2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
21 #include <cstddef> // For offsetof()
22 #include <cstring> // For std::memset, std::memcmp
33 #include "syzygy/tbprobe.h"
39 Key psq[PIECE_NB][SQUARE_NB];
40 Key enpassant[FILE_NB];
41 Key castling[CASTLING_RIGHT_NB];
47 const string PieceToChar(" PNBRQK pnbrqk");
49 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
50 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
54 /// operator<<(Position) returns an ASCII representation of the position
56 std::ostream& operator<<(std::ostream& os, const Position& pos) {
58 os << "\n +---+---+---+---+---+---+---+---+\n";
60 for (Rank r = RANK_8; r >= RANK_1; --r)
62 for (File f = FILE_A; f <= FILE_H; ++f)
63 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
65 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
68 os << " a b c d e f g h\n"
69 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
70 << std::setfill('0') << std::setw(16) << pos.key()
71 << std::setfill(' ') << std::dec << "\nCheckers: ";
73 for (Bitboard b = pos.checkers(); b; )
74 os << UCI::square(pop_lsb(&b)) << " ";
76 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
77 && !pos.can_castle(ANY_CASTLING))
80 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
83 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
84 Tablebases::ProbeState s1, s2;
85 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
86 int dtz = Tablebases::probe_dtz(p, &s2);
87 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
88 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
95 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
96 // situations. Description of the algorithm in the following paper:
97 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
99 // First and second hash functions for indexing the cuckoo tables
100 inline int H1(Key h) { return h & 0x1fff; }
101 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
103 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
105 Move cuckooMove[8192];
108 /// Position::init() initializes at startup the various arrays used to compute hash keys
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)
122 Zobrist::castling[cr] = rng.rand<Key>();
124 Zobrist::side = rng.rand<Key>();
125 Zobrist::noPawns = rng.rand<Key>();
127 // Prepare the cuckoo tables
128 std::memset(cuckoo, 0, sizeof(cuckoo));
129 std::memset(cuckooMove, 0, sizeof(cuckooMove));
131 for (Piece pc : Pieces)
132 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
133 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
134 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
136 Move move = make_move(s1, s2);
137 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
141 std::swap(cuckoo[i], key);
142 std::swap(cuckooMove[i], move);
143 if (move == MOVE_NONE) // Arrived at empty slot?
145 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
149 assert(count == 3668);
153 /// Position::set() initializes the position object with the given FEN string.
154 /// This function is not very robust - make sure that input FENs are correct,
155 /// this is assumed to be the responsibility of the GUI.
157 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
159 A FEN string defines a particular position using only the ASCII character set.
161 A FEN string contains six fields separated by a space. The fields are:
163 1) Piece placement (from white's perspective). Each rank is described, starting
164 with rank 8 and ending with rank 1. Within each rank, the contents of each
165 square are described from file A through file H. Following the Standard
166 Algebraic Notation (SAN), each piece is identified by a single letter taken
167 from the standard English names. White pieces are designated using upper-case
168 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
169 noted using digits 1 through 8 (the number of blank squares), and "/"
172 2) Active color. "w" means white moves next, "b" means black.
174 3) Castling availability. If neither side can castle, this is "-". Otherwise,
175 this has one or more letters: "K" (White can castle kingside), "Q" (White
176 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
177 can castle queenside).
179 4) En passant target square (in algebraic notation). If there's no en passant
180 target square, this is "-". If a pawn has just made a 2-square move, this
181 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
182 if there is a pawn in position to make an en passant capture, and if there really
183 is a pawn that might have advanced two squares.
185 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
186 or capture. This is used to determine if a draw can be claimed under the
189 6) Fullmove number. The number of the full move. It starts at 1, and is
190 incremented after Black's move.
193 unsigned char col, row, token;
196 std::istringstream ss(fenStr);
198 std::memset(this, 0, sizeof(Position));
199 std::memset(si, 0, sizeof(StateInfo));
204 // 1. Piece placement
205 while ((ss >> token) && !isspace(token))
208 sq += (token - '0') * EAST; // Advance the given number of files
210 else if (token == '/')
213 else if ((idx = PieceToChar.find(token)) != string::npos) {
214 put_piece(Piece(idx), sq);
221 sideToMove = (token == 'w' ? WHITE : BLACK);
224 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
225 // Shredder-FEN that uses the letters of the columns on which the rooks began
226 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
227 // if an inner rook is associated with the castling right, the castling tag is
228 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
229 while ((ss >> token) && !isspace(token))
232 Color c = islower(token) ? BLACK : WHITE;
233 Piece rook = make_piece(c, ROOK);
235 token = char(toupper(token));
238 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
240 else if (token == 'Q')
241 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
243 else if (token >= 'A' && token <= 'H')
244 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
249 set_castling_right(c, rsq);
252 // 4. En passant square.
253 // Ignore if square is invalid or not on side to move relative rank 6.
254 bool enpassant = false;
256 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
257 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
259 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
261 // En passant square will be considered only if
262 // a) side to move have a pawn threatening epSquare
263 // b) there is an enemy pawn in front of epSquare
264 // c) there is no piece on epSquare or behind epSquare
265 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
266 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
267 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
271 st->epSquare = SQ_NONE;
273 // 5-6. Halfmove clock and fullmove number
274 ss >> std::skipws >> st->rule50 >> gamePly;
276 // Convert from fullmove starting from 1 to gamePly starting from 0,
277 // handle also common incorrect FEN with fullmove = 0.
278 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
280 chess960 = isChess960;
283 st->accumulator.state[WHITE] = Eval::NNUE::INIT;
284 st->accumulator.state[BLACK] = Eval::NNUE::INIT;
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)
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] = pawn_attacks_bb(~sideToMove, ksq);
323 si->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
324 si->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
325 si->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
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[0] == 'K');
380 string sides[] = { code.substr(code.find('K', 1)), // Weak
381 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
383 assert(sides[0].length() > 0 && sides[0].length() < 8);
384 assert(sides[1].length() > 0 && sides[1].length() < 8);
386 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
388 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
389 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
391 return set(fenStr, false, si, nullptr);
395 /// Position::fen() returns a FEN representation of the position. In case of
396 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
398 const string Position::fen() const {
401 std::ostringstream ss;
403 for (Rank r = RANK_8; r >= RANK_1; --r)
405 for (File f = FILE_A; f <= FILE_H; ++f)
407 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
414 ss << PieceToChar[piece_on(make_square(f, r))];
421 ss << (sideToMove == WHITE ? " w " : " b ");
423 if (can_castle(WHITE_OO))
424 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
426 if (can_castle(WHITE_OOO))
427 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
429 if (can_castle(BLACK_OO))
430 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
432 if (can_castle(BLACK_OOO))
433 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
435 if (!can_castle(ANY_CASTLING))
438 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
439 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
445 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
446 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
447 /// slider if removing that piece from the board would result in a position where
448 /// square 's' is attacked. For example, a king-attack blocking piece can be either
449 /// a pinned or a discovered check piece, according if its color is the opposite
450 /// or the same of the color of the slider.
452 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
454 Bitboard blockers = 0;
457 // Snipers are sliders that attack 's' when a piece and other snipers are removed
458 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
459 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
460 Bitboard occupancy = pieces() ^ snipers;
464 Square sniperSq = pop_lsb(&snipers);
465 Bitboard b = between_bb(s, sniperSq) & occupancy;
467 if (b && !more_than_one(b))
470 if (b & pieces(color_of(piece_on(s))))
478 /// Position::attackers_to() computes a bitboard of all pieces which attack a
479 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
481 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
483 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
484 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
485 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
486 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
487 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
488 | (attacks_bb<KING>(s) & pieces(KING));
492 /// Position::legal() tests whether a pseudo-legal move is legal
494 bool Position::legal(Move m) const {
498 Color us = sideToMove;
499 Square from = from_sq(m);
500 Square to = to_sq(m);
502 assert(color_of(moved_piece(m)) == us);
503 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
505 // En passant captures are a tricky special case. Because they are rather
506 // uncommon, we do it simply by testing whether the king is attacked after
508 if (type_of(m) == EN_PASSANT)
510 Square ksq = square<KING>(us);
511 Square capsq = to - pawn_push(us);
512 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
514 assert(to == ep_square());
515 assert(moved_piece(m) == make_piece(us, PAWN));
516 assert(piece_on(capsq) == make_piece(~us, PAWN));
517 assert(piece_on(to) == NO_PIECE);
519 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
520 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
523 // Castling moves generation does not check if the castling path is clear of
524 // enemy attacks, it is delayed at a later time: now!
525 if (type_of(m) == CASTLING)
527 // After castling, the rook and king final positions are the same in
528 // Chess960 as they would be in standard chess.
529 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
530 Direction step = to > from ? WEST : EAST;
532 for (Square s = to; s != from; s += step)
533 if (attackers_to(s) & pieces(~us))
536 // In case of Chess960, verify if the Rook blocks some checks
537 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
538 return !chess960 || !(blockers_for_king(us) & to_sq(m));
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 // yet we skip the legality check of MoveList<LEGAL>().
566 if (type_of(m) != NORMAL)
567 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
568 : MoveList<NON_EVASIONS>(*this).contains(m);
570 // Is not a promotion, so promotion piece must be empty
571 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
574 // If the 'from' square is not occupied by a piece belonging to the side to
575 // move, the move is obviously not legal.
576 if (pc == NO_PIECE || color_of(pc) != us)
579 // The destination square cannot be occupied by a friendly piece
583 // Handle the special case of a pawn move
584 if (type_of(pc) == PAWN)
586 // We have already handled promotion moves, so destination
587 // cannot be on the 8th/1st rank.
588 if ((Rank8BB | Rank1BB) & to)
591 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
592 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
593 && !( (from + 2 * pawn_push(us) == to) // Not a double push
594 && (relative_rank(us, from) == RANK_2)
596 && empty(to - pawn_push(us))))
599 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
602 // Evasions generator already takes care to avoid some kind of illegal moves
603 // and legal() relies on this. We therefore have to take care that the same
604 // kind of moves are filtered out here.
607 if (type_of(pc) != KING)
609 // Double check? In this case a king move is required
610 if (more_than_one(checkers()))
613 // Our move must be a blocking evasion or a capture of the checking piece
614 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
617 // In case of king moves under check we have to remove king so as to catch
618 // invalid moves like b1a1 when opposite queen is on c1.
619 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
627 /// Position::gives_check() tests whether a pseudo-legal move gives a check
629 bool Position::gives_check(Move m) const {
632 assert(color_of(moved_piece(m)) == sideToMove);
634 Square from = from_sq(m);
635 Square to = to_sq(m);
637 // Is there a direct check?
638 if (check_squares(type_of(piece_on(from))) & to)
641 // Is there a discovered check?
642 if ( (blockers_for_king(~sideToMove) & from)
643 && !aligned(from, to, square<KING>(~sideToMove)))
652 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
654 // En passant capture with check? We have already handled the case
655 // of direct checks and ordinary discovered check, so the only case we
656 // need to handle is the unusual case of a discovered check through
657 // the captured pawn.
660 Square capsq = make_square(file_of(to), rank_of(from));
661 Bitboard b = (pieces() ^ from ^ capsq) | to;
663 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
664 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
668 // Castling is encoded as 'king captures the rook'
669 Square ksq = square<KING>(~sideToMove);
670 Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);
672 return (attacks_bb<ROOK>(rto) & ksq)
673 && (attacks_bb<ROOK>(rto, pieces() ^ from ^ to) & ksq);
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.
705 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
706 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
707 auto& dp = st->dirtyPiece;
710 Color us = sideToMove;
712 Square from = from_sq(m);
713 Square to = to_sq(m);
714 Piece pc = piece_on(from);
715 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
717 assert(color_of(pc) == us);
718 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
719 assert(type_of(captured) != KING);
721 if (type_of(m) == CASTLING)
723 assert(pc == make_piece(us, KING));
724 assert(captured == make_piece(us, ROOK));
727 do_castling<true>(us, from, to, rfrom, rto);
729 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
737 // If the captured piece is a pawn, update pawn hash key, otherwise
738 // update non-pawn material.
739 if (type_of(captured) == PAWN)
741 if (type_of(m) == EN_PASSANT)
743 capsq -= pawn_push(us);
745 assert(pc == make_piece(us, PAWN));
746 assert(to == st->epSquare);
747 assert(relative_rank(us, to) == RANK_6);
748 assert(piece_on(to) == NO_PIECE);
749 assert(piece_on(capsq) == make_piece(them, PAWN));
752 st->pawnKey ^= Zobrist::psq[captured][capsq];
755 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
759 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
760 dp.piece[1] = captured;
765 // Update board and piece lists
768 if (type_of(m) == EN_PASSANT)
769 board[capsq] = NO_PIECE;
771 // Update material hash key and prefetch access to materialTable
772 k ^= Zobrist::psq[captured][capsq];
773 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
774 prefetch(thisThread->materialTable[st->materialKey]);
776 // Reset rule 50 counter
781 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
783 // Reset en passant square
784 if (st->epSquare != SQ_NONE)
786 k ^= Zobrist::enpassant[file_of(st->epSquare)];
787 st->epSquare = SQ_NONE;
790 // Update castling rights if needed
791 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
793 k ^= Zobrist::castling[st->castlingRights];
794 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
795 k ^= Zobrist::castling[st->castlingRights];
798 // Move the piece. The tricky Chess960 castling is handled earlier
799 if (type_of(m) != CASTLING)
808 move_piece(from, to);
811 // If the moving piece is a pawn do some special extra work
812 if (type_of(pc) == PAWN)
814 // Set en passant square if the moved pawn can be captured
815 if ( (int(to) ^ int(from)) == 16
816 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
818 st->epSquare = to - pawn_push(us);
819 k ^= Zobrist::enpassant[file_of(st->epSquare)];
822 else if (type_of(m) == PROMOTION)
824 Piece promotion = make_piece(us, promotion_type(m));
826 assert(relative_rank(us, to) == RANK_8);
827 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
830 put_piece(promotion, to);
834 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
836 dp.piece[dp.dirty_num] = promotion;
837 dp.from[dp.dirty_num] = SQ_NONE;
838 dp.to[dp.dirty_num] = to;
843 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
844 st->pawnKey ^= Zobrist::psq[pc][to];
845 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
846 ^ Zobrist::psq[pc][pieceCount[pc]];
849 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
852 // Update pawn hash key
853 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
855 // Reset rule 50 draw counter
860 st->capturedPiece = captured;
862 // Update the key with the final value
865 // Calculate checkers bitboard (if move gives check)
866 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
868 sideToMove = ~sideToMove;
870 // Update king attacks used for fast check detection
873 // Calculate the repetition info. It is the ply distance from the previous
874 // occurrence of the same position, negative in the 3-fold case, or zero
875 // if the position was not repeated.
877 int end = std::min(st->rule50, st->pliesFromNull);
880 StateInfo* stp = st->previous->previous;
881 for (int i = 4; i <= end; i += 2)
883 stp = stp->previous->previous;
884 if (stp->key == st->key)
886 st->repetition = stp->repetition ? -i : i;
896 /// Position::undo_move() unmakes a move. When it returns, the position should
897 /// be restored to exactly the same state as before the move was made.
899 void Position::undo_move(Move m) {
903 sideToMove = ~sideToMove;
905 Color us = sideToMove;
906 Square from = from_sq(m);
907 Square to = to_sq(m);
908 Piece pc = piece_on(to);
910 assert(empty(from) || type_of(m) == CASTLING);
911 assert(type_of(st->capturedPiece) != KING);
913 if (type_of(m) == PROMOTION)
915 assert(relative_rank(us, to) == RANK_8);
916 assert(type_of(pc) == promotion_type(m));
917 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
920 pc = make_piece(us, PAWN);
924 if (type_of(m) == CASTLING)
927 do_castling<false>(us, from, to, rfrom, rto);
931 move_piece(to, from); // Put the piece back at the source square
933 if (st->capturedPiece)
937 if (type_of(m) == EN_PASSANT)
939 capsq -= pawn_push(us);
941 assert(type_of(pc) == PAWN);
942 assert(to == st->previous->epSquare);
943 assert(relative_rank(us, to) == RANK_6);
944 assert(piece_on(capsq) == NO_PIECE);
945 assert(st->capturedPiece == make_piece(~us, PAWN));
948 put_piece(st->capturedPiece, capsq); // Restore the captured piece
952 // Finally point our state pointer back to the previous state
960 /// Position::do_castling() is a helper used to do/undo a castling move. This
961 /// is a bit tricky in Chess960 where from/to squares can overlap.
963 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
965 bool kingSide = to > from;
966 rfrom = to; // Castling is encoded as "king captures friendly rook"
967 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
968 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
970 if (Do && Eval::useNNUE)
972 auto& dp = st->dirtyPiece;
973 dp.piece[0] = make_piece(us, KING);
976 dp.piece[1] = make_piece(us, ROOK);
982 // Remove both pieces first since squares could overlap in Chess960
983 remove_piece(Do ? from : to);
984 remove_piece(Do ? rfrom : rto);
985 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
986 put_piece(make_piece(us, KING), Do ? to : from);
987 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
991 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
992 /// the side to move without executing any move on the board.
994 void Position::do_null_move(StateInfo& newSt) {
997 assert(&newSt != st);
999 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1001 newSt.previous = st;
1004 st->dirtyPiece.dirty_num = 0;
1005 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1006 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
1007 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
1009 if (st->epSquare != SQ_NONE)
1011 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1012 st->epSquare = SQ_NONE;
1015 st->key ^= Zobrist::side;
1016 prefetch(TT.first_entry(key()));
1019 st->pliesFromNull = 0;
1021 sideToMove = ~sideToMove;
1027 assert(pos_is_ok());
1030 void Position::undo_null_move() {
1032 assert(!checkers());
1035 sideToMove = ~sideToMove;
1039 /// Position::key_after() computes the new hash key after the given move. Needed
1040 /// for speculative prefetch. It doesn't recognize special moves like castling,
1041 /// en passant and promotions.
1043 Key Position::key_after(Move m) const {
1045 Square from = from_sq(m);
1046 Square to = to_sq(m);
1047 Piece pc = piece_on(from);
1048 Piece captured = piece_on(to);
1049 Key k = st->key ^ Zobrist::side;
1052 k ^= Zobrist::psq[captured][to];
1054 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1058 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1059 /// SEE value of move is greater or equal to the given threshold. We'll use an
1060 /// algorithm similar to alpha-beta pruning with a null window.
1062 bool Position::see_ge(Move m, Value threshold) const {
1066 // Only deal with normal moves, assume others pass a simple SEE
1067 if (type_of(m) != NORMAL)
1068 return VALUE_ZERO >= threshold;
1070 Square from = from_sq(m), to = to_sq(m);
1072 int swap = PieceValue[MG][piece_on(to)] - threshold;
1076 swap = PieceValue[MG][piece_on(from)] - swap;
1080 Bitboard occupied = pieces() ^ from ^ to;
1081 Color stm = color_of(piece_on(from));
1082 Bitboard attackers = attackers_to(to, occupied);
1083 Bitboard stmAttackers, bb;
1089 attackers &= occupied;
1091 // If stm has no more attackers then give up: stm loses
1092 if (!(stmAttackers = attackers & pieces(stm)))
1095 // Don't allow pinned pieces to attack (except the king) as long as
1096 // there are pinners on their original square.
1097 if (pinners(~stm) & occupied)
1098 stmAttackers &= ~blockers_for_king(stm);
1105 // Locate and remove the next least valuable attacker, and add to
1106 // the bitboard 'attackers' any X-ray attackers behind it.
1107 if ((bb = stmAttackers & pieces(PAWN)))
1109 if ((swap = PawnValueMg - swap) < res)
1112 occupied ^= lsb(bb);
1113 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1116 else if ((bb = stmAttackers & pieces(KNIGHT)))
1118 if ((swap = KnightValueMg - swap) < res)
1121 occupied ^= lsb(bb);
1124 else if ((bb = stmAttackers & pieces(BISHOP)))
1126 if ((swap = BishopValueMg - swap) < res)
1129 occupied ^= lsb(bb);
1130 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1133 else if ((bb = stmAttackers & pieces(ROOK)))
1135 if ((swap = RookValueMg - swap) < res)
1138 occupied ^= lsb(bb);
1139 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1142 else if ((bb = stmAttackers & pieces(QUEEN)))
1144 if ((swap = QueenValueMg - swap) < res)
1147 occupied ^= lsb(bb);
1148 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1149 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1153 // If we "capture" with the king but opponent still has attackers,
1154 // reverse the result.
1155 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1162 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1163 /// or by repetition. It does not detect stalemates.
1165 bool Position::is_draw(int ply) const {
1167 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1170 // Return a draw score if a position repeats once earlier but strictly
1171 // after the root, or repeats twice before or at the root.
1172 return st->repetition && st->repetition < ply;
1176 // Position::has_repeated() tests whether there has been at least one repetition
1177 // of positions since the last capture or pawn move.
1179 bool Position::has_repeated() const {
1181 StateInfo* stc = st;
1182 int end = std::min(st->rule50, st->pliesFromNull);
1185 if (stc->repetition)
1188 stc = stc->previous;
1194 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1195 /// or an earlier position has a move that directly reaches the current position.
1197 bool Position::has_game_cycle(int ply) const {
1201 int end = std::min(st->rule50, st->pliesFromNull);
1206 Key originalKey = st->key;
1207 StateInfo* stp = st->previous;
1209 for (int i = 3; i <= end; i += 2)
1211 stp = stp->previous->previous;
1213 Key moveKey = originalKey ^ stp->key;
1214 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1215 || (j = H2(moveKey), cuckoo[j] == moveKey))
1217 Move move = cuckooMove[j];
1218 Square s1 = from_sq(move);
1219 Square s2 = to_sq(move);
1221 if (!(between_bb(s1, s2) & pieces()))
1226 // For nodes before or at the root, check that the move is a
1227 // repetition rather than a move to the current position.
1228 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1229 // the same location, so we have to select which square to check.
1230 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1233 // For repetitions before or at the root, require one more
1234 if (stp->repetition)
1243 /// Position::flip() flips position with the white and black sides reversed. This
1244 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1246 void Position::flip() {
1249 std::stringstream ss(fen());
1251 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1253 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1254 f.insert(0, token + (f.empty() ? " " : "/"));
1257 ss >> token; // Active color
1258 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1260 ss >> token; // Castling availability
1263 std::transform(f.begin(), f.end(), f.begin(),
1264 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1266 ss >> token; // En passant square
1267 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1269 std::getline(ss, token); // Half and full moves
1272 set(f, is_chess960(), st, this_thread());
1274 assert(pos_is_ok());
1278 /// Position::pos_is_ok() performs some consistency checks for the
1279 /// position object and raises an asserts if something wrong is detected.
1280 /// This is meant to be helpful when debugging.
1282 bool Position::pos_is_ok() const {
1284 constexpr bool Fast = true; // Quick (default) or full check?
1286 if ( (sideToMove != WHITE && sideToMove != BLACK)
1287 || piece_on(square<KING>(WHITE)) != W_KING
1288 || piece_on(square<KING>(BLACK)) != B_KING
1289 || ( ep_square() != SQ_NONE
1290 && relative_rank(sideToMove, ep_square()) != RANK_6))
1291 assert(0 && "pos_is_ok: Default");
1296 if ( pieceCount[W_KING] != 1
1297 || pieceCount[B_KING] != 1
1298 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1299 assert(0 && "pos_is_ok: Kings");
1301 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1302 || pieceCount[W_PAWN] > 8
1303 || pieceCount[B_PAWN] > 8)
1304 assert(0 && "pos_is_ok: Pawns");
1306 if ( (pieces(WHITE) & pieces(BLACK))
1307 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1308 || popcount(pieces(WHITE)) > 16
1309 || popcount(pieces(BLACK)) > 16)
1310 assert(0 && "pos_is_ok: Bitboards");
1312 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1313 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1314 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1315 assert(0 && "pos_is_ok: Bitboards");
1318 ASSERT_ALIGNED(&si, Eval::NNUE::kCacheLineSize);
1321 if (std::memcmp(&si, st, sizeof(StateInfo)))
1322 assert(0 && "pos_is_ok: State");
1324 for (Piece pc : Pieces)
1325 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1326 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1327 assert(0 && "pos_is_ok: Pieces");
1329 for (Color c : { WHITE, BLACK })
1330 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1332 if (!can_castle(cr))
1335 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1336 || castlingRightsMask[castlingRookSquare[cr]] != cr
1337 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1338 assert(0 && "pos_is_ok: Castling");