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 that when moving the castling rook we do
537 // not discover some hidden checker.
538 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
540 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
543 // If the moving piece is a king, check whether the destination square is
544 // attacked by the opponent.
545 if (type_of(piece_on(from)) == KING)
546 return !(attackers_to(to) & pieces(~us));
548 // A non-king move is legal if and only if it is not pinned or it
549 // is moving along the ray towards or away from the king.
550 return !(blockers_for_king(us) & from)
551 || aligned(from, to, square<KING>(us));
555 /// Position::pseudo_legal() takes a random move and tests whether the move is
556 /// pseudo legal. It is used to validate moves from TT that can be corrupted
557 /// due to SMP concurrent access or hash position key aliasing.
559 bool Position::pseudo_legal(const Move m) const {
561 Color us = sideToMove;
562 Square from = from_sq(m);
563 Square to = to_sq(m);
564 Piece pc = moved_piece(m);
566 // Use a slower but simpler function for uncommon cases
567 // yet we skip the legality check of MoveList<LEGAL>().
568 if (type_of(m) != NORMAL)
569 return checkers() ? MoveList< EVASIONS>(*this).contains(m)
570 : MoveList<NON_EVASIONS>(*this).contains(m);
572 // Is not a promotion, so promotion piece must be empty
573 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
576 // If the 'from' square is not occupied by a piece belonging to the side to
577 // move, the move is obviously not legal.
578 if (pc == NO_PIECE || color_of(pc) != us)
581 // The destination square cannot be occupied by a friendly piece
585 // Handle the special case of a pawn move
586 if (type_of(pc) == PAWN)
588 // We have already handled promotion moves, so destination
589 // cannot be on the 8th/1st rank.
590 if ((Rank8BB | Rank1BB) & to)
593 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
594 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
595 && !( (from + 2 * pawn_push(us) == to) // Not a double push
596 && (relative_rank(us, from) == RANK_2)
598 && empty(to - pawn_push(us))))
601 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
604 // Evasions generator already takes care to avoid some kind of illegal moves
605 // and legal() relies on this. We therefore have to take care that the same
606 // kind of moves are filtered out here.
609 if (type_of(pc) != KING)
611 // Double check? In this case a king move is required
612 if (more_than_one(checkers()))
615 // Our move must be a blocking evasion or a capture of the checking piece
616 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
619 // In case of king moves under check we have to remove king so as to catch
620 // invalid moves like b1a1 when opposite queen is on c1.
621 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
629 /// Position::gives_check() tests whether a pseudo-legal move gives a check
631 bool Position::gives_check(Move m) const {
634 assert(color_of(moved_piece(m)) == sideToMove);
636 Square from = from_sq(m);
637 Square to = to_sq(m);
639 // Is there a direct check?
640 if (check_squares(type_of(piece_on(from))) & to)
643 // Is there a discovered check?
644 if ( (blockers_for_king(~sideToMove) & from)
645 && !aligned(from, to, square<KING>(~sideToMove)))
654 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
656 // En passant capture with check? We have already handled the case
657 // of direct checks and ordinary discovered check, so the only case we
658 // need to handle is the unusual case of a discovered check through
659 // the captured pawn.
662 Square capsq = make_square(file_of(to), rank_of(from));
663 Bitboard b = (pieces() ^ from ^ capsq) | to;
665 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
666 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
671 Square rfrom = to; // Castling is encoded as 'king captures the rook'
672 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
673 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
675 return (attacks_bb<ROOK>(rto) & square<KING>(~sideToMove))
676 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
685 /// Position::do_move() makes a move, and saves all information necessary
686 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
687 /// moves should be filtered out before this function is called.
689 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
692 assert(&newSt != st);
694 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
695 Key k = st->key ^ Zobrist::side;
697 // Copy some fields of the old state to our new StateInfo object except the
698 // ones which are going to be recalculated from scratch anyway and then switch
699 // our state pointer to point to the new (ready to be updated) state.
700 std::memcpy(&newSt, st, offsetof(StateInfo, key));
704 // Increment ply counters. In particular, rule50 will be reset to zero later on
705 // in case of a capture or a pawn move.
711 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
712 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
713 auto& dp = st->dirtyPiece;
716 Color us = sideToMove;
718 Square from = from_sq(m);
719 Square to = to_sq(m);
720 Piece pc = piece_on(from);
721 Piece captured = type_of(m) == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
723 assert(color_of(pc) == us);
724 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
725 assert(type_of(captured) != KING);
727 if (type_of(m) == CASTLING)
729 assert(pc == make_piece(us, KING));
730 assert(captured == make_piece(us, ROOK));
733 do_castling<true>(us, from, to, rfrom, rto);
735 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
743 // If the captured piece is a pawn, update pawn hash key, otherwise
744 // update non-pawn material.
745 if (type_of(captured) == PAWN)
747 if (type_of(m) == EN_PASSANT)
749 capsq -= pawn_push(us);
751 assert(pc == make_piece(us, PAWN));
752 assert(to == st->epSquare);
753 assert(relative_rank(us, to) == RANK_6);
754 assert(piece_on(to) == NO_PIECE);
755 assert(piece_on(capsq) == make_piece(them, PAWN));
758 st->pawnKey ^= Zobrist::psq[captured][capsq];
761 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
765 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
766 dp.piece[1] = captured;
771 // Update board and piece lists
774 if (type_of(m) == EN_PASSANT)
775 board[capsq] = NO_PIECE;
777 // Update material hash key and prefetch access to materialTable
778 k ^= Zobrist::psq[captured][capsq];
779 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
780 prefetch(thisThread->materialTable[st->materialKey]);
782 // Reset rule 50 counter
787 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
789 // Reset en passant square
790 if (st->epSquare != SQ_NONE)
792 k ^= Zobrist::enpassant[file_of(st->epSquare)];
793 st->epSquare = SQ_NONE;
796 // Update castling rights if needed
797 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
799 k ^= Zobrist::castling[st->castlingRights];
800 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
801 k ^= Zobrist::castling[st->castlingRights];
804 // Move the piece. The tricky Chess960 castling is handled earlier
805 if (type_of(m) != CASTLING)
814 move_piece(from, to);
817 // If the moving piece is a pawn do some special extra work
818 if (type_of(pc) == PAWN)
820 // Set en passant square if the moved pawn can be captured
821 if ( (int(to) ^ int(from)) == 16
822 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
824 st->epSquare = to - pawn_push(us);
825 k ^= Zobrist::enpassant[file_of(st->epSquare)];
828 else if (type_of(m) == PROMOTION)
830 Piece promotion = make_piece(us, promotion_type(m));
832 assert(relative_rank(us, to) == RANK_8);
833 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
836 put_piece(promotion, to);
840 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
842 dp.piece[dp.dirty_num] = promotion;
843 dp.from[dp.dirty_num] = SQ_NONE;
844 dp.to[dp.dirty_num] = to;
849 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
850 st->pawnKey ^= Zobrist::psq[pc][to];
851 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
852 ^ Zobrist::psq[pc][pieceCount[pc]];
855 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
858 // Update pawn hash key
859 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
861 // Reset rule 50 draw counter
866 st->capturedPiece = captured;
868 // Update the key with the final value
871 // Calculate checkers bitboard (if move gives check)
872 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
874 sideToMove = ~sideToMove;
876 // Update king attacks used for fast check detection
879 // Calculate the repetition info. It is the ply distance from the previous
880 // occurrence of the same position, negative in the 3-fold case, or zero
881 // if the position was not repeated.
883 int end = std::min(st->rule50, st->pliesFromNull);
886 StateInfo* stp = st->previous->previous;
887 for (int i = 4; i <= end; i += 2)
889 stp = stp->previous->previous;
890 if (stp->key == st->key)
892 st->repetition = stp->repetition ? -i : i;
902 /// Position::undo_move() unmakes a move. When it returns, the position should
903 /// be restored to exactly the same state as before the move was made.
905 void Position::undo_move(Move m) {
909 sideToMove = ~sideToMove;
911 Color us = sideToMove;
912 Square from = from_sq(m);
913 Square to = to_sq(m);
914 Piece pc = piece_on(to);
916 assert(empty(from) || type_of(m) == CASTLING);
917 assert(type_of(st->capturedPiece) != KING);
919 if (type_of(m) == PROMOTION)
921 assert(relative_rank(us, to) == RANK_8);
922 assert(type_of(pc) == promotion_type(m));
923 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
926 pc = make_piece(us, PAWN);
930 if (type_of(m) == CASTLING)
933 do_castling<false>(us, from, to, rfrom, rto);
937 move_piece(to, from); // Put the piece back at the source square
939 if (st->capturedPiece)
943 if (type_of(m) == EN_PASSANT)
945 capsq -= pawn_push(us);
947 assert(type_of(pc) == PAWN);
948 assert(to == st->previous->epSquare);
949 assert(relative_rank(us, to) == RANK_6);
950 assert(piece_on(capsq) == NO_PIECE);
951 assert(st->capturedPiece == make_piece(~us, PAWN));
954 put_piece(st->capturedPiece, capsq); // Restore the captured piece
958 // Finally point our state pointer back to the previous state
966 /// Position::do_castling() is a helper used to do/undo a castling move. This
967 /// is a bit tricky in Chess960 where from/to squares can overlap.
969 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
971 bool kingSide = to > from;
972 rfrom = to; // Castling is encoded as "king captures friendly rook"
973 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
974 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
976 if (Do && Eval::useNNUE)
978 auto& dp = st->dirtyPiece;
979 dp.piece[0] = make_piece(us, KING);
982 dp.piece[1] = make_piece(us, ROOK);
988 // Remove both pieces first since squares could overlap in Chess960
989 remove_piece(Do ? from : to);
990 remove_piece(Do ? rfrom : rto);
991 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
992 put_piece(make_piece(us, KING), Do ? to : from);
993 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
997 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
998 /// the side to move without executing any move on the board.
1000 void Position::do_null_move(StateInfo& newSt) {
1002 assert(!checkers());
1003 assert(&newSt != st);
1005 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1007 newSt.previous = st;
1010 st->dirtyPiece.dirty_num = 0;
1011 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1012 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
1013 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
1015 if (st->epSquare != SQ_NONE)
1017 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1018 st->epSquare = SQ_NONE;
1021 st->key ^= Zobrist::side;
1022 prefetch(TT.first_entry(key()));
1025 st->pliesFromNull = 0;
1027 sideToMove = ~sideToMove;
1033 assert(pos_is_ok());
1036 void Position::undo_null_move() {
1038 assert(!checkers());
1041 sideToMove = ~sideToMove;
1045 /// Position::key_after() computes the new hash key after the given move. Needed
1046 /// for speculative prefetch. It doesn't recognize special moves like castling,
1047 /// en passant and promotions.
1049 Key Position::key_after(Move m) const {
1051 Square from = from_sq(m);
1052 Square to = to_sq(m);
1053 Piece pc = piece_on(from);
1054 Piece captured = piece_on(to);
1055 Key k = st->key ^ Zobrist::side;
1058 k ^= Zobrist::psq[captured][to];
1060 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1064 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1065 /// SEE value of move is greater or equal to the given threshold. We'll use an
1066 /// algorithm similar to alpha-beta pruning with a null window.
1068 bool Position::see_ge(Move m, Value threshold) const {
1072 // Only deal with normal moves, assume others pass a simple SEE
1073 if (type_of(m) != NORMAL)
1074 return VALUE_ZERO >= threshold;
1076 Square from = from_sq(m), to = to_sq(m);
1078 int swap = PieceValue[MG][piece_on(to)] - threshold;
1082 swap = PieceValue[MG][piece_on(from)] - swap;
1086 Bitboard occupied = pieces() ^ from ^ to;
1087 Color stm = color_of(piece_on(from));
1088 Bitboard attackers = attackers_to(to, occupied);
1089 Bitboard stmAttackers, bb;
1095 attackers &= occupied;
1097 // If stm has no more attackers then give up: stm loses
1098 if (!(stmAttackers = attackers & pieces(stm)))
1101 // Don't allow pinned pieces to attack (except the king) as long as
1102 // there are pinners on their original square.
1103 if (pinners(~stm) & occupied)
1104 stmAttackers &= ~blockers_for_king(stm);
1111 // Locate and remove the next least valuable attacker, and add to
1112 // the bitboard 'attackers' any X-ray attackers behind it.
1113 if ((bb = stmAttackers & pieces(PAWN)))
1115 if ((swap = PawnValueMg - swap) < res)
1118 occupied ^= lsb(bb);
1119 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1122 else if ((bb = stmAttackers & pieces(KNIGHT)))
1124 if ((swap = KnightValueMg - swap) < res)
1127 occupied ^= lsb(bb);
1130 else if ((bb = stmAttackers & pieces(BISHOP)))
1132 if ((swap = BishopValueMg - swap) < res)
1135 occupied ^= lsb(bb);
1136 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1139 else if ((bb = stmAttackers & pieces(ROOK)))
1141 if ((swap = RookValueMg - swap) < res)
1144 occupied ^= lsb(bb);
1145 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1148 else if ((bb = stmAttackers & pieces(QUEEN)))
1150 if ((swap = QueenValueMg - swap) < res)
1153 occupied ^= lsb(bb);
1154 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1155 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1159 // If we "capture" with the king but opponent still has attackers,
1160 // reverse the result.
1161 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1168 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1169 /// or by repetition. It does not detect stalemates.
1171 bool Position::is_draw(int ply) const {
1173 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1176 // Return a draw score if a position repeats once earlier but strictly
1177 // after the root, or repeats twice before or at the root.
1178 return st->repetition && st->repetition < ply;
1182 // Position::has_repeated() tests whether there has been at least one repetition
1183 // of positions since the last capture or pawn move.
1185 bool Position::has_repeated() const {
1187 StateInfo* stc = st;
1188 int end = std::min(st->rule50, st->pliesFromNull);
1191 if (stc->repetition)
1194 stc = stc->previous;
1200 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1201 /// or an earlier position has a move that directly reaches the current position.
1203 bool Position::has_game_cycle(int ply) const {
1207 int end = std::min(st->rule50, st->pliesFromNull);
1212 Key originalKey = st->key;
1213 StateInfo* stp = st->previous;
1215 for (int i = 3; i <= end; i += 2)
1217 stp = stp->previous->previous;
1219 Key moveKey = originalKey ^ stp->key;
1220 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1221 || (j = H2(moveKey), cuckoo[j] == moveKey))
1223 Move move = cuckooMove[j];
1224 Square s1 = from_sq(move);
1225 Square s2 = to_sq(move);
1227 if (!(between_bb(s1, s2) & pieces()))
1232 // For nodes before or at the root, check that the move is a
1233 // repetition rather than a move to the current position.
1234 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1235 // the same location, so we have to select which square to check.
1236 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1239 // For repetitions before or at the root, require one more
1240 if (stp->repetition)
1249 /// Position::flip() flips position with the white and black sides reversed. This
1250 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1252 void Position::flip() {
1255 std::stringstream ss(fen());
1257 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1259 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1260 f.insert(0, token + (f.empty() ? " " : "/"));
1263 ss >> token; // Active color
1264 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1266 ss >> token; // Castling availability
1269 std::transform(f.begin(), f.end(), f.begin(),
1270 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1272 ss >> token; // En passant square
1273 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1275 std::getline(ss, token); // Half and full moves
1278 set(f, is_chess960(), st, this_thread());
1280 assert(pos_is_ok());
1284 /// Position::pos_is_ok() performs some consistency checks for the
1285 /// position object and raises an asserts if something wrong is detected.
1286 /// This is meant to be helpful when debugging.
1288 bool Position::pos_is_ok() const {
1290 constexpr bool Fast = true; // Quick (default) or full check?
1292 if ( (sideToMove != WHITE && sideToMove != BLACK)
1293 || piece_on(square<KING>(WHITE)) != W_KING
1294 || piece_on(square<KING>(BLACK)) != B_KING
1295 || ( ep_square() != SQ_NONE
1296 && relative_rank(sideToMove, ep_square()) != RANK_6))
1297 assert(0 && "pos_is_ok: Default");
1302 if ( pieceCount[W_KING] != 1
1303 || pieceCount[B_KING] != 1
1304 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1305 assert(0 && "pos_is_ok: Kings");
1307 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1308 || pieceCount[W_PAWN] > 8
1309 || pieceCount[B_PAWN] > 8)
1310 assert(0 && "pos_is_ok: Pawns");
1312 if ( (pieces(WHITE) & pieces(BLACK))
1313 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1314 || popcount(pieces(WHITE)) > 16
1315 || popcount(pieces(BLACK)) > 16)
1316 assert(0 && "pos_is_ok: Bitboards");
1318 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1319 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1320 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1321 assert(0 && "pos_is_ok: Bitboards");
1324 ASSERT_ALIGNED(&si, Eval::NNUE::kCacheLineSize);
1327 if (std::memcmp(&si, st, sizeof(StateInfo)))
1328 assert(0 && "pos_is_ok: State");
1330 for (Piece pc : Pieces)
1331 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1332 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1333 assert(0 && "pos_is_ok: Pieces");
1335 for (Color c : { WHITE, BLACK })
1336 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1338 if (!can_castle(cr))
1341 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1342 || castlingRightsMask[castlingRookSquare[cr]] != cr
1343 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1344 assert(0 && "pos_is_ok: Castling");