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) 2015-2018 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"
40 extern Score psq[PIECE_NB][SQUARE_NB];
45 Key psq[PIECE_NB][SQUARE_NB];
46 Key enpassant[FILE_NB];
47 Key castling[CASTLING_RIGHT_NB];
53 const string PieceToChar(" PNBRQK pnbrqk");
55 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
56 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
58 // min_attacker() is a helper function used by see_ge() to locate the least
59 // valuable attacker for the side to move, remove the attacker we just found
60 // from the bitboards and scan for new X-ray attacks behind it.
63 PieceType min_attacker(const Bitboard* byTypeBB, Square to, Bitboard stmAttackers,
64 Bitboard& occupied, Bitboard& attackers) {
66 Bitboard b = stmAttackers & byTypeBB[Pt];
68 return min_attacker<Pt + 1>(byTypeBB, to, stmAttackers, occupied, attackers);
70 occupied ^= lsb(b); // Remove the attacker from occupied
72 // Add any X-ray attack behind the just removed piece. For instance with
73 // rooks in a8 and a7 attacking a1, after removing a7 we add rook in a8.
74 // Note that new added attackers can be of any color.
75 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
76 attackers |= attacks_bb<BISHOP>(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]);
78 if (Pt == ROOK || Pt == QUEEN)
79 attackers |= attacks_bb<ROOK>(to, occupied) & (byTypeBB[ROOK] | byTypeBB[QUEEN]);
81 // X-ray may add already processed pieces because byTypeBB[] is constant: in
82 // the rook example, now attackers contains _again_ rook in a7, so remove it.
83 attackers &= occupied;
88 PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
89 return KING; // No need to update bitboards: it is the last cycle
95 /// operator<<(Position) returns an ASCII representation of the position
97 std::ostream& operator<<(std::ostream& os, const Position& pos) {
99 os << "\n +---+---+---+---+---+---+---+---+\n";
101 for (Rank r = RANK_8; r >= RANK_1; --r)
103 for (File f = FILE_A; f <= FILE_H; ++f)
104 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
106 os << " |\n +---+---+---+---+---+---+---+---+\n";
109 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
110 << std::setfill('0') << std::setw(16) << pos.key()
111 << std::setfill(' ') << std::dec << "\nCheckers: ";
113 for (Bitboard b = pos.checkers(); b; )
114 os << UCI::square(pop_lsb(&b)) << " ";
116 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
117 && !pos.can_castle(ANY_CASTLING))
121 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
122 Tablebases::ProbeState s1, s2;
123 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
124 int dtz = Tablebases::probe_dtz(p, &s2);
125 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
126 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
133 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
134 // situations. Description of the algorithm in the following paper:
135 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
137 // First and second hash functions for indexing the cuckoo tables
138 inline int H1(Key h) { return h & 0x1fff; }
139 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
141 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
143 Move cuckooMove[8192];
146 /// Position::init() initializes at startup the various arrays used to compute
149 void Position::init() {
153 for (Piece pc : Pieces)
154 for (Square s = SQ_A1; s <= SQ_H8; ++s)
155 Zobrist::psq[pc][s] = rng.rand<Key>();
157 for (File f = FILE_A; f <= FILE_H; ++f)
158 Zobrist::enpassant[f] = rng.rand<Key>();
160 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
162 Zobrist::castling[cr] = 0;
166 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
167 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
171 Zobrist::side = rng.rand<Key>();
172 Zobrist::noPawns = rng.rand<Key>();
174 // Prepare the cuckoo tables
176 for (Piece pc : Pieces)
177 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
178 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
179 if (PseudoAttacks[type_of(pc)][s1] & s2)
181 Move move = make_move(s1, s2);
182 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
186 std::swap(cuckoo[i], key);
187 std::swap(cuckooMove[i], move);
188 if (move == 0) // Arrived at empty slot ?
190 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
194 assert(count == 3668);
198 /// Position::set() initializes the position object with the given FEN string.
199 /// This function is not very robust - make sure that input FENs are correct,
200 /// this is assumed to be the responsibility of the GUI.
202 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
204 A FEN string defines a particular position using only the ASCII character set.
206 A FEN string contains six fields separated by a space. The fields are:
208 1) Piece placement (from white's perspective). Each rank is described, starting
209 with rank 8 and ending with rank 1. Within each rank, the contents of each
210 square are described from file A through file H. Following the Standard
211 Algebraic Notation (SAN), each piece is identified by a single letter taken
212 from the standard English names. White pieces are designated using upper-case
213 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
214 noted using digits 1 through 8 (the number of blank squares), and "/"
217 2) Active color. "w" means white moves next, "b" means black.
219 3) Castling availability. If neither side can castle, this is "-". Otherwise,
220 this has one or more letters: "K" (White can castle kingside), "Q" (White
221 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
222 can castle queenside).
224 4) En passant target square (in algebraic notation). If there's no en passant
225 target square, this is "-". If a pawn has just made a 2-square move, this
226 is the position "behind" the pawn. This is recorded only if there is a pawn
227 in position to make an en passant capture, and if there really is a pawn
228 that might have advanced two squares.
230 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
231 or capture. This is used to determine if a draw can be claimed under the
234 6) Fullmove number. The number of the full move. It starts at 1, and is
235 incremented after Black's move.
238 unsigned char col, row, token;
241 std::istringstream ss(fenStr);
243 std::memset(this, 0, sizeof(Position));
244 std::memset(si, 0, sizeof(StateInfo));
245 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
250 // 1. Piece placement
251 while ((ss >> token) && !isspace(token))
254 sq += (token - '0') * EAST; // Advance the given number of files
256 else if (token == '/')
259 else if ((idx = PieceToChar.find(token)) != string::npos)
261 put_piece(Piece(idx), sq);
268 sideToMove = (token == 'w' ? WHITE : BLACK);
271 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
272 // Shredder-FEN that uses the letters of the columns on which the rooks began
273 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
274 // if an inner rook is associated with the castling right, the castling tag is
275 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
276 while ((ss >> token) && !isspace(token))
279 Color c = islower(token) ? BLACK : WHITE;
280 Piece rook = make_piece(c, ROOK);
282 token = char(toupper(token));
285 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
287 else if (token == 'Q')
288 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
290 else if (token >= 'A' && token <= 'H')
291 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
296 set_castling_right(c, rsq);
299 // 4. En passant square. Ignore if no pawn capture is possible
300 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
301 && ((ss >> row) && (row == '3' || row == '6')))
303 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
305 if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
306 || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
307 st->epSquare = SQ_NONE;
310 st->epSquare = SQ_NONE;
312 // 5-6. Halfmove clock and fullmove number
313 ss >> std::skipws >> st->rule50 >> gamePly;
315 // Convert from fullmove starting from 1 to gamePly starting from 0,
316 // handle also common incorrect FEN with fullmove = 0.
317 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
319 chess960 = isChess960;
329 /// Position::set_castling_right() is a helper function used to set castling
330 /// rights given the corresponding color and the rook starting square.
332 void Position::set_castling_right(Color c, Square rfrom) {
334 Square kfrom = square<KING>(c);
335 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
336 CastlingRight cr = (c | cs);
338 st->castlingRights |= cr;
339 castlingRightsMask[kfrom] |= cr;
340 castlingRightsMask[rfrom] |= cr;
341 castlingRookSquare[cr] = rfrom;
343 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
344 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
346 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
347 if (s != kfrom && s != rfrom)
348 castlingPath[cr] |= s;
350 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
351 if (s != kfrom && s != rfrom)
352 castlingPath[cr] |= s;
356 /// Position::set_check_info() sets king attacks to detect if a move gives check
358 void Position::set_check_info(StateInfo* si) const {
360 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
361 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
363 Square ksq = square<KING>(~sideToMove);
365 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
366 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
367 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
368 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
369 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
370 si->checkSquares[KING] = 0;
374 /// Position::set_state() computes the hash keys of the position, and other
375 /// data that once computed is updated incrementally as moves are made.
376 /// The function is only used when a new position is set up, and to verify
377 /// the correctness of the StateInfo data when running in debug mode.
379 void Position::set_state(StateInfo* si) const {
381 si->key = si->materialKey = 0;
382 si->pawnKey = Zobrist::noPawns;
383 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
384 si->psq = SCORE_ZERO;
385 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
389 for (Bitboard b = pieces(); b; )
391 Square s = pop_lsb(&b);
392 Piece pc = piece_on(s);
393 si->key ^= Zobrist::psq[pc][s];
394 si->psq += PSQT::psq[pc][s];
397 if (si->epSquare != SQ_NONE)
398 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
400 if (sideToMove == BLACK)
401 si->key ^= Zobrist::side;
403 si->key ^= Zobrist::castling[si->castlingRights];
405 for (Bitboard b = pieces(PAWN); b; )
407 Square s = pop_lsb(&b);
408 si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
411 for (Piece pc : Pieces)
413 if (type_of(pc) != PAWN && type_of(pc) != KING)
414 si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
416 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
417 si->materialKey ^= Zobrist::psq[pc][cnt];
422 /// Position::set() is an overload to initialize the position object with
423 /// the given endgame code string like "KBPKN". It is mainly a helper to
424 /// get the material key out of an endgame code.
426 Position& Position::set(const string& code, Color c, StateInfo* si) {
428 assert(code.length() > 0 && code.length() < 8);
429 assert(code[0] == 'K');
431 string sides[] = { code.substr(code.find('K', 1)), // Weak
432 code.substr(0, code.find('K', 1)) }; // Strong
434 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
436 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
437 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
439 return set(fenStr, false, si, nullptr);
443 /// Position::fen() returns a FEN representation of the position. In case of
444 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
446 const string Position::fen() const {
449 std::ostringstream ss;
451 for (Rank r = RANK_8; r >= RANK_1; --r)
453 for (File f = FILE_A; f <= FILE_H; ++f)
455 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
462 ss << PieceToChar[piece_on(make_square(f, r))];
469 ss << (sideToMove == WHITE ? " w " : " b ");
471 if (can_castle(WHITE_OO))
472 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | KING_SIDE))) : 'K');
474 if (can_castle(WHITE_OOO))
475 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
477 if (can_castle(BLACK_OO))
478 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | KING_SIDE))) : 'k');
480 if (can_castle(BLACK_OOO))
481 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
483 if (!can_castle(WHITE) && !can_castle(BLACK))
486 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
487 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
493 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
494 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
495 /// slider if removing that piece from the board would result in a position where
496 /// square 's' is attacked. For example, a king-attack blocking piece can be either
497 /// a pinned or a discovered check piece, according if its color is the opposite
498 /// or the same of the color of the slider.
500 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
502 Bitboard blockers = 0;
505 // Snipers are sliders that attack 's' when a piece is removed
506 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
507 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
511 Square sniperSq = pop_lsb(&snipers);
512 Bitboard b = between_bb(s, sniperSq) & pieces();
514 if (b && !more_than_one(b))
517 if (b & pieces(color_of(piece_on(s))))
525 /// Position::attackers_to() computes a bitboard of all pieces which attack a
526 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
528 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
530 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
531 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
532 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
533 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
534 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
535 | (attacks_from<KING>(s) & pieces(KING));
539 /// Position::legal() tests whether a pseudo-legal move is legal
541 bool Position::legal(Move m) const {
545 Color us = sideToMove;
546 Square from = from_sq(m);
548 assert(color_of(moved_piece(m)) == us);
549 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
551 // En passant captures are a tricky special case. Because they are rather
552 // uncommon, we do it simply by testing whether the king is attacked after
554 if (type_of(m) == ENPASSANT)
556 Square ksq = square<KING>(us);
557 Square to = to_sq(m);
558 Square capsq = to - pawn_push(us);
559 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
561 assert(to == ep_square());
562 assert(moved_piece(m) == make_piece(us, PAWN));
563 assert(piece_on(capsq) == make_piece(~us, PAWN));
564 assert(piece_on(to) == NO_PIECE);
566 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
567 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
570 // If the moving piece is a king, check whether the destination
571 // square is attacked by the opponent. Castling moves are checked
572 // for legality during move generation.
573 if (type_of(piece_on(from)) == KING)
574 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
576 // A non-king move is legal if and only if it is not pinned or it
577 // is moving along the ray towards or away from the king.
578 return !(blockers_for_king(us) & from)
579 || aligned(from, to_sq(m), square<KING>(us));
583 /// Position::pseudo_legal() takes a random move and tests whether the move is
584 /// pseudo legal. It is used to validate moves from TT that can be corrupted
585 /// due to SMP concurrent access or hash position key aliasing.
587 bool Position::pseudo_legal(const Move m) const {
589 Color us = sideToMove;
590 Square from = from_sq(m);
591 Square to = to_sq(m);
592 Piece pc = moved_piece(m);
594 // Use a slower but simpler function for uncommon cases
595 if (type_of(m) != NORMAL)
596 return MoveList<LEGAL>(*this).contains(m);
598 // Is not a promotion, so promotion piece must be empty
599 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
602 // If the 'from' square is not occupied by a piece belonging to the side to
603 // move, the move is obviously not legal.
604 if (pc == NO_PIECE || color_of(pc) != us)
607 // The destination square cannot be occupied by a friendly piece
611 // Handle the special case of a pawn move
612 if (type_of(pc) == PAWN)
614 // We have already handled promotion moves, so destination
615 // cannot be on the 8th/1st rank.
616 if (rank_of(to) == relative_rank(us, RANK_8))
619 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
620 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
621 && !( (from + 2 * pawn_push(us) == to) // Not a double push
622 && (rank_of(from) == relative_rank(us, RANK_2))
624 && empty(to - pawn_push(us))))
627 else if (!(attacks_from(type_of(pc), from) & to))
630 // Evasions generator already takes care to avoid some kind of illegal moves
631 // and legal() relies on this. We therefore have to take care that the same
632 // kind of moves are filtered out here.
635 if (type_of(pc) != KING)
637 // Double check? In this case a king move is required
638 if (more_than_one(checkers()))
641 // Our move must be a blocking evasion or a capture of the checking piece
642 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
645 // In case of king moves under check we have to remove king so as to catch
646 // invalid moves like b1a1 when opposite queen is on c1.
647 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
655 /// Position::gives_check() tests whether a pseudo-legal move gives a check
657 bool Position::gives_check(Move m) const {
660 assert(color_of(moved_piece(m)) == sideToMove);
662 Square from = from_sq(m);
663 Square to = to_sq(m);
665 // Is there a direct check?
666 if (st->checkSquares[type_of(piece_on(from))] & to)
669 // Is there a discovered check?
670 if ( (st->blockersForKing[~sideToMove] & from)
671 && !aligned(from, to, square<KING>(~sideToMove)))
680 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
682 // En passant capture with check? We have already handled the case
683 // of direct checks and ordinary discovered check, so the only case we
684 // need to handle is the unusual case of a discovered check through
685 // the captured pawn.
688 Square capsq = make_square(file_of(to), rank_of(from));
689 Bitboard b = (pieces() ^ from ^ capsq) | to;
691 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
692 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
697 Square rfrom = to; // Castling is encoded as 'King captures the rook'
698 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
699 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
701 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
702 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
711 /// Position::do_move() makes a move, and saves all information necessary
712 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
713 /// moves should be filtered out before this function is called.
715 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
718 assert(&newSt != st);
720 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
721 Key k = st->key ^ Zobrist::side;
723 // Copy some fields of the old state to our new StateInfo object except the
724 // ones which are going to be recalculated from scratch anyway and then switch
725 // our state pointer to point to the new (ready to be updated) state.
726 std::memcpy(&newSt, st, offsetof(StateInfo, key));
730 // Increment ply counters. In particular, rule50 will be reset to zero later on
731 // in case of a capture or a pawn move.
736 Color us = sideToMove;
738 Square from = from_sq(m);
739 Square to = to_sq(m);
740 Piece pc = piece_on(from);
741 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
743 assert(color_of(pc) == us);
744 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
745 assert(type_of(captured) != KING);
747 if (type_of(m) == CASTLING)
749 assert(pc == make_piece(us, KING));
750 assert(captured == make_piece(us, ROOK));
753 do_castling<true>(us, from, to, rfrom, rto);
755 st->psq += PSQT::psq[captured][rto] - PSQT::psq[captured][rfrom];
756 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
764 // If the captured piece is a pawn, update pawn hash key, otherwise
765 // update non-pawn material.
766 if (type_of(captured) == PAWN)
768 if (type_of(m) == ENPASSANT)
770 capsq -= pawn_push(us);
772 assert(pc == make_piece(us, PAWN));
773 assert(to == st->epSquare);
774 assert(relative_rank(us, to) == RANK_6);
775 assert(piece_on(to) == NO_PIECE);
776 assert(piece_on(capsq) == make_piece(them, PAWN));
778 board[capsq] = NO_PIECE; // Not done by remove_piece()
781 st->pawnKey ^= Zobrist::psq[captured][capsq];
784 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
786 // Update board and piece lists
787 remove_piece(captured, capsq);
789 // Update material hash key and prefetch access to materialTable
790 k ^= Zobrist::psq[captured][capsq];
791 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
792 prefetch(thisThread->materialTable[st->materialKey]);
794 // Update incremental scores
795 st->psq -= PSQT::psq[captured][capsq];
797 // Reset rule 50 counter
802 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
804 // Reset en passant square
805 if (st->epSquare != SQ_NONE)
807 k ^= Zobrist::enpassant[file_of(st->epSquare)];
808 st->epSquare = SQ_NONE;
811 // Update castling rights if needed
812 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
814 int cr = castlingRightsMask[from] | castlingRightsMask[to];
815 k ^= Zobrist::castling[st->castlingRights & cr];
816 st->castlingRights &= ~cr;
819 // Move the piece. The tricky Chess960 castling is handled earlier
820 if (type_of(m) != CASTLING)
821 move_piece(pc, from, to);
823 // If the moving piece is a pawn do some special extra work
824 if (type_of(pc) == PAWN)
826 // Set en-passant square if the moved pawn can be captured
827 if ( (int(to) ^ int(from)) == 16
828 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
830 st->epSquare = to - pawn_push(us);
831 k ^= Zobrist::enpassant[file_of(st->epSquare)];
834 else if (type_of(m) == PROMOTION)
836 Piece promotion = make_piece(us, promotion_type(m));
838 assert(relative_rank(us, to) == RANK_8);
839 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
841 remove_piece(pc, to);
842 put_piece(promotion, to);
845 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
846 st->pawnKey ^= Zobrist::psq[pc][to];
847 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
848 ^ Zobrist::psq[pc][pieceCount[pc]];
850 // Update incremental score
851 st->psq += PSQT::psq[promotion][to] - PSQT::psq[pc][to];
854 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
857 // Update pawn hash key and prefetch access to pawnsTable
858 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
859 prefetch2(thisThread->pawnsTable[st->pawnKey]);
861 // Reset rule 50 draw counter
865 // Update incremental scores
866 st->psq += PSQT::psq[pc][to] - PSQT::psq[pc][from];
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 // all 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 = st->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 from = from_sq(move);
1197 Square to = to_sq(move);
1199 if (!(between_bb(from, to) & pieces()))
1201 // Take care to reverse the move in the no-progress case (opponent to move)
1203 move = make_move(to, from);
1208 // For repetitions before or at the root, require one more
1209 StateInfo* next_stp = stp;
1210 for (int k = i + 2; k <= end; k += 2)
1212 next_stp = next_stp->previous->previous;
1213 if (next_stp->key == stp->key)
1223 /// Position::flip() flips position with the white and black sides reversed. This
1224 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1226 void Position::flip() {
1229 std::stringstream ss(fen());
1231 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1233 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1234 f.insert(0, token + (f.empty() ? " " : "/"));
1237 ss >> token; // Active color
1238 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1240 ss >> token; // Castling availability
1243 std::transform(f.begin(), f.end(), f.begin(),
1244 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1246 ss >> token; // En passant square
1247 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1249 std::getline(ss, token); // Half and full moves
1252 set(f, is_chess960(), st, this_thread());
1254 assert(pos_is_ok());
1258 /// Position::pos_is_ok() performs some consistency checks for the
1259 /// position object and raises an asserts if something wrong is detected.
1260 /// This is meant to be helpful when debugging.
1262 bool Position::pos_is_ok() const {
1264 constexpr bool Fast = true; // Quick (default) or full check?
1266 if ( (sideToMove != WHITE && sideToMove != BLACK)
1267 || piece_on(square<KING>(WHITE)) != W_KING
1268 || piece_on(square<KING>(BLACK)) != B_KING
1269 || ( ep_square() != SQ_NONE
1270 && relative_rank(sideToMove, ep_square()) != RANK_6))
1271 assert(0 && "pos_is_ok: Default");
1276 if ( pieceCount[W_KING] != 1
1277 || pieceCount[B_KING] != 1
1278 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1279 assert(0 && "pos_is_ok: Kings");
1281 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1282 || pieceCount[W_PAWN] > 8
1283 || pieceCount[B_PAWN] > 8)
1284 assert(0 && "pos_is_ok: Pawns");
1286 if ( (pieces(WHITE) & pieces(BLACK))
1287 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1288 || popcount(pieces(WHITE)) > 16
1289 || popcount(pieces(BLACK)) > 16)
1290 assert(0 && "pos_is_ok: Bitboards");
1292 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1293 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1294 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1295 assert(0 && "pos_is_ok: Bitboards");
1299 if (std::memcmp(&si, st, sizeof(StateInfo)))
1300 assert(0 && "pos_is_ok: State");
1302 for (Piece pc : Pieces)
1304 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1305 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1306 assert(0 && "pos_is_ok: Pieces");
1308 for (int i = 0; i < pieceCount[pc]; ++i)
1309 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1310 assert(0 && "pos_is_ok: Index");
1313 for (Color c = WHITE; c <= BLACK; ++c)
1314 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1316 if (!can_castle(c | s))
1319 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1320 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1321 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1322 assert(0 && "pos_is_ok: Castling");