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;
291 /// Position::set_castling_right() is a helper function used to set castling
292 /// rights given the corresponding color and the rook starting square.
294 void Position::set_castling_right(Color c, Square rfrom) {
296 Square kfrom = square<KING>(c);
297 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
299 st->castlingRights |= cr;
300 castlingRightsMask[kfrom] |= cr;
301 castlingRightsMask[rfrom] |= cr;
302 castlingRookSquare[cr] = rfrom;
304 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
305 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
307 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto))
312 /// Position::set_check_info() sets king attacks to detect if a move gives check
314 void Position::set_check_info() const {
316 st->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), st->pinners[BLACK]);
317 st->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), st->pinners[WHITE]);
319 Square ksq = square<KING>(~sideToMove);
321 st->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
322 st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
323 st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
324 st->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
325 st->checkSquares[QUEEN] = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
326 st->checkSquares[KING] = 0;
330 /// Position::set_state() computes the hash keys of the position, and other
331 /// data that once computed is updated incrementally as moves are made.
332 /// The function is only used when a new position is set up
334 void Position::set_state() const {
336 st->key = st->materialKey = 0;
337 st->pawnKey = Zobrist::noPawns;
338 st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
339 st->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 st->key ^= Zobrist::psq[pc][s];
349 if (type_of(pc) == PAWN)
350 st->pawnKey ^= Zobrist::psq[pc][s];
352 else if (type_of(pc) != KING)
353 st->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
356 if (st->epSquare != SQ_NONE)
357 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
359 if (sideToMove == BLACK)
360 st->key ^= Zobrist::side;
362 st->key ^= Zobrist::castling[st->castlingRights];
364 for (Piece pc : Pieces)
365 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
366 st->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 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
571 // If the 'from' square is not occupied by a piece belonging to the side to
572 // move, the move is obviously not legal.
573 if (pc == NO_PIECE || color_of(pc) != us)
576 // The destination square cannot be occupied by a friendly piece
580 // Handle the special case of a pawn move
581 if (type_of(pc) == PAWN)
583 // We have already handled promotion moves, so destination
584 // cannot be on the 8th/1st rank.
585 if ((Rank8BB | Rank1BB) & to)
588 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
589 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
590 && !( (from + 2 * pawn_push(us) == to) // Not a double push
591 && (relative_rank(us, from) == RANK_2)
593 && empty(to - pawn_push(us))))
596 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
599 // Evasions generator already takes care to avoid some kind of illegal moves
600 // and legal() relies on this. We therefore have to take care that the same
601 // kind of moves are filtered out here.
604 if (type_of(pc) != KING)
606 // Double check? In this case a king move is required
607 if (more_than_one(checkers()))
610 // Our move must be a blocking interposition or a capture of the checking piece
611 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
614 // In case of king moves under check we have to remove king so as to catch
615 // invalid moves like b1a1 when opposite queen is on c1.
616 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
624 /// Position::gives_check() tests whether a pseudo-legal move gives a check
626 bool Position::gives_check(Move m) const {
629 assert(color_of(moved_piece(m)) == sideToMove);
631 Square from = from_sq(m);
632 Square to = to_sq(m);
634 // Is there a direct check?
635 if (check_squares(type_of(piece_on(from))) & to)
638 // Is there a discovered check?
639 if (blockers_for_king(~sideToMove) & from)
640 return !aligned(from, to, square<KING>(~sideToMove))
641 || type_of(m) == CASTLING;
649 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
651 // En passant capture with check? We have already handled the case
652 // of direct checks and ordinary discovered check, so the only case we
653 // need to handle is the unusual case of a discovered check through
654 // the captured pawn.
657 Square capsq = make_square(file_of(to), rank_of(from));
658 Bitboard b = (pieces() ^ from ^ capsq) | to;
660 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
661 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
665 // Castling is encoded as 'king captures the rook'
666 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
668 return check_squares(ROOK) & rto;
674 /// Position::do_move() makes a move, and saves all information necessary
675 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
676 /// moves should be filtered out before this function is called.
678 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
681 assert(&newSt != st);
683 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
684 Key k = st->key ^ Zobrist::side;
686 // Copy some fields of the old state to our new StateInfo object except the
687 // ones which are going to be recalculated from scratch anyway and then switch
688 // our state pointer to point to the new (ready to be updated) state.
689 std::memcpy(&newSt, st, offsetof(StateInfo, key));
693 // Increment ply counters. In particular, rule50 will be reset to zero later on
694 // in case of a capture or a pawn move.
700 st->accumulator.computed[WHITE] = false;
701 st->accumulator.computed[BLACK] = false;
702 auto& dp = st->dirtyPiece;
705 Color us = sideToMove;
707 Square from = from_sq(m);
708 Square to = to_sq(m);
709 Piece pc = piece_on(from);
710 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
712 assert(color_of(pc) == us);
713 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
714 assert(type_of(captured) != KING);
716 if (type_of(m) == CASTLING)
718 assert(pc == make_piece(us, KING));
719 assert(captured == make_piece(us, ROOK));
722 do_castling<true>(us, from, to, rfrom, rto);
724 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
732 // If the captured piece is a pawn, update pawn hash key, otherwise
733 // update non-pawn material.
734 if (type_of(captured) == PAWN)
736 if (type_of(m) == EN_PASSANT)
738 capsq -= pawn_push(us);
740 assert(pc == make_piece(us, PAWN));
741 assert(to == st->epSquare);
742 assert(relative_rank(us, to) == RANK_6);
743 assert(piece_on(to) == NO_PIECE);
744 assert(piece_on(capsq) == make_piece(them, PAWN));
747 st->pawnKey ^= Zobrist::psq[captured][capsq];
750 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
752 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
753 dp.piece[1] = captured;
757 // Update board and piece lists
760 // Update material hash key and prefetch access to materialTable
761 k ^= Zobrist::psq[captured][capsq];
762 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
764 // Reset rule 50 counter
769 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
771 // Reset en passant square
772 if (st->epSquare != SQ_NONE)
774 k ^= Zobrist::enpassant[file_of(st->epSquare)];
775 st->epSquare = SQ_NONE;
778 // Update castling rights if needed
779 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
781 k ^= Zobrist::castling[st->castlingRights];
782 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
783 k ^= Zobrist::castling[st->castlingRights];
786 // Move the piece. The tricky Chess960 castling is handled earlier
787 if (type_of(m) != CASTLING)
793 move_piece(from, to);
796 // If the moving piece is a pawn do some special extra work
797 if (type_of(pc) == PAWN)
799 // Set en passant square if the moved pawn can be captured
800 if ( (int(to) ^ int(from)) == 16
801 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
803 st->epSquare = to - pawn_push(us);
804 k ^= Zobrist::enpassant[file_of(st->epSquare)];
807 else if (type_of(m) == PROMOTION)
809 Piece promotion = make_piece(us, promotion_type(m));
811 assert(relative_rank(us, to) == RANK_8);
812 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
815 put_piece(promotion, to);
817 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
819 dp.piece[dp.dirty_num] = promotion;
820 dp.from[dp.dirty_num] = SQ_NONE;
821 dp.to[dp.dirty_num] = to;
825 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
826 st->pawnKey ^= Zobrist::psq[pc][to];
827 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
828 ^ Zobrist::psq[pc][pieceCount[pc]];
831 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
834 // Update pawn hash key
835 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
837 // Reset rule 50 draw counter
842 st->capturedPiece = captured;
844 // Update the key with the final value
847 // Calculate checkers bitboard (if move gives check)
848 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
850 sideToMove = ~sideToMove;
852 // Update king attacks used for fast check detection
855 // Calculate the repetition info. It is the ply distance from the previous
856 // occurrence of the same position, negative in the 3-fold case, or zero
857 // if the position was not repeated.
859 int end = std::min(st->rule50, st->pliesFromNull);
862 StateInfo* stp = st->previous->previous;
863 for (int i = 4; i <= end; i += 2)
865 stp = stp->previous->previous;
866 if (stp->key == st->key)
868 st->repetition = stp->repetition ? -i : i;
878 /// Position::undo_move() unmakes a move. When it returns, the position should
879 /// be restored to exactly the same state as before the move was made.
881 void Position::undo_move(Move m) {
885 sideToMove = ~sideToMove;
887 Color us = sideToMove;
888 Square from = from_sq(m);
889 Square to = to_sq(m);
890 Piece pc = piece_on(to);
892 assert(empty(from) || type_of(m) == CASTLING);
893 assert(type_of(st->capturedPiece) != KING);
895 if (type_of(m) == PROMOTION)
897 assert(relative_rank(us, to) == RANK_8);
898 assert(type_of(pc) == promotion_type(m));
899 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
902 pc = make_piece(us, PAWN);
906 if (type_of(m) == CASTLING)
909 do_castling<false>(us, from, to, rfrom, rto);
913 move_piece(to, from); // Put the piece back at the source square
915 if (st->capturedPiece)
919 if (type_of(m) == EN_PASSANT)
921 capsq -= pawn_push(us);
923 assert(type_of(pc) == PAWN);
924 assert(to == st->previous->epSquare);
925 assert(relative_rank(us, to) == RANK_6);
926 assert(piece_on(capsq) == NO_PIECE);
927 assert(st->capturedPiece == make_piece(~us, PAWN));
930 put_piece(st->capturedPiece, capsq); // Restore the captured piece
934 // Finally point our state pointer back to the previous state
942 /// Position::do_castling() is a helper used to do/undo a castling move. This
943 /// is a bit tricky in Chess960 where from/to squares can overlap.
945 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
947 bool kingSide = to > from;
948 rfrom = to; // Castling is encoded as "king captures friendly rook"
949 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
950 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
954 auto& dp = st->dirtyPiece;
955 dp.piece[0] = make_piece(us, KING);
958 dp.piece[1] = make_piece(us, ROOK);
964 // Remove both pieces first since squares could overlap in Chess960
965 remove_piece(Do ? from : to);
966 remove_piece(Do ? rfrom : rto);
967 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
968 put_piece(make_piece(us, KING), Do ? to : from);
969 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
973 /// Position::do_null_move() is used to do a "null move": it flips
974 /// the side to move without executing any move on the board.
976 void Position::do_null_move(StateInfo& newSt) {
979 assert(&newSt != st);
981 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
986 st->dirtyPiece.dirty_num = 0;
987 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
988 st->accumulator.computed[WHITE] = false;
989 st->accumulator.computed[BLACK] = false;
991 if (st->epSquare != SQ_NONE)
993 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
994 st->epSquare = SQ_NONE;
997 st->key ^= Zobrist::side;
999 prefetch(TT.first_entry(key()));
1001 st->pliesFromNull = 0;
1003 sideToMove = ~sideToMove;
1009 assert(pos_is_ok());
1013 /// Position::undo_null_move() must be used to undo a "null move"
1015 void Position::undo_null_move() {
1017 assert(!checkers());
1020 sideToMove = ~sideToMove;
1024 /// Position::key_after() computes the new hash key after the given move. Needed
1025 /// for speculative prefetch. It doesn't recognize special moves like castling,
1026 /// en passant and promotions.
1028 Key Position::key_after(Move m) const {
1030 Square from = from_sq(m);
1031 Square to = to_sq(m);
1032 Piece pc = piece_on(from);
1033 Piece captured = piece_on(to);
1034 Key k = st->key ^ Zobrist::side;
1037 k ^= Zobrist::psq[captured][to];
1039 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1041 return (captured || type_of(pc) == PAWN)
1042 ? k : adjust_key50<true>(k);
1046 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1047 /// SEE value of move is greater or equal to the given threshold. We'll use an
1048 /// algorithm similar to alpha-beta pruning with a null window.
1050 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1054 // Only deal with normal moves, assume others pass a simple SEE
1055 if (type_of(m) != NORMAL)
1056 return VALUE_ZERO >= threshold;
1058 Square from = from_sq(m), to = to_sq(m);
1060 int swap = PieceValue[MG][piece_on(to)] - threshold;
1064 swap = PieceValue[MG][piece_on(from)] - swap;
1068 assert(color_of(piece_on(from)) == sideToMove);
1069 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1070 Color stm = sideToMove;
1071 Bitboard attackers = attackers_to(to, occupied);
1072 Bitboard stmAttackers, bb;
1078 attackers &= occupied;
1080 // If stm has no more attackers then give up: stm loses
1081 if (!(stmAttackers = attackers & pieces(stm)))
1084 // Don't allow pinned pieces to attack as long as there are
1085 // pinners on their original square.
1086 if (pinners(~stm) & occupied)
1088 stmAttackers &= ~blockers_for_king(stm);
1096 // Locate and remove the next least valuable attacker, and add to
1097 // the bitboard 'attackers' any X-ray attackers behind it.
1098 if ((bb = stmAttackers & pieces(PAWN)))
1100 occupied ^= least_significant_square_bb(bb);
1101 if ((swap = PawnValueMg - swap) < res)
1104 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1107 else if ((bb = stmAttackers & pieces(KNIGHT)))
1109 occupied ^= least_significant_square_bb(bb);
1110 if ((swap = KnightValueMg - swap) < res)
1114 else if ((bb = stmAttackers & pieces(BISHOP)))
1116 occupied ^= least_significant_square_bb(bb);
1117 if ((swap = BishopValueMg - swap) < res)
1120 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1123 else if ((bb = stmAttackers & pieces(ROOK)))
1125 occupied ^= least_significant_square_bb(bb);
1126 if ((swap = RookValueMg - swap) < res)
1129 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1132 else if ((bb = stmAttackers & pieces(QUEEN)))
1134 occupied ^= least_significant_square_bb(bb);
1135 if ((swap = QueenValueMg - swap) < res)
1138 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1139 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1143 // If we "capture" with the king but opponent still has attackers,
1144 // reverse the result.
1145 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1151 bool Position::see_ge(Move m, Value threshold) const {
1153 return see_ge(m, occupied, threshold);
1157 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1158 /// or by repetition. It does not detect stalemates.
1160 bool Position::is_draw(int ply) const {
1162 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1165 // Return a draw score if a position repeats once earlier but strictly
1166 // after the root, or repeats twice before or at the root.
1167 return st->repetition && st->repetition < ply;
1171 // Position::has_repeated() tests whether there has been at least one repetition
1172 // of positions since the last capture or pawn move.
1174 bool Position::has_repeated() const {
1176 StateInfo* stc = st;
1177 int end = std::min(st->rule50, st->pliesFromNull);
1180 if (stc->repetition)
1183 stc = stc->previous;
1189 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1190 /// or an earlier position has a move that directly reaches the current position.
1192 bool Position::has_game_cycle(int ply) const {
1196 int end = std::min(st->rule50, st->pliesFromNull);
1201 Key originalKey = st->key;
1202 StateInfo* stp = st->previous;
1204 for (int i = 3; i <= end; i += 2)
1206 stp = stp->previous->previous;
1208 Key moveKey = originalKey ^ stp->key;
1209 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1210 || (j = H2(moveKey), cuckoo[j] == moveKey))
1212 Move move = cuckooMove[j];
1213 Square s1 = from_sq(move);
1214 Square s2 = to_sq(move);
1216 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1221 // For nodes before or at the root, check that the move is a
1222 // repetition rather than a move to the current position.
1223 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1224 // the same location, so we have to select which square to check.
1225 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1228 // For repetitions before or at the root, require one more
1229 if (stp->repetition)
1238 /// Position::flip() flips position with the white and black sides reversed. This
1239 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1241 void Position::flip() {
1244 std::stringstream ss(fen());
1246 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1248 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1249 f.insert(0, token + (f.empty() ? " " : "/"));
1252 ss >> token; // Active color
1253 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1255 ss >> token; // Castling availability
1258 std::transform(f.begin(), f.end(), f.begin(),
1259 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1261 ss >> token; // En passant square
1262 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1264 std::getline(ss, token); // Half and full moves
1267 set(f, is_chess960(), st, this_thread());
1269 assert(pos_is_ok());
1273 /// Position::pos_is_ok() performs some consistency checks for the
1274 /// position object and raises an asserts if something wrong is detected.
1275 /// This is meant to be helpful when debugging.
1277 bool Position::pos_is_ok() const {
1279 constexpr bool Fast = true; // Quick (default) or full check?
1281 if ( (sideToMove != WHITE && sideToMove != BLACK)
1282 || piece_on(square<KING>(WHITE)) != W_KING
1283 || piece_on(square<KING>(BLACK)) != B_KING
1284 || ( ep_square() != SQ_NONE
1285 && relative_rank(sideToMove, ep_square()) != RANK_6))
1286 assert(0 && "pos_is_ok: Default");
1291 if ( pieceCount[W_KING] != 1
1292 || pieceCount[B_KING] != 1
1293 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1294 assert(0 && "pos_is_ok: Kings");
1296 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1297 || pieceCount[W_PAWN] > 8
1298 || pieceCount[B_PAWN] > 8)
1299 assert(0 && "pos_is_ok: Pawns");
1301 if ( (pieces(WHITE) & pieces(BLACK))
1302 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1303 || popcount(pieces(WHITE)) > 16
1304 || popcount(pieces(BLACK)) > 16)
1305 assert(0 && "pos_is_ok: Bitboards");
1307 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1308 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1309 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1310 assert(0 && "pos_is_ok: Bitboards");
1313 for (Piece pc : Pieces)
1314 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1315 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1316 assert(0 && "pos_is_ok: Pieces");
1318 for (Color c : { WHITE, BLACK })
1319 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1321 if (!can_castle(cr))
1324 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1325 || castlingRightsMask[castlingRookSquare[cr]] != cr
1326 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1327 assert(0 && "pos_is_ok: Castling");
1333 } // namespace Stockfish