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) 2015-2019 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/>.
22 #include <cstddef> // For offsetof()
23 #include <cstring> // For std::memset, std::memcmp
34 #include "syzygy/tbprobe.h"
40 Key psq[PIECE_NB][SQUARE_NB];
41 Key enpassant[FILE_NB];
42 Key castling[CASTLING_RIGHT_NB];
48 const string PieceToChar(" PNBRQK pnbrqk");
50 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
51 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
53 // min_attacker() is a helper function used by see_ge() to locate the least
54 // valuable attacker for the side to move, remove the attacker we just found
55 // from the bitboards and scan for new X-ray attacks behind it.
58 PieceType min_attacker(const Bitboard* byTypeBB, Square to, Bitboard stmAttackers,
59 Bitboard& occupied, Bitboard& attackers) {
61 Bitboard b = stmAttackers & byTypeBB[Pt];
63 return min_attacker<Pt + 1>(byTypeBB, to, stmAttackers, occupied, attackers);
65 occupied ^= lsb(b); // Remove the attacker from occupied
67 // Add any X-ray attack behind the just removed piece. For instance with
68 // rooks in a8 and a7 attacking a1, after removing a7 we add rook in a8.
69 // Note that new added attackers can be of any color.
70 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
71 attackers |= attacks_bb<BISHOP>(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]);
73 if (Pt == ROOK || Pt == QUEEN)
74 attackers |= attacks_bb<ROOK>(to, occupied) & (byTypeBB[ROOK] | byTypeBB[QUEEN]);
76 // X-ray may add already processed pieces because byTypeBB[] is constant: in
77 // the rook example, now attackers contains _again_ rook in a7, so remove it.
78 attackers &= occupied;
83 PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
84 return KING; // No need to update bitboards: it is the last cycle
90 /// operator<<(Position) returns an ASCII representation of the position
92 std::ostream& operator<<(std::ostream& os, const Position& pos) {
94 os << "\n +---+---+---+---+---+---+---+---+\n";
96 for (Rank r = RANK_8; r >= RANK_1; --r)
98 for (File f = FILE_A; f <= FILE_H; ++f)
99 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
101 os << " |\n +---+---+---+---+---+---+---+---+\n";
104 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
105 << std::setfill('0') << std::setw(16) << pos.key()
106 << std::setfill(' ') << std::dec << "\nCheckers: ";
108 for (Bitboard b = pos.checkers(); b; )
109 os << UCI::square(pop_lsb(&b)) << " ";
111 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
112 && !pos.can_castle(ANY_CASTLING))
116 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
117 Tablebases::ProbeState s1, s2;
118 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
119 int dtz = Tablebases::probe_dtz(p, &s2);
120 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
121 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
128 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
129 // situations. Description of the algorithm in the following paper:
130 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
132 // First and second hash functions for indexing the cuckoo tables
133 inline int H1(Key h) { return h & 0x1fff; }
134 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
136 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
138 Move cuckooMove[8192];
141 /// Position::init() initializes at startup the various arrays used to compute
144 void Position::init() {
148 for (Piece pc : Pieces)
149 for (Square s = SQ_A1; s <= SQ_H8; ++s)
150 Zobrist::psq[pc][s] = rng.rand<Key>();
152 for (File f = FILE_A; f <= FILE_H; ++f)
153 Zobrist::enpassant[f] = rng.rand<Key>();
155 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
157 Zobrist::castling[cr] = 0;
161 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
162 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
166 Zobrist::side = rng.rand<Key>();
167 Zobrist::noPawns = rng.rand<Key>();
169 // Prepare the cuckoo tables
170 std::memset(cuckoo, 0, sizeof(cuckoo));
171 std::memset(cuckooMove, 0, sizeof(cuckooMove));
173 for (Piece pc : Pieces)
174 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
175 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
176 if (PseudoAttacks[type_of(pc)][s1] & s2)
178 Move move = make_move(s1, s2);
179 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
183 std::swap(cuckoo[i], key);
184 std::swap(cuckooMove[i], move);
185 if (move == MOVE_NONE) // Arrived at empty slot?
187 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
191 assert(count == 3668);
195 /// Position::set() initializes the position object with the given FEN string.
196 /// This function is not very robust - make sure that input FENs are correct,
197 /// this is assumed to be the responsibility of the GUI.
199 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
201 A FEN string defines a particular position using only the ASCII character set.
203 A FEN string contains six fields separated by a space. The fields are:
205 1) Piece placement (from white's perspective). Each rank is described, starting
206 with rank 8 and ending with rank 1. Within each rank, the contents of each
207 square are described from file A through file H. Following the Standard
208 Algebraic Notation (SAN), each piece is identified by a single letter taken
209 from the standard English names. White pieces are designated using upper-case
210 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
211 noted using digits 1 through 8 (the number of blank squares), and "/"
214 2) Active color. "w" means white moves next, "b" means black.
216 3) Castling availability. If neither side can castle, this is "-". Otherwise,
217 this has one or more letters: "K" (White can castle kingside), "Q" (White
218 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
219 can castle queenside).
221 4) En passant target square (in algebraic notation). If there's no en passant
222 target square, this is "-". If a pawn has just made a 2-square move, this
223 is the position "behind" the pawn. This is recorded only if there is a pawn
224 in position to make an en passant capture, and if there really is a pawn
225 that might have advanced two squares.
227 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
228 or capture. This is used to determine if a draw can be claimed under the
231 6) Fullmove number. The number of the full move. It starts at 1, and is
232 incremented after Black's move.
235 unsigned char col, row, token;
238 std::istringstream ss(fenStr);
240 std::memset(this, 0, sizeof(Position));
241 std::memset(si, 0, sizeof(StateInfo));
242 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
247 // 1. Piece placement
248 while ((ss >> token) && !isspace(token))
251 sq += (token - '0') * EAST; // Advance the given number of files
253 else if (token == '/')
256 else if ((idx = PieceToChar.find(token)) != string::npos)
258 put_piece(Piece(idx), sq);
265 sideToMove = (token == 'w' ? WHITE : BLACK);
268 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
269 // Shredder-FEN that uses the letters of the columns on which the rooks began
270 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
271 // if an inner rook is associated with the castling right, the castling tag is
272 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
273 while ((ss >> token) && !isspace(token))
276 Color c = islower(token) ? BLACK : WHITE;
277 Piece rook = make_piece(c, ROOK);
279 token = char(toupper(token));
282 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
284 else if (token == 'Q')
285 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
287 else if (token >= 'A' && token <= 'H')
288 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
293 set_castling_right(c, rsq);
296 // 4. En passant square. Ignore if no pawn capture is possible
297 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
298 && ((ss >> row) && (row == '3' || row == '6')))
300 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
302 if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
303 || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
304 st->epSquare = SQ_NONE;
307 st->epSquare = SQ_NONE;
309 // 5-6. Halfmove clock and fullmove number
310 ss >> std::skipws >> st->rule50 >> gamePly;
312 // Convert from fullmove starting from 1 to gamePly starting from 0,
313 // handle also common incorrect FEN with fullmove = 0.
314 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
316 chess960 = isChess960;
326 /// Position::set_castling_right() is a helper function used to set castling
327 /// rights given the corresponding color and the rook starting square.
329 void Position::set_castling_right(Color c, Square rfrom) {
331 Square kfrom = square<KING>(c);
332 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
333 CastlingRight cr = (c | cs);
335 st->castlingRights |= cr;
336 castlingRightsMask[kfrom] |= cr;
337 castlingRightsMask[rfrom] |= cr;
338 castlingRookSquare[cr] = rfrom;
340 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
341 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
343 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
344 & ~(square_bb(kfrom) | rfrom);
348 /// Position::set_check_info() sets king attacks to detect if a move gives check
350 void Position::set_check_info(StateInfo* si) const {
352 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
353 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
355 Square ksq = square<KING>(~sideToMove);
357 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
358 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
359 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
360 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
361 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
362 si->checkSquares[KING] = 0;
366 /// Position::set_state() computes the hash keys of the position, and other
367 /// data that once computed is updated incrementally as moves are made.
368 /// The function is only used when a new position is set up, and to verify
369 /// the correctness of the StateInfo data when running in debug mode.
371 void Position::set_state(StateInfo* si) const {
373 si->key = si->materialKey = 0;
374 si->pawnKey = Zobrist::noPawns;
375 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
376 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
380 for (Bitboard b = pieces(); b; )
382 Square s = pop_lsb(&b);
383 Piece pc = piece_on(s);
384 si->key ^= Zobrist::psq[pc][s];
387 if (si->epSquare != SQ_NONE)
388 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
390 if (sideToMove == BLACK)
391 si->key ^= Zobrist::side;
393 si->key ^= Zobrist::castling[si->castlingRights];
395 for (Bitboard b = pieces(PAWN); b; )
397 Square s = pop_lsb(&b);
398 si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
401 for (Piece pc : Pieces)
403 if (type_of(pc) != PAWN && type_of(pc) != KING)
404 si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
406 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
407 si->materialKey ^= Zobrist::psq[pc][cnt];
412 /// Position::set() is an overload to initialize the position object with
413 /// the given endgame code string like "KBPKN". It is mainly a helper to
414 /// get the material key out of an endgame code.
416 Position& Position::set(const string& code, Color c, StateInfo* si) {
418 assert(code.length() > 0 && code.length() < 8);
419 assert(code[0] == 'K');
421 string sides[] = { code.substr(code.find('K', 1)), // Weak
422 code.substr(0, code.find('K', 1)) }; // Strong
424 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
426 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
427 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
429 return set(fenStr, false, si, nullptr);
433 /// Position::fen() returns a FEN representation of the position. In case of
434 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
436 const string Position::fen() const {
439 std::ostringstream ss;
441 for (Rank r = RANK_8; r >= RANK_1; --r)
443 for (File f = FILE_A; f <= FILE_H; ++f)
445 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
452 ss << PieceToChar[piece_on(make_square(f, r))];
459 ss << (sideToMove == WHITE ? " w " : " b ");
461 if (can_castle(WHITE_OO))
462 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
464 if (can_castle(WHITE_OOO))
465 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
467 if (can_castle(BLACK_OO))
468 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
470 if (can_castle(BLACK_OOO))
471 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
473 if (!can_castle(ANY_CASTLING))
476 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
477 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
483 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
484 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
485 /// slider if removing that piece from the board would result in a position where
486 /// square 's' is attacked. For example, a king-attack blocking piece can be either
487 /// a pinned or a discovered check piece, according if its color is the opposite
488 /// or the same of the color of the slider.
490 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
492 Bitboard blockers = 0;
495 // Snipers are sliders that attack 's' when a piece and other snipers are removed
496 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
497 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
498 Bitboard occupancy = pieces() & ~snipers;
502 Square sniperSq = pop_lsb(&snipers);
503 Bitboard b = between_bb(s, sniperSq) & occupancy;
505 if (b && !more_than_one(b))
508 if (b & pieces(color_of(piece_on(s))))
516 /// Position::attackers_to() computes a bitboard of all pieces which attack a
517 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
519 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
521 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
522 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
523 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
524 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
525 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
526 | (attacks_from<KING>(s) & pieces(KING));
530 /// Position::legal() tests whether a pseudo-legal move is legal
532 bool Position::legal(Move m) const {
536 Color us = sideToMove;
537 Square from = from_sq(m);
538 Square to = to_sq(m);
540 assert(color_of(moved_piece(m)) == us);
541 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
543 // En passant captures are a tricky special case. Because they are rather
544 // uncommon, we do it simply by testing whether the king is attacked after
546 if (type_of(m) == ENPASSANT)
548 Square ksq = square<KING>(us);
549 Square capsq = to - pawn_push(us);
550 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
552 assert(to == ep_square());
553 assert(moved_piece(m) == make_piece(us, PAWN));
554 assert(piece_on(capsq) == make_piece(~us, PAWN));
555 assert(piece_on(to) == NO_PIECE);
557 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
558 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
561 // Castling moves generation does not check if the castling path is clear of
562 // enemy attacks, it is delayed at a later time: now!
563 if (type_of(m) == CASTLING)
565 // After castling, the rook and king final positions are the same in
566 // Chess960 as they would be in standard chess.
567 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
568 Direction step = to > from ? WEST : EAST;
570 for (Square s = to; s != from; s += step)
571 if (attackers_to(s) & pieces(~us))
574 // In case of Chess960, verify that when moving the castling rook we do
575 // not discover some hidden checker.
576 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
578 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
581 // If the moving piece is a king, check whether the destination square is
582 // attacked by the opponent.
583 if (type_of(piece_on(from)) == KING)
584 return !(attackers_to(to) & pieces(~us));
586 // A non-king move is legal if and only if it is not pinned or it
587 // is moving along the ray towards or away from the king.
588 return !(blockers_for_king(us) & from)
589 || aligned(from, to, square<KING>(us));
593 /// Position::pseudo_legal() takes a random move and tests whether the move is
594 /// pseudo legal. It is used to validate moves from TT that can be corrupted
595 /// due to SMP concurrent access or hash position key aliasing.
597 bool Position::pseudo_legal(const Move m) const {
599 Color us = sideToMove;
600 Square from = from_sq(m);
601 Square to = to_sq(m);
602 Piece pc = moved_piece(m);
604 // Use a slower but simpler function for uncommon cases
605 if (type_of(m) != NORMAL)
606 return MoveList<LEGAL>(*this).contains(m);
608 // Is not a promotion, so promotion piece must be empty
609 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
612 // If the 'from' square is not occupied by a piece belonging to the side to
613 // move, the move is obviously not legal.
614 if (pc == NO_PIECE || color_of(pc) != us)
617 // The destination square cannot be occupied by a friendly piece
621 // Handle the special case of a pawn move
622 if (type_of(pc) == PAWN)
624 // We have already handled promotion moves, so destination
625 // cannot be on the 8th/1st rank.
626 if ((Rank8BB | Rank1BB) & to)
629 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
630 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
631 && !( (from + 2 * pawn_push(us) == to) // Not a double push
632 && (rank_of(from) == relative_rank(us, RANK_2))
634 && empty(to - pawn_push(us))))
637 else if (!(attacks_from(type_of(pc), from) & to))
640 // Evasions generator already takes care to avoid some kind of illegal moves
641 // and legal() relies on this. We therefore have to take care that the same
642 // kind of moves are filtered out here.
645 if (type_of(pc) != KING)
647 // Double check? In this case a king move is required
648 if (more_than_one(checkers()))
651 // Our move must be a blocking evasion or a capture of the checking piece
652 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
655 // In case of king moves under check we have to remove king so as to catch
656 // invalid moves like b1a1 when opposite queen is on c1.
657 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
665 /// Position::gives_check() tests whether a pseudo-legal move gives a check
667 bool Position::gives_check(Move m) const {
670 assert(color_of(moved_piece(m)) == sideToMove);
672 Square from = from_sq(m);
673 Square to = to_sq(m);
675 // Is there a direct check?
676 if (st->checkSquares[type_of(piece_on(from))] & to)
679 // Is there a discovered check?
680 if ( (st->blockersForKing[~sideToMove] & from)
681 && !aligned(from, to, square<KING>(~sideToMove)))
690 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
692 // En passant capture with check? We have already handled the case
693 // of direct checks and ordinary discovered check, so the only case we
694 // need to handle is the unusual case of a discovered check through
695 // the captured pawn.
698 Square capsq = make_square(file_of(to), rank_of(from));
699 Bitboard b = (pieces() ^ from ^ capsq) | to;
701 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
702 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
707 Square rfrom = to; // Castling is encoded as 'King captures the rook'
708 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
709 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
711 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
712 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
721 /// Position::do_move() makes a move, and saves all information necessary
722 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
723 /// moves should be filtered out before this function is called.
725 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
728 assert(&newSt != st);
730 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
731 Key k = st->key ^ Zobrist::side;
733 // Copy some fields of the old state to our new StateInfo object except the
734 // ones which are going to be recalculated from scratch anyway and then switch
735 // our state pointer to point to the new (ready to be updated) state.
736 std::memcpy(&newSt, st, offsetof(StateInfo, key));
740 // Increment ply counters. In particular, rule50 will be reset to zero later on
741 // in case of a capture or a pawn move.
746 Color us = sideToMove;
748 Square from = from_sq(m);
749 Square to = to_sq(m);
750 Piece pc = piece_on(from);
751 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
753 assert(color_of(pc) == us);
754 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
755 assert(type_of(captured) != KING);
757 if (type_of(m) == CASTLING)
759 assert(pc == make_piece(us, KING));
760 assert(captured == make_piece(us, ROOK));
763 do_castling<true>(us, from, to, rfrom, rto);
765 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
773 // If the captured piece is a pawn, update pawn hash key, otherwise
774 // update non-pawn material.
775 if (type_of(captured) == PAWN)
777 if (type_of(m) == ENPASSANT)
779 capsq -= pawn_push(us);
781 assert(pc == make_piece(us, PAWN));
782 assert(to == st->epSquare);
783 assert(relative_rank(us, to) == RANK_6);
784 assert(piece_on(to) == NO_PIECE);
785 assert(piece_on(capsq) == make_piece(them, PAWN));
787 board[capsq] = NO_PIECE; // Not done by remove_piece()
790 st->pawnKey ^= Zobrist::psq[captured][capsq];
793 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
795 // Update board and piece lists
796 remove_piece(captured, capsq);
798 // Update material hash key and prefetch access to materialTable
799 k ^= Zobrist::psq[captured][capsq];
800 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
801 prefetch(thisThread->materialTable[st->materialKey]);
803 // Reset rule 50 counter
808 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
810 // Reset en passant square
811 if (st->epSquare != SQ_NONE)
813 k ^= Zobrist::enpassant[file_of(st->epSquare)];
814 st->epSquare = SQ_NONE;
817 // Update castling rights if needed
818 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
820 int cr = castlingRightsMask[from] | castlingRightsMask[to];
821 k ^= Zobrist::castling[st->castlingRights & cr];
822 st->castlingRights &= ~cr;
825 // Move the piece. The tricky Chess960 castling is handled earlier
826 if (type_of(m) != CASTLING)
827 move_piece(pc, from, to);
829 // If the moving piece is a pawn do some special extra work
830 if (type_of(pc) == PAWN)
832 // Set en-passant square if the moved pawn can be captured
833 if ( (int(to) ^ int(from)) == 16
834 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
836 st->epSquare = to - pawn_push(us);
837 k ^= Zobrist::enpassant[file_of(st->epSquare)];
840 else if (type_of(m) == PROMOTION)
842 Piece promotion = make_piece(us, promotion_type(m));
844 assert(relative_rank(us, to) == RANK_8);
845 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
847 remove_piece(pc, to);
848 put_piece(promotion, to);
851 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
852 st->pawnKey ^= Zobrist::psq[pc][to];
853 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
854 ^ Zobrist::psq[pc][pieceCount[pc]];
857 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
860 // Update pawn hash key and prefetch access to pawnsTable
861 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
862 prefetch2(thisThread->pawnsTable[st->pawnKey]);
864 // Reset rule 50 draw counter
869 st->capturedPiece = captured;
871 // Update the key with the final value
874 // Calculate checkers bitboard (if move gives check)
875 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
877 sideToMove = ~sideToMove;
879 // Update king attacks used for fast check detection
886 /// Position::undo_move() unmakes a move. When it returns, the position should
887 /// be restored to exactly the same state as before the move was made.
889 void Position::undo_move(Move m) {
893 sideToMove = ~sideToMove;
895 Color us = sideToMove;
896 Square from = from_sq(m);
897 Square to = to_sq(m);
898 Piece pc = piece_on(to);
900 assert(empty(from) || type_of(m) == CASTLING);
901 assert(type_of(st->capturedPiece) != KING);
903 if (type_of(m) == PROMOTION)
905 assert(relative_rank(us, to) == RANK_8);
906 assert(type_of(pc) == promotion_type(m));
907 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
909 remove_piece(pc, to);
910 pc = make_piece(us, PAWN);
914 if (type_of(m) == CASTLING)
917 do_castling<false>(us, from, to, rfrom, rto);
921 move_piece(pc, to, from); // Put the piece back at the source square
923 if (st->capturedPiece)
927 if (type_of(m) == ENPASSANT)
929 capsq -= pawn_push(us);
931 assert(type_of(pc) == PAWN);
932 assert(to == st->previous->epSquare);
933 assert(relative_rank(us, to) == RANK_6);
934 assert(piece_on(capsq) == NO_PIECE);
935 assert(st->capturedPiece == make_piece(~us, PAWN));
938 put_piece(st->capturedPiece, capsq); // Restore the captured piece
942 // Finally point our state pointer back to the previous state
950 /// Position::do_castling() is a helper used to do/undo a castling move. This
951 /// is a bit tricky in Chess960 where from/to squares can overlap.
953 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
955 bool kingSide = to > from;
956 rfrom = to; // Castling is encoded as "king captures friendly rook"
957 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
958 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
960 // Remove both pieces first since squares could overlap in Chess960
961 remove_piece(make_piece(us, KING), Do ? from : to);
962 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
963 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
964 put_piece(make_piece(us, KING), Do ? to : from);
965 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
969 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
970 /// the side to move without executing any move on the board.
972 void Position::do_null_move(StateInfo& newSt) {
975 assert(&newSt != st);
977 std::memcpy(&newSt, st, sizeof(StateInfo));
981 if (st->epSquare != SQ_NONE)
983 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
984 st->epSquare = SQ_NONE;
987 st->key ^= Zobrist::side;
988 prefetch(TT.first_entry(st->key));
991 st->pliesFromNull = 0;
993 sideToMove = ~sideToMove;
1000 void Position::undo_null_move() {
1002 assert(!checkers());
1005 sideToMove = ~sideToMove;
1009 /// Position::key_after() computes the new hash key after the given move. Needed
1010 /// for speculative prefetch. It doesn't recognize special moves like castling,
1011 /// en-passant and promotions.
1013 Key Position::key_after(Move m) const {
1015 Square from = from_sq(m);
1016 Square to = to_sq(m);
1017 Piece pc = piece_on(from);
1018 Piece captured = piece_on(to);
1019 Key k = st->key ^ Zobrist::side;
1022 k ^= Zobrist::psq[captured][to];
1024 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1028 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1029 /// SEE value of move is greater or equal to the given threshold. We'll use an
1030 /// algorithm similar to alpha-beta pruning with a null window.
1032 bool Position::see_ge(Move m, Value threshold) const {
1036 // Only deal with normal moves, assume others pass a simple see
1037 if (type_of(m) != NORMAL)
1038 return VALUE_ZERO >= threshold;
1040 Bitboard stmAttackers;
1041 Square from = from_sq(m), to = to_sq(m);
1042 PieceType nextVictim = type_of(piece_on(from));
1043 Color us = color_of(piece_on(from));
1044 Color stm = ~us; // First consider opponent's move
1045 Value balance; // Values of the pieces taken by us minus opponent's ones
1047 // The opponent may be able to recapture so this is the best result
1049 balance = PieceValue[MG][piece_on(to)] - threshold;
1051 if (balance < VALUE_ZERO)
1054 // Now assume the worst possible result: that the opponent can
1055 // capture our piece for free.
1056 balance -= PieceValue[MG][nextVictim];
1058 // If it is enough (like in PxQ) then return immediately. Note that
1059 // in case nextVictim == KING we always return here, this is ok
1060 // if the given move is legal.
1061 if (balance >= VALUE_ZERO)
1064 // Find all attackers to the destination square, with the moving piece
1065 // removed, but possibly an X-ray attacker added behind it.
1066 Bitboard occupied = pieces() ^ from ^ to;
1067 Bitboard attackers = attackers_to(to, occupied) & occupied;
1071 stmAttackers = attackers & pieces(stm);
1073 // Don't allow pinned pieces to attack (except the king) as long as
1074 // any pinners are on their original square.
1075 if (st->pinners[~stm] & occupied)
1076 stmAttackers &= ~st->blockersForKing[stm];
1078 // If stm has no more attackers then give up: stm loses
1082 // Locate and remove the next least valuable attacker, and add to
1083 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1084 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1086 stm = ~stm; // Switch side to move
1088 // Negamax the balance with alpha = balance, beta = balance+1 and
1089 // add nextVictim's value.
1091 // (balance, balance+1) -> (-balance-1, -balance)
1093 assert(balance < VALUE_ZERO);
1095 balance = -balance - 1 - PieceValue[MG][nextVictim];
1097 // If balance is still non-negative after giving away nextVictim then we
1098 // win. The only thing to be careful about it is that we should revert
1099 // stm if we captured with the king when the opponent still has attackers.
1100 if (balance >= VALUE_ZERO)
1102 if (nextVictim == KING && (attackers & pieces(stm)))
1106 assert(nextVictim != KING);
1108 return us != stm; // We break the above loop when stm loses
1112 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1113 /// or by repetition. It does not detect stalemates.
1115 bool Position::is_draw(int ply) const {
1117 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1120 int end = std::min(st->rule50, st->pliesFromNull);
1125 StateInfo* stp = st->previous->previous;
1128 for (int i = 4; i <= end; i += 2)
1130 stp = stp->previous->previous;
1132 // Return a draw score if a position repeats once earlier but strictly
1133 // after the root, or repeats twice before or at the root.
1134 if ( stp->key == st->key
1135 && ++cnt + (ply > i) == 2)
1143 // Position::has_repeated() tests whether there has been at least one repetition
1144 // of positions since the last capture or pawn move.
1146 bool Position::has_repeated() const {
1148 StateInfo* stc = st;
1151 int i = 4, end = std::min(stc->rule50, stc->pliesFromNull);
1156 StateInfo* stp = stc->previous->previous;
1159 stp = stp->previous->previous;
1161 if (stp->key == stc->key)
1167 stc = stc->previous;
1172 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1173 /// or an earlier position has a move that directly reaches the current position.
1175 bool Position::has_game_cycle(int ply) const {
1179 int end = std::min(st->rule50, st->pliesFromNull);
1184 Key originalKey = st->key;
1185 StateInfo* stp = st->previous;
1187 for (int i = 3; i <= end; i += 2)
1189 stp = stp->previous->previous;
1191 Key moveKey = originalKey ^ stp->key;
1192 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1193 || (j = H2(moveKey), cuckoo[j] == moveKey))
1195 Move move = cuckooMove[j];
1196 Square s1 = from_sq(move);
1197 Square s2 = to_sq(move);
1199 if (!(between_bb(s1, s2) & pieces()))
1201 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in the same
1202 // location. We select the legal one by reversing the move variable if necessary.
1204 move = make_move(s2, s1);
1209 // For repetitions before or at the root, require one more
1210 StateInfo* next_stp = stp;
1211 for (int k = i + 2; k <= end; k += 2)
1213 next_stp = next_stp->previous->previous;
1214 if (next_stp->key == stp->key)
1224 /// Position::flip() flips position with the white and black sides reversed. This
1225 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1227 void Position::flip() {
1230 std::stringstream ss(fen());
1232 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1234 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1235 f.insert(0, token + (f.empty() ? " " : "/"));
1238 ss >> token; // Active color
1239 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1241 ss >> token; // Castling availability
1244 std::transform(f.begin(), f.end(), f.begin(),
1245 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1247 ss >> token; // En passant square
1248 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1250 std::getline(ss, token); // Half and full moves
1253 set(f, is_chess960(), st, this_thread());
1255 assert(pos_is_ok());
1259 /// Position::pos_is_ok() performs some consistency checks for the
1260 /// position object and raises an asserts if something wrong is detected.
1261 /// This is meant to be helpful when debugging.
1263 bool Position::pos_is_ok() const {
1265 constexpr bool Fast = true; // Quick (default) or full check?
1267 if ( (sideToMove != WHITE && sideToMove != BLACK)
1268 || piece_on(square<KING>(WHITE)) != W_KING
1269 || piece_on(square<KING>(BLACK)) != B_KING
1270 || ( ep_square() != SQ_NONE
1271 && relative_rank(sideToMove, ep_square()) != RANK_6))
1272 assert(0 && "pos_is_ok: Default");
1277 if ( pieceCount[W_KING] != 1
1278 || pieceCount[B_KING] != 1
1279 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1280 assert(0 && "pos_is_ok: Kings");
1282 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1283 || pieceCount[W_PAWN] > 8
1284 || pieceCount[B_PAWN] > 8)
1285 assert(0 && "pos_is_ok: Pawns");
1287 if ( (pieces(WHITE) & pieces(BLACK))
1288 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1289 || popcount(pieces(WHITE)) > 16
1290 || popcount(pieces(BLACK)) > 16)
1291 assert(0 && "pos_is_ok: Bitboards");
1293 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1294 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1295 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1296 assert(0 && "pos_is_ok: Bitboards");
1300 if (std::memcmp(&si, st, sizeof(StateInfo)))
1301 assert(0 && "pos_is_ok: State");
1303 for (Piece pc : Pieces)
1305 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1306 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1307 assert(0 && "pos_is_ok: Pieces");
1309 for (int i = 0; i < pieceCount[pc]; ++i)
1310 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1311 assert(0 && "pos_is_ok: Index");
1314 for (Color c = WHITE; c <= BLACK; ++c)
1315 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1317 if (!can_castle(c | s))
1320 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1321 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1322 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1323 assert(0 && "pos_is_ok: Castling");