2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
23 #include <cstddef> // For offsetof()
24 #include <cstring> // For std::memset, std::memcmp
35 #include "syzygy/tbprobe.h"
41 Key psq[PIECE_NB][SQUARE_NB];
42 Key enpassant[FILE_NB];
43 Key castling[CASTLING_RIGHT_NB];
49 const string PieceToChar(" PNBRQK pnbrqk");
51 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
52 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
56 /// operator<<(Position) returns an ASCII representation of the position
58 std::ostream& operator<<(std::ostream& os, const Position& pos) {
60 os << "\n +---+---+---+---+---+---+---+---+\n";
62 for (Rank r = RANK_8; r >= RANK_1; --r)
64 for (File f = FILE_A; f <= FILE_H; ++f)
65 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
67 os << " |\n +---+---+---+---+---+---+---+---+\n";
70 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
71 << std::setfill('0') << std::setw(16) << pos.key()
72 << std::setfill(' ') << std::dec << "\nCheckers: ";
74 for (Bitboard b = pos.checkers(); b; )
75 os << UCI::square(pop_lsb(&b)) << " ";
77 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
78 && !pos.can_castle(ANY_CASTLING))
82 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
83 Tablebases::ProbeState s1, s2;
84 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
85 int dtz = Tablebases::probe_dtz(p, &s2);
86 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
87 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
94 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
95 // situations. Description of the algorithm in the following paper:
96 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
98 // First and second hash functions for indexing the cuckoo tables
99 inline int H1(Key h) { return h & 0x1fff; }
100 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
102 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
104 Move cuckooMove[8192];
107 /// Position::init() initializes at startup the various arrays used to compute
110 void Position::init() {
114 for (Piece pc : Pieces)
115 for (Square s = SQ_A1; s <= SQ_H8; ++s)
116 Zobrist::psq[pc][s] = rng.rand<Key>();
118 for (File f = FILE_A; f <= FILE_H; ++f)
119 Zobrist::enpassant[f] = rng.rand<Key>();
121 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
123 Zobrist::castling[cr] = 0;
127 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
128 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
132 Zobrist::side = rng.rand<Key>();
133 Zobrist::noPawns = rng.rand<Key>();
135 // Prepare the cuckoo tables
136 std::memset(cuckoo, 0, sizeof(cuckoo));
137 std::memset(cuckooMove, 0, sizeof(cuckooMove));
139 for (Piece pc : Pieces)
140 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
141 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
142 if (PseudoAttacks[type_of(pc)][s1] & s2)
144 Move move = make_move(s1, s2);
145 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
149 std::swap(cuckoo[i], key);
150 std::swap(cuckooMove[i], move);
151 if (move == MOVE_NONE) // Arrived at empty slot?
153 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
157 assert(count == 3668);
161 /// Position::set() initializes the position object with the given FEN string.
162 /// This function is not very robust - make sure that input FENs are correct,
163 /// this is assumed to be the responsibility of the GUI.
165 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
167 A FEN string defines a particular position using only the ASCII character set.
169 A FEN string contains six fields separated by a space. The fields are:
171 1) Piece placement (from white's perspective). Each rank is described, starting
172 with rank 8 and ending with rank 1. Within each rank, the contents of each
173 square are described from file A through file H. Following the Standard
174 Algebraic Notation (SAN), each piece is identified by a single letter taken
175 from the standard English names. White pieces are designated using upper-case
176 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
177 noted using digits 1 through 8 (the number of blank squares), and "/"
180 2) Active color. "w" means white moves next, "b" means black.
182 3) Castling availability. If neither side can castle, this is "-". Otherwise,
183 this has one or more letters: "K" (White can castle kingside), "Q" (White
184 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
185 can castle queenside).
187 4) En passant target square (in algebraic notation). If there's no en passant
188 target square, this is "-". If a pawn has just made a 2-square move, this
189 is the position "behind" the pawn. This is recorded only if there is a pawn
190 in position to make an en passant capture, and if there really is a pawn
191 that might have advanced two squares.
193 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
194 or capture. This is used to determine if a draw can be claimed under the
197 6) Fullmove number. The number of the full move. It starts at 1, and is
198 incremented after Black's move.
201 unsigned char col, row, token;
204 std::istringstream ss(fenStr);
206 std::memset(this, 0, sizeof(Position));
207 std::memset(si, 0, sizeof(StateInfo));
208 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
213 // 1. Piece placement
214 while ((ss >> token) && !isspace(token))
217 sq += (token - '0') * EAST; // Advance the given number of files
219 else if (token == '/')
222 else if ((idx = PieceToChar.find(token)) != string::npos)
224 put_piece(Piece(idx), sq);
231 sideToMove = (token == 'w' ? WHITE : BLACK);
234 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
235 // Shredder-FEN that uses the letters of the columns on which the rooks began
236 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
237 // if an inner rook is associated with the castling right, the castling tag is
238 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
239 while ((ss >> token) && !isspace(token))
242 Color c = islower(token) ? BLACK : WHITE;
243 Piece rook = make_piece(c, ROOK);
245 token = char(toupper(token));
248 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
250 else if (token == 'Q')
251 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
253 else if (token >= 'A' && token <= 'H')
254 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
259 set_castling_right(c, rsq);
262 // 4. En passant square. Ignore if no pawn capture is possible
263 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
264 && ((ss >> row) && (row == '3' || row == '6')))
266 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
268 if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
269 || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
270 st->epSquare = SQ_NONE;
273 st->epSquare = SQ_NONE;
275 // 5-6. Halfmove clock and fullmove number
276 ss >> std::skipws >> st->rule50 >> gamePly;
278 // Convert from fullmove starting from 1 to gamePly starting from 0,
279 // handle also common incorrect FEN with fullmove = 0.
280 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
282 chess960 = isChess960;
290 /// Position::set_castling_right() is a helper function used to set castling
291 /// rights given the corresponding color and the rook starting square.
293 void Position::set_castling_right(Color c, Square rfrom) {
295 Square kfrom = square<KING>(c);
296 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
298 st->castlingRights |= cr;
299 castlingRightsMask[kfrom] |= cr;
300 castlingRightsMask[rfrom] |= cr;
301 castlingRookSquare[cr] = rfrom;
303 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
304 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
306 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
307 & ~(square_bb(kfrom) | rfrom);
311 /// Position::set_check_info() sets king attacks to detect if a move gives check
313 void Position::set_check_info(StateInfo* si) const {
315 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
316 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
318 Square ksq = square<KING>(~sideToMove);
320 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
321 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
322 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
323 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
324 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
325 si->checkSquares[KING] = 0;
329 /// Position::set_state() computes the hash keys of the position, and other
330 /// data that once computed is updated incrementally as moves are made.
331 /// The function is only used when a new position is set up, and to verify
332 /// the correctness of the StateInfo data when running in debug mode.
334 void Position::set_state(StateInfo* si) const {
336 si->key = si->materialKey = 0;
337 si->pawnKey = Zobrist::noPawns;
338 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
339 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
343 for (Bitboard b = pieces(); b; )
345 Square s = pop_lsb(&b);
346 Piece pc = piece_on(s);
347 si->key ^= Zobrist::psq[pc][s];
349 if (type_of(pc) == PAWN)
350 si->pawnKey ^= Zobrist::psq[pc][s];
352 else if (type_of(pc) != KING)
353 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
356 if (si->epSquare != SQ_NONE)
357 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
359 if (sideToMove == BLACK)
360 si->key ^= Zobrist::side;
362 si->key ^= Zobrist::castling[si->castlingRights];
364 for (Piece pc : Pieces)
365 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
366 si->materialKey ^= Zobrist::psq[pc][cnt];
370 /// Position::set() is an overload to initialize the position object with
371 /// the given endgame code string like "KBPKN". It is mainly a helper to
372 /// get the material key out of an endgame code.
374 Position& Position::set(const string& code, Color c, StateInfo* si) {
376 assert(code.length() > 0 && code.length() < 8);
377 assert(code[0] == 'K');
379 string sides[] = { code.substr(code.find('K', 1)), // Weak
380 code.substr(0, code.find('K', 1)) }; // Strong
382 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
384 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
385 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
387 return set(fenStr, false, si, nullptr);
391 /// Position::fen() returns a FEN representation of the position. In case of
392 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
394 const string Position::fen() const {
397 std::ostringstream ss;
399 for (Rank r = RANK_8; r >= RANK_1; --r)
401 for (File f = FILE_A; f <= FILE_H; ++f)
403 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
410 ss << PieceToChar[piece_on(make_square(f, r))];
417 ss << (sideToMove == WHITE ? " w " : " b ");
419 if (can_castle(WHITE_OO))
420 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
422 if (can_castle(WHITE_OOO))
423 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
425 if (can_castle(BLACK_OO))
426 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
428 if (can_castle(BLACK_OOO))
429 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
431 if (!can_castle(ANY_CASTLING))
434 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
435 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
441 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
442 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
443 /// slider if removing that piece from the board would result in a position where
444 /// square 's' is attacked. For example, a king-attack blocking piece can be either
445 /// a pinned or a discovered check piece, according if its color is the opposite
446 /// or the same of the color of the slider.
448 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
450 Bitboard blockers = 0;
453 // Snipers are sliders that attack 's' when a piece and other snipers are removed
454 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
455 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
456 Bitboard occupancy = pieces() ^ snipers;
460 Square sniperSq = pop_lsb(&snipers);
461 Bitboard b = between_bb(s, sniperSq) & occupancy;
463 if (b && !more_than_one(b))
466 if (b & pieces(color_of(piece_on(s))))
474 /// Position::attackers_to() computes a bitboard of all pieces which attack a
475 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
477 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
479 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
480 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
481 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
482 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
483 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
484 | (attacks_from<KING>(s) & pieces(KING));
488 /// Position::legal() tests whether a pseudo-legal move is legal
490 bool Position::legal(Move m) const {
494 Color us = sideToMove;
495 Square from = from_sq(m);
496 Square to = to_sq(m);
498 assert(color_of(moved_piece(m)) == us);
499 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
501 // En passant captures are a tricky special case. Because they are rather
502 // uncommon, we do it simply by testing whether the king is attacked after
504 if (type_of(m) == ENPASSANT)
506 Square ksq = square<KING>(us);
507 Square capsq = to - pawn_push(us);
508 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
510 assert(to == ep_square());
511 assert(moved_piece(m) == make_piece(us, PAWN));
512 assert(piece_on(capsq) == make_piece(~us, PAWN));
513 assert(piece_on(to) == NO_PIECE);
515 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
516 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
519 // Castling moves generation does not check if the castling path is clear of
520 // enemy attacks, it is delayed at a later time: now!
521 if (type_of(m) == CASTLING)
523 // After castling, the rook and king final positions are the same in
524 // Chess960 as they would be in standard chess.
525 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
526 Direction step = to > from ? WEST : EAST;
528 for (Square s = to; s != from; s += step)
529 if (attackers_to(s) & pieces(~us))
532 // In case of Chess960, verify that when moving the castling rook we do
533 // not discover some hidden checker.
534 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
536 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
539 // If the moving piece is a king, check whether the destination square is
540 // attacked by the opponent.
541 if (type_of(piece_on(from)) == KING)
542 return !(attackers_to(to) & pieces(~us));
544 // A non-king move is legal if and only if it is not pinned or it
545 // is moving along the ray towards or away from the king.
546 return !(blockers_for_king(us) & from)
547 || aligned(from, to, square<KING>(us));
551 /// Position::pseudo_legal() takes a random move and tests whether the move is
552 /// pseudo legal. It is used to validate moves from TT that can be corrupted
553 /// due to SMP concurrent access or hash position key aliasing.
555 bool Position::pseudo_legal(const Move m) const {
557 Color us = sideToMove;
558 Square from = from_sq(m);
559 Square to = to_sq(m);
560 Piece pc = moved_piece(m);
562 // Use a slower but simpler function for uncommon cases
563 if (type_of(m) != NORMAL)
564 return MoveList<LEGAL>(*this).contains(m);
566 // Is not a promotion, so promotion piece must be empty
567 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
570 // If the 'from' square is not occupied by a piece belonging to the side to
571 // move, the move is obviously not legal.
572 if (pc == NO_PIECE || color_of(pc) != us)
575 // The destination square cannot be occupied by a friendly piece
579 // Handle the special case of a pawn move
580 if (type_of(pc) == PAWN)
582 // We have already handled promotion moves, so destination
583 // cannot be on the 8th/1st rank.
584 if ((Rank8BB | Rank1BB) & to)
587 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
588 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
589 && !( (from + 2 * pawn_push(us) == to) // Not a double push
590 && (rank_of(from) == relative_rank(us, RANK_2))
592 && empty(to - pawn_push(us))))
595 else if (!(attacks_from(type_of(pc), from) & to))
598 // Evasions generator already takes care to avoid some kind of illegal moves
599 // and legal() relies on this. We therefore have to take care that the same
600 // kind of moves are filtered out here.
603 if (type_of(pc) != KING)
605 // Double check? In this case a king move is required
606 if (more_than_one(checkers()))
609 // Our move must be a blocking evasion or a capture of the checking piece
610 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
613 // In case of king moves under check we have to remove king so as to catch
614 // invalid moves like b1a1 when opposite queen is on c1.
615 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
623 /// Position::gives_check() tests whether a pseudo-legal move gives a check
625 bool Position::gives_check(Move m) const {
628 assert(color_of(moved_piece(m)) == sideToMove);
630 Square from = from_sq(m);
631 Square to = to_sq(m);
633 // Is there a direct check?
634 if (st->checkSquares[type_of(piece_on(from))] & to)
637 // Is there a discovered check?
638 if ( (st->blockersForKing[~sideToMove] & from)
639 && !aligned(from, to, square<KING>(~sideToMove)))
648 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
650 // En passant capture with check? We have already handled the case
651 // of direct checks and ordinary discovered check, so the only case we
652 // need to handle is the unusual case of a discovered check through
653 // the captured pawn.
656 Square capsq = make_square(file_of(to), rank_of(from));
657 Bitboard b = (pieces() ^ from ^ capsq) | to;
659 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
660 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
665 Square rfrom = to; // Castling is encoded as 'King captures the rook'
666 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
667 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
669 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
670 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
679 /// Position::do_move() makes a move, and saves all information necessary
680 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
681 /// moves should be filtered out before this function is called.
683 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
686 assert(&newSt != st);
688 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
689 Key k = st->key ^ Zobrist::side;
691 // Copy some fields of the old state to our new StateInfo object except the
692 // ones which are going to be recalculated from scratch anyway and then switch
693 // our state pointer to point to the new (ready to be updated) state.
694 std::memcpy(&newSt, st, offsetof(StateInfo, key));
698 // Increment ply counters. In particular, rule50 will be reset to zero later on
699 // in case of a capture or a pawn move.
704 Color us = sideToMove;
706 Square from = from_sq(m);
707 Square to = to_sq(m);
708 Piece pc = piece_on(from);
709 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
711 assert(color_of(pc) == us);
712 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
713 assert(type_of(captured) != KING);
715 if (type_of(m) == CASTLING)
717 assert(pc == make_piece(us, KING));
718 assert(captured == make_piece(us, ROOK));
721 do_castling<true>(us, from, to, rfrom, rto);
723 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
731 // If the captured piece is a pawn, update pawn hash key, otherwise
732 // update non-pawn material.
733 if (type_of(captured) == PAWN)
735 if (type_of(m) == ENPASSANT)
737 capsq -= pawn_push(us);
739 assert(pc == make_piece(us, PAWN));
740 assert(to == st->epSquare);
741 assert(relative_rank(us, to) == RANK_6);
742 assert(piece_on(to) == NO_PIECE);
743 assert(piece_on(capsq) == make_piece(them, PAWN));
745 board[capsq] = NO_PIECE; // Not done by remove_piece()
748 st->pawnKey ^= Zobrist::psq[captured][capsq];
751 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
753 // Update board and piece lists
754 remove_piece(captured, capsq);
756 // Update material hash key and prefetch access to materialTable
757 k ^= Zobrist::psq[captured][capsq];
758 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
759 prefetch(thisThread->materialTable[st->materialKey]);
761 // Reset rule 50 counter
766 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
768 // Reset en passant square
769 if (st->epSquare != SQ_NONE)
771 k ^= Zobrist::enpassant[file_of(st->epSquare)];
772 st->epSquare = SQ_NONE;
775 // Update castling rights if needed
776 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
778 int cr = castlingRightsMask[from] | castlingRightsMask[to];
779 k ^= Zobrist::castling[st->castlingRights & cr];
780 st->castlingRights &= ~cr;
783 // Move the piece. The tricky Chess960 castling is handled earlier
784 if (type_of(m) != CASTLING)
785 move_piece(pc, from, to);
787 // If the moving piece is a pawn do some special extra work
788 if (type_of(pc) == PAWN)
790 // Set en-passant square if the moved pawn can be captured
791 if ( (int(to) ^ int(from)) == 16
792 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
794 st->epSquare = to - pawn_push(us);
795 k ^= Zobrist::enpassant[file_of(st->epSquare)];
798 else if (type_of(m) == PROMOTION)
800 Piece promotion = make_piece(us, promotion_type(m));
802 assert(relative_rank(us, to) == RANK_8);
803 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
805 remove_piece(pc, to);
806 put_piece(promotion, to);
809 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
810 st->pawnKey ^= Zobrist::psq[pc][to];
811 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
812 ^ Zobrist::psq[pc][pieceCount[pc]];
815 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
818 // Update pawn hash key
819 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
821 // Reset rule 50 draw counter
826 st->capturedPiece = captured;
828 // Update the key with the final value
831 // Calculate checkers bitboard (if move gives check)
832 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
834 sideToMove = ~sideToMove;
836 // Update king attacks used for fast check detection
839 // Calculate the repetition info. It is the ply distance from the previous
840 // occurrence of the same position, negative in the 3-fold case, or zero
841 // if the position was not repeated.
843 int end = std::min(st->rule50, st->pliesFromNull);
846 StateInfo* stp = st->previous->previous;
847 for (int i = 4; i <= end; i += 2)
849 stp = stp->previous->previous;
850 if (stp->key == st->key)
852 st->repetition = stp->repetition ? -i : i;
862 /// Position::undo_move() unmakes a move. When it returns, the position should
863 /// be restored to exactly the same state as before the move was made.
865 void Position::undo_move(Move m) {
869 sideToMove = ~sideToMove;
871 Color us = sideToMove;
872 Square from = from_sq(m);
873 Square to = to_sq(m);
874 Piece pc = piece_on(to);
876 assert(empty(from) || type_of(m) == CASTLING);
877 assert(type_of(st->capturedPiece) != KING);
879 if (type_of(m) == PROMOTION)
881 assert(relative_rank(us, to) == RANK_8);
882 assert(type_of(pc) == promotion_type(m));
883 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
885 remove_piece(pc, to);
886 pc = make_piece(us, PAWN);
890 if (type_of(m) == CASTLING)
893 do_castling<false>(us, from, to, rfrom, rto);
897 move_piece(pc, to, from); // Put the piece back at the source square
899 if (st->capturedPiece)
903 if (type_of(m) == ENPASSANT)
905 capsq -= pawn_push(us);
907 assert(type_of(pc) == PAWN);
908 assert(to == st->previous->epSquare);
909 assert(relative_rank(us, to) == RANK_6);
910 assert(piece_on(capsq) == NO_PIECE);
911 assert(st->capturedPiece == make_piece(~us, PAWN));
914 put_piece(st->capturedPiece, capsq); // Restore the captured piece
918 // Finally point our state pointer back to the previous state
926 /// Position::do_castling() is a helper used to do/undo a castling move. This
927 /// is a bit tricky in Chess960 where from/to squares can overlap.
929 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
931 bool kingSide = to > from;
932 rfrom = to; // Castling is encoded as "king captures friendly rook"
933 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
934 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
936 // Remove both pieces first since squares could overlap in Chess960
937 remove_piece(make_piece(us, KING), Do ? from : to);
938 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
939 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
940 put_piece(make_piece(us, KING), Do ? to : from);
941 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
945 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
946 /// the side to move without executing any move on the board.
948 void Position::do_null_move(StateInfo& newSt) {
951 assert(&newSt != st);
953 std::memcpy(&newSt, st, sizeof(StateInfo));
957 if (st->epSquare != SQ_NONE)
959 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
960 st->epSquare = SQ_NONE;
963 st->key ^= Zobrist::side;
964 prefetch(TT.first_entry(st->key));
967 st->pliesFromNull = 0;
969 sideToMove = ~sideToMove;
978 void Position::undo_null_move() {
983 sideToMove = ~sideToMove;
987 /// Position::key_after() computes the new hash key after the given move. Needed
988 /// for speculative prefetch. It doesn't recognize special moves like castling,
989 /// en-passant and promotions.
991 Key Position::key_after(Move m) const {
993 Square from = from_sq(m);
994 Square to = to_sq(m);
995 Piece pc = piece_on(from);
996 Piece captured = piece_on(to);
997 Key k = st->key ^ Zobrist::side;
1000 k ^= Zobrist::psq[captured][to];
1002 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1006 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1007 /// SEE value of move is greater or equal to the given threshold. We'll use an
1008 /// algorithm similar to alpha-beta pruning with a null window.
1010 bool Position::see_ge(Move m, Value threshold) const {
1014 // Only deal with normal moves, assume others pass a simple see
1015 if (type_of(m) != NORMAL)
1016 return VALUE_ZERO >= threshold;
1018 Square from = from_sq(m), to = to_sq(m);
1020 int swap = PieceValue[MG][piece_on(to)] - threshold;
1024 swap = PieceValue[MG][piece_on(from)] - swap;
1028 Bitboard occupied = pieces() ^ from ^ to;
1029 Color stm = color_of(piece_on(from));
1030 Bitboard attackers = attackers_to(to, occupied);
1031 Bitboard stmAttackers, bb;
1037 attackers &= occupied;
1039 // If stm has no more attackers then give up: stm loses
1040 if (!(stmAttackers = attackers & pieces(stm)))
1043 // Don't allow pinned pieces to attack (except the king) as long as
1044 // there are pinners on their original square.
1045 if (st->pinners[~stm] & occupied)
1046 stmAttackers &= ~st->blockersForKing[stm];
1053 // Locate and remove the next least valuable attacker, and add to
1054 // the bitboard 'attackers' any X-ray attackers behind it.
1055 if ((bb = stmAttackers & pieces(PAWN)))
1057 if ((swap = PawnValueMg - swap) < res)
1060 occupied ^= lsb(bb);
1061 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1064 else if ((bb = stmAttackers & pieces(KNIGHT)))
1066 if ((swap = KnightValueMg - swap) < res)
1069 occupied ^= lsb(bb);
1072 else if ((bb = stmAttackers & pieces(BISHOP)))
1074 if ((swap = BishopValueMg - swap) < res)
1077 occupied ^= lsb(bb);
1078 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1081 else if ((bb = stmAttackers & pieces(ROOK)))
1083 if ((swap = RookValueMg - swap) < res)
1086 occupied ^= lsb(bb);
1087 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1090 else if ((bb = stmAttackers & pieces(QUEEN)))
1092 if ((swap = QueenValueMg - swap) < res)
1095 occupied ^= lsb(bb);
1096 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1097 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1101 // If we "capture" with the king but opponent still has attackers,
1102 // reverse the result.
1103 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1109 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1110 /// or by repetition. It does not detect stalemates.
1112 bool Position::is_draw(int ply) const {
1114 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1117 // Return a draw score if a position repeats once earlier but strictly
1118 // after the root, or repeats twice before or at the root.
1119 if (st->repetition && st->repetition < ply)
1126 // Position::has_repeated() tests whether there has been at least one repetition
1127 // of positions since the last capture or pawn move.
1129 bool Position::has_repeated() const {
1131 StateInfo* stc = st;
1132 int end = std::min(st->rule50, st->pliesFromNull);
1135 if (stc->repetition)
1138 stc = stc->previous;
1144 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1145 /// or an earlier position has a move that directly reaches the current position.
1147 bool Position::has_game_cycle(int ply) const {
1151 int end = std::min(st->rule50, st->pliesFromNull);
1156 Key originalKey = st->key;
1157 StateInfo* stp = st->previous;
1159 for (int i = 3; i <= end; i += 2)
1161 stp = stp->previous->previous;
1163 Key moveKey = originalKey ^ stp->key;
1164 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1165 || (j = H2(moveKey), cuckoo[j] == moveKey))
1167 Move move = cuckooMove[j];
1168 Square s1 = from_sq(move);
1169 Square s2 = to_sq(move);
1171 if (!(between_bb(s1, s2) & pieces()))
1176 // For nodes before or at the root, check that the move is a
1177 // repetition rather than a move to the current position.
1178 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1179 // the same location, so we have to select which square to check.
1180 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1183 // For repetitions before or at the root, require one more
1184 if (stp->repetition)
1193 /// Position::flip() flips position with the white and black sides reversed. This
1194 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1196 void Position::flip() {
1199 std::stringstream ss(fen());
1201 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1203 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1204 f.insert(0, token + (f.empty() ? " " : "/"));
1207 ss >> token; // Active color
1208 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1210 ss >> token; // Castling availability
1213 std::transform(f.begin(), f.end(), f.begin(),
1214 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1216 ss >> token; // En passant square
1217 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1219 std::getline(ss, token); // Half and full moves
1222 set(f, is_chess960(), st, this_thread());
1224 assert(pos_is_ok());
1228 /// Position::pos_is_ok() performs some consistency checks for the
1229 /// position object and raises an asserts if something wrong is detected.
1230 /// This is meant to be helpful when debugging.
1232 bool Position::pos_is_ok() const {
1234 constexpr bool Fast = true; // Quick (default) or full check?
1236 if ( (sideToMove != WHITE && sideToMove != BLACK)
1237 || piece_on(square<KING>(WHITE)) != W_KING
1238 || piece_on(square<KING>(BLACK)) != B_KING
1239 || ( ep_square() != SQ_NONE
1240 && relative_rank(sideToMove, ep_square()) != RANK_6))
1241 assert(0 && "pos_is_ok: Default");
1246 if ( pieceCount[W_KING] != 1
1247 || pieceCount[B_KING] != 1
1248 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1249 assert(0 && "pos_is_ok: Kings");
1251 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1252 || pieceCount[W_PAWN] > 8
1253 || pieceCount[B_PAWN] > 8)
1254 assert(0 && "pos_is_ok: Pawns");
1256 if ( (pieces(WHITE) & pieces(BLACK))
1257 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1258 || popcount(pieces(WHITE)) > 16
1259 || popcount(pieces(BLACK)) > 16)
1260 assert(0 && "pos_is_ok: Bitboards");
1262 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1263 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1264 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1265 assert(0 && "pos_is_ok: Bitboards");
1269 if (std::memcmp(&si, st, sizeof(StateInfo)))
1270 assert(0 && "pos_is_ok: State");
1272 for (Piece pc : Pieces)
1274 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1275 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1276 assert(0 && "pos_is_ok: Pieces");
1278 for (int i = 0; i < pieceCount[pc]; ++i)
1279 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1280 assert(0 && "pos_is_ok: Index");
1283 for (Color c : { WHITE, BLACK })
1284 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1286 if (!can_castle(cr))
1289 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1290 || castlingRightsMask[castlingRookSquare[cr]] != cr
1291 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1292 assert(0 && "pos_is_ok: Castling");