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.
58 template<PieceType Pt>
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<PieceType(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;
325 /// Position::set_castling_right() is a helper function used to set castling
326 /// rights given the corresponding color and the rook starting square.
328 void Position::set_castling_right(Color c, Square rfrom) {
330 Square kfrom = square<KING>(c);
331 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
333 st->castlingRights |= cr;
334 castlingRightsMask[kfrom] |= cr;
335 castlingRightsMask[rfrom] |= cr;
336 castlingRookSquare[cr] = rfrom;
338 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
339 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
341 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
342 & ~(square_bb(kfrom) | rfrom);
346 /// Position::set_check_info() sets king attacks to detect if a move gives check
348 void Position::set_check_info(StateInfo* si) const {
350 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
351 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
353 Square ksq = square<KING>(~sideToMove);
355 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
356 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
357 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
358 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
359 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
360 si->checkSquares[KING] = 0;
364 /// Position::set_state() computes the hash keys of the position, and other
365 /// data that once computed is updated incrementally as moves are made.
366 /// The function is only used when a new position is set up, and to verify
367 /// the correctness of the StateInfo data when running in debug mode.
369 void Position::set_state(StateInfo* si) const {
371 si->key = si->materialKey = 0;
372 si->pawnKey = Zobrist::noPawns;
373 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
374 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
378 for (Bitboard b = pieces(); b; )
380 Square s = pop_lsb(&b);
381 Piece pc = piece_on(s);
382 si->key ^= Zobrist::psq[pc][s];
384 if (type_of(pc) == PAWN)
385 si->pawnKey ^= Zobrist::psq[pc][s];
387 else if (type_of(pc) != KING)
388 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
391 if (si->epSquare != SQ_NONE)
392 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
394 if (sideToMove == BLACK)
395 si->key ^= Zobrist::side;
397 si->key ^= Zobrist::castling[si->castlingRights];
399 for (Piece pc : Pieces)
400 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
401 si->materialKey ^= Zobrist::psq[pc][cnt];
405 /// Position::set() is an overload to initialize the position object with
406 /// the given endgame code string like "KBPKN". It is mainly a helper to
407 /// get the material key out of an endgame code.
409 Position& Position::set(const string& code, Color c, StateInfo* si) {
411 assert(code.length() > 0 && code.length() < 8);
412 assert(code[0] == 'K');
414 string sides[] = { code.substr(code.find('K', 1)), // Weak
415 code.substr(0, code.find('K', 1)) }; // Strong
417 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
419 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
420 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
422 return set(fenStr, false, si, nullptr);
426 /// Position::fen() returns a FEN representation of the position. In case of
427 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
429 const string Position::fen() const {
432 std::ostringstream ss;
434 for (Rank r = RANK_8; r >= RANK_1; --r)
436 for (File f = FILE_A; f <= FILE_H; ++f)
438 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
445 ss << PieceToChar[piece_on(make_square(f, r))];
452 ss << (sideToMove == WHITE ? " w " : " b ");
454 if (can_castle(WHITE_OO))
455 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
457 if (can_castle(WHITE_OOO))
458 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
460 if (can_castle(BLACK_OO))
461 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
463 if (can_castle(BLACK_OOO))
464 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
466 if (!can_castle(ANY_CASTLING))
469 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
470 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
476 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
477 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
478 /// slider if removing that piece from the board would result in a position where
479 /// square 's' is attacked. For example, a king-attack blocking piece can be either
480 /// a pinned or a discovered check piece, according if its color is the opposite
481 /// or the same of the color of the slider.
483 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
485 Bitboard blockers = 0;
488 // Snipers are sliders that attack 's' when a piece and other snipers are removed
489 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
490 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
491 Bitboard occupancy = pieces() ^ snipers;
495 Square sniperSq = pop_lsb(&snipers);
496 Bitboard b = between_bb(s, sniperSq) & occupancy;
498 if (b && !more_than_one(b))
501 if (b & pieces(color_of(piece_on(s))))
509 /// Position::attackers_to() computes a bitboard of all pieces which attack a
510 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
512 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
514 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
515 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
516 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
517 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
518 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
519 | (attacks_from<KING>(s) & pieces(KING));
523 /// Position::legal() tests whether a pseudo-legal move is legal
525 bool Position::legal(Move m) const {
529 Color us = sideToMove;
530 Square from = from_sq(m);
531 Square to = to_sq(m);
533 assert(color_of(moved_piece(m)) == us);
534 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
536 // En passant captures are a tricky special case. Because they are rather
537 // uncommon, we do it simply by testing whether the king is attacked after
539 if (type_of(m) == ENPASSANT)
541 Square ksq = square<KING>(us);
542 Square capsq = to - pawn_push(us);
543 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
545 assert(to == ep_square());
546 assert(moved_piece(m) == make_piece(us, PAWN));
547 assert(piece_on(capsq) == make_piece(~us, PAWN));
548 assert(piece_on(to) == NO_PIECE);
550 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
551 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
554 // Castling moves generation does not check if the castling path is clear of
555 // enemy attacks, it is delayed at a later time: now!
556 if (type_of(m) == CASTLING)
558 // After castling, the rook and king final positions are the same in
559 // Chess960 as they would be in standard chess.
560 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
561 Direction step = to > from ? WEST : EAST;
563 for (Square s = to; s != from; s += step)
564 if (attackers_to(s) & pieces(~us))
567 // In case of Chess960, verify that when moving the castling rook we do
568 // not discover some hidden checker.
569 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
571 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
574 // If the moving piece is a king, check whether the destination square is
575 // attacked by the opponent.
576 if (type_of(piece_on(from)) == KING)
577 return !(attackers_to(to) & pieces(~us));
579 // A non-king move is legal if and only if it is not pinned or it
580 // is moving along the ray towards or away from the king.
581 return !(blockers_for_king(us) & from)
582 || aligned(from, to, square<KING>(us));
586 /// Position::pseudo_legal() takes a random move and tests whether the move is
587 /// pseudo legal. It is used to validate moves from TT that can be corrupted
588 /// due to SMP concurrent access or hash position key aliasing.
590 bool Position::pseudo_legal(const Move m) const {
592 Color us = sideToMove;
593 Square from = from_sq(m);
594 Square to = to_sq(m);
595 Piece pc = moved_piece(m);
597 // Use a slower but simpler function for uncommon cases
598 if (type_of(m) != NORMAL)
599 return MoveList<LEGAL>(*this).contains(m);
601 // Is not a promotion, so promotion piece must be empty
602 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
605 // If the 'from' square is not occupied by a piece belonging to the side to
606 // move, the move is obviously not legal.
607 if (pc == NO_PIECE || color_of(pc) != us)
610 // The destination square cannot be occupied by a friendly piece
614 // Handle the special case of a pawn move
615 if (type_of(pc) == PAWN)
617 // We have already handled promotion moves, so destination
618 // cannot be on the 8th/1st rank.
619 if ((Rank8BB | Rank1BB) & to)
622 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
623 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
624 && !( (from + 2 * pawn_push(us) == to) // Not a double push
625 && (rank_of(from) == relative_rank(us, RANK_2))
627 && empty(to - pawn_push(us))))
630 else if (!(attacks_from(type_of(pc), from) & to))
633 // Evasions generator already takes care to avoid some kind of illegal moves
634 // and legal() relies on this. We therefore have to take care that the same
635 // kind of moves are filtered out here.
638 if (type_of(pc) != KING)
640 // Double check? In this case a king move is required
641 if (more_than_one(checkers()))
644 // Our move must be a blocking evasion or a capture of the checking piece
645 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
648 // In case of king moves under check we have to remove king so as to catch
649 // invalid moves like b1a1 when opposite queen is on c1.
650 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
658 /// Position::gives_check() tests whether a pseudo-legal move gives a check
660 bool Position::gives_check(Move m) const {
663 assert(color_of(moved_piece(m)) == sideToMove);
665 Square from = from_sq(m);
666 Square to = to_sq(m);
668 // Is there a direct check?
669 if (st->checkSquares[type_of(piece_on(from))] & to)
672 // Is there a discovered check?
673 if ( (st->blockersForKing[~sideToMove] & from)
674 && !aligned(from, to, square<KING>(~sideToMove)))
683 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
685 // En passant capture with check? We have already handled the case
686 // of direct checks and ordinary discovered check, so the only case we
687 // need to handle is the unusual case of a discovered check through
688 // the captured pawn.
691 Square capsq = make_square(file_of(to), rank_of(from));
692 Bitboard b = (pieces() ^ from ^ capsq) | to;
694 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
695 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
700 Square rfrom = to; // Castling is encoded as 'King captures the rook'
701 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
702 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
704 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
705 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
714 /// Position::do_move() makes a move, and saves all information necessary
715 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
716 /// moves should be filtered out before this function is called.
718 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
721 assert(&newSt != st);
723 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
724 Key k = st->key ^ Zobrist::side;
726 // Copy some fields of the old state to our new StateInfo object except the
727 // ones which are going to be recalculated from scratch anyway and then switch
728 // our state pointer to point to the new (ready to be updated) state.
729 std::memcpy(&newSt, st, offsetof(StateInfo, key));
733 // Increment ply counters. In particular, rule50 will be reset to zero later on
734 // in case of a capture or a pawn move.
739 Color us = sideToMove;
741 Square from = from_sq(m);
742 Square to = to_sq(m);
743 Piece pc = piece_on(from);
744 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
746 assert(color_of(pc) == us);
747 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
748 assert(type_of(captured) != KING);
750 if (type_of(m) == CASTLING)
752 assert(pc == make_piece(us, KING));
753 assert(captured == make_piece(us, ROOK));
756 do_castling<true>(us, from, to, rfrom, rto);
758 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
766 // If the captured piece is a pawn, update pawn hash key, otherwise
767 // update non-pawn material.
768 if (type_of(captured) == PAWN)
770 if (type_of(m) == ENPASSANT)
772 capsq -= pawn_push(us);
774 assert(pc == make_piece(us, PAWN));
775 assert(to == st->epSquare);
776 assert(relative_rank(us, to) == RANK_6);
777 assert(piece_on(to) == NO_PIECE);
778 assert(piece_on(capsq) == make_piece(them, PAWN));
780 board[capsq] = NO_PIECE; // Not done by remove_piece()
783 st->pawnKey ^= Zobrist::psq[captured][capsq];
786 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
788 // Update board and piece lists
789 remove_piece(captured, capsq);
791 // Update material hash key and prefetch access to materialTable
792 k ^= Zobrist::psq[captured][capsq];
793 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
794 prefetch(thisThread->materialTable[st->materialKey]);
796 // Reset rule 50 counter
801 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
803 // Reset en passant square
804 if (st->epSquare != SQ_NONE)
806 k ^= Zobrist::enpassant[file_of(st->epSquare)];
807 st->epSquare = SQ_NONE;
810 // Update castling rights if needed
811 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
813 int cr = castlingRightsMask[from] | castlingRightsMask[to];
814 k ^= Zobrist::castling[st->castlingRights & cr];
815 st->castlingRights &= ~cr;
818 // Move the piece. The tricky Chess960 castling is handled earlier
819 if (type_of(m) != CASTLING)
820 move_piece(pc, from, to);
822 // If the moving piece is a pawn do some special extra work
823 if (type_of(pc) == PAWN)
825 // Set en-passant square if the moved pawn can be captured
826 if ( (int(to) ^ int(from)) == 16
827 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
829 st->epSquare = to - pawn_push(us);
830 k ^= Zobrist::enpassant[file_of(st->epSquare)];
833 else if (type_of(m) == PROMOTION)
835 Piece promotion = make_piece(us, promotion_type(m));
837 assert(relative_rank(us, to) == RANK_8);
838 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
840 remove_piece(pc, to);
841 put_piece(promotion, to);
844 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
845 st->pawnKey ^= Zobrist::psq[pc][to];
846 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
847 ^ Zobrist::psq[pc][pieceCount[pc]];
850 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
853 // Update pawn hash key and prefetch access to pawnsTable
854 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
856 // Reset rule 50 draw counter
861 st->capturedPiece = captured;
863 // Update the key with the final value
866 // Calculate checkers bitboard (if move gives check)
867 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
869 sideToMove = ~sideToMove;
871 // Update king attacks used for fast check detection
874 // Calculate the repetition info. It is the ply distance from the previous
875 // occurrence of the same position, negative in the 3-fold case, or zero
876 // if the position was not repeated.
878 int end = std::min(st->rule50, st->pliesFromNull);
881 StateInfo* stp = st->previous->previous;
882 for (int i = 4; i <= end; i += 2)
884 stp = stp->previous->previous;
885 if (stp->key == st->key)
887 st->repetition = stp->repetition ? -i : i;
897 /// Position::undo_move() unmakes a move. When it returns, the position should
898 /// be restored to exactly the same state as before the move was made.
900 void Position::undo_move(Move m) {
904 sideToMove = ~sideToMove;
906 Color us = sideToMove;
907 Square from = from_sq(m);
908 Square to = to_sq(m);
909 Piece pc = piece_on(to);
911 assert(empty(from) || type_of(m) == CASTLING);
912 assert(type_of(st->capturedPiece) != KING);
914 if (type_of(m) == PROMOTION)
916 assert(relative_rank(us, to) == RANK_8);
917 assert(type_of(pc) == promotion_type(m));
918 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
920 remove_piece(pc, to);
921 pc = make_piece(us, PAWN);
925 if (type_of(m) == CASTLING)
928 do_castling<false>(us, from, to, rfrom, rto);
932 move_piece(pc, to, from); // Put the piece back at the source square
934 if (st->capturedPiece)
938 if (type_of(m) == ENPASSANT)
940 capsq -= pawn_push(us);
942 assert(type_of(pc) == PAWN);
943 assert(to == st->previous->epSquare);
944 assert(relative_rank(us, to) == RANK_6);
945 assert(piece_on(capsq) == NO_PIECE);
946 assert(st->capturedPiece == make_piece(~us, PAWN));
949 put_piece(st->capturedPiece, capsq); // Restore the captured piece
953 // Finally point our state pointer back to the previous state
961 /// Position::do_castling() is a helper used to do/undo a castling move. This
962 /// is a bit tricky in Chess960 where from/to squares can overlap.
964 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
966 bool kingSide = to > from;
967 rfrom = to; // Castling is encoded as "king captures friendly rook"
968 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
969 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
971 // Remove both pieces first since squares could overlap in Chess960
972 remove_piece(make_piece(us, KING), Do ? from : to);
973 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
974 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
975 put_piece(make_piece(us, KING), Do ? to : from);
976 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
980 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
981 /// the side to move without executing any move on the board.
983 void Position::do_null_move(StateInfo& newSt) {
986 assert(&newSt != st);
988 std::memcpy(&newSt, st, sizeof(StateInfo));
992 if (st->epSquare != SQ_NONE)
994 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
995 st->epSquare = SQ_NONE;
998 st->key ^= Zobrist::side;
999 prefetch(TT.first_entry(st->key));
1002 st->pliesFromNull = 0;
1004 sideToMove = ~sideToMove;
1010 assert(pos_is_ok());
1013 void Position::undo_null_move() {
1015 assert(!checkers());
1018 sideToMove = ~sideToMove;
1022 /// Position::key_after() computes the new hash key after the given move. Needed
1023 /// for speculative prefetch. It doesn't recognize special moves like castling,
1024 /// en-passant and promotions.
1026 Key Position::key_after(Move m) const {
1028 Square from = from_sq(m);
1029 Square to = to_sq(m);
1030 Piece pc = piece_on(from);
1031 Piece captured = piece_on(to);
1032 Key k = st->key ^ Zobrist::side;
1035 k ^= Zobrist::psq[captured][to];
1037 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1041 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1042 /// SEE value of move is greater or equal to the given threshold. We'll use an
1043 /// algorithm similar to alpha-beta pruning with a null window.
1045 bool Position::see_ge(Move m, Value threshold) const {
1049 // Only deal with normal moves, assume others pass a simple see
1050 if (type_of(m) != NORMAL)
1051 return VALUE_ZERO >= threshold;
1053 Bitboard stmAttackers;
1054 Square from = from_sq(m), to = to_sq(m);
1055 PieceType nextVictim = type_of(piece_on(from));
1056 Color us = color_of(piece_on(from));
1057 Color stm = ~us; // First consider opponent's move
1058 Value balance; // Values of the pieces taken by us minus opponent's ones
1060 // The opponent may be able to recapture so this is the best result
1062 balance = PieceValue[MG][piece_on(to)] - threshold;
1064 if (balance < VALUE_ZERO)
1067 // Now assume the worst possible result: that the opponent can
1068 // capture our piece for free.
1069 balance -= PieceValue[MG][nextVictim];
1071 // If it is enough (like in PxQ) then return immediately. Note that
1072 // in case nextVictim == KING we always return here, this is ok
1073 // if the given move is legal.
1074 if (balance >= VALUE_ZERO)
1077 // Find all attackers to the destination square, with the moving piece
1078 // removed, but possibly an X-ray attacker added behind it.
1079 Bitboard occupied = pieces() ^ from ^ to;
1080 Bitboard attackers = attackers_to(to, occupied) & occupied;
1084 stmAttackers = attackers & pieces(stm);
1086 // Don't allow pinned pieces to attack (except the king) as long as
1087 // any pinners are on their original square.
1088 if (st->pinners[~stm] & occupied)
1089 stmAttackers &= ~st->blockersForKing[stm];
1091 // If stm has no more attackers then give up: stm loses
1095 // Locate and remove the next least valuable attacker, and add to
1096 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1097 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1099 stm = ~stm; // Switch side to move
1101 // Negamax the balance with alpha = balance, beta = balance+1 and
1102 // add nextVictim's value.
1104 // (balance, balance+1) -> (-balance-1, -balance)
1106 assert(balance < VALUE_ZERO);
1108 balance = -balance - 1 - PieceValue[MG][nextVictim];
1110 // If balance is still non-negative after giving away nextVictim then we
1111 // win. The only thing to be careful about it is that we should revert
1112 // stm if we captured with the king when the opponent still has attackers.
1113 if (balance >= VALUE_ZERO)
1115 if (nextVictim == KING && (attackers & pieces(stm)))
1119 assert(nextVictim != KING);
1121 return us != stm; // We break the above loop when stm loses
1125 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1126 /// or by repetition. It does not detect stalemates.
1128 bool Position::is_draw(int ply) const {
1130 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1133 // Return a draw score if a position repeats once earlier but strictly
1134 // after the root, or repeats twice before or at the root.
1135 if (st->repetition && st->repetition < ply)
1142 // Position::has_repeated() tests whether there has been at least one repetition
1143 // of positions since the last capture or pawn move.
1145 bool Position::has_repeated() const {
1147 StateInfo* stc = st;
1148 int end = std::min(st->rule50, st->pliesFromNull);
1151 if (stc->repetition)
1154 stc = stc->previous;
1160 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1161 /// or an earlier position has a move that directly reaches the current position.
1163 bool Position::has_game_cycle(int ply) const {
1167 int end = std::min(st->rule50, st->pliesFromNull);
1172 Key originalKey = st->key;
1173 StateInfo* stp = st->previous;
1175 for (int i = 3; i <= end; i += 2)
1177 stp = stp->previous->previous;
1179 Key moveKey = originalKey ^ stp->key;
1180 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1181 || (j = H2(moveKey), cuckoo[j] == moveKey))
1183 Move move = cuckooMove[j];
1184 Square s1 = from_sq(move);
1185 Square s2 = to_sq(move);
1187 if (!(between_bb(s1, s2) & pieces()))
1192 // For nodes before or at the root, check that the move is a
1193 // repetition rather than a move to the current position.
1194 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1195 // the same location, so we have to select which square to check.
1196 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1199 // For repetitions before or at the root, require one more
1200 if (stp->repetition)
1209 /// Position::flip() flips position with the white and black sides reversed. This
1210 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1212 void Position::flip() {
1215 std::stringstream ss(fen());
1217 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1219 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1220 f.insert(0, token + (f.empty() ? " " : "/"));
1223 ss >> token; // Active color
1224 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1226 ss >> token; // Castling availability
1229 std::transform(f.begin(), f.end(), f.begin(),
1230 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1232 ss >> token; // En passant square
1233 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1235 std::getline(ss, token); // Half and full moves
1238 set(f, is_chess960(), st, this_thread());
1240 assert(pos_is_ok());
1244 /// Position::pos_is_ok() performs some consistency checks for the
1245 /// position object and raises an asserts if something wrong is detected.
1246 /// This is meant to be helpful when debugging.
1248 bool Position::pos_is_ok() const {
1250 constexpr bool Fast = true; // Quick (default) or full check?
1252 if ( (sideToMove != WHITE && sideToMove != BLACK)
1253 || piece_on(square<KING>(WHITE)) != W_KING
1254 || piece_on(square<KING>(BLACK)) != B_KING
1255 || ( ep_square() != SQ_NONE
1256 && relative_rank(sideToMove, ep_square()) != RANK_6))
1257 assert(0 && "pos_is_ok: Default");
1262 if ( pieceCount[W_KING] != 1
1263 || pieceCount[B_KING] != 1
1264 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1265 assert(0 && "pos_is_ok: Kings");
1267 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1268 || pieceCount[W_PAWN] > 8
1269 || pieceCount[B_PAWN] > 8)
1270 assert(0 && "pos_is_ok: Pawns");
1272 if ( (pieces(WHITE) & pieces(BLACK))
1273 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1274 || popcount(pieces(WHITE)) > 16
1275 || popcount(pieces(BLACK)) > 16)
1276 assert(0 && "pos_is_ok: Bitboards");
1278 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1279 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1280 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1281 assert(0 && "pos_is_ok: Bitboards");
1285 if (std::memcmp(&si, st, sizeof(StateInfo)))
1286 assert(0 && "pos_is_ok: State");
1288 for (Piece pc : Pieces)
1290 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1291 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1292 assert(0 && "pos_is_ok: Pieces");
1294 for (int i = 0; i < pieceCount[pc]; ++i)
1295 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1296 assert(0 && "pos_is_ok: Index");
1299 for (Color c : { WHITE, BLACK })
1300 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1302 if (!can_castle(cr))
1305 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1306 || castlingRightsMask[castlingRookSquare[cr]] != cr
1307 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1308 assert(0 && "pos_is_ok: Castling");