2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
23 #include <cstddef> // For offsetof()
24 #include <cstring> // For std::memset, std::memcmp
35 #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))
83 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
84 Tablebases::ProbeState s1, s2;
85 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
86 int dtz = Tablebases::probe_dtz(p, &s2);
87 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
88 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
95 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
96 // situations. Description of the algorithm in the following paper:
97 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
99 // First and second hash functions for indexing the cuckoo tables
100 inline int H1(Key h) { return h & 0x1fff; }
101 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
103 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
105 Move cuckooMove[8192];
108 /// Position::init() initializes at startup the various arrays used to compute hash keys
110 void Position::init() {
114 for (Piece pc : Pieces)
115 for (Square s = SQ_A1; s <= SQ_H8; ++s)
116 Zobrist::psq[pc][s] = rng.rand<Key>();
118 for (File f = FILE_A; f <= FILE_H; ++f)
119 Zobrist::enpassant[f] = rng.rand<Key>();
121 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
122 Zobrist::castling[cr] = rng.rand<Key>();
124 Zobrist::side = rng.rand<Key>();
125 Zobrist::noPawns = rng.rand<Key>();
127 // Prepare the cuckoo tables
128 std::memset(cuckoo, 0, sizeof(cuckoo));
129 std::memset(cuckooMove, 0, sizeof(cuckooMove));
131 for (Piece pc : Pieces)
132 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
133 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
134 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
136 Move move = make_move(s1, s2);
137 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
141 std::swap(cuckoo[i], key);
142 std::swap(cuckooMove[i], move);
143 if (move == MOVE_NONE) // Arrived at empty slot?
145 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
149 assert(count == 3668);
153 /// Position::set() initializes the position object with the given FEN string.
154 /// This function is not very robust - make sure that input FENs are correct,
155 /// this is assumed to be the responsibility of the GUI.
157 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
159 A FEN string defines a particular position using only the ASCII character set.
161 A FEN string contains six fields separated by a space. The fields are:
163 1) Piece placement (from white's perspective). Each rank is described, starting
164 with rank 8 and ending with rank 1. Within each rank, the contents of each
165 square are described from file A through file H. Following the Standard
166 Algebraic Notation (SAN), each piece is identified by a single letter taken
167 from the standard English names. White pieces are designated using upper-case
168 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
169 noted using digits 1 through 8 (the number of blank squares), and "/"
172 2) Active color. "w" means white moves next, "b" means black.
174 3) Castling availability. If neither side can castle, this is "-". Otherwise,
175 this has one or more letters: "K" (White can castle kingside), "Q" (White
176 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
177 can castle queenside).
179 4) En passant target square (in algebraic notation). If there's no en passant
180 target square, this is "-". If a pawn has just made a 2-square move, this
181 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
182 if there is a pawn in position to make an en passant capture, and if there really
183 is a pawn that might have advanced two squares.
185 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
186 or capture. This is used to determine if a draw can be claimed under the
189 6) Fullmove number. The number of the full move. It starts at 1, and is
190 incremented after Black's move.
193 unsigned char col, row, token;
196 std::istringstream ss(fenStr);
198 std::memset(this, 0, sizeof(Position));
199 std::memset(si, 0, sizeof(StateInfo));
200 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
205 // 1. Piece placement
206 while ((ss >> token) && !isspace(token))
209 sq += (token - '0') * EAST; // Advance the given number of files
211 else if (token == '/')
214 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;
292 /// Position::set_castling_right() is a helper function used to set castling
293 /// rights given the corresponding color and the rook starting square.
295 void Position::set_castling_right(Color c, Square rfrom) {
297 Square kfrom = square<KING>(c);
298 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
300 st->castlingRights |= cr;
301 castlingRightsMask[kfrom] |= cr;
302 castlingRightsMask[rfrom] |= cr;
303 castlingRookSquare[cr] = rfrom;
305 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
306 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
308 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
313 /// Position::set_check_info() sets king attacks to detect if a move gives check
315 void Position::set_check_info(StateInfo* si) const {
317 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
318 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
320 Square ksq = square<KING>(~sideToMove);
322 si->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
323 si->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
324 si->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
325 si->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
326 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
327 si->checkSquares[KING] = 0;
331 /// Position::set_state() computes the hash keys of the position, and other
332 /// data that once computed is updated incrementally as moves are made.
333 /// The function is only used when a new position is set up, and to verify
334 /// the correctness of the StateInfo data when running in debug mode.
336 void Position::set_state(StateInfo* si) const {
338 si->key = si->materialKey = 0;
339 si->pawnKey = Zobrist::noPawns;
340 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
341 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
345 for (Bitboard b = pieces(); b; )
347 Square s = pop_lsb(&b);
348 Piece pc = piece_on(s);
349 si->key ^= Zobrist::psq[pc][s];
351 if (type_of(pc) == PAWN)
352 si->pawnKey ^= Zobrist::psq[pc][s];
354 else if (type_of(pc) != KING)
355 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
358 if (si->epSquare != SQ_NONE)
359 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
361 if (sideToMove == BLACK)
362 si->key ^= Zobrist::side;
364 si->key ^= Zobrist::castling[si->castlingRights];
366 for (Piece pc : Pieces)
367 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
368 si->materialKey ^= Zobrist::psq[pc][cnt];
372 /// Position::set() is an overload to initialize the position object with
373 /// the given endgame code string like "KBPKN". It is mainly a helper to
374 /// get the material key out of an endgame code.
376 Position& Position::set(const string& code, Color c, StateInfo* si) {
378 assert(code[0] == 'K');
380 string sides[] = { code.substr(code.find('K', 1)), // Weak
381 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
383 assert(sides[0].length() > 0 && sides[0].length() < 8);
384 assert(sides[1].length() > 0 && sides[1].length() < 8);
386 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
388 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
389 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
391 return set(fenStr, false, si, nullptr);
395 /// Position::fen() returns a FEN representation of the position. In case of
396 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
398 const string Position::fen() const {
401 std::ostringstream ss;
403 for (Rank r = RANK_8; r >= RANK_1; --r)
405 for (File f = FILE_A; f <= FILE_H; ++f)
407 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
414 ss << PieceToChar[piece_on(make_square(f, r))];
421 ss << (sideToMove == WHITE ? " w " : " b ");
423 if (can_castle(WHITE_OO))
424 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
426 if (can_castle(WHITE_OOO))
427 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
429 if (can_castle(BLACK_OO))
430 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
432 if (can_castle(BLACK_OOO))
433 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
435 if (!can_castle(ANY_CASTLING))
438 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
439 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
445 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
446 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
447 /// slider if removing that piece from the board would result in a position where
448 /// square 's' is attacked. For example, a king-attack blocking piece can be either
449 /// a pinned or a discovered check piece, according if its color is the opposite
450 /// or the same of the color of the slider.
452 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
454 Bitboard blockers = 0;
457 // Snipers are sliders that attack 's' when a piece and other snipers are removed
458 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
459 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
460 Bitboard occupancy = pieces() ^ snipers;
464 Square sniperSq = pop_lsb(&snipers);
465 Bitboard b = between_bb(s, sniperSq) & occupancy;
467 if (b && !more_than_one(b))
470 if (b & pieces(color_of(piece_on(s))))
478 /// Position::attackers_to() computes a bitboard of all pieces which attack a
479 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
481 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
483 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
484 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
485 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
486 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
487 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
488 | (attacks_bb<KING>(s) & pieces(KING));
492 /// Position::legal() tests whether a pseudo-legal move is legal
494 bool Position::legal(Move m) const {
498 Color us = sideToMove;
499 Square from = from_sq(m);
500 Square to = to_sq(m);
502 assert(color_of(moved_piece(m)) == us);
503 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
505 // En passant captures are a tricky special case. Because they are rather
506 // uncommon, we do it simply by testing whether the king is attacked after
508 if (type_of(m) == ENPASSANT)
510 Square ksq = square<KING>(us);
511 Square capsq = to - pawn_push(us);
512 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
514 assert(to == ep_square());
515 assert(moved_piece(m) == make_piece(us, PAWN));
516 assert(piece_on(capsq) == make_piece(~us, PAWN));
517 assert(piece_on(to) == NO_PIECE);
519 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
520 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
523 // Castling moves generation does not check if the castling path is clear of
524 // enemy attacks, it is delayed at a later time: now!
525 if (type_of(m) == CASTLING)
527 // After castling, the rook and king final positions are the same in
528 // Chess960 as they would be in standard chess.
529 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
530 Direction step = to > from ? WEST : EAST;
532 for (Square s = to; s != from; s += step)
533 if (attackers_to(s) & pieces(~us))
536 // In case of Chess960, verify that when moving the castling rook we do
537 // not discover some hidden checker.
538 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
540 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
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(~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 if (type_of(m) != NORMAL)
568 return MoveList<LEGAL>(*this).contains(m);
570 // Is not a promotion, so promotion piece must be empty
571 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
574 // If the 'from' square is not occupied by a piece belonging to the side to
575 // move, the move is obviously not legal.
576 if (pc == NO_PIECE || color_of(pc) != us)
579 // The destination square cannot be occupied by a friendly piece
583 // Handle the special case of a pawn move
584 if (type_of(pc) == PAWN)
586 // We have already handled promotion moves, so destination
587 // cannot be on the 8th/1st rank.
588 if ((Rank8BB | Rank1BB) & to)
591 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
592 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
593 && !( (from + 2 * pawn_push(us) == to) // Not a double push
594 && (relative_rank(us, from) == RANK_2)
596 && empty(to - pawn_push(us))))
599 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
602 // Evasions generator already takes care to avoid some kind of illegal moves
603 // and legal() relies on this. We therefore have to take care that the same
604 // kind of moves are filtered out here.
607 if (type_of(pc) != KING)
609 // Double check? In this case a king move is required
610 if (more_than_one(checkers()))
613 // Our move must be a blocking evasion or a capture of the checking piece
614 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
617 // In case of king moves under check we have to remove king so as to catch
618 // invalid moves like b1a1 when opposite queen is on c1.
619 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
627 /// Position::gives_check() tests whether a pseudo-legal move gives a check
629 bool Position::gives_check(Move m) const {
632 assert(color_of(moved_piece(m)) == sideToMove);
634 Square from = from_sq(m);
635 Square to = to_sq(m);
637 // Is there a direct check?
638 if (check_squares(type_of(piece_on(from))) & to)
641 // Is there a discovered check?
642 if ( (blockers_for_king(~sideToMove) & from)
643 && !aligned(from, to, square<KING>(~sideToMove)))
652 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
654 // En passant capture with check? We have already handled the case
655 // of direct checks and ordinary discovered check, so the only case we
656 // need to handle is the unusual case of a discovered check through
657 // the captured pawn.
660 Square capsq = make_square(file_of(to), rank_of(from));
661 Bitboard b = (pieces() ^ from ^ capsq) | to;
663 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
664 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
669 Square rfrom = to; // Castling is encoded as 'king captures the rook'
670 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
671 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
673 return (attacks_bb<ROOK>(rto) & square<KING>(~sideToMove))
674 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
683 /// Position::do_move() makes a move, and saves all information necessary
684 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
685 /// moves should be filtered out before this function is called.
687 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
690 assert(&newSt != st);
692 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
693 Key k = st->key ^ Zobrist::side;
695 // Copy some fields of the old state to our new StateInfo object except the
696 // ones which are going to be recalculated from scratch anyway and then switch
697 // our state pointer to point to the new (ready to be updated) state.
698 std::memcpy(&newSt, st, offsetof(StateInfo, key));
702 // Increment ply counters. In particular, rule50 will be reset to zero later on
703 // in case of a capture or a pawn move.
708 Color us = sideToMove;
710 Square from = from_sq(m);
711 Square to = to_sq(m);
712 Piece pc = piece_on(from);
713 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
715 assert(color_of(pc) == us);
716 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
717 assert(type_of(captured) != KING);
719 if (type_of(m) == CASTLING)
721 assert(pc == make_piece(us, KING));
722 assert(captured == make_piece(us, ROOK));
725 do_castling<true>(us, from, to, rfrom, rto);
727 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
735 // If the captured piece is a pawn, update pawn hash key, otherwise
736 // update non-pawn material.
737 if (type_of(captured) == PAWN)
739 if (type_of(m) == ENPASSANT)
741 capsq -= pawn_push(us);
743 assert(pc == make_piece(us, PAWN));
744 assert(to == st->epSquare);
745 assert(relative_rank(us, to) == RANK_6);
746 assert(piece_on(to) == NO_PIECE);
747 assert(piece_on(capsq) == make_piece(them, PAWN));
750 st->pawnKey ^= Zobrist::psq[captured][capsq];
753 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
755 // Update board and piece lists
758 if (type_of(m) == ENPASSANT)
759 board[capsq] = NO_PIECE;
761 // Update material hash key and prefetch access to materialTable
762 k ^= Zobrist::psq[captured][capsq];
763 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
764 prefetch(thisThread->materialTable[st->materialKey]);
766 // Reset rule 50 counter
771 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
773 // Reset en passant square
774 if (st->epSquare != SQ_NONE)
776 k ^= Zobrist::enpassant[file_of(st->epSquare)];
777 st->epSquare = SQ_NONE;
780 // Update castling rights if needed
781 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
783 k ^= Zobrist::castling[st->castlingRights];
784 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
785 k ^= Zobrist::castling[st->castlingRights];
788 // Move the piece. The tricky Chess960 castling is handled earlier
789 if (type_of(m) != CASTLING)
790 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);
814 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
815 st->pawnKey ^= Zobrist::psq[pc][to];
816 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
817 ^ Zobrist::psq[pc][pieceCount[pc]];
820 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
823 // Update pawn hash key
824 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
826 // Reset rule 50 draw counter
831 st->capturedPiece = captured;
833 // Update the key with the final value
836 // Calculate checkers bitboard (if move gives check)
837 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
839 sideToMove = ~sideToMove;
841 // Update king attacks used for fast check detection
844 // Calculate the repetition info. It is the ply distance from the previous
845 // occurrence of the same position, negative in the 3-fold case, or zero
846 // if the position was not repeated.
848 int end = std::min(st->rule50, st->pliesFromNull);
851 StateInfo* stp = st->previous->previous;
852 for (int i = 4; i <= end; i += 2)
854 stp = stp->previous->previous;
855 if (stp->key == st->key)
857 st->repetition = stp->repetition ? -i : i;
867 /// Position::undo_move() unmakes a move. When it returns, the position should
868 /// be restored to exactly the same state as before the move was made.
870 void Position::undo_move(Move m) {
874 sideToMove = ~sideToMove;
876 Color us = sideToMove;
877 Square from = from_sq(m);
878 Square to = to_sq(m);
879 Piece pc = piece_on(to);
881 assert(empty(from) || type_of(m) == CASTLING);
882 assert(type_of(st->capturedPiece) != KING);
884 if (type_of(m) == PROMOTION)
886 assert(relative_rank(us, to) == RANK_8);
887 assert(type_of(pc) == promotion_type(m));
888 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
891 pc = make_piece(us, PAWN);
895 if (type_of(m) == CASTLING)
898 do_castling<false>(us, from, to, rfrom, rto);
902 move_piece(to, from); // Put the piece back at the source square
904 if (st->capturedPiece)
908 if (type_of(m) == ENPASSANT)
910 capsq -= pawn_push(us);
912 assert(type_of(pc) == PAWN);
913 assert(to == st->previous->epSquare);
914 assert(relative_rank(us, to) == RANK_6);
915 assert(piece_on(capsq) == NO_PIECE);
916 assert(st->capturedPiece == make_piece(~us, PAWN));
919 put_piece(st->capturedPiece, capsq); // Restore the captured piece
923 // Finally point our state pointer back to the previous state
931 /// Position::do_castling() is a helper used to do/undo a castling move. This
932 /// is a bit tricky in Chess960 where from/to squares can overlap.
934 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
936 bool kingSide = to > from;
937 rfrom = to; // Castling is encoded as "king captures friendly rook"
938 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
939 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
941 // Remove both pieces first since squares could overlap in Chess960
942 remove_piece(Do ? from : to);
943 remove_piece(Do ? rfrom : rto);
944 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
945 put_piece(make_piece(us, KING), Do ? to : from);
946 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
950 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
951 /// the side to move without executing any move on the board.
953 void Position::do_null_move(StateInfo& newSt) {
956 assert(&newSt != st);
958 std::memcpy(&newSt, st, sizeof(StateInfo));
962 if (st->epSquare != SQ_NONE)
964 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
965 st->epSquare = SQ_NONE;
968 st->key ^= Zobrist::side;
969 prefetch(TT.first_entry(st->key));
972 st->pliesFromNull = 0;
974 sideToMove = ~sideToMove;
983 void Position::undo_null_move() {
988 sideToMove = ~sideToMove;
992 /// Position::key_after() computes the new hash key after the given move. Needed
993 /// for speculative prefetch. It doesn't recognize special moves like castling,
994 /// en-passant and promotions.
996 Key Position::key_after(Move m) const {
998 Square from = from_sq(m);
999 Square to = to_sq(m);
1000 Piece pc = piece_on(from);
1001 Piece captured = piece_on(to);
1002 Key k = st->key ^ Zobrist::side;
1005 k ^= Zobrist::psq[captured][to];
1007 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1011 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1012 /// SEE value of move is greater or equal to the given threshold. We'll use an
1013 /// algorithm similar to alpha-beta pruning with a null window.
1015 bool Position::see_ge(Move m, Value threshold) const {
1019 // Only deal with normal moves, assume others pass a simple see
1020 if (type_of(m) != NORMAL)
1021 return VALUE_ZERO >= threshold;
1023 Square from = from_sq(m), to = to_sq(m);
1025 int swap = PieceValue[MG][piece_on(to)] - threshold;
1029 swap = PieceValue[MG][piece_on(from)] - swap;
1033 Bitboard occupied = pieces() ^ from ^ to;
1034 Color stm = color_of(piece_on(from));
1035 Bitboard attackers = attackers_to(to, occupied);
1036 Bitboard stmAttackers, bb;
1042 attackers &= occupied;
1044 // If stm has no more attackers then give up: stm loses
1045 if (!(stmAttackers = attackers & pieces(stm)))
1048 // Don't allow pinned pieces to attack (except the king) as long as
1049 // there are pinners on their original square.
1050 if (st->pinners[~stm] & occupied)
1051 stmAttackers &= ~st->blockersForKing[stm];
1058 // Locate and remove the next least valuable attacker, and add to
1059 // the bitboard 'attackers' any X-ray attackers behind it.
1060 if ((bb = stmAttackers & pieces(PAWN)))
1062 if ((swap = PawnValueMg - swap) < res)
1065 occupied ^= lsb(bb);
1066 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1069 else if ((bb = stmAttackers & pieces(KNIGHT)))
1071 if ((swap = KnightValueMg - swap) < res)
1074 occupied ^= lsb(bb);
1077 else if ((bb = stmAttackers & pieces(BISHOP)))
1079 if ((swap = BishopValueMg - swap) < res)
1082 occupied ^= lsb(bb);
1083 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1086 else if ((bb = stmAttackers & pieces(ROOK)))
1088 if ((swap = RookValueMg - swap) < res)
1091 occupied ^= lsb(bb);
1092 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1095 else if ((bb = stmAttackers & pieces(QUEEN)))
1097 if ((swap = QueenValueMg - swap) < res)
1100 occupied ^= lsb(bb);
1101 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1102 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1106 // If we "capture" with the king but opponent still has attackers,
1107 // reverse the result.
1108 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1115 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1116 /// or by repetition. It does not detect stalemates.
1118 bool Position::is_draw(int ply) const {
1120 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1123 // Return a draw score if a position repeats once earlier but strictly
1124 // after the root, or repeats twice before or at the root.
1125 return st->repetition && st->repetition < ply;
1129 // Position::has_repeated() tests whether there has been at least one repetition
1130 // of positions since the last capture or pawn move.
1132 bool Position::has_repeated() const {
1134 StateInfo* stc = st;
1135 int end = std::min(st->rule50, st->pliesFromNull);
1138 if (stc->repetition)
1141 stc = stc->previous;
1147 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1148 /// or an earlier position has a move that directly reaches the current position.
1150 bool Position::has_game_cycle(int ply) const {
1154 int end = std::min(st->rule50, st->pliesFromNull);
1159 Key originalKey = st->key;
1160 StateInfo* stp = st->previous;
1162 for (int i = 3; i <= end; i += 2)
1164 stp = stp->previous->previous;
1166 Key moveKey = originalKey ^ stp->key;
1167 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1168 || (j = H2(moveKey), cuckoo[j] == moveKey))
1170 Move move = cuckooMove[j];
1171 Square s1 = from_sq(move);
1172 Square s2 = to_sq(move);
1174 if (!(between_bb(s1, s2) & pieces()))
1179 // For nodes before or at the root, check that the move is a
1180 // repetition rather than a move to the current position.
1181 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1182 // the same location, so we have to select which square to check.
1183 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1186 // For repetitions before or at the root, require one more
1187 if (stp->repetition)
1196 /// Position::flip() flips position with the white and black sides reversed. This
1197 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1199 void Position::flip() {
1202 std::stringstream ss(fen());
1204 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1206 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1207 f.insert(0, token + (f.empty() ? " " : "/"));
1210 ss >> token; // Active color
1211 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1213 ss >> token; // Castling availability
1216 std::transform(f.begin(), f.end(), f.begin(),
1217 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1219 ss >> token; // En passant square
1220 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1222 std::getline(ss, token); // Half and full moves
1225 set(f, is_chess960(), st, this_thread());
1227 assert(pos_is_ok());
1231 /// Position::pos_is_ok() performs some consistency checks for the
1232 /// position object and raises an asserts if something wrong is detected.
1233 /// This is meant to be helpful when debugging.
1235 bool Position::pos_is_ok() const {
1237 constexpr bool Fast = true; // Quick (default) or full check?
1239 if ( (sideToMove != WHITE && sideToMove != BLACK)
1240 || piece_on(square<KING>(WHITE)) != W_KING
1241 || piece_on(square<KING>(BLACK)) != B_KING
1242 || ( ep_square() != SQ_NONE
1243 && relative_rank(sideToMove, ep_square()) != RANK_6))
1244 assert(0 && "pos_is_ok: Default");
1249 if ( pieceCount[W_KING] != 1
1250 || pieceCount[B_KING] != 1
1251 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1252 assert(0 && "pos_is_ok: Kings");
1254 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1255 || pieceCount[W_PAWN] > 8
1256 || pieceCount[B_PAWN] > 8)
1257 assert(0 && "pos_is_ok: Pawns");
1259 if ( (pieces(WHITE) & pieces(BLACK))
1260 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1261 || popcount(pieces(WHITE)) > 16
1262 || popcount(pieces(BLACK)) > 16)
1263 assert(0 && "pos_is_ok: Bitboards");
1265 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1266 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1267 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1268 assert(0 && "pos_is_ok: Bitboards");
1272 if (std::memcmp(&si, st, sizeof(StateInfo)))
1273 assert(0 && "pos_is_ok: State");
1275 for (Piece pc : Pieces)
1277 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1278 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1279 assert(0 && "pos_is_ok: Pieces");
1281 for (int i = 0; i < pieceCount[pc]; ++i)
1282 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1283 assert(0 && "pos_is_ok: Index");
1286 for (Color c : { WHITE, BLACK })
1287 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1289 if (!can_castle(cr))
1292 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1293 || castlingRightsMask[castlingRookSquare[cr]] != cr
1294 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1295 assert(0 && "pos_is_ok: Castling");