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/>.
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 };
54 // min_attacker() is a helper function used by see_ge() to locate the least
55 // valuable attacker for the side to move, remove the attacker we just found
56 // from the bitboards and scan for new X-ray attacks behind it.
59 PieceType min_attacker(const Bitboard* byTypeBB, Square to, Bitboard stmAttackers,
60 Bitboard& occupied, Bitboard& attackers) {
62 Bitboard b = stmAttackers & byTypeBB[Pt];
64 return min_attacker<Pt + 1>(byTypeBB, to, stmAttackers, occupied, attackers);
66 occupied ^= lsb(b); // Remove the attacker from occupied
68 // Add any X-ray attack behind the just removed piece. For instance with
69 // rooks in a8 and a7 attacking a1, after removing a7 we add rook in a8.
70 // Note that new added attackers can be of any color.
71 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
72 attackers |= attacks_bb<BISHOP>(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]);
74 if (Pt == ROOK || Pt == QUEEN)
75 attackers |= attacks_bb<ROOK>(to, occupied) & (byTypeBB[ROOK] | byTypeBB[QUEEN]);
77 // X-ray may add already processed pieces because byTypeBB[] is constant: in
78 // the rook example, now attackers contains _again_ rook in a7, so remove it.
79 attackers &= occupied;
84 PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
85 return KING; // No need to update bitboards: it is the last cycle
91 /// operator<<(Position) returns an ASCII representation of the position
93 std::ostream& operator<<(std::ostream& os, const Position& pos) {
95 os << "\n +---+---+---+---+---+---+---+---+\n";
97 for (Rank r = RANK_8; r >= RANK_1; --r)
99 for (File f = FILE_A; f <= FILE_H; ++f)
100 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
102 os << " |\n +---+---+---+---+---+---+---+---+\n";
105 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
106 << std::setfill('0') << std::setw(16) << pos.key()
107 << std::setfill(' ') << std::dec << "\nCheckers: ";
109 for (Bitboard b = pos.checkers(); b; )
110 os << UCI::square(pop_lsb(&b)) << " ";
112 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
113 && !pos.can_castle(ANY_CASTLING))
117 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
118 Tablebases::ProbeState s1, s2;
119 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
120 int dtz = Tablebases::probe_dtz(p, &s2);
121 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
122 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
129 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
130 // situations. Description of the algorithm in the following paper:
131 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
133 // First and second hash functions for indexing the cuckoo tables
134 inline int H1(Key h) { return h & 0x1fff; }
135 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
137 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
139 Move cuckooMove[8192];
142 /// Position::init() initializes at startup the various arrays used to compute
145 void Position::init() {
149 for (Piece pc : Pieces)
150 for (Square s = SQ_A1; s <= SQ_H8; ++s)
151 Zobrist::psq[pc][s] = rng.rand<Key>();
153 for (File f = FILE_A; f <= FILE_H; ++f)
154 Zobrist::enpassant[f] = rng.rand<Key>();
156 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
158 Zobrist::castling[cr] = 0;
162 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
163 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
167 Zobrist::side = rng.rand<Key>();
168 Zobrist::noPawns = rng.rand<Key>();
170 // Prepare the cuckoo tables
171 std::memset(cuckoo, 0, sizeof(cuckoo));
172 std::memset(cuckooMove, 0, sizeof(cuckooMove));
174 for (Piece pc : Pieces)
175 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
176 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
177 if (PseudoAttacks[type_of(pc)][s1] & s2)
179 Move move = make_move(s1, s2);
180 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
184 std::swap(cuckoo[i], key);
185 std::swap(cuckooMove[i], move);
186 if (move == MOVE_NONE) // Arrived at empty slot?
188 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
192 assert(count == 3668);
196 /// Position::set() initializes the position object with the given FEN string.
197 /// This function is not very robust - make sure that input FENs are correct,
198 /// this is assumed to be the responsibility of the GUI.
200 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
202 A FEN string defines a particular position using only the ASCII character set.
204 A FEN string contains six fields separated by a space. The fields are:
206 1) Piece placement (from white's perspective). Each rank is described, starting
207 with rank 8 and ending with rank 1. Within each rank, the contents of each
208 square are described from file A through file H. Following the Standard
209 Algebraic Notation (SAN), each piece is identified by a single letter taken
210 from the standard English names. White pieces are designated using upper-case
211 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
212 noted using digits 1 through 8 (the number of blank squares), and "/"
215 2) Active color. "w" means white moves next, "b" means black.
217 3) Castling availability. If neither side can castle, this is "-". Otherwise,
218 this has one or more letters: "K" (White can castle kingside), "Q" (White
219 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
220 can castle queenside).
222 4) En passant target square (in algebraic notation). If there's no en passant
223 target square, this is "-". If a pawn has just made a 2-square move, this
224 is the position "behind" the pawn. This is recorded only if there is a pawn
225 in position to make an en passant capture, and if there really is a pawn
226 that might have advanced two squares.
228 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
229 or capture. This is used to determine if a draw can be claimed under the
232 6) Fullmove number. The number of the full move. It starts at 1, and is
233 incremented after Black's move.
236 unsigned char col, row, token;
239 std::istringstream ss(fenStr);
241 std::memset(this, 0, sizeof(Position));
242 std::memset(si, 0, sizeof(StateInfo));
243 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
248 // 1. Piece placement
249 while ((ss >> token) && !isspace(token))
252 sq += (token - '0') * EAST; // Advance the given number of files
254 else if (token == '/')
257 else if ((idx = PieceToChar.find(token)) != string::npos)
259 put_piece(Piece(idx), sq);
266 sideToMove = (token == 'w' ? WHITE : BLACK);
269 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
270 // Shredder-FEN that uses the letters of the columns on which the rooks began
271 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
272 // if an inner rook is associated with the castling right, the castling tag is
273 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
274 while ((ss >> token) && !isspace(token))
277 Color c = islower(token) ? BLACK : WHITE;
278 Piece rook = make_piece(c, ROOK);
280 token = char(toupper(token));
283 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
285 else if (token == 'Q')
286 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
288 else if (token >= 'A' && token <= 'H')
289 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
294 set_castling_right(c, rsq);
297 // 4. En passant square. Ignore if no pawn capture is possible
298 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
299 && ((ss >> row) && (row == '3' || row == '6')))
301 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
303 if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
304 || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
305 st->epSquare = SQ_NONE;
308 st->epSquare = SQ_NONE;
310 // 5-6. Halfmove clock and fullmove number
311 ss >> std::skipws >> st->rule50 >> gamePly;
313 // Convert from fullmove starting from 1 to gamePly starting from 0,
314 // handle also common incorrect FEN with fullmove = 0.
315 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
317 chess960 = isChess960;
327 /// Position::set_castling_right() is a helper function used to set castling
328 /// rights given the corresponding color and the rook starting square.
330 void Position::set_castling_right(Color c, Square rfrom) {
332 Square kfrom = square<KING>(c);
333 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
334 CastlingRight cr = (c | cs);
336 st->castlingRights |= cr;
337 castlingRightsMask[kfrom] |= cr;
338 castlingRightsMask[rfrom] |= cr;
339 castlingRookSquare[cr] = rfrom;
341 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
342 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
344 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
345 & ~(square_bb(kfrom) | rfrom);
349 /// Position::set_check_info() sets king attacks to detect if a move gives check
351 void Position::set_check_info(StateInfo* si) const {
353 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
354 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
356 Square ksq = square<KING>(~sideToMove);
358 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
359 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
360 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
361 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
362 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
363 si->checkSquares[KING] = 0;
367 /// Position::set_state() computes the hash keys of the position, and other
368 /// data that once computed is updated incrementally as moves are made.
369 /// The function is only used when a new position is set up, and to verify
370 /// the correctness of the StateInfo data when running in debug mode.
372 void Position::set_state(StateInfo* si) const {
374 si->key = si->materialKey = 0;
375 si->pawnKey = Zobrist::noPawns;
376 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
377 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
381 for (Bitboard b = pieces(); b; )
383 Square s = pop_lsb(&b);
384 Piece pc = piece_on(s);
385 si->key ^= Zobrist::psq[pc][s];
388 if (si->epSquare != SQ_NONE)
389 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
391 if (sideToMove == BLACK)
392 si->key ^= Zobrist::side;
394 si->key ^= Zobrist::castling[si->castlingRights];
396 for (Bitboard b = pieces(PAWN); b; )
398 Square s = pop_lsb(&b);
399 si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
402 for (Piece pc : Pieces)
404 if (type_of(pc) != PAWN && type_of(pc) != KING)
405 si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
407 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
408 si->materialKey ^= Zobrist::psq[pc][cnt];
413 /// Position::set() is an overload to initialize the position object with
414 /// the given endgame code string like "KBPKN". It is mainly a helper to
415 /// get the material key out of an endgame code.
417 Position& Position::set(const string& code, Color c, StateInfo* si) {
419 assert(code.length() > 0 && code.length() < 8);
420 assert(code[0] == 'K');
422 string sides[] = { code.substr(code.find('K', 1)), // Weak
423 code.substr(0, code.find('K', 1)) }; // Strong
425 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
427 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
428 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
430 return set(fenStr, false, si, nullptr);
434 /// Position::fen() returns a FEN representation of the position. In case of
435 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
437 const string Position::fen() const {
440 std::ostringstream ss;
442 for (Rank r = RANK_8; r >= RANK_1; --r)
444 for (File f = FILE_A; f <= FILE_H; ++f)
446 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
453 ss << PieceToChar[piece_on(make_square(f, r))];
460 ss << (sideToMove == WHITE ? " w " : " b ");
462 if (can_castle(WHITE_OO))
463 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
465 if (can_castle(WHITE_OOO))
466 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
468 if (can_castle(BLACK_OO))
469 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
471 if (can_castle(BLACK_OOO))
472 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
474 if (!can_castle(ANY_CASTLING))
477 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
478 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
484 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
485 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
486 /// slider if removing that piece from the board would result in a position where
487 /// square 's' is attacked. For example, a king-attack blocking piece can be either
488 /// a pinned or a discovered check piece, according if its color is the opposite
489 /// or the same of the color of the slider.
491 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
493 Bitboard blockers = 0;
496 // Snipers are sliders that attack 's' when a piece and other snipers are removed
497 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
498 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
499 Bitboard occupancy = pieces() & ~snipers;
503 Square sniperSq = pop_lsb(&snipers);
504 Bitboard b = between_bb(s, sniperSq) & occupancy;
506 if (b && !more_than_one(b))
509 if (b & pieces(color_of(piece_on(s))))
517 /// Position::attackers_to() computes a bitboard of all pieces which attack a
518 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
520 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
522 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
523 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
524 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
525 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
526 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
527 | (attacks_from<KING>(s) & pieces(KING));
531 /// Position::legal() tests whether a pseudo-legal move is legal
533 bool Position::legal(Move m) const {
537 Color us = sideToMove;
538 Square from = from_sq(m);
539 Square to = to_sq(m);
541 assert(color_of(moved_piece(m)) == us);
542 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
544 // En passant captures are a tricky special case. Because they are rather
545 // uncommon, we do it simply by testing whether the king is attacked after
547 if (type_of(m) == ENPASSANT)
549 Square ksq = square<KING>(us);
550 Square capsq = to - pawn_push(us);
551 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
553 assert(to == ep_square());
554 assert(moved_piece(m) == make_piece(us, PAWN));
555 assert(piece_on(capsq) == make_piece(~us, PAWN));
556 assert(piece_on(to) == NO_PIECE);
558 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
559 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
562 // Castling moves generation does not check if the castling path is clear of
563 // enemy attacks, it is delayed at a later time: now!
564 if (type_of(m) == CASTLING)
566 // After castling, the rook and king final positions are the same in
567 // Chess960 as they would be in standard chess.
568 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
569 Direction step = to > from ? WEST : EAST;
571 for (Square s = to; s != from; s += step)
572 if (attackers_to(s) & pieces(~us))
575 // In case of Chess960, verify that when moving the castling rook we do
576 // not discover some hidden checker.
577 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
579 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
582 // If the moving piece is a king, check whether the destination square is
583 // attacked by the opponent.
584 if (type_of(piece_on(from)) == KING)
585 return !(attackers_to(to) & pieces(~us));
587 // A non-king move is legal if and only if it is not pinned or it
588 // is moving along the ray towards or away from the king.
589 return !(blockers_for_king(us) & from)
590 || aligned(from, to, square<KING>(us));
594 /// Position::pseudo_legal() takes a random move and tests whether the move is
595 /// pseudo legal. It is used to validate moves from TT that can be corrupted
596 /// due to SMP concurrent access or hash position key aliasing.
598 bool Position::pseudo_legal(const Move m) const {
600 Color us = sideToMove;
601 Square from = from_sq(m);
602 Square to = to_sq(m);
603 Piece pc = moved_piece(m);
605 // Use a slower but simpler function for uncommon cases
606 if (type_of(m) != NORMAL)
607 return MoveList<LEGAL>(*this).contains(m);
609 // Is not a promotion, so promotion piece must be empty
610 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
613 // If the 'from' square is not occupied by a piece belonging to the side to
614 // move, the move is obviously not legal.
615 if (pc == NO_PIECE || color_of(pc) != us)
618 // The destination square cannot be occupied by a friendly piece
622 // Handle the special case of a pawn move
623 if (type_of(pc) == PAWN)
625 // We have already handled promotion moves, so destination
626 // cannot be on the 8th/1st rank.
627 if ((Rank8BB | Rank1BB) & to)
630 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
631 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
632 && !( (from + 2 * pawn_push(us) == to) // Not a double push
633 && (rank_of(from) == relative_rank(us, RANK_2))
635 && empty(to - pawn_push(us))))
638 else if (!(attacks_from(type_of(pc), from) & to))
641 // Evasions generator already takes care to avoid some kind of illegal moves
642 // and legal() relies on this. We therefore have to take care that the same
643 // kind of moves are filtered out here.
646 if (type_of(pc) != KING)
648 // Double check? In this case a king move is required
649 if (more_than_one(checkers()))
652 // Our move must be a blocking evasion or a capture of the checking piece
653 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
656 // In case of king moves under check we have to remove king so as to catch
657 // invalid moves like b1a1 when opposite queen is on c1.
658 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
666 /// Position::gives_check() tests whether a pseudo-legal move gives a check
668 bool Position::gives_check(Move m) const {
671 assert(color_of(moved_piece(m)) == sideToMove);
673 Square from = from_sq(m);
674 Square to = to_sq(m);
676 // Is there a direct check?
677 if (st->checkSquares[type_of(piece_on(from))] & to)
680 // Is there a discovered check?
681 if ( (st->blockersForKing[~sideToMove] & from)
682 && !aligned(from, to, square<KING>(~sideToMove)))
691 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
693 // En passant capture with check? We have already handled the case
694 // of direct checks and ordinary discovered check, so the only case we
695 // need to handle is the unusual case of a discovered check through
696 // the captured pawn.
699 Square capsq = make_square(file_of(to), rank_of(from));
700 Bitboard b = (pieces() ^ from ^ capsq) | to;
702 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
703 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
708 Square rfrom = to; // Castling is encoded as 'King captures the rook'
709 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
710 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
712 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
713 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
722 /// Position::do_move() makes a move, and saves all information necessary
723 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
724 /// moves should be filtered out before this function is called.
726 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
729 assert(&newSt != st);
731 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
732 Key k = st->key ^ Zobrist::side;
734 // Copy some fields of the old state to our new StateInfo object except the
735 // ones which are going to be recalculated from scratch anyway and then switch
736 // our state pointer to point to the new (ready to be updated) state.
737 std::memcpy(&newSt, st, offsetof(StateInfo, key));
741 // Increment ply counters. In particular, rule50 will be reset to zero later on
742 // in case of a capture or a pawn move.
747 Color us = sideToMove;
749 Square from = from_sq(m);
750 Square to = to_sq(m);
751 Piece pc = piece_on(from);
752 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
754 assert(color_of(pc) == us);
755 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
756 assert(type_of(captured) != KING);
758 if (type_of(m) == CASTLING)
760 assert(pc == make_piece(us, KING));
761 assert(captured == make_piece(us, ROOK));
764 do_castling<true>(us, from, to, rfrom, rto);
766 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
774 // If the captured piece is a pawn, update pawn hash key, otherwise
775 // update non-pawn material.
776 if (type_of(captured) == PAWN)
778 if (type_of(m) == ENPASSANT)
780 capsq -= pawn_push(us);
782 assert(pc == make_piece(us, PAWN));
783 assert(to == st->epSquare);
784 assert(relative_rank(us, to) == RANK_6);
785 assert(piece_on(to) == NO_PIECE);
786 assert(piece_on(capsq) == make_piece(them, PAWN));
788 board[capsq] = NO_PIECE; // Not done by remove_piece()
791 st->pawnKey ^= Zobrist::psq[captured][capsq];
794 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
796 // Update board and piece lists
797 remove_piece(captured, capsq);
799 // Update material hash key and prefetch access to materialTable
800 k ^= Zobrist::psq[captured][capsq];
801 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
802 prefetch(thisThread->materialTable[st->materialKey]);
804 // Reset rule 50 counter
809 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
811 // Reset en passant square
812 if (st->epSquare != SQ_NONE)
814 k ^= Zobrist::enpassant[file_of(st->epSquare)];
815 st->epSquare = SQ_NONE;
818 // Update castling rights if needed
819 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
821 int cr = castlingRightsMask[from] | castlingRightsMask[to];
822 k ^= Zobrist::castling[st->castlingRights & cr];
823 st->castlingRights &= ~cr;
826 // Move the piece. The tricky Chess960 castling is handled earlier
827 if (type_of(m) != CASTLING)
828 move_piece(pc, from, to);
830 // If the moving piece is a pawn do some special extra work
831 if (type_of(pc) == PAWN)
833 // Set en-passant square if the moved pawn can be captured
834 if ( (int(to) ^ int(from)) == 16
835 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
837 st->epSquare = to - pawn_push(us);
838 k ^= Zobrist::enpassant[file_of(st->epSquare)];
841 else if (type_of(m) == PROMOTION)
843 Piece promotion = make_piece(us, promotion_type(m));
845 assert(relative_rank(us, to) == RANK_8);
846 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
848 remove_piece(pc, to);
849 put_piece(promotion, to);
852 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
853 st->pawnKey ^= Zobrist::psq[pc][to];
854 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
855 ^ Zobrist::psq[pc][pieceCount[pc]];
858 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
861 // Update pawn hash key and prefetch access to pawnsTable
862 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
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
882 // Calculate the repetition info. It is the ply distance from the previous
883 // occurrence of the same position, negative in the 3-fold case, or zero
884 // if the position was not repeated.
886 int end = std::min(st->rule50, st->pliesFromNull);
889 StateInfo* stp = st->previous->previous;
890 for (int i=4; i <= end; i += 2)
892 stp = stp->previous->previous;
893 if (stp->key == st->key)
895 st->repetition = stp->repetition ? -i : i;
905 /// Position::undo_move() unmakes a move. When it returns, the position should
906 /// be restored to exactly the same state as before the move was made.
908 void Position::undo_move(Move m) {
912 sideToMove = ~sideToMove;
914 Color us = sideToMove;
915 Square from = from_sq(m);
916 Square to = to_sq(m);
917 Piece pc = piece_on(to);
919 assert(empty(from) || type_of(m) == CASTLING);
920 assert(type_of(st->capturedPiece) != KING);
922 if (type_of(m) == PROMOTION)
924 assert(relative_rank(us, to) == RANK_8);
925 assert(type_of(pc) == promotion_type(m));
926 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
928 remove_piece(pc, to);
929 pc = make_piece(us, PAWN);
933 if (type_of(m) == CASTLING)
936 do_castling<false>(us, from, to, rfrom, rto);
940 move_piece(pc, to, from); // Put the piece back at the source square
942 if (st->capturedPiece)
946 if (type_of(m) == ENPASSANT)
948 capsq -= pawn_push(us);
950 assert(type_of(pc) == PAWN);
951 assert(to == st->previous->epSquare);
952 assert(relative_rank(us, to) == RANK_6);
953 assert(piece_on(capsq) == NO_PIECE);
954 assert(st->capturedPiece == make_piece(~us, PAWN));
957 put_piece(st->capturedPiece, capsq); // Restore the captured piece
961 // Finally point our state pointer back to the previous state
969 /// Position::do_castling() is a helper used to do/undo a castling move. This
970 /// is a bit tricky in Chess960 where from/to squares can overlap.
972 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
974 bool kingSide = to > from;
975 rfrom = to; // Castling is encoded as "king captures friendly rook"
976 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
977 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
979 // Remove both pieces first since squares could overlap in Chess960
980 remove_piece(make_piece(us, KING), Do ? from : to);
981 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
982 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
983 put_piece(make_piece(us, KING), Do ? to : from);
984 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
988 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
989 /// the side to move without executing any move on the board.
991 void Position::do_null_move(StateInfo& newSt) {
994 assert(&newSt != st);
996 std::memcpy(&newSt, st, sizeof(StateInfo));
1000 if (st->epSquare != SQ_NONE)
1002 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1003 st->epSquare = SQ_NONE;
1006 st->key ^= Zobrist::side;
1007 prefetch(TT.first_entry(st->key));
1010 st->pliesFromNull = 0;
1012 sideToMove = ~sideToMove;
1018 assert(pos_is_ok());
1021 void Position::undo_null_move() {
1023 assert(!checkers());
1026 sideToMove = ~sideToMove;
1030 /// Position::key_after() computes the new hash key after the given move. Needed
1031 /// for speculative prefetch. It doesn't recognize special moves like castling,
1032 /// en-passant and promotions.
1034 Key Position::key_after(Move m) const {
1036 Square from = from_sq(m);
1037 Square to = to_sq(m);
1038 Piece pc = piece_on(from);
1039 Piece captured = piece_on(to);
1040 Key k = st->key ^ Zobrist::side;
1043 k ^= Zobrist::psq[captured][to];
1045 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1049 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1050 /// SEE value of move is greater or equal to the given threshold. We'll use an
1051 /// algorithm similar to alpha-beta pruning with a null window.
1053 bool Position::see_ge(Move m, Value threshold) const {
1057 // Only deal with normal moves, assume others pass a simple see
1058 if (type_of(m) != NORMAL)
1059 return VALUE_ZERO >= threshold;
1061 Bitboard stmAttackers;
1062 Square from = from_sq(m), to = to_sq(m);
1063 PieceType nextVictim = type_of(piece_on(from));
1064 Color us = color_of(piece_on(from));
1065 Color stm = ~us; // First consider opponent's move
1066 Value balance; // Values of the pieces taken by us minus opponent's ones
1068 // The opponent may be able to recapture so this is the best result
1070 balance = PieceValue[MG][piece_on(to)] - threshold;
1072 if (balance < VALUE_ZERO)
1075 // Now assume the worst possible result: that the opponent can
1076 // capture our piece for free.
1077 balance -= PieceValue[MG][nextVictim];
1079 // If it is enough (like in PxQ) then return immediately. Note that
1080 // in case nextVictim == KING we always return here, this is ok
1081 // if the given move is legal.
1082 if (balance >= VALUE_ZERO)
1085 // Find all attackers to the destination square, with the moving piece
1086 // removed, but possibly an X-ray attacker added behind it.
1087 Bitboard occupied = pieces() ^ from ^ to;
1088 Bitboard attackers = attackers_to(to, occupied) & occupied;
1092 stmAttackers = attackers & pieces(stm);
1094 // Don't allow pinned pieces to attack (except the king) as long as
1095 // any pinners are on their original square.
1096 if (st->pinners[~stm] & occupied)
1097 stmAttackers &= ~st->blockersForKing[stm];
1099 // If stm has no more attackers then give up: stm loses
1103 // Locate and remove the next least valuable attacker, and add to
1104 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1105 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1107 stm = ~stm; // Switch side to move
1109 // Negamax the balance with alpha = balance, beta = balance+1 and
1110 // add nextVictim's value.
1112 // (balance, balance+1) -> (-balance-1, -balance)
1114 assert(balance < VALUE_ZERO);
1116 balance = -balance - 1 - PieceValue[MG][nextVictim];
1118 // If balance is still non-negative after giving away nextVictim then we
1119 // win. The only thing to be careful about it is that we should revert
1120 // stm if we captured with the king when the opponent still has attackers.
1121 if (balance >= VALUE_ZERO)
1123 if (nextVictim == KING && (attackers & pieces(stm)))
1127 assert(nextVictim != KING);
1129 return us != stm; // We break the above loop when stm loses
1133 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1134 /// or by repetition. It does not detect stalemates.
1136 bool Position::is_draw(int ply) const {
1138 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1141 // Return a draw score if a position repeats once earlier but strictly
1142 // after the root, or repeats twice before or at the root.
1143 if (st->repetition && st->repetition < ply)
1150 // Position::has_repeated() tests whether there has been at least one repetition
1151 // of positions since the last capture or pawn move.
1153 bool Position::has_repeated() const {
1155 StateInfo* stc = st;
1156 int end = std::min(st->rule50, st->pliesFromNull);
1159 if (stc->repetition)
1162 stc = stc->previous;
1168 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1169 /// or an earlier position has a move that directly reaches the current position.
1171 bool Position::has_game_cycle(int ply) const {
1175 int end = std::min(st->rule50, st->pliesFromNull);
1180 Key originalKey = st->key;
1181 StateInfo* stp = st->previous;
1183 for (int i = 3; i <= end; i += 2)
1185 stp = stp->previous->previous;
1187 Key moveKey = originalKey ^ stp->key;
1188 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1189 || (j = H2(moveKey), cuckoo[j] == moveKey))
1191 Move move = cuckooMove[j];
1192 Square s1 = from_sq(move);
1193 Square s2 = to_sq(move);
1195 if (!(between_bb(s1, s2) & pieces()))
1197 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in the same
1198 // location. We select the legal one by reversing the move variable if necessary.
1200 move = make_move(s2, s1);
1205 // For nodes before or at the root, check that the move is a repetition one
1206 // rather than a move to the current position
1207 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1210 // For repetitions before or at the root, require one more
1211 if (stp->repetition)
1220 /// Position::flip() flips position with the white and black sides reversed. This
1221 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1223 void Position::flip() {
1226 std::stringstream ss(fen());
1228 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1230 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1231 f.insert(0, token + (f.empty() ? " " : "/"));
1234 ss >> token; // Active color
1235 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1237 ss >> token; // Castling availability
1240 std::transform(f.begin(), f.end(), f.begin(),
1241 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1243 ss >> token; // En passant square
1244 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1246 std::getline(ss, token); // Half and full moves
1249 set(f, is_chess960(), st, this_thread());
1251 assert(pos_is_ok());
1255 /// Position::pos_is_ok() performs some consistency checks for the
1256 /// position object and raises an asserts if something wrong is detected.
1257 /// This is meant to be helpful when debugging.
1259 bool Position::pos_is_ok() const {
1261 constexpr bool Fast = true; // Quick (default) or full check?
1263 if ( (sideToMove != WHITE && sideToMove != BLACK)
1264 || piece_on(square<KING>(WHITE)) != W_KING
1265 || piece_on(square<KING>(BLACK)) != B_KING
1266 || ( ep_square() != SQ_NONE
1267 && relative_rank(sideToMove, ep_square()) != RANK_6))
1268 assert(0 && "pos_is_ok: Default");
1273 if ( pieceCount[W_KING] != 1
1274 || pieceCount[B_KING] != 1
1275 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1276 assert(0 && "pos_is_ok: Kings");
1278 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1279 || pieceCount[W_PAWN] > 8
1280 || pieceCount[B_PAWN] > 8)
1281 assert(0 && "pos_is_ok: Pawns");
1283 if ( (pieces(WHITE) & pieces(BLACK))
1284 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1285 || popcount(pieces(WHITE)) > 16
1286 || popcount(pieces(BLACK)) > 16)
1287 assert(0 && "pos_is_ok: Bitboards");
1289 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1290 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1291 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1292 assert(0 && "pos_is_ok: Bitboards");
1296 if (std::memcmp(&si, st, sizeof(StateInfo)))
1297 assert(0 && "pos_is_ok: State");
1299 for (Piece pc : Pieces)
1301 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1302 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1303 assert(0 && "pos_is_ok: Pieces");
1305 for (int i = 0; i < pieceCount[pc]; ++i)
1306 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1307 assert(0 && "pos_is_ok: Index");
1310 for (Color c = WHITE; c <= BLACK; ++c)
1311 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1313 if (!can_castle(c | s))
1316 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1317 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1318 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1319 assert(0 && "pos_is_ok: Castling");