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;
292 /// Position::set_castling_right() is a helper function used to set castling
293 /// rights given the corresponding color and the rook starting square.
295 void Position::set_castling_right(Color c, Square rfrom) {
297 Square kfrom = square<KING>(c);
298 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
300 st->castlingRights |= cr;
301 castlingRightsMask[kfrom] |= cr;
302 castlingRightsMask[rfrom] |= cr;
303 castlingRookSquare[cr] = rfrom;
305 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
306 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
308 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
309 & ~(square_bb(kfrom) | rfrom);
313 /// Position::set_check_info() sets king attacks to detect if a move gives check
315 void Position::set_check_info(StateInfo* si) const {
317 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
318 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
320 Square ksq = square<KING>(~sideToMove);
322 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
323 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
324 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
325 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
326 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
327 si->checkSquares[KING] = 0;
331 /// Position::set_state() computes the hash keys of the position, and other
332 /// data that once computed is updated incrementally as moves are made.
333 /// The function is only used when a new position is set up, and to verify
334 /// the correctness of the StateInfo data when running in debug mode.
336 void Position::set_state(StateInfo* si) const {
338 si->key = si->materialKey = 0;
339 si->pawnKey = Zobrist::noPawns;
340 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
341 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
345 for (Bitboard b = pieces(); b; )
347 Square s = pop_lsb(&b);
348 Piece pc = piece_on(s);
349 si->key ^= Zobrist::psq[pc][s];
351 if (type_of(pc) == PAWN)
352 si->pawnKey ^= Zobrist::psq[pc][s];
354 else if (type_of(pc) != KING)
355 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
358 if (si->epSquare != SQ_NONE)
359 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
361 if (sideToMove == BLACK)
362 si->key ^= Zobrist::side;
364 si->key ^= Zobrist::castling[si->castlingRights];
366 for (Piece pc : Pieces)
367 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
368 si->materialKey ^= Zobrist::psq[pc][cnt];
372 /// Position::set() is an overload to initialize the position object with
373 /// the given endgame code string like "KBPKN". It is mainly a helper to
374 /// get the material key out of an endgame code.
376 Position& Position::set(const string& code, Color c, StateInfo* si) {
378 assert(code.length() > 0 && code.length() < 8);
379 assert(code[0] == 'K');
381 string sides[] = { code.substr(code.find('K', 1)), // Weak
382 code.substr(0, code.find('K', 1)) }; // Strong
384 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
386 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
387 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
389 return set(fenStr, false, si, nullptr);
393 /// Position::fen() returns a FEN representation of the position. In case of
394 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
396 const string Position::fen() const {
399 std::ostringstream ss;
401 for (Rank r = RANK_8; r >= RANK_1; --r)
403 for (File f = FILE_A; f <= FILE_H; ++f)
405 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
412 ss << PieceToChar[piece_on(make_square(f, r))];
419 ss << (sideToMove == WHITE ? " w " : " b ");
421 if (can_castle(WHITE_OO))
422 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
424 if (can_castle(WHITE_OOO))
425 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
427 if (can_castle(BLACK_OO))
428 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
430 if (can_castle(BLACK_OOO))
431 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
433 if (!can_castle(ANY_CASTLING))
436 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
437 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
443 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
444 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
445 /// slider if removing that piece from the board would result in a position where
446 /// square 's' is attacked. For example, a king-attack blocking piece can be either
447 /// a pinned or a discovered check piece, according if its color is the opposite
448 /// or the same of the color of the slider.
450 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
452 Bitboard blockers = 0;
455 // Snipers are sliders that attack 's' when a piece and other snipers are removed
456 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
457 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
458 Bitboard occupancy = pieces() ^ snipers;
462 Square sniperSq = pop_lsb(&snipers);
463 Bitboard b = between_bb(s, sniperSq) & occupancy;
465 if (b && !more_than_one(b))
468 if (b & pieces(color_of(piece_on(s))))
476 /// Position::attackers_to() computes a bitboard of all pieces which attack a
477 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
479 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
481 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
482 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
483 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
484 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
485 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
486 | (attacks_from<KING>(s) & pieces(KING));
490 /// Position::legal() tests whether a pseudo-legal move is legal
492 bool Position::legal(Move m) const {
496 Color us = sideToMove;
497 Square from = from_sq(m);
498 Square to = to_sq(m);
500 assert(color_of(moved_piece(m)) == us);
501 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
503 // En passant captures are a tricky special case. Because they are rather
504 // uncommon, we do it simply by testing whether the king is attacked after
506 if (type_of(m) == ENPASSANT)
508 Square ksq = square<KING>(us);
509 Square capsq = to - pawn_push(us);
510 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
512 assert(to == ep_square());
513 assert(moved_piece(m) == make_piece(us, PAWN));
514 assert(piece_on(capsq) == make_piece(~us, PAWN));
515 assert(piece_on(to) == NO_PIECE);
517 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
518 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
521 // Castling moves generation does not check if the castling path is clear of
522 // enemy attacks, it is delayed at a later time: now!
523 if (type_of(m) == CASTLING)
525 // After castling, the rook and king final positions are the same in
526 // Chess960 as they would be in standard chess.
527 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
528 Direction step = to > from ? WEST : EAST;
530 for (Square s = to; s != from; s += step)
531 if (attackers_to(s) & pieces(~us))
534 // In case of Chess960, verify that when moving the castling rook we do
535 // not discover some hidden checker.
536 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
538 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
541 // If the moving piece is a king, check whether the destination square is
542 // attacked by the opponent.
543 if (type_of(piece_on(from)) == KING)
544 return !(attackers_to(to) & pieces(~us));
546 // A non-king move is legal if and only if it is not pinned or it
547 // is moving along the ray towards or away from the king.
548 return !(blockers_for_king(us) & from)
549 || aligned(from, to, square<KING>(us));
553 /// Position::pseudo_legal() takes a random move and tests whether the move is
554 /// pseudo legal. It is used to validate moves from TT that can be corrupted
555 /// due to SMP concurrent access or hash position key aliasing.
557 bool Position::pseudo_legal(const Move m) const {
559 Color us = sideToMove;
560 Square from = from_sq(m);
561 Square to = to_sq(m);
562 Piece pc = moved_piece(m);
564 // Use a slower but simpler function for uncommon cases
565 if (type_of(m) != NORMAL)
566 return MoveList<LEGAL>(*this).contains(m);
568 // Is not a promotion, so promotion piece must be empty
569 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
572 // If the 'from' square is not occupied by a piece belonging to the side to
573 // move, the move is obviously not legal.
574 if (pc == NO_PIECE || color_of(pc) != us)
577 // The destination square cannot be occupied by a friendly piece
581 // Handle the special case of a pawn move
582 if (type_of(pc) == PAWN)
584 // We have already handled promotion moves, so destination
585 // cannot be on the 8th/1st rank.
586 if ((Rank8BB | Rank1BB) & to)
589 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
590 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
591 && !( (from + 2 * pawn_push(us) == to) // Not a double push
592 && (rank_of(from) == relative_rank(us, RANK_2))
594 && empty(to - pawn_push(us))))
597 else if (!(attacks_from(type_of(pc), from) & to))
600 // Evasions generator already takes care to avoid some kind of illegal moves
601 // and legal() relies on this. We therefore have to take care that the same
602 // kind of moves are filtered out here.
605 if (type_of(pc) != KING)
607 // Double check? In this case a king move is required
608 if (more_than_one(checkers()))
611 // Our move must be a blocking evasion or a capture of the checking piece
612 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
615 // In case of king moves under check we have to remove king so as to catch
616 // invalid moves like b1a1 when opposite queen is on c1.
617 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
625 /// Position::gives_check() tests whether a pseudo-legal move gives a check
627 bool Position::gives_check(Move m) const {
630 assert(color_of(moved_piece(m)) == sideToMove);
632 Square from = from_sq(m);
633 Square to = to_sq(m);
635 // Is there a direct check?
636 if (st->checkSquares[type_of(piece_on(from))] & to)
639 // Is there a discovered check?
640 if ( (st->blockersForKing[~sideToMove] & from)
641 && !aligned(from, to, square<KING>(~sideToMove)))
650 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
652 // En passant capture with check? We have already handled the case
653 // of direct checks and ordinary discovered check, so the only case we
654 // need to handle is the unusual case of a discovered check through
655 // the captured pawn.
658 Square capsq = make_square(file_of(to), rank_of(from));
659 Bitboard b = (pieces() ^ from ^ capsq) | to;
661 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
662 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
667 Square rfrom = to; // Castling is encoded as 'King captures the rook'
668 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
669 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
671 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
672 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
681 /// Position::do_move() makes a move, and saves all information necessary
682 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
683 /// moves should be filtered out before this function is called.
685 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
688 assert(&newSt != st);
690 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
691 Key k = st->key ^ Zobrist::side;
693 // Copy some fields of the old state to our new StateInfo object except the
694 // ones which are going to be recalculated from scratch anyway and then switch
695 // our state pointer to point to the new (ready to be updated) state.
696 std::memcpy(&newSt, st, offsetof(StateInfo, key));
700 // Increment ply counters. In particular, rule50 will be reset to zero later on
701 // in case of a capture or a pawn move.
706 Color us = sideToMove;
708 Square from = from_sq(m);
709 Square to = to_sq(m);
710 Piece pc = piece_on(from);
711 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
713 assert(color_of(pc) == us);
714 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
715 assert(type_of(captured) != KING);
717 if (type_of(m) == CASTLING)
719 assert(pc == make_piece(us, KING));
720 assert(captured == make_piece(us, ROOK));
723 do_castling<true>(us, from, to, rfrom, rto);
725 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
733 // If the captured piece is a pawn, update pawn hash key, otherwise
734 // update non-pawn material.
735 if (type_of(captured) == PAWN)
737 if (type_of(m) == ENPASSANT)
739 capsq -= pawn_push(us);
741 assert(pc == make_piece(us, PAWN));
742 assert(to == st->epSquare);
743 assert(relative_rank(us, to) == RANK_6);
744 assert(piece_on(to) == NO_PIECE);
745 assert(piece_on(capsq) == make_piece(them, PAWN));
748 st->pawnKey ^= Zobrist::psq[captured][capsq];
751 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
753 // Update board and piece lists
756 if (type_of(m) == ENPASSANT)
757 board[capsq] = NO_PIECE;
759 // Update material hash key and prefetch access to materialTable
760 k ^= Zobrist::psq[captured][capsq];
761 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
762 prefetch(thisThread->materialTable[st->materialKey]);
764 // Reset rule 50 counter
769 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
771 // Reset en passant square
772 if (st->epSquare != SQ_NONE)
774 k ^= Zobrist::enpassant[file_of(st->epSquare)];
775 st->epSquare = SQ_NONE;
778 // Update castling rights if needed
779 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
781 int cr = castlingRightsMask[from] | castlingRightsMask[to];
782 k ^= Zobrist::castling[st->castlingRights & cr];
783 st->castlingRights &= ~cr;
786 // Move the piece. The tricky Chess960 castling is handled earlier
787 if (type_of(m) != CASTLING)
788 move_piece(from, to);
790 // If the moving piece is a pawn do some special extra work
791 if (type_of(pc) == PAWN)
793 // Set en-passant square if the moved pawn can be captured
794 if ( (int(to) ^ int(from)) == 16
795 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
797 st->epSquare = to - pawn_push(us);
798 k ^= Zobrist::enpassant[file_of(st->epSquare)];
801 else if (type_of(m) == PROMOTION)
803 Piece promotion = make_piece(us, promotion_type(m));
805 assert(relative_rank(us, to) == RANK_8);
806 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
809 put_piece(promotion, to);
812 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
813 st->pawnKey ^= Zobrist::psq[pc][to];
814 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
815 ^ Zobrist::psq[pc][pieceCount[pc]];
818 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
821 // Update pawn hash key
822 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
824 // Reset rule 50 draw counter
829 st->capturedPiece = captured;
831 // Update the key with the final value
834 // Calculate checkers bitboard (if move gives check)
835 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
837 sideToMove = ~sideToMove;
839 // Update king attacks used for fast check detection
842 // Calculate the repetition info. It is the ply distance from the previous
843 // occurrence of the same position, negative in the 3-fold case, or zero
844 // if the position was not repeated.
846 int end = std::min(st->rule50, st->pliesFromNull);
849 StateInfo* stp = st->previous->previous;
850 for (int i = 4; i <= end; i += 2)
852 stp = stp->previous->previous;
853 if (stp->key == st->key)
855 st->repetition = stp->repetition ? -i : i;
865 /// Position::undo_move() unmakes a move. When it returns, the position should
866 /// be restored to exactly the same state as before the move was made.
868 void Position::undo_move(Move m) {
872 sideToMove = ~sideToMove;
874 Color us = sideToMove;
875 Square from = from_sq(m);
876 Square to = to_sq(m);
877 Piece pc = piece_on(to);
879 assert(empty(from) || type_of(m) == CASTLING);
880 assert(type_of(st->capturedPiece) != KING);
882 if (type_of(m) == PROMOTION)
884 assert(relative_rank(us, to) == RANK_8);
885 assert(type_of(pc) == promotion_type(m));
886 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
889 pc = make_piece(us, PAWN);
893 if (type_of(m) == CASTLING)
896 do_castling<false>(us, from, to, rfrom, rto);
900 move_piece(to, from); // Put the piece back at the source square
902 if (st->capturedPiece)
906 if (type_of(m) == ENPASSANT)
908 capsq -= pawn_push(us);
910 assert(type_of(pc) == PAWN);
911 assert(to == st->previous->epSquare);
912 assert(relative_rank(us, to) == RANK_6);
913 assert(piece_on(capsq) == NO_PIECE);
914 assert(st->capturedPiece == make_piece(~us, PAWN));
917 put_piece(st->capturedPiece, capsq); // Restore the captured piece
921 // Finally point our state pointer back to the previous state
929 /// Position::do_castling() is a helper used to do/undo a castling move. This
930 /// is a bit tricky in Chess960 where from/to squares can overlap.
932 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
934 bool kingSide = to > from;
935 rfrom = to; // Castling is encoded as "king captures friendly rook"
936 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
937 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
939 // Remove both pieces first since squares could overlap in Chess960
940 remove_piece(Do ? from : to);
941 remove_piece(Do ? rfrom : rto);
942 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
943 put_piece(make_piece(us, KING), Do ? to : from);
944 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
948 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
949 /// the side to move without executing any move on the board.
951 void Position::do_null_move(StateInfo& newSt) {
954 assert(&newSt != st);
956 std::memcpy(&newSt, st, sizeof(StateInfo));
960 if (st->epSquare != SQ_NONE)
962 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
963 st->epSquare = SQ_NONE;
966 st->key ^= Zobrist::side;
967 prefetch(TT.first_entry(st->key));
970 st->pliesFromNull = 0;
972 sideToMove = ~sideToMove;
981 void Position::undo_null_move() {
986 sideToMove = ~sideToMove;
990 /// Position::key_after() computes the new hash key after the given move. Needed
991 /// for speculative prefetch. It doesn't recognize special moves like castling,
992 /// en-passant and promotions.
994 Key Position::key_after(Move m) const {
996 Square from = from_sq(m);
997 Square to = to_sq(m);
998 Piece pc = piece_on(from);
999 Piece captured = piece_on(to);
1000 Key k = st->key ^ Zobrist::side;
1003 k ^= Zobrist::psq[captured][to];
1005 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1009 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1010 /// SEE value of move is greater or equal to the given threshold. We'll use an
1011 /// algorithm similar to alpha-beta pruning with a null window.
1013 bool Position::see_ge(Move m, Value threshold) const {
1017 // Only deal with normal moves, assume others pass a simple see
1018 if (type_of(m) != NORMAL)
1019 return VALUE_ZERO >= threshold;
1021 Square from = from_sq(m), to = to_sq(m);
1023 int swap = PieceValue[MG][piece_on(to)] - threshold;
1027 swap = PieceValue[MG][piece_on(from)] - swap;
1031 Bitboard occupied = pieces() ^ from ^ to;
1032 Color stm = color_of(piece_on(from));
1033 Bitboard attackers = attackers_to(to, occupied);
1034 Bitboard stmAttackers, bb;
1040 attackers &= occupied;
1042 // If stm has no more attackers then give up: stm loses
1043 if (!(stmAttackers = attackers & pieces(stm)))
1046 // Don't allow pinned pieces to attack (except the king) as long as
1047 // there are pinners on their original square.
1048 if (st->pinners[~stm] & occupied)
1049 stmAttackers &= ~st->blockersForKing[stm];
1056 // Locate and remove the next least valuable attacker, and add to
1057 // the bitboard 'attackers' any X-ray attackers behind it.
1058 if ((bb = stmAttackers & pieces(PAWN)))
1060 if ((swap = PawnValueMg - swap) < res)
1063 occupied ^= lsb(bb);
1064 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1067 else if ((bb = stmAttackers & pieces(KNIGHT)))
1069 if ((swap = KnightValueMg - swap) < res)
1072 occupied ^= lsb(bb);
1075 else if ((bb = stmAttackers & pieces(BISHOP)))
1077 if ((swap = BishopValueMg - swap) < res)
1080 occupied ^= lsb(bb);
1081 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1084 else if ((bb = stmAttackers & pieces(ROOK)))
1086 if ((swap = RookValueMg - swap) < res)
1089 occupied ^= lsb(bb);
1090 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1093 else if ((bb = stmAttackers & pieces(QUEEN)))
1095 if ((swap = QueenValueMg - swap) < res)
1098 occupied ^= lsb(bb);
1099 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1100 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1104 // If we "capture" with the king but opponent still has attackers,
1105 // reverse the result.
1106 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
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 // Return a draw score if a position repeats once earlier but strictly
1121 // after the root, or repeats twice before or at the root.
1122 if (st->repetition && st->repetition < ply)
1129 // Position::has_repeated() tests whether there has been at least one repetition
1130 // of positions since the last capture or pawn move.
1132 bool Position::has_repeated() const {
1134 StateInfo* stc = st;
1135 int end = std::min(st->rule50, st->pliesFromNull);
1138 if (stc->repetition)
1141 stc = stc->previous;
1147 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1148 /// or an earlier position has a move that directly reaches the current position.
1150 bool Position::has_game_cycle(int ply) const {
1154 int end = std::min(st->rule50, st->pliesFromNull);
1159 Key originalKey = st->key;
1160 StateInfo* stp = st->previous;
1162 for (int i = 3; i <= end; i += 2)
1164 stp = stp->previous->previous;
1166 Key moveKey = originalKey ^ stp->key;
1167 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1168 || (j = H2(moveKey), cuckoo[j] == moveKey))
1170 Move move = cuckooMove[j];
1171 Square s1 = from_sq(move);
1172 Square s2 = to_sq(move);
1174 if (!(between_bb(s1, s2) & pieces()))
1179 // For nodes before or at the root, check that the move is a
1180 // repetition rather than a move to the current position.
1181 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1182 // the same location, so we have to select which square to check.
1183 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1186 // For repetitions before or at the root, require one more
1187 if (stp->repetition)
1196 /// Position::flip() flips position with the white and black sides reversed. This
1197 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1199 void Position::flip() {
1202 std::stringstream ss(fen());
1204 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1206 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1207 f.insert(0, token + (f.empty() ? " " : "/"));
1210 ss >> token; // Active color
1211 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1213 ss >> token; // Castling availability
1216 std::transform(f.begin(), f.end(), f.begin(),
1217 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1219 ss >> token; // En passant square
1220 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1222 std::getline(ss, token); // Half and full moves
1225 set(f, is_chess960(), st, this_thread());
1227 assert(pos_is_ok());
1231 /// Position::pos_is_ok() performs some consistency checks for the
1232 /// position object and raises an asserts if something wrong is detected.
1233 /// This is meant to be helpful when debugging.
1235 bool Position::pos_is_ok() const {
1237 constexpr bool Fast = true; // Quick (default) or full check?
1239 if ( (sideToMove != WHITE && sideToMove != BLACK)
1240 || piece_on(square<KING>(WHITE)) != W_KING
1241 || piece_on(square<KING>(BLACK)) != B_KING
1242 || ( ep_square() != SQ_NONE
1243 && relative_rank(sideToMove, ep_square()) != RANK_6))
1244 assert(0 && "pos_is_ok: Default");
1249 if ( pieceCount[W_KING] != 1
1250 || pieceCount[B_KING] != 1
1251 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1252 assert(0 && "pos_is_ok: Kings");
1254 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1255 || pieceCount[W_PAWN] > 8
1256 || pieceCount[B_PAWN] > 8)
1257 assert(0 && "pos_is_ok: Pawns");
1259 if ( (pieces(WHITE) & pieces(BLACK))
1260 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1261 || popcount(pieces(WHITE)) > 16
1262 || popcount(pieces(BLACK)) > 16)
1263 assert(0 && "pos_is_ok: Bitboards");
1265 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1266 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1267 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1268 assert(0 && "pos_is_ok: Bitboards");
1272 if (std::memcmp(&si, st, sizeof(StateInfo)))
1273 assert(0 && "pos_is_ok: State");
1275 for (Piece pc : Pieces)
1277 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1278 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1279 assert(0 && "pos_is_ok: Pieces");
1281 for (int i = 0; i < pieceCount[pc]; ++i)
1282 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1283 assert(0 && "pos_is_ok: Index");
1286 for (Color c : { WHITE, BLACK })
1287 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1289 if (!can_castle(cr))
1292 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1293 || castlingRightsMask[castlingRookSquare[cr]] != cr
1294 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1295 assert(0 && "pos_is_ok: Castling");