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 return !aligned(from, to, square<KING>(~sideToMove))
643 || type_of(m) == CASTLING;
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 rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
670 return check_squares(ROOK) & rto;
676 /// Position::do_move() makes a move, and saves all information necessary
677 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
678 /// moves should be filtered out before this function is called.
680 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
683 assert(&newSt != st);
685 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
686 Key k = st->key ^ Zobrist::side;
688 // Copy some fields of the old state to our new StateInfo object except the
689 // ones which are going to be recalculated from scratch anyway and then switch
690 // our state pointer to point to the new (ready to be updated) state.
691 std::memcpy(&newSt, st, offsetof(StateInfo, key));
695 // Increment ply counters. In particular, rule50 will be reset to zero later on
696 // in case of a capture or a pawn move.
702 st->accumulator.computed[WHITE] = false;
703 st->accumulator.computed[BLACK] = false;
704 auto& dp = st->dirtyPiece;
707 Color us = sideToMove;
709 Square from = from_sq(m);
710 Square to = to_sq(m);
711 Piece pc = piece_on(from);
712 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
714 assert(color_of(pc) == us);
715 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
716 assert(type_of(captured) != KING);
718 if (type_of(m) == CASTLING)
720 assert(pc == make_piece(us, KING));
721 assert(captured == make_piece(us, ROOK));
724 do_castling<true>(us, from, to, rfrom, rto);
726 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
734 // If the captured piece is a pawn, update pawn hash key, otherwise
735 // update non-pawn material.
736 if (type_of(captured) == PAWN)
738 if (type_of(m) == EN_PASSANT)
740 capsq -= pawn_push(us);
742 assert(pc == make_piece(us, PAWN));
743 assert(to == st->epSquare);
744 assert(relative_rank(us, to) == RANK_6);
745 assert(piece_on(to) == NO_PIECE);
746 assert(piece_on(capsq) == make_piece(them, PAWN));
749 st->pawnKey ^= Zobrist::psq[captured][capsq];
752 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
754 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
755 dp.piece[1] = captured;
759 // Update board and piece lists
762 // Update material hash key and prefetch access to materialTable
763 k ^= Zobrist::psq[captured][capsq];
764 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
766 // Reset rule 50 counter
771 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
773 // Reset en passant square
774 if (st->epSquare != SQ_NONE)
776 k ^= Zobrist::enpassant[file_of(st->epSquare)];
777 st->epSquare = SQ_NONE;
780 // Update castling rights if needed
781 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
783 k ^= Zobrist::castling[st->castlingRights];
784 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
785 k ^= Zobrist::castling[st->castlingRights];
788 // Move the piece. The tricky Chess960 castling is handled earlier
789 if (type_of(m) != CASTLING)
795 move_piece(from, to);
798 // If the moving piece is a pawn do some special extra work
799 if (type_of(pc) == PAWN)
801 // Set en passant square if the moved pawn can be captured
802 if ( (int(to) ^ int(from)) == 16
803 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
805 st->epSquare = to - pawn_push(us);
806 k ^= Zobrist::enpassant[file_of(st->epSquare)];
809 else if (type_of(m) == PROMOTION)
811 Piece promotion = make_piece(us, promotion_type(m));
813 assert(relative_rank(us, to) == RANK_8);
814 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
817 put_piece(promotion, to);
819 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
821 dp.piece[dp.dirty_num] = promotion;
822 dp.from[dp.dirty_num] = SQ_NONE;
823 dp.to[dp.dirty_num] = to;
827 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
828 st->pawnKey ^= Zobrist::psq[pc][to];
829 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
830 ^ Zobrist::psq[pc][pieceCount[pc]];
833 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
836 // Update pawn hash key
837 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
839 // Reset rule 50 draw counter
844 st->capturedPiece = captured;
846 // Update the key with the final value
849 // Calculate checkers bitboard (if move gives check)
850 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
852 sideToMove = ~sideToMove;
854 // Update king attacks used for fast check detection
857 // Calculate the repetition info. It is the ply distance from the previous
858 // occurrence of the same position, negative in the 3-fold case, or zero
859 // if the position was not repeated.
861 int end = std::min(st->rule50, st->pliesFromNull);
864 StateInfo* stp = st->previous->previous;
865 for (int i = 4; i <= end; i += 2)
867 stp = stp->previous->previous;
868 if (stp->key == st->key)
870 st->repetition = stp->repetition ? -i : i;
880 /// Position::undo_move() unmakes a move. When it returns, the position should
881 /// be restored to exactly the same state as before the move was made.
883 void Position::undo_move(Move m) {
887 sideToMove = ~sideToMove;
889 Color us = sideToMove;
890 Square from = from_sq(m);
891 Square to = to_sq(m);
892 Piece pc = piece_on(to);
894 assert(empty(from) || type_of(m) == CASTLING);
895 assert(type_of(st->capturedPiece) != KING);
897 if (type_of(m) == PROMOTION)
899 assert(relative_rank(us, to) == RANK_8);
900 assert(type_of(pc) == promotion_type(m));
901 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
904 pc = make_piece(us, PAWN);
908 if (type_of(m) == CASTLING)
911 do_castling<false>(us, from, to, rfrom, rto);
915 move_piece(to, from); // Put the piece back at the source square
917 if (st->capturedPiece)
921 if (type_of(m) == EN_PASSANT)
923 capsq -= pawn_push(us);
925 assert(type_of(pc) == PAWN);
926 assert(to == st->previous->epSquare);
927 assert(relative_rank(us, to) == RANK_6);
928 assert(piece_on(capsq) == NO_PIECE);
929 assert(st->capturedPiece == make_piece(~us, PAWN));
932 put_piece(st->capturedPiece, capsq); // Restore the captured piece
936 // Finally point our state pointer back to the previous state
944 /// Position::do_castling() is a helper used to do/undo a castling move. This
945 /// is a bit tricky in Chess960 where from/to squares can overlap.
947 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
949 bool kingSide = to > from;
950 rfrom = to; // Castling is encoded as "king captures friendly rook"
951 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
952 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
956 auto& dp = st->dirtyPiece;
957 dp.piece[0] = make_piece(us, KING);
960 dp.piece[1] = make_piece(us, ROOK);
966 // Remove both pieces first since squares could overlap in Chess960
967 remove_piece(Do ? from : to);
968 remove_piece(Do ? rfrom : rto);
969 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
970 put_piece(make_piece(us, KING), Do ? to : from);
971 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
975 /// Position::do_null_move() is used to do a "null move": it flips
976 /// the side to move without executing any move on the board.
978 void Position::do_null_move(StateInfo& newSt) {
981 assert(&newSt != st);
983 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
988 st->dirtyPiece.dirty_num = 0;
989 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
990 st->accumulator.computed[WHITE] = false;
991 st->accumulator.computed[BLACK] = false;
993 if (st->epSquare != SQ_NONE)
995 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
996 st->epSquare = SQ_NONE;
999 st->key ^= Zobrist::side;
1001 prefetch(TT.first_entry(key()));
1003 st->pliesFromNull = 0;
1005 sideToMove = ~sideToMove;
1011 assert(pos_is_ok());
1015 /// Position::undo_null_move() must be used to undo a "null move"
1017 void Position::undo_null_move() {
1019 assert(!checkers());
1022 sideToMove = ~sideToMove;
1026 /// Position::key_after() computes the new hash key after the given move. Needed
1027 /// for speculative prefetch. It doesn't recognize special moves like castling,
1028 /// en passant and promotions.
1030 Key Position::key_after(Move m) const {
1032 Square from = from_sq(m);
1033 Square to = to_sq(m);
1034 Piece pc = piece_on(from);
1035 Piece captured = piece_on(to);
1036 Key k = st->key ^ Zobrist::side;
1039 k ^= Zobrist::psq[captured][to];
1041 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1043 return (captured || type_of(pc) == PAWN)
1044 ? k : adjust_key50<true>(k);
1048 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1049 /// SEE value of move is greater or equal to the given threshold. We'll use an
1050 /// algorithm similar to alpha-beta pruning with a null window.
1052 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1056 // Only deal with normal moves, assume others pass a simple SEE
1057 if (type_of(m) != NORMAL)
1058 return VALUE_ZERO >= threshold;
1060 Square from = from_sq(m), to = to_sq(m);
1062 int swap = PieceValue[MG][piece_on(to)] - threshold;
1066 swap = PieceValue[MG][piece_on(from)] - swap;
1070 assert(color_of(piece_on(from)) == sideToMove);
1071 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1072 Color stm = sideToMove;
1073 Bitboard attackers = attackers_to(to, occupied);
1074 Bitboard stmAttackers, bb;
1080 attackers &= occupied;
1082 // If stm has no more attackers then give up: stm loses
1083 if (!(stmAttackers = attackers & pieces(stm)))
1086 // Don't allow pinned pieces to attack as long as there are
1087 // pinners on their original square.
1088 if (pinners(~stm) & occupied)
1090 stmAttackers &= ~blockers_for_king(stm);
1098 // Locate and remove the next least valuable attacker, and add to
1099 // the bitboard 'attackers' any X-ray attackers behind it.
1100 if ((bb = stmAttackers & pieces(PAWN)))
1102 occupied ^= least_significant_square_bb(bb);
1103 if ((swap = PawnValueMg - swap) < res)
1106 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1109 else if ((bb = stmAttackers & pieces(KNIGHT)))
1111 occupied ^= least_significant_square_bb(bb);
1112 if ((swap = KnightValueMg - swap) < res)
1116 else if ((bb = stmAttackers & pieces(BISHOP)))
1118 occupied ^= least_significant_square_bb(bb);
1119 if ((swap = BishopValueMg - swap) < res)
1122 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1125 else if ((bb = stmAttackers & pieces(ROOK)))
1127 occupied ^= least_significant_square_bb(bb);
1128 if ((swap = RookValueMg - swap) < res)
1131 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1134 else if ((bb = stmAttackers & pieces(QUEEN)))
1136 occupied ^= least_significant_square_bb(bb);
1137 if ((swap = QueenValueMg - swap) < res)
1140 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1141 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1145 // If we "capture" with the king but opponent still has attackers,
1146 // reverse the result.
1147 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1153 bool Position::see_ge(Move m, Value threshold) const {
1155 return see_ge(m, occupied, threshold);
1159 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1160 /// or by repetition. It does not detect stalemates.
1162 bool Position::is_draw(int ply) const {
1164 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1167 // Return a draw score if a position repeats once earlier but strictly
1168 // after the root, or repeats twice before or at the root.
1169 return st->repetition && st->repetition < ply;
1173 // Position::has_repeated() tests whether there has been at least one repetition
1174 // of positions since the last capture or pawn move.
1176 bool Position::has_repeated() const {
1178 StateInfo* stc = st;
1179 int end = std::min(st->rule50, st->pliesFromNull);
1182 if (stc->repetition)
1185 stc = stc->previous;
1191 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1192 /// or an earlier position has a move that directly reaches the current position.
1194 bool Position::has_game_cycle(int ply) const {
1198 int end = std::min(st->rule50, st->pliesFromNull);
1203 Key originalKey = st->key;
1204 StateInfo* stp = st->previous;
1206 for (int i = 3; i <= end; i += 2)
1208 stp = stp->previous->previous;
1210 Key moveKey = originalKey ^ stp->key;
1211 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1212 || (j = H2(moveKey), cuckoo[j] == moveKey))
1214 Move move = cuckooMove[j];
1215 Square s1 = from_sq(move);
1216 Square s2 = to_sq(move);
1218 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1223 // For nodes before or at the root, check that the move is a
1224 // repetition rather than a move to the current position.
1225 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1226 // the same location, so we have to select which square to check.
1227 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1230 // For repetitions before or at the root, require one more
1231 if (stp->repetition)
1240 /// Position::flip() flips position with the white and black sides reversed. This
1241 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1243 void Position::flip() {
1246 std::stringstream ss(fen());
1248 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1250 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1251 f.insert(0, token + (f.empty() ? " " : "/"));
1254 ss >> token; // Active color
1255 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1257 ss >> token; // Castling availability
1260 std::transform(f.begin(), f.end(), f.begin(),
1261 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1263 ss >> token; // En passant square
1264 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1266 std::getline(ss, token); // Half and full moves
1269 set(f, is_chess960(), st, this_thread());
1271 assert(pos_is_ok());
1275 /// Position::pos_is_ok() performs some consistency checks for the
1276 /// position object and raises an asserts if something wrong is detected.
1277 /// This is meant to be helpful when debugging.
1279 bool Position::pos_is_ok() const {
1281 constexpr bool Fast = true; // Quick (default) or full check?
1283 if ( (sideToMove != WHITE && sideToMove != BLACK)
1284 || piece_on(square<KING>(WHITE)) != W_KING
1285 || piece_on(square<KING>(BLACK)) != B_KING
1286 || ( ep_square() != SQ_NONE
1287 && relative_rank(sideToMove, ep_square()) != RANK_6))
1288 assert(0 && "pos_is_ok: Default");
1293 if ( pieceCount[W_KING] != 1
1294 || pieceCount[B_KING] != 1
1295 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1296 assert(0 && "pos_is_ok: Kings");
1298 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1299 || pieceCount[W_PAWN] > 8
1300 || pieceCount[B_PAWN] > 8)
1301 assert(0 && "pos_is_ok: Pawns");
1303 if ( (pieces(WHITE) & pieces(BLACK))
1304 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1305 || popcount(pieces(WHITE)) > 16
1306 || popcount(pieces(BLACK)) > 16)
1307 assert(0 && "pos_is_ok: Bitboards");
1309 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1310 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1311 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1312 assert(0 && "pos_is_ok: Bitboards");
1315 for (Piece pc : Pieces)
1316 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1317 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1318 assert(0 && "pos_is_ok: Pieces");
1320 for (Color c : { WHITE, BLACK })
1321 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1323 if (!can_castle(cr))
1326 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1327 || castlingRightsMask[castlingRookSquare[cr]] != cr
1328 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1329 assert(0 && "pos_is_ok: Castling");
1335 } // namespace Stockfish