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->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
340 st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
344 for (Bitboard b = pieces(); b; )
346 Square s = pop_lsb(b);
347 Piece pc = piece_on(s);
348 st->key ^= Zobrist::psq[pc][s];
350 if (type_of(pc) != KING && type_of(pc) != PAWN)
351 st->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
354 if (st->epSquare != SQ_NONE)
355 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
357 if (sideToMove == BLACK)
358 st->key ^= Zobrist::side;
360 st->key ^= Zobrist::castling[st->castlingRights];
362 for (Piece pc : Pieces)
363 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
364 st->materialKey ^= Zobrist::psq[pc][cnt];
368 /// Position::set() is an overload to initialize the position object with
369 /// the given endgame code string like "KBPKN". It is mainly a helper to
370 /// get the material key out of an endgame code.
372 Position& Position::set(const string& code, Color c, StateInfo* si) {
374 assert(code[0] == 'K');
376 string sides[] = { code.substr(code.find('K', 1)), // Weak
377 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
379 assert(sides[0].length() > 0 && sides[0].length() < 8);
380 assert(sides[1].length() > 0 && sides[1].length() < 8);
382 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
384 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
385 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
387 return set(fenStr, false, si, nullptr);
391 /// Position::fen() returns a FEN representation of the position. In case of
392 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
394 string Position::fen() const {
397 std::ostringstream ss;
399 for (Rank r = RANK_8; r >= RANK_1; --r)
401 for (File f = FILE_A; f <= FILE_H; ++f)
403 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
410 ss << PieceToChar[piece_on(make_square(f, r))];
417 ss << (sideToMove == WHITE ? " w " : " b ");
419 if (can_castle(WHITE_OO))
420 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
422 if (can_castle(WHITE_OOO))
423 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
425 if (can_castle(BLACK_OO))
426 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
428 if (can_castle(BLACK_OOO))
429 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
431 if (!can_castle(ANY_CASTLING))
434 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
435 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
441 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
442 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
443 /// slider if removing that piece from the board would result in a position where
444 /// square 's' is attacked. For example, a king-attack blocking piece can be either
445 /// a pinned or a discovered check piece, according if its color is the opposite
446 /// or the same of the color of the slider.
448 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
450 Bitboard blockers = 0;
453 // Snipers are sliders that attack 's' when a piece and other snipers are removed
454 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
455 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
456 Bitboard occupancy = pieces() ^ snipers;
460 Square sniperSq = pop_lsb(snipers);
461 Bitboard b = between_bb(s, sniperSq) & occupancy;
463 if (b && !more_than_one(b))
466 if (b & pieces(color_of(piece_on(s))))
474 /// Position::attackers_to() computes a bitboard of all pieces which attack a
475 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
477 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
479 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
480 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
481 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
482 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
483 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
484 | (attacks_bb<KING>(s) & pieces(KING));
488 /// Position::legal() tests whether a pseudo-legal move is legal
490 bool Position::legal(Move m) const {
494 Color us = sideToMove;
495 Square from = from_sq(m);
496 Square to = to_sq(m);
498 assert(color_of(moved_piece(m)) == us);
499 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
501 // En passant captures are a tricky special case. Because they are rather
502 // uncommon, we do it simply by testing whether the king is attacked after
504 if (type_of(m) == EN_PASSANT)
506 Square ksq = square<KING>(us);
507 Square capsq = to - pawn_push(us);
508 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
510 assert(to == ep_square());
511 assert(moved_piece(m) == make_piece(us, PAWN));
512 assert(piece_on(capsq) == make_piece(~us, PAWN));
513 assert(piece_on(to) == NO_PIECE);
515 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
516 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
519 // Castling moves generation does not check if the castling path is clear of
520 // enemy attacks, it is delayed at a later time: now!
521 if (type_of(m) == CASTLING)
523 // After castling, the rook and king final positions are the same in
524 // Chess960 as they would be in standard chess.
525 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
526 Direction step = to > from ? WEST : EAST;
528 for (Square s = to; s != from; s += step)
529 if (attackers_to(s) & pieces(~us))
532 // In case of Chess960, verify if the Rook blocks some checks
533 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
534 return !chess960 || !(blockers_for_king(us) & to_sq(m));
537 // If the moving piece is a king, check whether the destination square is
538 // attacked by the opponent.
539 if (type_of(piece_on(from)) == KING)
540 return !(attackers_to(to, pieces() ^ from) & pieces(~us));
542 // A non-king move is legal if and only if it is not pinned or it
543 // is moving along the ray towards or away from the king.
544 return !(blockers_for_king(us) & from)
545 || aligned(from, to, square<KING>(us));
549 /// Position::pseudo_legal() takes a random move and tests whether the move is
550 /// pseudo legal. It is used to validate moves from TT that can be corrupted
551 /// due to SMP concurrent access or hash position key aliasing.
553 bool Position::pseudo_legal(const Move m) const {
555 Color us = sideToMove;
556 Square from = from_sq(m);
557 Square to = to_sq(m);
558 Piece pc = moved_piece(m);
560 // Use a slower but simpler function for uncommon cases
561 // yet we skip the legality check of MoveList<LEGAL>().
562 if (type_of(m) != NORMAL)
563 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
564 : MoveList<NON_EVASIONS>(*this).contains(m);
566 // Is not a promotion, so promotion piece must be empty
567 assert(promotion_type(m) - KNIGHT == NO_PIECE_TYPE);
569 // If the 'from' square is not occupied by a piece belonging to the side to
570 // move, the move is obviously not legal.
571 if (pc == NO_PIECE || color_of(pc) != us)
574 // The destination square cannot be occupied by a friendly piece
578 // Handle the special case of a pawn move
579 if (type_of(pc) == PAWN)
581 // We have already handled promotion moves, so destination
582 // cannot be on the 8th/1st rank.
583 if ((Rank8BB | Rank1BB) & to)
586 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
587 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
588 && !( (from + 2 * pawn_push(us) == to) // Not a double push
589 && (relative_rank(us, from) == RANK_2)
591 && empty(to - pawn_push(us))))
594 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
597 // Evasions generator already takes care to avoid some kind of illegal moves
598 // and legal() relies on this. We therefore have to take care that the same
599 // kind of moves are filtered out here.
602 if (type_of(pc) != KING)
604 // Double check? In this case a king move is required
605 if (more_than_one(checkers()))
608 // Our move must be a blocking interposition or a capture of the checking piece
609 if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
612 // In case of king moves under check we have to remove king so as to catch
613 // invalid moves like b1a1 when opposite queen is on c1.
614 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
622 /// Position::gives_check() tests whether a pseudo-legal move gives a check
624 bool Position::gives_check(Move m) const {
627 assert(color_of(moved_piece(m)) == sideToMove);
629 Square from = from_sq(m);
630 Square to = to_sq(m);
632 // Is there a direct check?
633 if (check_squares(type_of(piece_on(from))) & to)
636 // Is there a discovered check?
637 if (blockers_for_king(~sideToMove) & from)
638 return !aligned(from, to, square<KING>(~sideToMove))
639 || type_of(m) == CASTLING;
647 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
649 // En passant capture with check? We have already handled the case
650 // of direct checks and ordinary discovered check, so the only case we
651 // need to handle is the unusual case of a discovered check through
652 // the captured pawn.
655 Square capsq = make_square(file_of(to), rank_of(from));
656 Bitboard b = (pieces() ^ from ^ capsq) | to;
658 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
659 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
663 // Castling is encoded as 'king captures the rook'
664 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
666 return check_squares(ROOK) & rto;
672 /// Position::do_move() makes a move, and saves all information necessary
673 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
674 /// moves should be filtered out before this function is called.
676 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
679 assert(&newSt != st);
681 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
682 Key k = st->key ^ Zobrist::side;
684 // Copy some fields of the old state to our new StateInfo object except the
685 // ones which are going to be recalculated from scratch anyway and then switch
686 // our state pointer to point to the new (ready to be updated) state.
687 std::memcpy(&newSt, st, offsetof(StateInfo, key));
691 // Increment ply counters. In particular, rule50 will be reset to zero later on
692 // in case of a capture or a pawn move.
698 st->accumulator.computed[WHITE] = false;
699 st->accumulator.computed[BLACK] = false;
700 auto& dp = st->dirtyPiece;
703 Color us = sideToMove;
705 Square from = from_sq(m);
706 Square to = to_sq(m);
707 Piece pc = piece_on(from);
708 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
710 assert(color_of(pc) == us);
711 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
712 assert(type_of(captured) != KING);
714 if (type_of(m) == CASTLING)
716 assert(pc == make_piece(us, KING));
717 assert(captured == make_piece(us, ROOK));
720 do_castling<true>(us, from, to, rfrom, rto);
722 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
730 // If the captured piece is a pawn, update pawn hash key, otherwise
731 // update non-pawn material.
732 if (type_of(captured) == PAWN)
734 if (type_of(m) == EN_PASSANT)
736 capsq -= pawn_push(us);
738 assert(pc == make_piece(us, PAWN));
739 assert(to == st->epSquare);
740 assert(relative_rank(us, to) == RANK_6);
741 assert(piece_on(to) == NO_PIECE);
742 assert(piece_on(capsq) == make_piece(them, PAWN));
746 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
748 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
749 dp.piece[1] = captured;
753 // Update board and piece lists
756 // Update material hash key and prefetch access to materialTable
757 k ^= Zobrist::psq[captured][capsq];
758 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
760 // Reset rule 50 counter
765 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
767 // Reset en passant square
768 if (st->epSquare != SQ_NONE)
770 k ^= Zobrist::enpassant[file_of(st->epSquare)];
771 st->epSquare = SQ_NONE;
774 // Update castling rights if needed
775 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
777 k ^= Zobrist::castling[st->castlingRights];
778 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
779 k ^= Zobrist::castling[st->castlingRights];
782 // Move the piece. The tricky Chess960 castling is handled earlier
783 if (type_of(m) != CASTLING)
789 move_piece(from, to);
792 // If the moving piece is a pawn do some special extra work
793 if (type_of(pc) == PAWN)
795 // Set en passant square if the moved pawn can be captured
796 if ( (int(to) ^ int(from)) == 16
797 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
799 st->epSquare = to - pawn_push(us);
800 k ^= Zobrist::enpassant[file_of(st->epSquare)];
803 else if (type_of(m) == PROMOTION)
805 Piece promotion = make_piece(us, promotion_type(m));
807 assert(relative_rank(us, to) == RANK_8);
808 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
811 put_piece(promotion, to);
813 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
815 dp.piece[dp.dirty_num] = promotion;
816 dp.from[dp.dirty_num] = SQ_NONE;
817 dp.to[dp.dirty_num] = to;
821 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
822 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
823 ^ Zobrist::psq[pc][pieceCount[pc]];
826 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
829 // Reset rule 50 draw counter
834 st->capturedPiece = captured;
836 // Update the key with the final value
839 // Calculate checkers bitboard (if move gives check)
840 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
842 sideToMove = ~sideToMove;
844 // Update king attacks used for fast check detection
847 // Calculate the repetition info. It is the ply distance from the previous
848 // occurrence of the same position, negative in the 3-fold case, or zero
849 // if the position was not repeated.
851 int end = std::min(st->rule50, st->pliesFromNull);
854 StateInfo* stp = st->previous->previous;
855 for (int i = 4; i <= end; i += 2)
857 stp = stp->previous->previous;
858 if (stp->key == st->key)
860 st->repetition = stp->repetition ? -i : i;
870 /// Position::undo_move() unmakes a move. When it returns, the position should
871 /// be restored to exactly the same state as before the move was made.
873 void Position::undo_move(Move m) {
877 sideToMove = ~sideToMove;
879 Color us = sideToMove;
880 Square from = from_sq(m);
881 Square to = to_sq(m);
882 Piece pc = piece_on(to);
884 assert(empty(from) || type_of(m) == CASTLING);
885 assert(type_of(st->capturedPiece) != KING);
887 if (type_of(m) == PROMOTION)
889 assert(relative_rank(us, to) == RANK_8);
890 assert(type_of(pc) == promotion_type(m));
891 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
894 pc = make_piece(us, PAWN);
898 if (type_of(m) == CASTLING)
901 do_castling<false>(us, from, to, rfrom, rto);
905 move_piece(to, from); // Put the piece back at the source square
907 if (st->capturedPiece)
911 if (type_of(m) == EN_PASSANT)
913 capsq -= pawn_push(us);
915 assert(type_of(pc) == PAWN);
916 assert(to == st->previous->epSquare);
917 assert(relative_rank(us, to) == RANK_6);
918 assert(piece_on(capsq) == NO_PIECE);
919 assert(st->capturedPiece == make_piece(~us, PAWN));
922 put_piece(st->capturedPiece, capsq); // Restore the captured piece
926 // Finally point our state pointer back to the previous state
934 /// Position::do_castling() is a helper used to do/undo a castling move. This
935 /// is a bit tricky in Chess960 where from/to squares can overlap.
937 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
939 bool kingSide = to > from;
940 rfrom = to; // Castling is encoded as "king captures friendly rook"
941 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
942 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
946 auto& dp = st->dirtyPiece;
947 dp.piece[0] = make_piece(us, KING);
950 dp.piece[1] = make_piece(us, ROOK);
956 // Remove both pieces first since squares could overlap in Chess960
957 remove_piece(Do ? from : to);
958 remove_piece(Do ? rfrom : rto);
959 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
960 put_piece(make_piece(us, KING), Do ? to : from);
961 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
965 /// Position::do_null_move() is used to do a "null move": it flips
966 /// the side to move without executing any move on the board.
968 void Position::do_null_move(StateInfo& newSt) {
971 assert(&newSt != st);
973 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
978 st->dirtyPiece.dirty_num = 0;
979 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
980 st->accumulator.computed[WHITE] = false;
981 st->accumulator.computed[BLACK] = false;
983 if (st->epSquare != SQ_NONE)
985 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
986 st->epSquare = SQ_NONE;
989 st->key ^= Zobrist::side;
991 prefetch(TT.first_entry(key()));
993 st->pliesFromNull = 0;
995 sideToMove = ~sideToMove;
1001 assert(pos_is_ok());
1005 /// Position::undo_null_move() must be used to undo a "null move"
1007 void Position::undo_null_move() {
1009 assert(!checkers());
1012 sideToMove = ~sideToMove;
1016 /// Position::key_after() computes the new hash key after the given move. Needed
1017 /// for speculative prefetch. It doesn't recognize special moves like castling,
1018 /// en passant and promotions.
1020 Key Position::key_after(Move m) const {
1022 Square from = from_sq(m);
1023 Square to = to_sq(m);
1024 Piece pc = piece_on(from);
1025 Piece captured = piece_on(to);
1026 Key k = st->key ^ Zobrist::side;
1029 k ^= Zobrist::psq[captured][to];
1031 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1033 return (captured || type_of(pc) == PAWN)
1034 ? k : adjust_key50<true>(k);
1038 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1039 /// SEE value of move is greater or equal to the given threshold. We'll use an
1040 /// algorithm similar to alpha-beta pruning with a null window.
1042 bool Position::see_ge(Move m, Bitboard& occupied, Value threshold) const {
1046 // Only deal with normal moves, assume others pass a simple SEE
1047 if (type_of(m) != NORMAL)
1048 return VALUE_ZERO >= threshold;
1050 Square from = from_sq(m), to = to_sq(m);
1052 int swap = PieceValue[MG][piece_on(to)] - threshold;
1056 swap = PieceValue[MG][piece_on(from)] - swap;
1060 assert(color_of(piece_on(from)) == sideToMove);
1061 occupied = pieces() ^ from ^ to; // xoring to is important for pinned piece logic
1062 Color stm = sideToMove;
1063 Bitboard attackers = attackers_to(to, occupied);
1064 Bitboard stmAttackers, bb;
1070 attackers &= occupied;
1072 // If stm has no more attackers then give up: stm loses
1073 if (!(stmAttackers = attackers & pieces(stm)))
1076 // Don't allow pinned pieces to attack as long as there are
1077 // pinners on their original square.
1078 if (pinners(~stm) & occupied)
1080 stmAttackers &= ~blockers_for_king(stm);
1088 // Locate and remove the next least valuable attacker, and add to
1089 // the bitboard 'attackers' any X-ray attackers behind it.
1090 if ((bb = stmAttackers & pieces(PAWN)))
1092 occupied ^= least_significant_square_bb(bb);
1093 if ((swap = PawnValueMg - swap) < res)
1096 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1099 else if ((bb = stmAttackers & pieces(KNIGHT)))
1101 occupied ^= least_significant_square_bb(bb);
1102 if ((swap = KnightValueMg - swap) < res)
1106 else if ((bb = stmAttackers & pieces(BISHOP)))
1108 occupied ^= least_significant_square_bb(bb);
1109 if ((swap = BishopValueMg - swap) < res)
1112 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1115 else if ((bb = stmAttackers & pieces(ROOK)))
1117 occupied ^= least_significant_square_bb(bb);
1118 if ((swap = RookValueMg - swap) < res)
1121 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1124 else if ((bb = stmAttackers & pieces(QUEEN)))
1126 occupied ^= least_significant_square_bb(bb);
1127 if ((swap = QueenValueMg - swap) < res)
1130 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1131 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1135 // If we "capture" with the king but opponent still has attackers,
1136 // reverse the result.
1137 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1143 bool Position::see_ge(Move m, Value threshold) const {
1145 return see_ge(m, occupied, threshold);
1149 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1150 /// or by repetition. It does not detect stalemates.
1152 bool Position::is_draw(int ply) const {
1154 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1157 // Return a draw score if a position repeats once earlier but strictly
1158 // after the root, or repeats twice before or at the root.
1159 return st->repetition && st->repetition < ply;
1163 // Position::has_repeated() tests whether there has been at least one repetition
1164 // of positions since the last capture or pawn move.
1166 bool Position::has_repeated() const {
1168 StateInfo* stc = st;
1169 int end = std::min(st->rule50, st->pliesFromNull);
1172 if (stc->repetition)
1175 stc = stc->previous;
1181 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1182 /// or an earlier position has a move that directly reaches the current position.
1184 bool Position::has_game_cycle(int ply) const {
1188 int end = std::min(st->rule50, st->pliesFromNull);
1193 Key originalKey = st->key;
1194 StateInfo* stp = st->previous;
1196 for (int i = 3; i <= end; i += 2)
1198 stp = stp->previous->previous;
1200 Key moveKey = originalKey ^ stp->key;
1201 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1202 || (j = H2(moveKey), cuckoo[j] == moveKey))
1204 Move move = cuckooMove[j];
1205 Square s1 = from_sq(move);
1206 Square s2 = to_sq(move);
1208 if (!((between_bb(s1, s2) ^ s2) & pieces()))
1213 // For nodes before or at the root, check that the move is a
1214 // repetition rather than a move to the current position.
1215 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1216 // the same location, so we have to select which square to check.
1217 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1220 // For repetitions before or at the root, require one more
1221 if (stp->repetition)
1230 /// Position::flip() flips position with the white and black sides reversed. This
1231 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1233 void Position::flip() {
1236 std::stringstream ss(fen());
1238 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1240 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1241 f.insert(0, token + (f.empty() ? " " : "/"));
1244 ss >> token; // Active color
1245 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1247 ss >> token; // Castling availability
1250 std::transform(f.begin(), f.end(), f.begin(),
1251 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1253 ss >> token; // En passant square
1254 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1256 std::getline(ss, token); // Half and full moves
1259 set(f, is_chess960(), st, this_thread());
1261 assert(pos_is_ok());
1265 /// Position::pos_is_ok() performs some consistency checks for the
1266 /// position object and raises an asserts if something wrong is detected.
1267 /// This is meant to be helpful when debugging.
1269 bool Position::pos_is_ok() const {
1271 constexpr bool Fast = true; // Quick (default) or full check?
1273 if ( (sideToMove != WHITE && sideToMove != BLACK)
1274 || piece_on(square<KING>(WHITE)) != W_KING
1275 || piece_on(square<KING>(BLACK)) != B_KING
1276 || ( ep_square() != SQ_NONE
1277 && relative_rank(sideToMove, ep_square()) != RANK_6))
1278 assert(0 && "pos_is_ok: Default");
1283 if ( pieceCount[W_KING] != 1
1284 || pieceCount[B_KING] != 1
1285 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1286 assert(0 && "pos_is_ok: Kings");
1288 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1289 || pieceCount[W_PAWN] > 8
1290 || pieceCount[B_PAWN] > 8)
1291 assert(0 && "pos_is_ok: Pawns");
1293 if ( (pieces(WHITE) & pieces(BLACK))
1294 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1295 || popcount(pieces(WHITE)) > 16
1296 || popcount(pieces(BLACK)) > 16)
1297 assert(0 && "pos_is_ok: Bitboards");
1299 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1300 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1301 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1302 assert(0 && "pos_is_ok: Bitboards");
1305 for (Piece pc : Pieces)
1306 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1307 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1308 assert(0 && "pos_is_ok: Pieces");
1310 for (Color c : { WHITE, BLACK })
1311 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1313 if (!can_castle(cr))
1316 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1317 || castlingRightsMask[castlingRookSquare[cr]] != cr
1318 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1319 assert(0 && "pos_is_ok: Castling");
1325 } // namespace Stockfish