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;
285 st->accumulator.state[WHITE] = Eval::NNUE::INIT;
286 st->accumulator.state[BLACK] = Eval::NNUE::INIT;
294 /// Position::set_castling_right() is a helper function used to set castling
295 /// rights given the corresponding color and the rook starting square.
297 void Position::set_castling_right(Color c, Square rfrom) {
299 Square kfrom = square<KING>(c);
300 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
302 st->castlingRights |= cr;
303 castlingRightsMask[kfrom] |= cr;
304 castlingRightsMask[rfrom] |= cr;
305 castlingRookSquare[cr] = rfrom;
307 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
308 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
310 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto))
315 /// Position::set_check_info() sets king attacks to detect if a move gives check
317 void Position::set_check_info(StateInfo* si) const {
319 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
320 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
322 Square ksq = square<KING>(~sideToMove);
324 si->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
325 si->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
326 si->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
327 si->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
328 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
329 si->checkSquares[KING] = 0;
333 /// Position::set_state() computes the hash keys of the position, and other
334 /// data that once computed is updated incrementally as moves are made.
335 /// The function is only used when a new position is set up, and to verify
336 /// the correctness of the StateInfo data when running in debug mode.
338 void Position::set_state(StateInfo* si) const {
340 si->key = si->materialKey = 0;
341 si->pawnKey = Zobrist::noPawns;
342 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
343 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
347 for (Bitboard b = pieces(); b; )
349 Square s = pop_lsb(b);
350 Piece pc = piece_on(s);
351 si->key ^= Zobrist::psq[pc][s];
353 if (type_of(pc) == PAWN)
354 si->pawnKey ^= Zobrist::psq[pc][s];
356 else if (type_of(pc) != KING)
357 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
360 if (si->epSquare != SQ_NONE)
361 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
363 if (sideToMove == BLACK)
364 si->key ^= Zobrist::side;
366 si->key ^= Zobrist::castling[si->castlingRights];
368 for (Piece pc : Pieces)
369 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
370 si->materialKey ^= Zobrist::psq[pc][cnt];
374 /// Position::set() is an overload to initialize the position object with
375 /// the given endgame code string like "KBPKN". It is mainly a helper to
376 /// get the material key out of an endgame code.
378 Position& Position::set(const string& code, Color c, StateInfo* si) {
380 assert(code[0] == 'K');
382 string sides[] = { code.substr(code.find('K', 1)), // Weak
383 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
385 assert(sides[0].length() > 0 && sides[0].length() < 8);
386 assert(sides[1].length() > 0 && sides[1].length() < 8);
388 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
390 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
391 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
393 return set(fenStr, false, si, nullptr);
397 /// Position::fen() returns a FEN representation of the position. In case of
398 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
400 string Position::fen() const {
403 std::ostringstream ss;
405 for (Rank r = RANK_8; r >= RANK_1; --r)
407 for (File f = FILE_A; f <= FILE_H; ++f)
409 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
416 ss << PieceToChar[piece_on(make_square(f, r))];
423 ss << (sideToMove == WHITE ? " w " : " b ");
425 if (can_castle(WHITE_OO))
426 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
428 if (can_castle(WHITE_OOO))
429 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
431 if (can_castle(BLACK_OO))
432 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
434 if (can_castle(BLACK_OOO))
435 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
437 if (!can_castle(ANY_CASTLING))
440 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
441 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
447 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
448 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
449 /// slider if removing that piece from the board would result in a position where
450 /// square 's' is attacked. For example, a king-attack blocking piece can be either
451 /// a pinned or a discovered check piece, according if its color is the opposite
452 /// or the same of the color of the slider.
454 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
456 Bitboard blockers = 0;
459 // Snipers are sliders that attack 's' when a piece and other snipers are removed
460 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
461 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
462 Bitboard occupancy = pieces() ^ snipers;
466 Square sniperSq = pop_lsb(snipers);
467 Bitboard b = between_bb(s, sniperSq) & occupancy;
469 if (b && !more_than_one(b))
472 if (b & pieces(color_of(piece_on(s))))
480 /// Position::attackers_to() computes a bitboard of all pieces which attack a
481 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
483 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
485 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
486 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
487 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
488 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
489 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
490 | (attacks_bb<KING>(s) & pieces(KING));
494 /// Position::legal() tests whether a pseudo-legal move is legal
496 bool Position::legal(Move m) const {
500 Color us = sideToMove;
501 Square from = from_sq(m);
502 Square to = to_sq(m);
504 assert(color_of(moved_piece(m)) == us);
505 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
507 // En passant captures are a tricky special case. Because they are rather
508 // uncommon, we do it simply by testing whether the king is attacked after
510 if (type_of(m) == EN_PASSANT)
512 Square ksq = square<KING>(us);
513 Square capsq = to - pawn_push(us);
514 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
516 assert(to == ep_square());
517 assert(moved_piece(m) == make_piece(us, PAWN));
518 assert(piece_on(capsq) == make_piece(~us, PAWN));
519 assert(piece_on(to) == NO_PIECE);
521 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
522 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
525 // Castling moves generation does not check if the castling path is clear of
526 // enemy attacks, it is delayed at a later time: now!
527 if (type_of(m) == CASTLING)
529 // After castling, the rook and king final positions are the same in
530 // Chess960 as they would be in standard chess.
531 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
532 Direction step = to > from ? WEST : EAST;
534 for (Square s = to; s != from; s += step)
535 if (attackers_to(s) & pieces(~us))
538 // In case of Chess960, verify if the Rook blocks some checks
539 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
540 return !chess960 || !(blockers_for_king(us) & to_sq(m));
543 // If the moving piece is a king, check whether the destination square is
544 // attacked by the opponent.
545 if (type_of(piece_on(from)) == KING)
546 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
548 // A non-king move is legal if and only if it is not pinned or it
549 // is moving along the ray towards or away from the king.
550 return !(blockers_for_king(us) & from)
551 || aligned(from, to, square<KING>(us));
555 /// Position::pseudo_legal() takes a random move and tests whether the move is
556 /// pseudo legal. It is used to validate moves from TT that can be corrupted
557 /// due to SMP concurrent access or hash position key aliasing.
559 bool Position::pseudo_legal(const Move m) const {
561 Color us = sideToMove;
562 Square from = from_sq(m);
563 Square to = to_sq(m);
564 Piece pc = moved_piece(m);
566 // Use a slower but simpler function for uncommon cases
567 // yet we skip the legality check of MoveList<LEGAL>().
568 if (type_of(m) != NORMAL)
569 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
570 : MoveList<NON_EVASIONS>(*this).contains(m);
572 // Is not a promotion, so promotion piece must be empty
573 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
576 // If the 'from' square is not occupied by a piece belonging to the side to
577 // move, the move is obviously not legal.
578 if (pc == NO_PIECE || color_of(pc) != us)
581 // The destination square cannot be occupied by a friendly piece
585 // Handle the special case of a pawn move
586 if (type_of(pc) == PAWN)
588 // We have already handled promotion moves, so destination
589 // cannot be on the 8th/1st rank.
590 if ((Rank8BB | Rank1BB) & to)
593 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
594 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
595 && !( (from + 2 * pawn_push(us) == to) // Not a double push
596 && (relative_rank(us, from) == RANK_2)
598 && empty(to - pawn_push(us))))
601 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
604 // Evasions generator already takes care to avoid some kind of illegal moves
605 // and legal() relies on this. We therefore have to take care that the same
606 // kind of moves are filtered out here.
609 if (type_of(pc) != KING)
611 // Double check? In this case a king move is required
612 if (more_than_one(checkers()))
615 // Our move must be a blocking interposition or a capture of the checking piece
616 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
619 // In case of king moves under check we have to remove king so as to catch
620 // invalid moves like b1a1 when opposite queen is on c1.
621 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
629 /// Position::gives_check() tests whether a pseudo-legal move gives a check
631 bool Position::gives_check(Move m) const {
634 assert(color_of(moved_piece(m)) == sideToMove);
636 Square from = from_sq(m);
637 Square to = to_sq(m);
639 // Is there a direct check?
640 if (check_squares(type_of(piece_on(from))) & to)
643 // Is there a discovered check?
644 if ( (blockers_for_king(~sideToMove) & from)
645 && !aligned(from, to, square<KING>(~sideToMove)))
654 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
656 // En passant capture with check? We have already handled the case
657 // of direct checks and ordinary discovered check, so the only case we
658 // need to handle is the unusual case of a discovered check through
659 // the captured pawn.
662 Square capsq = make_square(file_of(to), rank_of(from));
663 Bitboard b = (pieces() ^ from ^ capsq) | to;
665 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
666 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
670 // Castling is encoded as 'king captures the rook'
671 Square ksq = square<KING>(~sideToMove);
672 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
674 return (attacks_bb<ROOK>(rto) & ksq)
675 && (attacks_bb<ROOK>(rto, pieces() ^ from ^ to) & ksq);
681 /// Position::do_move() makes a move, and saves all information necessary
682 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
683 /// moves should be filtered out before this function is called.
685 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
688 assert(&newSt != st);
690 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
691 Key k = st->key ^ Zobrist::side;
693 // Copy some fields of the old state to our new StateInfo object except the
694 // ones which are going to be recalculated from scratch anyway and then switch
695 // our state pointer to point to the new (ready to be updated) state.
696 std::memcpy(&newSt, st, offsetof(StateInfo, key));
700 // Increment ply counters. In particular, rule50 will be reset to zero later on
701 // in case of a capture or a pawn move.
707 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
708 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
709 auto& dp = st->dirtyPiece;
712 Color us = sideToMove;
714 Square from = from_sq(m);
715 Square to = to_sq(m);
716 Piece pc = piece_on(from);
717 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
719 assert(color_of(pc) == us);
720 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
721 assert(type_of(captured) != KING);
723 if (type_of(m) == CASTLING)
725 assert(pc == make_piece(us, KING));
726 assert(captured == make_piece(us, ROOK));
729 do_castling<true>(us, from, to, rfrom, rto);
731 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
739 // If the captured piece is a pawn, update pawn hash key, otherwise
740 // update non-pawn material.
741 if (type_of(captured) == PAWN)
743 if (type_of(m) == EN_PASSANT)
745 capsq -= pawn_push(us);
747 assert(pc == make_piece(us, PAWN));
748 assert(to == st->epSquare);
749 assert(relative_rank(us, to) == RANK_6);
750 assert(piece_on(to) == NO_PIECE);
751 assert(piece_on(capsq) == make_piece(them, PAWN));
754 st->pawnKey ^= Zobrist::psq[captured][capsq];
757 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
761 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
762 dp.piece[1] = captured;
767 // Update board and piece lists
770 if (type_of(m) == EN_PASSANT)
771 board[capsq] = NO_PIECE;
773 // Update material hash key and prefetch access to materialTable
774 k ^= Zobrist::psq[captured][capsq];
775 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
776 prefetch(thisThread->materialTable[st->materialKey]);
778 // Reset rule 50 counter
783 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
785 // Reset en passant square
786 if (st->epSquare != SQ_NONE)
788 k ^= Zobrist::enpassant[file_of(st->epSquare)];
789 st->epSquare = SQ_NONE;
792 // Update castling rights if needed
793 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
795 k ^= Zobrist::castling[st->castlingRights];
796 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
797 k ^= Zobrist::castling[st->castlingRights];
800 // Move the piece. The tricky Chess960 castling is handled earlier
801 if (type_of(m) != CASTLING)
810 move_piece(from, to);
813 // If the moving piece is a pawn do some special extra work
814 if (type_of(pc) == PAWN)
816 // Set en passant square if the moved pawn can be captured
817 if ( (int(to) ^ int(from)) == 16
818 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
820 st->epSquare = to - pawn_push(us);
821 k ^= Zobrist::enpassant[file_of(st->epSquare)];
824 else if (type_of(m) == PROMOTION)
826 Piece promotion = make_piece(us, promotion_type(m));
828 assert(relative_rank(us, to) == RANK_8);
829 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
832 put_piece(promotion, to);
836 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
838 dp.piece[dp.dirty_num] = promotion;
839 dp.from[dp.dirty_num] = SQ_NONE;
840 dp.to[dp.dirty_num] = to;
845 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
846 st->pawnKey ^= Zobrist::psq[pc][to];
847 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
848 ^ Zobrist::psq[pc][pieceCount[pc]];
851 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
854 // Update pawn hash key
855 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
857 // Reset rule 50 draw counter
862 st->capturedPiece = captured;
864 // Update the key with the final value
867 // Calculate checkers bitboard (if move gives check)
868 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
870 sideToMove = ~sideToMove;
872 // Update king attacks used for fast check detection
875 // Calculate the repetition info. It is the ply distance from the previous
876 // occurrence of the same position, negative in the 3-fold case, or zero
877 // if the position was not repeated.
879 int end = std::min(st->rule50, st->pliesFromNull);
882 StateInfo* stp = st->previous->previous;
883 for (int i = 4; i <= end; i += 2)
885 stp = stp->previous->previous;
886 if (stp->key == st->key)
888 st->repetition = stp->repetition ? -i : i;
898 /// Position::undo_move() unmakes a move. When it returns, the position should
899 /// be restored to exactly the same state as before the move was made.
901 void Position::undo_move(Move m) {
905 sideToMove = ~sideToMove;
907 Color us = sideToMove;
908 Square from = from_sq(m);
909 Square to = to_sq(m);
910 Piece pc = piece_on(to);
912 assert(empty(from) || type_of(m) == CASTLING);
913 assert(type_of(st->capturedPiece) != KING);
915 if (type_of(m) == PROMOTION)
917 assert(relative_rank(us, to) == RANK_8);
918 assert(type_of(pc) == promotion_type(m));
919 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
922 pc = make_piece(us, PAWN);
926 if (type_of(m) == CASTLING)
929 do_castling<false>(us, from, to, rfrom, rto);
933 move_piece(to, from); // Put the piece back at the source square
935 if (st->capturedPiece)
939 if (type_of(m) == EN_PASSANT)
941 capsq -= pawn_push(us);
943 assert(type_of(pc) == PAWN);
944 assert(to == st->previous->epSquare);
945 assert(relative_rank(us, to) == RANK_6);
946 assert(piece_on(capsq) == NO_PIECE);
947 assert(st->capturedPiece == make_piece(~us, PAWN));
950 put_piece(st->capturedPiece, capsq); // Restore the captured piece
954 // Finally point our state pointer back to the previous state
962 /// Position::do_castling() is a helper used to do/undo a castling move. This
963 /// is a bit tricky in Chess960 where from/to squares can overlap.
965 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
967 bool kingSide = to > from;
968 rfrom = to; // Castling is encoded as "king captures friendly rook"
969 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
970 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
972 if (Do && Eval::useNNUE)
974 auto& dp = st->dirtyPiece;
975 dp.piece[0] = make_piece(us, KING);
978 dp.piece[1] = make_piece(us, ROOK);
984 // Remove both pieces first since squares could overlap in Chess960
985 remove_piece(Do ? from : to);
986 remove_piece(Do ? rfrom : rto);
987 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
988 put_piece(make_piece(us, KING), Do ? to : from);
989 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
993 /// Position::do_null_move() is used to do a "null move": it flips
994 /// the side to move without executing any move on the board.
996 void Position::do_null_move(StateInfo& newSt) {
999 assert(&newSt != st);
1001 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1003 newSt.previous = st;
1006 st->dirtyPiece.dirty_num = 0;
1007 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1008 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
1009 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
1011 if (st->epSquare != SQ_NONE)
1013 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1014 st->epSquare = SQ_NONE;
1017 st->key ^= Zobrist::side;
1018 prefetch(TT.first_entry(key()));
1021 st->pliesFromNull = 0;
1023 sideToMove = ~sideToMove;
1029 assert(pos_is_ok());
1033 /// Position::undo_null_move() must be used to undo a "null move"
1035 void Position::undo_null_move() {
1037 assert(!checkers());
1040 sideToMove = ~sideToMove;
1044 /// Position::key_after() computes the new hash key after the given move. Needed
1045 /// for speculative prefetch. It doesn't recognize special moves like castling,
1046 /// en passant and promotions.
1048 Key Position::key_after(Move m) const {
1050 Square from = from_sq(m);
1051 Square to = to_sq(m);
1052 Piece pc = piece_on(from);
1053 Piece captured = piece_on(to);
1054 Key k = st->key ^ Zobrist::side;
1057 k ^= Zobrist::psq[captured][to];
1059 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1063 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1064 /// SEE value of move is greater or equal to the given threshold. We'll use an
1065 /// algorithm similar to alpha-beta pruning with a null window.
1067 bool Position::see_ge(Move m, Value threshold) const {
1071 // Only deal with normal moves, assume others pass a simple SEE
1072 if (type_of(m) != NORMAL)
1073 return VALUE_ZERO >= threshold;
1075 Square from = from_sq(m), to = to_sq(m);
1077 int swap = PieceValue[MG][piece_on(to)] - threshold;
1081 swap = PieceValue[MG][piece_on(from)] - swap;
1085 Bitboard occupied = pieces() ^ from ^ to;
1086 Color stm = color_of(piece_on(from));
1087 Bitboard attackers = attackers_to(to, occupied);
1088 Bitboard stmAttackers, bb;
1094 attackers &= occupied;
1096 // If stm has no more attackers then give up: stm loses
1097 if (!(stmAttackers = attackers & pieces(stm)))
1100 // Don't allow pinned pieces to attack as long as there are
1101 // pinners on their original square.
1102 if (pinners(~stm) & occupied)
1103 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 if ((swap = PawnValueMg - swap) < res)
1117 occupied ^= least_significant_square_bb(bb);
1118 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1121 else if ((bb = stmAttackers & pieces(KNIGHT)))
1123 if ((swap = KnightValueMg - swap) < res)
1126 occupied ^= least_significant_square_bb(bb);
1129 else if ((bb = stmAttackers & pieces(BISHOP)))
1131 if ((swap = BishopValueMg - swap) < res)
1134 occupied ^= least_significant_square_bb(bb);
1135 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1138 else if ((bb = stmAttackers & pieces(ROOK)))
1140 if ((swap = RookValueMg - swap) < res)
1143 occupied ^= least_significant_square_bb(bb);
1144 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1147 else if ((bb = stmAttackers & pieces(QUEEN)))
1149 if ((swap = QueenValueMg - swap) < res)
1152 occupied ^= least_significant_square_bb(bb);
1153 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1154 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1158 // If we "capture" with the king but opponent still has attackers,
1159 // reverse the result.
1160 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1167 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1168 /// or by repetition. It does not detect stalemates.
1170 bool Position::is_draw(int ply) const {
1172 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1175 // Return a draw score if a position repeats once earlier but strictly
1176 // after the root, or repeats twice before or at the root.
1177 return st->repetition && st->repetition < ply;
1181 // Position::has_repeated() tests whether there has been at least one repetition
1182 // of positions since the last capture or pawn move.
1184 bool Position::has_repeated() const {
1186 StateInfo* stc = st;
1187 int end = std::min(st->rule50, st->pliesFromNull);
1190 if (stc->repetition)
1193 stc = stc->previous;
1199 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1200 /// or an earlier position has a move that directly reaches the current position.
1202 bool Position::has_game_cycle(int ply) const {
1206 int end = std::min(st->rule50, st->pliesFromNull);
1211 Key originalKey = st->key;
1212 StateInfo* stp = st->previous;
1214 for (int i = 3; i <= end; i += 2)
1216 stp = stp->previous->previous;
1218 Key moveKey = originalKey ^ stp->key;
1219 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1220 || (j = H2(moveKey), cuckoo[j] == moveKey))
1222 Move move = cuckooMove[j];
1223 Square s1 = from_sq(move);
1224 Square s2 = to_sq(move);
1226 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1231 // For nodes before or at the root, check that the move is a
1232 // repetition rather than a move to the current position.
1233 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1234 // the same location, so we have to select which square to check.
1235 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1238 // For repetitions before or at the root, require one more
1239 if (stp->repetition)
1248 /// Position::flip() flips position with the white and black sides reversed. This
1249 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1251 void Position::flip() {
1254 std::stringstream ss(fen());
1256 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1258 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1259 f.insert(0, token + (f.empty() ? " " : "/"));
1262 ss >> token; // Active color
1263 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1265 ss >> token; // Castling availability
1268 std::transform(f.begin(), f.end(), f.begin(),
1269 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1271 ss >> token; // En passant square
1272 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1274 std::getline(ss, token); // Half and full moves
1277 set(f, is_chess960(), st, this_thread());
1279 assert(pos_is_ok());
1283 /// Position::pos_is_ok() performs some consistency checks for the
1284 /// position object and raises an asserts if something wrong is detected.
1285 /// This is meant to be helpful when debugging.
1287 bool Position::pos_is_ok() const {
1289 constexpr bool Fast = true; // Quick (default) or full check?
1291 if ( (sideToMove != WHITE && sideToMove != BLACK)
1292 || piece_on(square<KING>(WHITE)) != W_KING
1293 || piece_on(square<KING>(BLACK)) != B_KING
1294 || ( ep_square() != SQ_NONE
1295 && relative_rank(sideToMove, ep_square()) != RANK_6))
1296 assert(0 && "pos_is_ok: Default");
1301 if ( pieceCount[W_KING] != 1
1302 || pieceCount[B_KING] != 1
1303 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1304 assert(0 && "pos_is_ok: Kings");
1306 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1307 || pieceCount[W_PAWN] > 8
1308 || pieceCount[B_PAWN] > 8)
1309 assert(0 && "pos_is_ok: Pawns");
1311 if ( (pieces(WHITE) & pieces(BLACK))
1312 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1313 || popcount(pieces(WHITE)) > 16
1314 || popcount(pieces(BLACK)) > 16)
1315 assert(0 && "pos_is_ok: Bitboards");
1317 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1318 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1319 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1320 assert(0 && "pos_is_ok: Bitboards");
1323 ASSERT_ALIGNED(&si, Eval::NNUE::CacheLineSize);
1326 if (std::memcmp(&si, st, sizeof(StateInfo)))
1327 assert(0 && "pos_is_ok: State");
1329 for (Piece pc : Pieces)
1330 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1331 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1332 assert(0 && "pos_is_ok: Pieces");
1334 for (Color c : { WHITE, BLACK })
1335 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1337 if (!can_castle(cr))
1340 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1341 || castlingRightsMask[castlingRookSquare[cr]] != cr
1342 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1343 assert(0 && "pos_is_ok: Castling");
1349 } // namespace Stockfish