered check?
641 if ( (blockers_for_king(~sideToMove) & from)
642 && !aligned(from, to, square<KING>(~sideToMove)))
651 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
653 // En passant capture with check? We have already handled the case
654 // of direct checks and ordinary discovered check, so the only case we
655 // need to handle is the unusual case of a discovered check through
656 // the captured pawn.
659 Square capsq = make_square(file_of(to), rank_of(from));
660 Bitboard b = (pieces() ^ from ^ capsq) | to;
662 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
663 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
667 // Castling is encoded as 'king captures the rook'
668 Square ksq = square<KING>(~sideToMove);
669 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
671 return (attacks_bb<ROOK>(rto) & ksq)
672 && (attacks_bb<ROOK>(rto, pieces() ^ from ^ to) & ksq);
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));
751 st->pawnKey ^= Zobrist::psq[captured][capsq];
754 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
758 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
759 dp.piece[1] = captured;
764 // Update board and piece lists
767 // Update material hash key and prefetch access to materialTable
768 k ^= Zobrist::psq[captured][capsq];
769 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
770 prefetch(thisThread->materialTable[st->materialKey]);
772 // Reset rule 50 counter
777 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
779 // Reset en passant square
780 if (st->epSquare != SQ_NONE)
782 k ^= Zobrist::enpassant[file_of(st->epSquare)];
783 st->epSquare = SQ_NONE;
786 // Update castling rights if needed
787 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
789 k ^= Zobrist::castling[st->castlingRights];
790 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
791 k ^= Zobrist::castling[st->castlingRights];
794 // Move the piece. The tricky Chess960 castling is handled earlier
795 if (type_of(m) != CASTLING)
804 move_piece(from, to);
807 // If the moving piece is a pawn do some special extra work
808 if (type_of(pc) == PAWN)
810 // Set en passant square if the moved pawn can be captured
811 if ( (int(to) ^ int(from)) == 16
812 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
814 st->epSquare = to - pawn_push(us);
815 k ^= Zobrist::enpassant[file_of(st->epSquare)];
818 else if (type_of(m) == PROMOTION)
820 Piece promotion = make_piece(us, promotion_type(m));
822 assert(relative_rank(us, to) == RANK_8);
823 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
826 put_piece(promotion, to);
830 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
832 dp.piece[dp.dirty_num] = promotion;
833 dp.from[dp.dirty_num] = SQ_NONE;
834 dp.to[dp.dirty_num] = to;
839 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
840 st->pawnKey ^= Zobrist::psq[pc][to];
841 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
842 ^ Zobrist::psq[pc][pieceCount[pc]];
845 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
848 // Update pawn hash key
849 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
851 // Reset rule 50 draw counter
856 st->capturedPiece = captured;
858 // Update the key with the final value
861 // Calculate checkers bitboard (if move gives check)
862 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
864 sideToMove = ~sideToMove;
866 // Update king attacks used for fast check detection
869 // Calculate the repetition info. It is the ply distance from the previous
870 // occurrence of the same position, negative in the 3-fold case, or zero
871 // if the position was not repeated.
873 int end = std::min(st->rule50, st->pliesFromNull);
876 StateInfo* stp = st->previous->previous;
877 for (int i = 4; i <= end; i += 2)
879 stp = stp->previous->previous;
880 if (stp->key == st->key)
882 st->repetition = stp->repetition ? -i : i;
892 /// Position::undo_move() unmakes a move. When it returns, the position should
893 /// be restored to exactly the same state as before the move was made.
895 void Position::undo_move(Move m) {
899 sideToMove = ~sideToMove;
901 Color us = sideToMove;
902 Square from = from_sq(m);
903 Square to = to_sq(m);
904 Piece pc = piece_on(to);
906 assert(empty(from) || type_of(m) == CASTLING);
907 assert(type_of(st->capturedPiece) != KING);
909 if (type_of(m) == PROMOTION)
911 assert(relative_rank(us, to) == RANK_8);
912 assert(type_of(pc) == promotion_type(m));
913 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
916 pc = make_piece(us, PAWN);
920 if (type_of(m) == CASTLING)
923 do_castling<false>(us, from, to, rfrom, rto);
927 move_piece(to, from); // Put the piece back at the source square
929 if (st->capturedPiece)
933 if (type_of(m) == EN_PASSANT)
935 capsq -= pawn_push(us);
937 assert(type_of(pc) == PAWN);
938 assert(to == st->previous->epSquare);
939 assert(relative_rank(us, to) == RANK_6);
940 assert(piece_on(capsq) == NO_PIECE);
941 assert(st->capturedPiece == make_piece(~us, PAWN));
944 put_piece(st->capturedPiece, capsq); // Restore the captured piece
948 // Finally point our state pointer back to the previous state
956 /// Position::do_castling() is a helper used to do/undo a castling move. This
957 /// is a bit tricky in Chess960 where from/to squares can overlap.
959 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
961 bool kingSide = to > from;
962 rfrom = to; // Castling is encoded as "king captures friendly rook"
963 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
964 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
966 if (Do && Eval::useNNUE)
968 auto& dp = st->dirtyPiece;
969 dp.piece[0] = make_piece(us, KING);
972 dp.piece[1] = make_piece(us, ROOK);
978 // Remove both pieces first since squares could overlap in Chess960
979 remove_piece(Do ? from : to);
980 remove_piece(Do ? rfrom : rto);
981 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
982 put_piece(make_piece(us, KING), Do ? to : from);
983 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
987 /// Position::do_null_move() is used to do a "null move": it flips
988 /// the side to move without executing any move on the board.
990 void Position::do_null_move(StateInfo& newSt) {
993 assert(&newSt != st);
995 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1000 st->dirtyPiece.dirty_num = 0;
1001 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1002 st->accumulator.computed[WHITE] = false;
1003 st->accumulator.computed[BLACK] = false;
1005 if (st->epSquare != SQ_NONE)
1007 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1008 st->epSquare = SQ_NONE;
1011 st->key ^= Zobrist::side;
1013 prefetch(TT.first_entry(key()));
1015 st->pliesFromNull = 0;
1017 sideToMove = ~sideToMove;
1023 assert(pos_is_ok());
1027 /// Position::undo_null_move() must be used to undo a "null move"
1029 void Position::undo_null_move() {
1031 assert(!checkers());
1034 sideToMove = ~sideToMove;
1038 /// Position::key_after() computes the new hash key after the given move. Needed
1039 /// for speculative prefetch. It doesn't recognize special moves like castling,
1040 /// en passant and promotions.
1042 Key Position::key_after(Move m) const {
1044 Square from = from_sq(m);
1045 Square to = to_sq(m);
1046 Piece pc = piece_on(from);
1047 Piece captured = piece_on(to);
1048 Key k = st->key ^ Zobrist::side;
1051 k ^= Zobrist::psq[captured][to];
1053 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1055 return (captured || type_of(pc) == PAWN)
1056 ? k : adjust_key50<true>(k);
1060 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1061 /// SEE value of move is greater or equal to the given threshold. We'll use an
1062 /// algorithm similar to alpha-beta pruning with a null window.
1064 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1068 // Only deal with normal moves, assume others pass a simple SEE
1069 if (type_of(m) != NORMAL)
1070 return VALUE_ZERO >= threshold;
1072 Square from = from_sq(m), to = to_sq(m);
1074 int swap = PieceValue[MG][piece_on(to)] - threshold;
1078 swap = PieceValue[MG][piece_on(from)] - swap;
1082 assert(color_of(piece_on(from)) == sideToMove);
1083 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1084 Color stm = sideToMove;
1085 Bitboard attackers = attackers_to(to, occupied);
1086 Bitboard stmAttackers, bb;
1092 attackers &= occupied;
1094 // If stm has no more attackers then give up: stm loses
1095 if (!(stmAttackers = attackers & pieces(stm)))
1098 // Don't allow pinned pieces to attack as long as there are
1099 // pinners on their original square.
1100 if (pinners(~stm) & occupied)
1102 stmAttackers &= ~blockers_for_king(stm);
1110 // Locate and remove the next least valuable attacker, and add to
1111 // the bitboard 'attackers' any X-ray attackers behind it.
1112 if ((bb = stmAttackers & pieces(PAWN)))
1114 occupied ^= least_significant_square_bb(bb);
1115 if ((swap = PawnValueMg - swap) < res)
1118 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1121 else if ((bb = stmAttackers & pieces(KNIGHT)))
1123 occupied ^= least_significant_square_bb(bb);
1124 if ((swap = KnightValueMg - swap) < res)
1128 else if ((bb = stmAttackers & pieces(BISHOP)))
1130 occupied ^= least_significant_square_bb(bb);
1131 if ((swap = BishopValueMg - swap) < res)
1134 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1137 else if ((bb = stmAttackers & pieces(ROOK)))
1139 occupied ^= least_significant_square_bb(bb);
1140 if ((swap = RookValueMg - swap) < res)
1143 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1146 else if ((bb = stmAttackers & pieces(QUEEN)))
1148 occupied ^= least_significant_square_bb(bb);
1149 if ((swap = QueenValueMg - swap) < res)
1152 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1153 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1157 // If we "capture" with the king but opponent still has attackers,
1158 // reverse the result.
1159 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1165 bool Position::see_ge(Move m, Value threshold) const {
1167 return see_ge(m, occupied, threshold);
1171 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1172 /// or by repetition. It does not detect stalemates.
1174 bool Position::is_draw(int ply) const {
1176 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1179 // Return a draw score if a position repeats once earlier but strictly
1180 // after the root, or repeats twice before or at the root.
1181 return st->repetition && st->repetition < ply;
1185 // Position::has_repeated() tests whether there has been at least one repetition
1186 // of positions since the last capture or pawn move.
1188 bool Position::has_repeated() const {
1190 StateInfo* stc = st;
1191 int end = std::min(st->rule50, st->pliesFromNull);
1194 if (stc->repetition)
1197 stc = stc->previous;
1203 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1204 /// or an earlier position has a move that directly reaches the current position.
1206 bool Position::has_game_cycle(int ply) const {
1210 int end = std::min(st->rule50, st->pliesFromNull);
1215 Key originalKey = st->key;
1216 StateInfo* stp = st->previous;
1218 for (int i = 3; i <= end; i += 2)
1220 stp = stp->previous->previous;
1222 Key moveKey = originalKey ^ stp->key;
1223 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1224 || (j = H2(moveKey), cuckoo[j] == moveKey))
1226 Move move = cuckooMove[j];
1227 Square s1 = from_sq(move);
1228 Square s2 = to_sq(move);
1230 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1235 // For nodes before or at the root, check that the move is a
1236 // repetition rather than a move to the current position.
1237 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1238 // the same location, so we have to select which square to check.
1239 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1242 // For repetitions before or at the root, require one more
1243 if (stp->repetition)
1252 /// Position::flip() flips position with the white and black sides reversed. This
1253 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1255 void Position::flip() {
1258 std::stringstream ss(fen());
1260 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1262 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1263 f.insert(0, token + (f.empty() ? " " : "/"));
1266 ss >> token; // Active color
1267 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1269 ss >> token; // Castling availability
1272 std::transform(f.begin(), f.end(), f.begin(),
1273 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1275 ss >> token; // En passant square
1276 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1278 std::getline(ss, token); // Half and full moves
1281 set(f, is_chess960(), st, this_thread());
1283 assert(pos_is_ok());
1287 /// Position::pos_is_ok() performs some consistency checks for the
1288 /// position object and raises an asserts if something wrong is detected.
1289 /// This is meant to be helpful when debugging.
1291 bool Position::pos_is_ok() const {
1293 constexpr bool Fast = true; // Quick (default) or full check?
1295 if ( (sideToMove != WHITE && sideToMove != BLACK)
1296 || piece_on(square<KING>(WHITE)) != W_KING
1297 || piece_on(square<KING>(BLACK)) != B_KING
1298 || ( ep_square() != SQ_NONE
1299 && relative_rank(sideToMove, ep_square()) != RANK_6))
1300 assert(0 && "pos_is_ok: Default");
1305 if ( pieceCount[W_KING] != 1
1306 || pieceCount[B_KING] != 1
1307 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1308 assert(0 && "pos_is_ok: Kings");
1310 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1311 || pieceCount[W_PAWN] > 8
1312 || pieceCount[B_PAWN] > 8)
1313 assert(0 && "pos_is_ok: Pawns");
1315 if ( (pieces(WHITE) & pieces(BLACK))
1316 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1317 || popcount(pieces(WHITE)) > 16
1318 || popcount(pieces(BLACK)) > 16)
1319 assert(0 && "pos_is_ok: Bitboards");
1321 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1322 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1323 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1324 assert(0 && "pos_is_ok: Bitboards");
1327 for (Piece pc : Pieces)
1328 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1329 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1330 assert(0 && "pos_is_ok: Pieces");
1332 for (Color c : { WHITE, BLACK })
1333 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1335 if (!can_castle(cr))
1338 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1339 || castlingRightsMask[castlingRookSquare[cr]] != cr
1340 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1341 assert(0 && "pos_is_ok: Castling");
1347 } // namespace Stockfish