2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2021 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
33 #include "syzygy/tbprobe.h"
41 Key psq[PIECE_NB][SQUARE_NB];
42 Key enpassant[FILE_NB];
43 Key castling[CASTLING_RIGHT_NB];
49 const string PieceToChar(" PNBRQK pnbrqk");
51 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
52 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
56 /// operator<<(Position) returns an ASCII representation of the position
58 std::ostream& operator<<(std::ostream& os, const Position& pos) {
60 os << "\n +---+---+---+---+---+---+---+---+\n";
62 for (Rank r = RANK_8; r >= RANK_1; --r)
64 for (File f = FILE_A; f <= FILE_H; ++f)
65 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
67 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
70 os << " a b c d e f g h\n"
71 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
72 << std::setfill('0') << std::setw(16) << pos.key()
73 << std::setfill(' ') << std::dec << "\nCheckers: ";
75 for (Bitboard b = pos.checkers(); b; )
76 os << UCI::square(pop_lsb(b)) << " ";
78 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
79 && !pos.can_castle(ANY_CASTLING))
82 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
85 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
86 Tablebases::ProbeState s1, s2;
87 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
88 int dtz = Tablebases::probe_dtz(p, &s2);
89 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
90 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
97 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
98 // situations. Description of the algorithm in the following paper:
99 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
101 // First and second hash functions for indexing the cuckoo tables
102 inline int H1(Key h) { return h & 0x1fff; }
103 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
105 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
107 Move cuckooMove[8192];
110 /// Position::init() initializes at startup the various arrays used to compute hash keys
112 void Position::init() {
116 for (Piece pc : Pieces)
117 for (Square s = SQ_A1; s <= SQ_H8; ++s)
118 Zobrist::psq[pc][s] = rng.rand<Key>();
120 for (File f = FILE_A; f <= FILE_H; ++f)
121 Zobrist::enpassant[f] = rng.rand<Key>();
123 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
124 Zobrist::castling[cr] = rng.rand<Key>();
126 Zobrist::side = rng.rand<Key>();
127 Zobrist::noPawns = rng.rand<Key>();
129 // Prepare the cuckoo tables
130 std::memset(cuckoo, 0, sizeof(cuckoo));
131 std::memset(cuckooMove, 0, sizeof(cuckooMove));
133 for (Piece pc : Pieces)
134 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
135 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
136 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
138 Move move = make_move(s1, s2);
139 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
143 std::swap(cuckoo[i], key);
144 std::swap(cuckooMove[i], move);
145 if (move == MOVE_NONE) // Arrived at empty slot?
147 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
151 assert(count == 3668);
155 /// Position::set() initializes the position object with the given FEN string.
156 /// This function is not very robust - make sure that input FENs are correct,
157 /// this is assumed to be the responsibility of the GUI.
159 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
161 A FEN string defines a particular position using only the ASCII character set.
163 A FEN string contains six fields separated by a space. The fields are:
165 1) Piece placement (from white's perspective). Each rank is described, starting
166 with rank 8 and ending with rank 1. Within each rank, the contents of each
167 square are described from file A through file H. Following the Standard
168 Algebraic Notation (SAN), each piece is identified by a single letter taken
169 from the standard English names. White pieces are designated using upper-case
170 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
171 noted using digits 1 through 8 (the number of blank squares), and "/"
174 2) Active color. "w" means white moves next, "b" means black.
176 3) Castling availability. If neither side can castle, this is "-". Otherwise,
177 this has one or more letters: "K" (White can castle kingside), "Q" (White
178 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
179 can castle queenside).
181 4) En passant target square (in algebraic notation). If there's no en passant
182 target square, this is "-". If a pawn has just made a 2-square move, this
183 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
184 if there is a pawn in position to make an en passant capture, and if there really
185 is a pawn that might have advanced two squares.
187 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
188 or capture. This is used to determine if a draw can be claimed under the
191 6) Fullmove number. The number of the full move. It starts at 1, and is
192 incremented after Black's move.
195 unsigned char col, row, token;
198 std::istringstream ss(fenStr);
200 std::memset(this, 0, sizeof(Position));
201 std::memset(si, 0, sizeof(StateInfo));
206 // 1. Piece placement
207 while ((ss >> token) && !isspace(token))
210 sq += (token - '0') * EAST; // Advance the given number of files
212 else if (token == '/')
215 else if ((idx = PieceToChar.find(token)) != string::npos) {
216 put_piece(Piece(idx), sq);
223 sideToMove = (token == 'w' ? WHITE : BLACK);
226 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
227 // Shredder-FEN that uses the letters of the columns on which the rooks began
228 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
229 // if an inner rook is associated with the castling right, the castling tag is
230 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
231 while ((ss >> token) && !isspace(token))
234 Color c = islower(token) ? BLACK : WHITE;
235 Piece rook = make_piece(c, ROOK);
237 token = char(toupper(token));
240 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
242 else if (token == 'Q')
243 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
245 else if (token >= 'A' && token <= 'H')
246 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
251 set_castling_right(c, rsq);
254 // 4. En passant square.
255 // Ignore if square is invalid or not on side to move relative rank 6.
256 bool enpassant = false;
258 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
259 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
261 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
263 // En passant square will be considered only if
264 // a) side to move have a pawn threatening epSquare
265 // b) there is an enemy pawn in front of epSquare
266 // c) there is no piece on epSquare or behind epSquare
267 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
268 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
269 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
273 st->epSquare = SQ_NONE;
275 // 5-6. Halfmove clock and fullmove number
276 ss >> std::skipws >> st->rule50 >> gamePly;
278 // Convert from fullmove starting from 1 to gamePly starting from 0,
279 // handle also common incorrect FEN with fullmove = 0.
280 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
282 chess960 = isChess960;
290 /// Position::set_castling_right() is a helper function used to set castling
291 /// rights given the corresponding color and the rook starting square.
293 void Position::set_castling_right(Color c, Square rfrom) {
295 Square kfrom = square<KING>(c);
296 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
298 st->castlingRights |= cr;
299 castlingRightsMask[kfrom] |= cr;
300 castlingRightsMask[rfrom] |= cr;
301 castlingRookSquare[cr] = rfrom;
303 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
304 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
306 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto))
311 /// Position::set_check_info() sets king attacks to detect if a move gives check
313 void Position::set_check_info(StateInfo* si) const {
315 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
316 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
318 Square ksq = square<KING>(~sideToMove);
320 si->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
321 si->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
322 si->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
323 si->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
324 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
325 si->checkSquares[KING] = 0;
329 /// Position::set_state() computes the hash keys of the position, and other
330 /// data that once computed is updated incrementally as moves are made.
331 /// The function is only used when a new position is set up, and to verify
332 /// the correctness of the StateInfo data when running in debug mode.
334 void Position::set_state(StateInfo* si) const {
336 si->key = si->materialKey = 0;
337 si->pawnKey = Zobrist::noPawns;
338 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
339 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
343 for (Bitboard b = pieces(); b; )
345 Square s = pop_lsb(b);
346 Piece pc = piece_on(s);
347 si->key ^= Zobrist::psq[pc][s];
349 if (type_of(pc) == PAWN)
350 si->pawnKey ^= Zobrist::psq[pc][s];
352 else if (type_of(pc) != KING)
353 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
356 if (si->epSquare != SQ_NONE)
357 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
359 if (sideToMove == BLACK)
360 si->key ^= Zobrist::side;
362 si->key ^= Zobrist::castling[si->castlingRights];
364 for (Piece pc : Pieces)
365 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
366 si->materialKey ^= Zobrist::psq[pc][cnt];
370 /// Position::set() is an overload to initialize the position object with
371 /// the given endgame code string like "KBPKN". It is mainly a helper to
372 /// get the material key out of an endgame code.
374 Position& Position::set(const string& code, Color c, StateInfo* si) {
376 assert(code[0] == 'K');
378 string sides[] = { code.substr(code.find('K', 1)), // Weak
379 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
381 assert(sides[0].length() > 0 && sides[0].length() < 8);
382 assert(sides[1].length() > 0 && sides[1].length() < 8);
384 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
386 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
387 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
389 return set(fenStr, false, si, nullptr);
393 /// Position::fen() returns a FEN representation of the position. In case of
394 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
396 string Position::fen() const {
399 std::ostringstream ss;
401 for (Rank r = RANK_8; r >= RANK_1; --r)
403 for (File f = FILE_A; f <= FILE_H; ++f)
405 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
412 ss << PieceToChar[piece_on(make_square(f, r))];
419 ss << (sideToMove == WHITE ? " w " : " b ");
421 if (can_castle(WHITE_OO))
422 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
424 if (can_castle(WHITE_OOO))
425 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
427 if (can_castle(BLACK_OO))
428 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
430 if (can_castle(BLACK_OOO))
431 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
433 if (!can_castle(ANY_CASTLING))
436 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
437 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
443 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
444 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
445 /// slider if removing that piece from the board would result in a position where
446 /// square 's' is attacked. For example, a king-attack blocking piece can be either
447 /// a pinned or a discovered check piece, according if its color is the opposite
448 /// or the same of the color of the slider.
450 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
452 Bitboard blockers = 0;
455 // Snipers are sliders that attack 's' when a piece and other snipers are removed
456 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
457 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
458 Bitboard occupancy = pieces() ^ snipers;
462 Square sniperSq = pop_lsb(snipers);
463 Bitboard b = between_bb(s, sniperSq) & occupancy;
465 if (b && !more_than_one(b))
468 if (b & pieces(color_of(piece_on(s))))
476 /// Position::attackers_to() computes a bitboard of all pieces which attack a
477 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
479 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
481 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
482 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
483 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
484 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
485 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
486 | (attacks_bb<KING>(s) & pieces(KING));
490 /// Position::legal() tests whether a pseudo-legal move is legal
492 bool Position::legal(Move m) const {
496 Color us = sideToMove;
497 Square from = from_sq(m);
498 Square to = to_sq(m);
500 assert(color_of(moved_piece(m)) == us);
501 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
503 // En passant captures are a tricky special case. Because they are rather
504 // uncommon, we do it simply by testing whether the king is attacked after
506 if (type_of(m) == EN_PASSANT)
508 Square ksq = square<KING>(us);
509 Square capsq = to - pawn_push(us);
510 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
512 assert(to == ep_square());
513 assert(moved_piece(m) == make_piece(us, PAWN));
514 assert(piece_on(capsq) == make_piece(~us, PAWN));
515 assert(piece_on(to) == NO_PIECE);
517 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
518 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
521 // Castling moves generation does not check if the castling path is clear of
522 // enemy attacks, it is delayed at a later time: now!
523 if (type_of(m) == CASTLING)
525 // After castling, the rook and king final positions are the same in
526 // Chess960 as they would be in standard chess.
527 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
528 Direction step = to > from ? WEST : EAST;
530 for (Square s = to; s != from; s += step)
531 if (attackers_to(s) & pieces(~us))
534 // In case of Chess960, verify if the Rook blocks some checks
535 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
536 return !chess960 || !(blockers_for_king(us) & to_sq(m));
539 // If the moving piece is a king, check whether the destination square is
540 // attacked by the opponent.
541 if (type_of(piece_on(from)) == KING)
542 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
544 // A non-king move is legal if and only if it is not pinned or it
545 // is moving along the ray towards or away from the king.
546 return !(blockers_for_king(us) & from)
547 || aligned(from, to, square<KING>(us));
551 /// Position::pseudo_legal() takes a random move and tests whether the move is
552 /// pseudo legal. It is used to validate moves from TT that can be corrupted
553 /// due to SMP concurrent access or hash position key aliasing.
555 bool Position::pseudo_legal(const Move m) const {
557 Color us = sideToMove;
558 Square from = from_sq(m);
559 Square to = to_sq(m);
560 Piece pc = moved_piece(m);
562 // Use a slower but simpler function for uncommon cases
563 // yet we skip the legality check of MoveList<LEGAL>().
564 if (type_of(m) != NORMAL)
565 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
566 : MoveList<NON_EVASIONS>(*this).contains(m);
568 // Is not a promotion, so promotion piece must be empty
569 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
572 // If the 'from' square is not occupied by a piece belonging to the side to
573 // move, the move is obviously not legal.
574 if (pc == NO_PIECE || color_of(pc) != us)
577 // The destination square cannot be occupied by a friendly piece
581 // Handle the special case of a pawn move
582 if (type_of(pc) == PAWN)
584 // We have already handled promotion moves, so destination
585 // cannot be on the 8th/1st rank.
586 if ((Rank8BB | Rank1BB) & to)
589 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
590 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
591 && !( (from + 2 * pawn_push(us) == to) // Not a double push
592 && (relative_rank(us, from) == RANK_2)
594 && empty(to - pawn_push(us))))
597 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
600 // Evasions generator already takes care to avoid some kind of illegal moves
601 // and legal() relies on this. We therefore have to take care that the same
602 // kind of moves are filtered out here.
605 if (type_of(pc) != KING)
607 // Double check? In this case a king move is required
608 if (more_than_one(checkers()))
611 // Our move must be a blocking interposition or a capture of the checking piece
612 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
615 // In case of king moves under check we have to remove king so as to catch
616 // invalid moves like b1a1 when opposite queen is on c1.
617 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
625 /// Position::gives_check() tests whether a pseudo-legal move gives a check
627 bool Position::gives_check(Move m) const {
630 assert(color_of(moved_piece(m)) == sideToMove);
632 Square from = from_sq(m);
633 Square to = to_sq(m);
635 // Is there a direct check?
636 if (check_squares(type_of(piece_on(from))) & to)
639 // Is there a discovered check?
640 if ( (blockers_for_king(~sideToMove) & from)
641 && !aligned(from, to, square<KING>(~sideToMove)))
650 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
652 // En passant capture with check? We have already handled the case
653 // of direct checks and ordinary discovered check, so the only case we
654 // need to handle is the unusual case of a discovered check through
655 // the captured pawn.
658 Square capsq = make_square(file_of(to), rank_of(from));
659 Bitboard b = (pieces() ^ from ^ capsq) | to;
661 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
662 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
666 // Castling is encoded as 'king captures the rook'
667 Square ksq = square<KING>(~sideToMove);
668 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
670 return (attacks_bb<ROOK>(rto) & ksq)
671 && (attacks_bb<ROOK>(rto, pieces() ^ from ^ to) & ksq);
677 /// Position::do_move() makes a move, and saves all information necessary
678 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
679 /// moves should be filtered out before this function is called.
681 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
684 assert(&newSt != st);
686 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
687 Key k = st->key ^ Zobrist::side;
689 // Copy some fields of the old state to our new StateInfo object except the
690 // ones which are going to be recalculated from scratch anyway and then switch
691 // our state pointer to point to the new (ready to be updated) state.
692 std::memcpy(&newSt, st, offsetof(StateInfo, key));
696 // Increment ply counters. In particular, rule50 will be reset to zero later on
697 // in case of a capture or a pawn move.
703 st->accumulator.computed[WHITE] = false;
704 st->accumulator.computed[BLACK] = false;
705 auto& dp = st->dirtyPiece;
708 Color us = sideToMove;
710 Square from = from_sq(m);
711 Square to = to_sq(m);
712 Piece pc = piece_on(from);
713 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
715 assert(color_of(pc) == us);
716 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
717 assert(type_of(captured) != KING);
719 if (type_of(m) == CASTLING)
721 assert(pc == make_piece(us, KING));
722 assert(captured == make_piece(us, ROOK));
725 do_castling<true>(us, from, to, rfrom, rto);
727 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
735 // If the captured piece is a pawn, update pawn hash key, otherwise
736 // update non-pawn material.
737 if (type_of(captured) == PAWN)
739 if (type_of(m) == EN_PASSANT)
741 capsq -= pawn_push(us);
743 assert(pc == make_piece(us, PAWN));
744 assert(to == st->epSquare);
745 assert(relative_rank(us, to) == RANK_6);
746 assert(piece_on(to) == NO_PIECE);
747 assert(piece_on(capsq) == make_piece(them, PAWN));
750 st->pawnKey ^= Zobrist::psq[captured][capsq];
753 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
757 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
758 dp.piece[1] = captured;
763 // Update board and piece lists
766 if (type_of(m) == EN_PASSANT)
767 board[capsq] = NO_PIECE;
769 // Update material hash key and prefetch access to materialTable
770 k ^= Zobrist::psq[captured][capsq];
771 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
772 prefetch(thisThread->materialTable[st->materialKey]);
774 // Reset rule 50 counter
779 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
781 // Reset en passant square
782 if (st->epSquare != SQ_NONE)
784 k ^= Zobrist::enpassant[file_of(st->epSquare)];
785 st->epSquare = SQ_NONE;
788 // Update castling rights if needed
789 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
791 k ^= Zobrist::castling[st->castlingRights];
792 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
793 k ^= Zobrist::castling[st->castlingRights];
796 // Move the piece. The tricky Chess960 castling is handled earlier
797 if (type_of(m) != CASTLING)
806 move_piece(from, to);
809 // If the moving piece is a pawn do some special extra work
810 if (type_of(pc) == PAWN)
812 // Set en passant square if the moved pawn can be captured
813 if ( (int(to) ^ int(from)) == 16
814 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
816 st->epSquare = to - pawn_push(us);
817 k ^= Zobrist::enpassant[file_of(st->epSquare)];
820 else if (type_of(m) == PROMOTION)
822 Piece promotion = make_piece(us, promotion_type(m));
824 assert(relative_rank(us, to) == RANK_8);
825 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
828 put_piece(promotion, to);
832 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
834 dp.piece[dp.dirty_num] = promotion;
835 dp.from[dp.dirty_num] = SQ_NONE;
836 dp.to[dp.dirty_num] = to;
841 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
842 st->pawnKey ^= Zobrist::psq[pc][to];
843 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
844 ^ Zobrist::psq[pc][pieceCount[pc]];
847 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
850 // Update pawn hash key
851 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
853 // Reset rule 50 draw counter
858 st->capturedPiece = captured;
860 // Update the key with the final value
863 // Calculate checkers bitboard (if move gives check)
864 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
866 sideToMove = ~sideToMove;
868 // Update king attacks used for fast check detection
871 // Calculate the repetition info. It is the ply distance from the previous
872 // occurrence of the same position, negative in the 3-fold case, or zero
873 // if the position was not repeated.
875 int end = std::min(st->rule50, st->pliesFromNull);
878 StateInfo* stp = st->previous->previous;
879 for (int i = 4; i <= end; i += 2)
881 stp = stp->previous->previous;
882 if (stp->key == st->key)
884 st->repetition = stp->repetition ? -i : i;
894 /// Position::undo_move() unmakes a move. When it returns, the position should
895 /// be restored to exactly the same state as before the move was made.
897 void Position::undo_move(Move m) {
901 sideToMove = ~sideToMove;
903 Color us = sideToMove;
904 Square from = from_sq(m);
905 Square to = to_sq(m);
906 Piece pc = piece_on(to);
908 assert(empty(from) || type_of(m) == CASTLING);
909 assert(type_of(st->capturedPiece) != KING);
911 if (type_of(m) == PROMOTION)
913 assert(relative_rank(us, to) == RANK_8);
914 assert(type_of(pc) == promotion_type(m));
915 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
918 pc = make_piece(us, PAWN);
922 if (type_of(m) == CASTLING)
925 do_castling<false>(us, from, to, rfrom, rto);
929 move_piece(to, from); // Put the piece back at the source square
931 if (st->capturedPiece)
935 if (type_of(m) == EN_PASSANT)
937 capsq -= pawn_push(us);
939 assert(type_of(pc) == PAWN);
940 assert(to == st->previous->epSquare);
941 assert(relative_rank(us, to) == RANK_6);
942 assert(piece_on(capsq) == NO_PIECE);
943 assert(st->capturedPiece == make_piece(~us, PAWN));
946 put_piece(st->capturedPiece, capsq); // Restore the captured piece
950 // Finally point our state pointer back to the previous state
958 /// Position::do_castling() is a helper used to do/undo a castling move. This
959 /// is a bit tricky in Chess960 where from/to squares can overlap.
961 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
963 bool kingSide = to > from;
964 rfrom = to; // Castling is encoded as "king captures friendly rook"
965 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
966 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
968 if (Do && Eval::useNNUE)
970 auto& dp = st->dirtyPiece;
971 dp.piece[0] = make_piece(us, KING);
974 dp.piece[1] = make_piece(us, ROOK);
980 // Remove both pieces first since squares could overlap in Chess960
981 remove_piece(Do ? from : to);
982 remove_piece(Do ? rfrom : rto);
983 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
984 put_piece(make_piece(us, KING), Do ? to : from);
985 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
989 /// Position::do_null_move() is used to do a "null move": it flips
990 /// the side to move without executing any move on the board.
992 void Position::do_null_move(StateInfo& newSt) {
995 assert(&newSt != st);
997 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1002 st->dirtyPiece.dirty_num = 0;
1003 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1004 st->accumulator.computed[WHITE] = false;
1005 st->accumulator.computed[BLACK] = false;
1007 if (st->epSquare != SQ_NONE)
1009 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1010 st->epSquare = SQ_NONE;
1013 st->key ^= Zobrist::side;
1015 prefetch(TT.first_entry(key()));
1017 st->pliesFromNull = 0;
1019 sideToMove = ~sideToMove;
1025 assert(pos_is_ok());
1029 /// Position::undo_null_move() must be used to undo a "null move"
1031 void Position::undo_null_move() {
1033 assert(!checkers());
1036 sideToMove = ~sideToMove;
1040 /// Position::key_after() computes the new hash key after the given move. Needed
1041 /// for speculative prefetch. It doesn't recognize special moves like castling,
1042 /// en passant and promotions.
1044 Key Position::key_after(Move m) const {
1046 Square from = from_sq(m);
1047 Square to = to_sq(m);
1048 Piece pc = piece_on(from);
1049 Piece captured = piece_on(to);
1050 Key k = st->key ^ Zobrist::side;
1053 k ^= Zobrist::psq[captured][to];
1055 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1059 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1060 /// SEE value of move is greater or equal to the given threshold. We'll use an
1061 /// algorithm similar to alpha-beta pruning with a null window.
1063 bool Position::see_ge(Move m, Value threshold) const {
1067 // Only deal with normal moves, assume others pass a simple SEE
1068 if (type_of(m) != NORMAL)
1069 return VALUE_ZERO >= threshold;
1071 Square from = from_sq(m), to = to_sq(m);
1073 int swap = PieceValue[MG][piece_on(to)] - threshold;
1077 swap = PieceValue[MG][piece_on(from)] - swap;
1081 assert(color_of(piece_on(from)) == sideToMove);
1082 Bitboard occupied = pieces() ^ from ^ to;
1083 Color stm = sideToMove;
1084 Bitboard attackers = attackers_to(to, occupied);
1085 Bitboard stmAttackers, bb;
1091 attackers &= occupied;
1093 // If stm has no more attackers then give up: stm loses
1094 if (!(stmAttackers = attackers & pieces(stm)))
1097 // Don't allow pinned pieces to attack as long as there are
1098 // pinners on their original square.
1099 if (pinners(~stm) & occupied)
1100 stmAttackers &= ~blockers_for_king(stm);
1107 // Locate and remove the next least valuable attacker, and add to
1108 // the bitboard 'attackers' any X-ray attackers behind it.
1109 if ((bb = stmAttackers & pieces(PAWN)))
1111 if ((swap = PawnValueMg - swap) < res)
1114 occupied ^= least_significant_square_bb(bb);
1115 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1118 else if ((bb = stmAttackers & pieces(KNIGHT)))
1120 if ((swap = KnightValueMg - swap) < res)
1123 occupied ^= least_significant_square_bb(bb);
1126 else if ((bb = stmAttackers & pieces(BISHOP)))
1128 if ((swap = BishopValueMg - swap) < res)
1131 occupied ^= least_significant_square_bb(bb);
1132 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1135 else if ((bb = stmAttackers & pieces(ROOK)))
1137 if ((swap = RookValueMg - swap) < res)
1140 occupied ^= least_significant_square_bb(bb);
1141 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1144 else if ((bb = stmAttackers & pieces(QUEEN)))
1146 if ((swap = QueenValueMg - swap) < res)
1149 occupied ^= least_significant_square_bb(bb);
1150 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1151 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1155 // If we "capture" with the king but opponent still has attackers,
1156 // reverse the result.
1157 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1164 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1165 /// or by repetition. It does not detect stalemates.
1167 bool Position::is_draw(int ply) const {
1169 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1172 // Return a draw score if a position repeats once earlier but strictly
1173 // after the root, or repeats twice before or at the root.
1174 return st->repetition && st->repetition < ply;
1178 // Position::has_repeated() tests whether there has been at least one repetition
1179 // of positions since the last capture or pawn move.
1181 bool Position::has_repeated() const {
1183 StateInfo* stc = st;
1184 int end = std::min(st->rule50, st->pliesFromNull);
1187 if (stc->repetition)
1190 stc = stc->previous;
1196 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1197 /// or an earlier position has a move that directly reaches the current position.
1199 bool Position::has_game_cycle(int ply) const {
1203 int end = std::min(st->rule50, st->pliesFromNull);
1208 Key originalKey = st->key;
1209 StateInfo* stp = st->previous;
1211 for (int i = 3; i <= end; i += 2)
1213 stp = stp->previous->previous;
1215 Key moveKey = originalKey ^ stp->key;
1216 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1217 || (j = H2(moveKey), cuckoo[j] == moveKey))
1219 Move move = cuckooMove[j];
1220 Square s1 = from_sq(move);
1221 Square s2 = to_sq(move);
1223 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1228 // For nodes before or at the root, check that the move is a
1229 // repetition rather than a move to the current position.
1230 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1231 // the same location, so we have to select which square to check.
1232 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1235 // For repetitions before or at the root, require one more
1236 if (stp->repetition)
1245 /// Position::flip() flips position with the white and black sides reversed. This
1246 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1248 void Position::flip() {
1251 std::stringstream ss(fen());
1253 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1255 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1256 f.insert(0, token + (f.empty() ? " " : "/"));
1259 ss >> token; // Active color
1260 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1262 ss >> token; // Castling availability
1265 std::transform(f.begin(), f.end(), f.begin(),
1266 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1268 ss >> token; // En passant square
1269 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1271 std::getline(ss, token); // Half and full moves
1274 set(f, is_chess960(), st, this_thread());
1276 assert(pos_is_ok());
1280 /// Position::pos_is_ok() performs some consistency checks for the
1281 /// position object and raises an asserts if something wrong is detected.
1282 /// This is meant to be helpful when debugging.
1284 bool Position::pos_is_ok() const {
1286 constexpr bool Fast = true; // Quick (default) or full check?
1288 if ( (sideToMove != WHITE && sideToMove != BLACK)
1289 || piece_on(square<KING>(WHITE)) != W_KING
1290 || piece_on(square<KING>(BLACK)) != B_KING
1291 || ( ep_square() != SQ_NONE
1292 && relative_rank(sideToMove, ep_square()) != RANK_6))
1293 assert(0 && "pos_is_ok: Default");
1298 if ( pieceCount[W_KING] != 1
1299 || pieceCount[B_KING] != 1
1300 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1301 assert(0 && "pos_is_ok: Kings");
1303 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1304 || pieceCount[W_PAWN] > 8
1305 || pieceCount[B_PAWN] > 8)
1306 assert(0 && "pos_is_ok: Pawns");
1308 if ( (pieces(WHITE) & pieces(BLACK))
1309 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1310 || popcount(pieces(WHITE)) > 16
1311 || popcount(pieces(BLACK)) > 16)
1312 assert(0 && "pos_is_ok: Bitboards");
1314 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1315 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1316 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1317 assert(0 && "pos_is_ok: Bitboards");
1320 ASSERT_ALIGNED(&si, Eval::NNUE::CacheLineSize);
1323 if (std::memcmp(&si, st, sizeof(StateInfo)))
1324 assert(0 && "pos_is_ok: State");
1326 for (Piece pc : Pieces)
1327 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1328 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1329 assert(0 && "pos_is_ok: Pieces");
1331 for (Color c : { WHITE, BLACK })
1332 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1334 if (!can_castle(cr))
1337 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1338 || castlingRightsMask[castlingRookSquare[cr]] != cr
1339 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1340 assert(0 && "pos_is_ok: Castling");
1346 } // namespace Stockfish