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));
747 board[capsq] = NO_PIECE; // Not done by remove_piece()
750 st->pawnKey ^= Zobrist::psq[captured][capsq];
753 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
755 // Update board and piece lists
756 remove_piece(captured, capsq);
758 // Update material hash key and prefetch access to materialTable
759 k ^= Zobrist::psq[captured][capsq];
760 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
761 prefetch(thisThread->materialTable[st->materialKey]);
763 // Reset rule 50 counter
768 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
770 // Reset en passant square
771 if (st->epSquare != SQ_NONE)
773 k ^= Zobrist::enpassant[file_of(st->epSquare)];
774 st->epSquare = SQ_NONE;
777 // Update castling rights if needed
778 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
780 int cr = castlingRightsMask[from] | castlingRightsMask[to];
781 k ^= Zobrist::castling[st->castlingRights & cr];
782 st->castlingRights &= ~cr;
785 // Move the piece. The tricky Chess960 castling is handled earlier
786 if (type_of(m) != CASTLING)
787 move_piece(pc, from, to);
789 // If the moving piece is a pawn do some special extra work
790 if (type_of(pc) == PAWN)
792 // Set en-passant square if the moved pawn can be captured
793 if ( (int(to) ^ int(from)) == 16
794 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
796 st->epSquare = to - pawn_push(us);
797 k ^= Zobrist::enpassant[file_of(st->epSquare)];
800 else if (type_of(m) == PROMOTION)
802 Piece promotion = make_piece(us, promotion_type(m));
804 assert(relative_rank(us, to) == RANK_8);
805 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
807 remove_piece(pc, to);
808 put_piece(promotion, to);
811 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
812 st->pawnKey ^= Zobrist::psq[pc][to];
813 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
814 ^ Zobrist::psq[pc][pieceCount[pc]];
817 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
820 // Update pawn hash key
821 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
823 // Reset rule 50 draw counter
828 st->capturedPiece = captured;
830 // Update the key with the final value
833 // Calculate checkers bitboard (if move gives check)
834 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
836 sideToMove = ~sideToMove;
838 // Update king attacks used for fast check detection
841 // Calculate the repetition info. It is the ply distance from the previous
842 // occurrence of the same position, negative in the 3-fold case, or zero
843 // if the position was not repeated.
845 int end = std::min(st->rule50, st->pliesFromNull);
848 StateInfo* stp = st->previous->previous;
849 for (int i = 4; i <= end; i += 2)
851 stp = stp->previous->previous;
852 if (stp->key == st->key)
854 st->repetition = stp->repetition ? -i : i;
864 /// Position::undo_move() unmakes a move. When it returns, the position should
865 /// be restored to exactly the same state as before the move was made.
867 void Position::undo_move(Move m) {
871 sideToMove = ~sideToMove;
873 Color us = sideToMove;
874 Square from = from_sq(m);
875 Square to = to_sq(m);
876 Piece pc = piece_on(to);
878 assert(empty(from) || type_of(m) == CASTLING);
879 assert(type_of(st->capturedPiece) != KING);
881 if (type_of(m) == PROMOTION)
883 assert(relative_rank(us, to) == RANK_8);
884 assert(type_of(pc) == promotion_type(m));
885 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
887 remove_piece(pc, to);
888 pc = make_piece(us, PAWN);
892 if (type_of(m) == CASTLING)
895 do_castling<false>(us, from, to, rfrom, rto);
899 move_piece(pc, to, from); // Put the piece back at the source square
901 if (st->capturedPiece)
905 if (type_of(m) == ENPASSANT)
907 capsq -= pawn_push(us);
909 assert(type_of(pc) == PAWN);
910 assert(to == st->previous->epSquare);
911 assert(relative_rank(us, to) == RANK_6);
912 assert(piece_on(capsq) == NO_PIECE);
913 assert(st->capturedPiece == make_piece(~us, PAWN));
916 put_piece(st->capturedPiece, capsq); // Restore the captured piece
920 // Finally point our state pointer back to the previous state
928 /// Position::do_castling() is a helper used to do/undo a castling move. This
929 /// is a bit tricky in Chess960 where from/to squares can overlap.
931 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
933 bool kingSide = to > from;
934 rfrom = to; // Castling is encoded as "king captures friendly rook"
935 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
936 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
938 // Remove both pieces first since squares could overlap in Chess960
939 remove_piece(make_piece(us, KING), Do ? from : to);
940 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
941 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
942 put_piece(make_piece(us, KING), Do ? to : from);
943 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
947 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
948 /// the side to move without executing any move on the board.
950 void Position::do_null_move(StateInfo& newSt) {
953 assert(&newSt != st);
955 std::memcpy(&newSt, st, sizeof(StateInfo));
959 if (st->epSquare != SQ_NONE)
961 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
962 st->epSquare = SQ_NONE;
965 st->key ^= Zobrist::side;
966 prefetch(TT.first_entry(st->key));
969 st->pliesFromNull = 0;
971 sideToMove = ~sideToMove;
980 void Position::undo_null_move() {
985 sideToMove = ~sideToMove;
989 /// Position::key_after() computes the new hash key after the given move. Needed
990 /// for speculative prefetch. It doesn't recognize special moves like castling,
991 /// en-passant and promotions.
993 Key Position::key_after(Move m) const {
995 Square from = from_sq(m);
996 Square to = to_sq(m);
997 Piece pc = piece_on(from);
998 Piece captured = piece_on(to);
999 Key k = st->key ^ Zobrist::side;
1002 k ^= Zobrist::psq[captured][to];
1004 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1008 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1009 /// SEE value of move is greater or equal to the given threshold. We'll use an
1010 /// algorithm similar to alpha-beta pruning with a null window.
1012 bool Position::see_ge(Move m, Value threshold) const {
1016 // Only deal with normal moves, assume others pass a simple see
1017 if (type_of(m) != NORMAL)
1018 return VALUE_ZERO >= threshold;
1020 Square from = from_sq(m), to = to_sq(m);
1022 int swap = PieceValue[MG][piece_on(to)] - threshold;
1026 swap = PieceValue[MG][piece_on(from)] - swap;
1030 Bitboard occupied = pieces() ^ from ^ to;
1031 Color stm = color_of(piece_on(from));
1032 Bitboard attackers = attackers_to(to, occupied);
1033 Bitboard stmAttackers, bb;
1039 attackers &= occupied;
1041 // If stm has no more attackers then give up: stm loses
1042 if (!(stmAttackers = attackers & pieces(stm)))
1045 // Don't allow pinned pieces to attack (except the king) as long as
1046 // there are pinners on their original square.
1047 if (st->pinners[~stm] & occupied)
1048 stmAttackers &= ~st->blockersForKing[stm];
1055 // Locate and remove the next least valuable attacker, and add to
1056 // the bitboard 'attackers' any X-ray attackers behind it.
1057 if ((bb = stmAttackers & pieces(PAWN)))
1059 if ((swap = PawnValueMg - swap) < res)
1062 occupied ^= lsb(bb);
1063 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1066 else if ((bb = stmAttackers & pieces(KNIGHT)))
1068 if ((swap = KnightValueMg - swap) < res)
1071 occupied ^= lsb(bb);
1074 else if ((bb = stmAttackers & pieces(BISHOP)))
1076 if ((swap = BishopValueMg - swap) < res)
1079 occupied ^= lsb(bb);
1080 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1083 else if ((bb = stmAttackers & pieces(ROOK)))
1085 if ((swap = RookValueMg - swap) < res)
1088 occupied ^= lsb(bb);
1089 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1092 else if ((bb = stmAttackers & pieces(QUEEN)))
1094 if ((swap = QueenValueMg - swap) < res)
1097 occupied ^= lsb(bb);
1098 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1099 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1103 // If we "capture" with the king but opponent still has attackers,
1104 // reverse the result.
1105 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1111 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1112 /// or by repetition. It does not detect stalemates.
1114 bool Position::is_draw(int ply) const {
1116 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1119 // Return a draw score if a position repeats once earlier but strictly
1120 // after the root, or repeats twice before or at the root.
1121 if (st->repetition && st->repetition < ply)
1128 // Position::has_repeated() tests whether there has been at least one repetition
1129 // of positions since the last capture or pawn move.
1131 bool Position::has_repeated() const {
1133 StateInfo* stc = st;
1134 int end = std::min(st->rule50, st->pliesFromNull);
1137 if (stc->repetition)
1140 stc = stc->previous;
1146 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1147 /// or an earlier position has a move that directly reaches the current position.
1149 bool Position::has_game_cycle(int ply) const {
1153 int end = std::min(st->rule50, st->pliesFromNull);
1158 Key originalKey = st->key;
1159 StateInfo* stp = st->previous;
1161 for (int i = 3; i <= end; i += 2)
1163 stp = stp->previous->previous;
1165 Key moveKey = originalKey ^ stp->key;
1166 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1167 || (j = H2(moveKey), cuckoo[j] == moveKey))
1169 Move move = cuckooMove[j];
1170 Square s1 = from_sq(move);
1171 Square s2 = to_sq(move);
1173 if (!(between_bb(s1, s2) & pieces()))
1178 // For nodes before or at the root, check that the move is a
1179 // repetition rather than a move to the current position.
1180 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1181 // the same location, so we have to select which square to check.
1182 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1185 // For repetitions before or at the root, require one more
1186 if (stp->repetition)
1195 /// Position::flip() flips position with the white and black sides reversed. This
1196 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1198 void Position::flip() {
1201 std::stringstream ss(fen());
1203 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1205 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1206 f.insert(0, token + (f.empty() ? " " : "/"));
1209 ss >> token; // Active color
1210 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1212 ss >> token; // Castling availability
1215 std::transform(f.begin(), f.end(), f.begin(),
1216 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1218 ss >> token; // En passant square
1219 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1221 std::getline(ss, token); // Half and full moves
1224 set(f, is_chess960(), st, this_thread());
1226 assert(pos_is_ok());
1230 /// Position::pos_is_ok() performs some consistency checks for the
1231 /// position object and raises an asserts if something wrong is detected.
1232 /// This is meant to be helpful when debugging.
1234 bool Position::pos_is_ok() const {
1236 constexpr bool Fast = true; // Quick (default) or full check?
1238 if ( (sideToMove != WHITE && sideToMove != BLACK)
1239 || piece_on(square<KING>(WHITE)) != W_KING
1240 || piece_on(square<KING>(BLACK)) != B_KING
1241 || ( ep_square() != SQ_NONE
1242 && relative_rank(sideToMove, ep_square()) != RANK_6))
1243 assert(0 && "pos_is_ok: Default");
1248 if ( pieceCount[W_KING] != 1
1249 || pieceCount[B_KING] != 1
1250 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1251 assert(0 && "pos_is_ok: Kings");
1253 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1254 || pieceCount[W_PAWN] > 8
1255 || pieceCount[B_PAWN] > 8)
1256 assert(0 && "pos_is_ok: Pawns");
1258 if ( (pieces(WHITE) & pieces(BLACK))
1259 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1260 || popcount(pieces(WHITE)) > 16
1261 || popcount(pieces(BLACK)) > 16)
1262 assert(0 && "pos_is_ok: Bitboards");
1264 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1265 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1266 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1267 assert(0 && "pos_is_ok: Bitboards");
1271 if (std::memcmp(&si, st, sizeof(StateInfo)))
1272 assert(0 && "pos_is_ok: State");
1274 for (Piece pc : Pieces)
1276 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1277 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1278 assert(0 && "pos_is_ok: Pieces");
1280 for (int i = 0; i < pieceCount[pc]; ++i)
1281 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1282 assert(0 && "pos_is_ok: Index");
1285 for (Color c : { WHITE, BLACK })
1286 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1288 if (!can_castle(cr))
1291 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1292 || castlingRightsMask[castlingRookSquare[cr]] != cr
1293 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1294 assert(0 && "pos_is_ok: Castling");