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) 2015-2019 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 };
54 // min_attacker() is a helper function used by see_ge() to locate the least
55 // valuable attacker for the side to move, remove the attacker we just found
56 // from the bitboards and scan for new X-ray attacks behind it.
59 PieceType min_attacker(const Bitboard* byTypeBB, Square to, Bitboard stmAttackers,
60 Bitboard& occupied, Bitboard& attackers) {
62 Bitboard b = stmAttackers & byTypeBB[Pt];
64 return min_attacker<Pt + 1>(byTypeBB, to, stmAttackers, occupied, attackers);
66 occupied ^= lsb(b); // Remove the attacker from occupied
68 // Add any X-ray attack behind the just removed piece. For instance with
69 // rooks in a8 and a7 attacking a1, after removing a7 we add rook in a8.
70 // Note that new added attackers can be of any color.
71 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
72 attackers |= attacks_bb<BISHOP>(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]);
74 if (Pt == ROOK || Pt == QUEEN)
75 attackers |= attacks_bb<ROOK>(to, occupied) & (byTypeBB[ROOK] | byTypeBB[QUEEN]);
77 // X-ray may add already processed pieces because byTypeBB[] is constant: in
78 // the rook example, now attackers contains _again_ rook in a7, so remove it.
79 attackers &= occupied;
84 PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
85 return KING; // No need to update bitboards: it is the last cycle
91 /// operator<<(Position) returns an ASCII representation of the position
93 std::ostream& operator<<(std::ostream& os, const Position& pos) {
95 os << "\n +---+---+---+---+---+---+---+---+\n";
97 for (Rank r = RANK_8; r >= RANK_1; --r)
99 for (File f = FILE_A; f <= FILE_H; ++f)
100 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
102 os << " |\n +---+---+---+---+---+---+---+---+\n";
105 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
106 << std::setfill('0') << std::setw(16) << pos.key()
107 << std::setfill(' ') << std::dec << "\nCheckers: ";
109 for (Bitboard b = pos.checkers(); b; )
110 os << UCI::square(pop_lsb(&b)) << " ";
112 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
113 && !pos.can_castle(ANY_CASTLING))
117 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
118 Tablebases::ProbeState s1, s2;
119 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
120 int dtz = Tablebases::probe_dtz(p, &s2);
121 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
122 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
129 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
130 // situations. Description of the algorithm in the following paper:
131 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
133 // First and second hash functions for indexing the cuckoo tables
134 inline int H1(Key h) { return h & 0x1fff; }
135 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
137 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
139 Move cuckooMove[8192];
142 /// Position::init() initializes at startup the various arrays used to compute
145 void Position::init() {
149 for (Piece pc : Pieces)
150 for (Square s = SQ_A1; s <= SQ_H8; ++s)
151 Zobrist::psq[pc][s] = rng.rand<Key>();
153 for (File f = FILE_A; f <= FILE_H; ++f)
154 Zobrist::enpassant[f] = rng.rand<Key>();
156 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
158 Zobrist::castling[cr] = 0;
162 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
163 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
167 Zobrist::side = rng.rand<Key>();
168 Zobrist::noPawns = rng.rand<Key>();
170 // Prepare the cuckoo tables
171 std::memset(cuckoo, 0, sizeof(cuckoo));
172 std::memset(cuckooMove, 0, sizeof(cuckooMove));
174 for (Piece pc : Pieces)
175 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
176 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
177 if (PseudoAttacks[type_of(pc)][s1] & s2)
179 Move move = make_move(s1, s2);
180 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
184 std::swap(cuckoo[i], key);
185 std::swap(cuckooMove[i], move);
186 if (move == 0) // Arrived at empty slot ?
188 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
192 assert(count == 3668);
196 /// Position::set() initializes the position object with the given FEN string.
197 /// This function is not very robust - make sure that input FENs are correct,
198 /// this is assumed to be the responsibility of the GUI.
200 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
202 A FEN string defines a particular position using only the ASCII character set.
204 A FEN string contains six fields separated by a space. The fields are:
206 1) Piece placement (from white's perspective). Each rank is described, starting
207 with rank 8 and ending with rank 1. Within each rank, the contents of each
208 square are described from file A through file H. Following the Standard
209 Algebraic Notation (SAN), each piece is identified by a single letter taken
210 from the standard English names. White pieces are designated using upper-case
211 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
212 noted using digits 1 through 8 (the number of blank squares), and "/"
215 2) Active color. "w" means white moves next, "b" means black.
217 3) Castling availability. If neither side can castle, this is "-". Otherwise,
218 this has one or more letters: "K" (White can castle kingside), "Q" (White
219 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
220 can castle queenside).
222 4) En passant target square (in algebraic notation). If there's no en passant
223 target square, this is "-". If a pawn has just made a 2-square move, this
224 is the position "behind" the pawn. This is recorded only if there is a pawn
225 in position to make an en passant capture, and if there really is a pawn
226 that might have advanced two squares.
228 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
229 or capture. This is used to determine if a draw can be claimed under the
232 6) Fullmove number. The number of the full move. It starts at 1, and is
233 incremented after Black's move.
236 unsigned char col, row, token;
239 std::istringstream ss(fenStr);
241 std::memset(this, 0, sizeof(Position));
242 std::memset(si, 0, sizeof(StateInfo));
243 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
248 // 1. Piece placement
249 while ((ss >> token) && !isspace(token))
252 sq += (token - '0') * EAST; // Advance the given number of files
254 else if (token == '/')
257 else if ((idx = PieceToChar.find(token)) != string::npos)
259 put_piece(Piece(idx), sq);
266 sideToMove = (token == 'w' ? WHITE : BLACK);
269 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
270 // Shredder-FEN that uses the letters of the columns on which the rooks began
271 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
272 // if an inner rook is associated with the castling right, the castling tag is
273 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
274 while ((ss >> token) && !isspace(token))
277 Color c = islower(token) ? BLACK : WHITE;
278 Piece rook = make_piece(c, ROOK);
280 token = char(toupper(token));
283 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
285 else if (token == 'Q')
286 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
288 else if (token >= 'A' && token <= 'H')
289 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
294 set_castling_right(c, rsq);
297 // 4. En passant square. Ignore if no pawn capture is possible
298 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
299 && ((ss >> row) && (row == '3' || row == '6')))
301 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
303 if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
304 || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
305 st->epSquare = SQ_NONE;
308 st->epSquare = SQ_NONE;
310 // 5-6. Halfmove clock and fullmove number
311 ss >> std::skipws >> st->rule50 >> gamePly;
313 // Convert from fullmove starting from 1 to gamePly starting from 0,
314 // handle also common incorrect FEN with fullmove = 0.
315 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
317 chess960 = isChess960;
327 /// Position::set_castling_right() is a helper function used to set castling
328 /// rights given the corresponding color and the rook starting square.
330 void Position::set_castling_right(Color c, Square rfrom) {
332 Square kfrom = square<KING>(c);
333 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
334 CastlingRight cr = (c | cs);
336 st->castlingRights |= cr;
337 castlingRightsMask[kfrom] |= cr;
338 castlingRightsMask[rfrom] |= cr;
339 castlingRookSquare[cr] = rfrom;
341 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
342 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
344 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
345 if (s != kfrom && s != rfrom)
346 castlingPath[cr] |= s;
348 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
349 if (s != kfrom && s != rfrom)
350 castlingPath[cr] |= s;
354 /// Position::set_check_info() sets king attacks to detect if a move gives check
356 void Position::set_check_info(StateInfo* si) const {
358 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
359 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
361 Square ksq = square<KING>(~sideToMove);
363 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
364 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
365 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
366 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
367 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
368 si->checkSquares[KING] = 0;
372 /// Position::set_state() computes the hash keys of the position, and other
373 /// data that once computed is updated incrementally as moves are made.
374 /// The function is only used when a new position is set up, and to verify
375 /// the correctness of the StateInfo data when running in debug mode.
377 void Position::set_state(StateInfo* si) const {
379 si->key = si->materialKey = 0;
380 si->pawnKey = Zobrist::noPawns;
381 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
382 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
386 for (Bitboard b = pieces(); b; )
388 Square s = pop_lsb(&b);
389 Piece pc = piece_on(s);
390 si->key ^= Zobrist::psq[pc][s];
393 if (si->epSquare != SQ_NONE)
394 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
396 if (sideToMove == BLACK)
397 si->key ^= Zobrist::side;
399 si->key ^= Zobrist::castling[si->castlingRights];
401 for (Bitboard b = pieces(PAWN); b; )
403 Square s = pop_lsb(&b);
404 si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
407 for (Piece pc : Pieces)
409 if (type_of(pc) != PAWN && type_of(pc) != KING)
410 si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
412 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
413 si->materialKey ^= Zobrist::psq[pc][cnt];
418 /// Position::set() is an overload to initialize the position object with
419 /// the given endgame code string like "KBPKN". It is mainly a helper to
420 /// get the material key out of an endgame code.
422 Position& Position::set(const string& code, Color c, StateInfo* si) {
424 assert(code.length() > 0 && code.length() < 8);
425 assert(code[0] == 'K');
427 string sides[] = { code.substr(code.find('K', 1)), // Weak
428 code.substr(0, code.find('K', 1)) }; // Strong
430 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
432 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
433 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
435 return set(fenStr, false, si, nullptr);
439 /// Position::fen() returns a FEN representation of the position. In case of
440 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
442 const string Position::fen() const {
445 std::ostringstream ss;
447 for (Rank r = RANK_8; r >= RANK_1; --r)
449 for (File f = FILE_A; f <= FILE_H; ++f)
451 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
458 ss << PieceToChar[piece_on(make_square(f, r))];
465 ss << (sideToMove == WHITE ? " w " : " b ");
467 if (can_castle(WHITE_OO))
468 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | KING_SIDE))) : 'K');
470 if (can_castle(WHITE_OOO))
471 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
473 if (can_castle(BLACK_OO))
474 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | KING_SIDE))) : 'k');
476 if (can_castle(BLACK_OOO))
477 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
479 if (!can_castle(WHITE) && !can_castle(BLACK))
482 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
483 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
489 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
490 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
491 /// slider if removing that piece from the board would result in a position where
492 /// square 's' is attacked. For example, a king-attack blocking piece can be either
493 /// a pinned or a discovered check piece, according if its color is the opposite
494 /// or the same of the color of the slider.
496 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
498 Bitboard blockers = 0;
501 // Snipers are sliders that attack 's' when a piece is removed
502 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
503 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
507 Square sniperSq = pop_lsb(&snipers);
508 Bitboard b = between_bb(s, sniperSq) & pieces();
510 if (b && !more_than_one(b))
513 if (b & pieces(color_of(piece_on(s))))
521 /// Position::attackers_to() computes a bitboard of all pieces which attack a
522 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
524 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
526 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
527 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
528 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
529 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
530 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
531 | (attacks_from<KING>(s) & pieces(KING));
535 /// Position::legal() tests whether a pseudo-legal move is legal
537 bool Position::legal(Move m) const {
541 Color us = sideToMove;
542 Square from = from_sq(m);
544 assert(color_of(moved_piece(m)) == us);
545 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
547 // En passant captures are a tricky special case. Because they are rather
548 // uncommon, we do it simply by testing whether the king is attacked after
550 if (type_of(m) == ENPASSANT)
552 Square ksq = square<KING>(us);
553 Square to = to_sq(m);
554 Square capsq = to - pawn_push(us);
555 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
557 assert(to == ep_square());
558 assert(moved_piece(m) == make_piece(us, PAWN));
559 assert(piece_on(capsq) == make_piece(~us, PAWN));
560 assert(piece_on(to) == NO_PIECE);
562 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
563 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
566 // If the moving piece is a king, check whether the destination
567 // square is attacked by the opponent. Castling moves are checked
568 // for legality during move generation.
569 if (type_of(piece_on(from)) == KING)
570 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
572 // A non-king move is legal if and only if it is not pinned or it
573 // is moving along the ray towards or away from the king.
574 return !(blockers_for_king(us) & from)
575 || aligned(from, to_sq(m), square<KING>(us));
579 /// Position::pseudo_legal() takes a random move and tests whether the move is
580 /// pseudo legal. It is used to validate moves from TT that can be corrupted
581 /// due to SMP concurrent access or hash position key aliasing.
582 /// MOVE_NONE is represented as SQ_A1 to SQ_A1 which is never pseudo_legal.
584 bool Position::pseudo_legal(const Move m) const {
586 Color us = sideToMove;
587 Square from = from_sq(m);
588 Square to = to_sq(m);
589 Piece pc = moved_piece(m);
591 // Use a slower but simpler function for uncommon cases
592 if (type_of(m) != NORMAL)
593 return MoveList<LEGAL>(*this).contains(m);
595 // Is not a promotion, so promotion piece must be empty
596 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
599 // If the 'from' square is not occupied by a piece belonging to the side to
600 // move, the move is obviously not legal.
601 if (pc == NO_PIECE || color_of(pc) != us)
604 // The destination square cannot be occupied by a friendly piece
608 // Handle the special case of a pawn move
609 if (type_of(pc) == PAWN)
611 // We have already handled promotion moves, so destination
612 // cannot be on the 8th/1st rank.
613 if (rank_of(to) == relative_rank(us, RANK_8))
616 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
617 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
618 && !( (from + 2 * pawn_push(us) == to) // Not a double push
619 && (rank_of(from) == relative_rank(us, RANK_2))
621 && empty(to - pawn_push(us))))
624 else if (!(attacks_from(type_of(pc), from) & to))
627 // Evasions generator already takes care to avoid some kind of illegal moves
628 // and legal() relies on this. We therefore have to take care that the same
629 // kind of moves are filtered out here.
632 if (type_of(pc) != KING)
634 // Double check? In this case a king move is required
635 if (more_than_one(checkers()))
638 // Our move must be a blocking evasion or a capture of the checking piece
639 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
642 // In case of king moves under check we have to remove king so as to catch
643 // invalid moves like b1a1 when opposite queen is on c1.
644 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
652 /// Position::gives_check() tests whether a pseudo-legal move gives a check
654 bool Position::gives_check(Move m) const {
657 assert(color_of(moved_piece(m)) == sideToMove);
659 Square from = from_sq(m);
660 Square to = to_sq(m);
662 // Is there a direct check?
663 if (st->checkSquares[type_of(piece_on(from))] & to)
666 // Is there a discovered check?
667 if ( (st->blockersForKing[~sideToMove] & from)
668 && !aligned(from, to, square<KING>(~sideToMove)))
677 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
679 // En passant capture with check? We have already handled the case
680 // of direct checks and ordinary discovered check, so the only case we
681 // need to handle is the unusual case of a discovered check through
682 // the captured pawn.
685 Square capsq = make_square(file_of(to), rank_of(from));
686 Bitboard b = (pieces() ^ from ^ capsq) | to;
688 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
689 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
694 Square rfrom = to; // Castling is encoded as 'King captures the rook'
695 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
696 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
698 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
699 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
708 /// Position::do_move() makes a move, and saves all information necessary
709 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
710 /// moves should be filtered out before this function is called.
712 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
715 assert(&newSt != st);
717 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
718 Key k = st->key ^ Zobrist::side;
720 // Copy some fields of the old state to our new StateInfo object except the
721 // ones which are going to be recalculated from scratch anyway and then switch
722 // our state pointer to point to the new (ready to be updated) state.
723 std::memcpy(&newSt, st, offsetof(StateInfo, key));
727 // Increment ply counters. In particular, rule50 will be reset to zero later on
728 // in case of a capture or a pawn move.
733 Color us = sideToMove;
735 Square from = from_sq(m);
736 Square to = to_sq(m);
737 Piece pc = piece_on(from);
738 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
740 assert(color_of(pc) == us);
741 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
742 assert(type_of(captured) != KING);
744 if (type_of(m) == CASTLING)
746 assert(pc == make_piece(us, KING));
747 assert(captured == make_piece(us, ROOK));
750 do_castling<true>(us, from, to, rfrom, rto);
752 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
760 // If the captured piece is a pawn, update pawn hash key, otherwise
761 // update non-pawn material.
762 if (type_of(captured) == PAWN)
764 if (type_of(m) == ENPASSANT)
766 capsq -= pawn_push(us);
768 assert(pc == make_piece(us, PAWN));
769 assert(to == st->epSquare);
770 assert(relative_rank(us, to) == RANK_6);
771 assert(piece_on(to) == NO_PIECE);
772 assert(piece_on(capsq) == make_piece(them, PAWN));
774 board[capsq] = NO_PIECE; // Not done by remove_piece()
777 st->pawnKey ^= Zobrist::psq[captured][capsq];
780 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
782 // Update board and piece lists
783 remove_piece(captured, capsq);
785 // Update material hash key and prefetch access to materialTable
786 k ^= Zobrist::psq[captured][capsq];
787 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
788 prefetch(thisThread->materialTable[st->materialKey]);
790 // Reset rule 50 counter
795 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
797 // Reset en passant square
798 if (st->epSquare != SQ_NONE)
800 k ^= Zobrist::enpassant[file_of(st->epSquare)];
801 st->epSquare = SQ_NONE;
804 // Update castling rights if needed
805 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
807 int cr = castlingRightsMask[from] | castlingRightsMask[to];
808 k ^= Zobrist::castling[st->castlingRights & cr];
809 st->castlingRights &= ~cr;
812 // Move the piece. The tricky Chess960 castling is handled earlier
813 if (type_of(m) != CASTLING)
814 move_piece(pc, from, to);
816 // If the moving piece is a pawn do some special extra work
817 if (type_of(pc) == PAWN)
819 // Set en-passant square if the moved pawn can be captured
820 if ( (int(to) ^ int(from)) == 16
821 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
823 st->epSquare = to - pawn_push(us);
824 k ^= Zobrist::enpassant[file_of(st->epSquare)];
827 else if (type_of(m) == PROMOTION)
829 Piece promotion = make_piece(us, promotion_type(m));
831 assert(relative_rank(us, to) == RANK_8);
832 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
834 remove_piece(pc, to);
835 put_piece(promotion, to);
838 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
839 st->pawnKey ^= Zobrist::psq[pc][to];
840 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
841 ^ Zobrist::psq[pc][pieceCount[pc]];
844 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
847 // Update pawn hash key and prefetch access to pawnsTable
848 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
849 prefetch2(thisThread->pawnsTable[st->pawnKey]);
851 // Reset rule 50 draw counter
856 st->capturedPiece = captured;
858 // Update the key with the final value
861 // Calculate checkers bitboard (if move gives check)
862 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
864 sideToMove = ~sideToMove;
866 // Update king attacks used for fast check detection
873 /// Position::undo_move() unmakes a move. When it returns, the position should
874 /// be restored to exactly the same state as before the move was made.
876 void Position::undo_move(Move m) {
880 sideToMove = ~sideToMove;
882 Color us = sideToMove;
883 Square from = from_sq(m);
884 Square to = to_sq(m);
885 Piece pc = piece_on(to);
887 assert(empty(from) || type_of(m) == CASTLING);
888 assert(type_of(st->capturedPiece) != KING);
890 if (type_of(m) == PROMOTION)
892 assert(relative_rank(us, to) == RANK_8);
893 assert(type_of(pc) == promotion_type(m));
894 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
896 remove_piece(pc, to);
897 pc = make_piece(us, PAWN);
901 if (type_of(m) == CASTLING)
904 do_castling<false>(us, from, to, rfrom, rto);
908 move_piece(pc, to, from); // Put the piece back at the source square
910 if (st->capturedPiece)
914 if (type_of(m) == ENPASSANT)
916 capsq -= pawn_push(us);
918 assert(type_of(pc) == PAWN);
919 assert(to == st->previous->epSquare);
920 assert(relative_rank(us, to) == RANK_6);
921 assert(piece_on(capsq) == NO_PIECE);
922 assert(st->capturedPiece == make_piece(~us, PAWN));
925 put_piece(st->capturedPiece, capsq); // Restore the captured piece
929 // Finally point our state pointer back to the previous state
937 /// Position::do_castling() is a helper used to do/undo a castling move. This
938 /// is a bit tricky in Chess960 where from/to squares can overlap.
940 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
942 bool kingSide = to > from;
943 rfrom = to; // Castling is encoded as "king captures friendly rook"
944 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
945 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
947 // Remove both pieces first since squares could overlap in Chess960
948 remove_piece(make_piece(us, KING), Do ? from : to);
949 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
950 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
951 put_piece(make_piece(us, KING), Do ? to : from);
952 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
956 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
957 /// the side to move without executing any move on the board.
959 void Position::do_null_move(StateInfo& newSt) {
962 assert(&newSt != st);
964 std::memcpy(&newSt, st, sizeof(StateInfo));
968 if (st->epSquare != SQ_NONE)
970 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
971 st->epSquare = SQ_NONE;
974 st->key ^= Zobrist::side;
975 prefetch(TT.first_entry(st->key));
978 st->pliesFromNull = 0;
980 sideToMove = ~sideToMove;
987 void Position::undo_null_move() {
992 sideToMove = ~sideToMove;
996 /// Position::key_after() computes the new hash key after the given move. Needed
997 /// for speculative prefetch. It doesn't recognize special moves like castling,
998 /// en-passant and promotions.
1000 Key Position::key_after(Move m) const {
1002 Square from = from_sq(m);
1003 Square to = to_sq(m);
1004 Piece pc = piece_on(from);
1005 Piece captured = piece_on(to);
1006 Key k = st->key ^ Zobrist::side;
1009 k ^= Zobrist::psq[captured][to];
1011 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1015 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1016 /// SEE value of move is greater or equal to the given threshold. We'll use an
1017 /// algorithm similar to alpha-beta pruning with a null window.
1019 bool Position::see_ge(Move m, Value threshold) const {
1023 // Only deal with normal moves, assume others pass a simple see
1024 if (type_of(m) != NORMAL)
1025 return VALUE_ZERO >= threshold;
1027 Bitboard stmAttackers;
1028 Square from = from_sq(m), to = to_sq(m);
1029 PieceType nextVictim = type_of(piece_on(from));
1030 Color us = color_of(piece_on(from));
1031 Color stm = ~us; // First consider opponent's move
1032 Value balance; // Values of the pieces taken by us minus opponent's ones
1034 // The opponent may be able to recapture so this is the best result
1036 balance = PieceValue[MG][piece_on(to)] - threshold;
1038 if (balance < VALUE_ZERO)
1041 // Now assume the worst possible result: that the opponent can
1042 // capture our piece for free.
1043 balance -= PieceValue[MG][nextVictim];
1045 // If it is enough (like in PxQ) then return immediately. Note that
1046 // in case nextVictim == KING we always return here, this is ok
1047 // if the given move is legal.
1048 if (balance >= VALUE_ZERO)
1051 // Find all attackers to the destination square, with the moving piece
1052 // removed, but possibly an X-ray attacker added behind it.
1053 Bitboard occupied = pieces() ^ from ^ to;
1054 Bitboard attackers = attackers_to(to, occupied) & occupied;
1058 stmAttackers = attackers & pieces(stm);
1060 // Don't allow pinned pieces to attack (except the king) as long as
1061 // all pinners are on their original square.
1062 if (!(st->pinners[~stm] & ~occupied))
1063 stmAttackers &= ~st->blockersForKing[stm];
1065 // If stm has no more attackers then give up: stm loses
1069 // Locate and remove the next least valuable attacker, and add to
1070 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1071 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1073 stm = ~stm; // Switch side to move
1075 // Negamax the balance with alpha = balance, beta = balance+1 and
1076 // add nextVictim's value.
1078 // (balance, balance+1) -> (-balance-1, -balance)
1080 assert(balance < VALUE_ZERO);
1082 balance = -balance - 1 - PieceValue[MG][nextVictim];
1084 // If balance is still non-negative after giving away nextVictim then we
1085 // win. The only thing to be careful about it is that we should revert
1086 // stm if we captured with the king when the opponent still has attackers.
1087 if (balance >= VALUE_ZERO)
1089 if (nextVictim == KING && (attackers & pieces(stm)))
1093 assert(nextVictim != KING);
1095 return us != stm; // We break the above loop when stm loses
1099 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1100 /// or by repetition. It does not detect stalemates.
1102 bool Position::is_draw(int ply) const {
1104 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1107 int end = std::min(st->rule50, st->pliesFromNull);
1112 StateInfo* stp = st->previous->previous;
1115 for (int i = 4; i <= end; i += 2)
1117 stp = stp->previous->previous;
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 ( stp->key == st->key
1122 && ++cnt + (ply > i) == 2)
1130 // Position::has_repeated() tests whether there has been at least one repetition
1131 // of positions since the last capture or pawn move.
1133 bool Position::has_repeated() const {
1135 StateInfo* stc = st;
1138 int i = 4, end = std::min(stc->rule50, stc->pliesFromNull);
1143 StateInfo* stp = stc->previous->previous;
1146 stp = stp->previous->previous;
1148 if (stp->key == stc->key)
1154 stc = stc->previous;
1159 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1160 /// or an earlier position has a move that directly reaches the current position.
1162 bool Position::has_game_cycle(int ply) const {
1166 int end = std::min(st->rule50, st->pliesFromNull);
1171 Key originalKey = st->key;
1172 StateInfo* stp = st->previous;
1174 for (int i = 3; i <= end; i += 2)
1176 stp = stp->previous->previous;
1178 Key moveKey = originalKey ^ stp->key;
1179 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1180 || (j = H2(moveKey), cuckoo[j] == moveKey))
1182 Move move = cuckooMove[j];
1183 Square s1 = from_sq(move);
1184 Square s2 = to_sq(move);
1186 if (!(between_bb(s1, s2) & pieces()))
1188 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in the same
1189 // location. We select the legal one by reversing the move variable if necessary.
1191 move = make_move(s2, s1);
1196 // For repetitions before or at the root, require one more
1197 StateInfo* next_stp = stp;
1198 for (int k = i + 2; k <= end; k += 2)
1200 next_stp = next_stp->previous->previous;
1201 if (next_stp->key == stp->key)
1211 /// Position::flip() flips position with the white and black sides reversed. This
1212 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1214 void Position::flip() {
1217 std::stringstream ss(fen());
1219 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1221 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1222 f.insert(0, token + (f.empty() ? " " : "/"));
1225 ss >> token; // Active color
1226 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1228 ss >> token; // Castling availability
1231 std::transform(f.begin(), f.end(), f.begin(),
1232 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1234 ss >> token; // En passant square
1235 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1237 std::getline(ss, token); // Half and full moves
1240 set(f, is_chess960(), st, this_thread());
1242 assert(pos_is_ok());
1246 /// Position::pos_is_ok() performs some consistency checks for the
1247 /// position object and raises an asserts if something wrong is detected.
1248 /// This is meant to be helpful when debugging.
1250 bool Position::pos_is_ok() const {
1252 constexpr bool Fast = true; // Quick (default) or full check?
1254 if ( (sideToMove != WHITE && sideToMove != BLACK)
1255 || piece_on(square<KING>(WHITE)) != W_KING
1256 || piece_on(square<KING>(BLACK)) != B_KING
1257 || ( ep_square() != SQ_NONE
1258 && relative_rank(sideToMove, ep_square()) != RANK_6))
1259 assert(0 && "pos_is_ok: Default");
1264 if ( pieceCount[W_KING] != 1
1265 || pieceCount[B_KING] != 1
1266 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1267 assert(0 && "pos_is_ok: Kings");
1269 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1270 || pieceCount[W_PAWN] > 8
1271 || pieceCount[B_PAWN] > 8)
1272 assert(0 && "pos_is_ok: Pawns");
1274 if ( (pieces(WHITE) & pieces(BLACK))
1275 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1276 || popcount(pieces(WHITE)) > 16
1277 || popcount(pieces(BLACK)) > 16)
1278 assert(0 && "pos_is_ok: Bitboards");
1280 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1281 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1282 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1283 assert(0 && "pos_is_ok: Bitboards");
1287 if (std::memcmp(&si, st, sizeof(StateInfo)))
1288 assert(0 && "pos_is_ok: State");
1290 for (Piece pc : Pieces)
1292 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1293 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1294 assert(0 && "pos_is_ok: Pieces");
1296 for (int i = 0; i < pieceCount[pc]; ++i)
1297 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1298 assert(0 && "pos_is_ok: Index");
1301 for (Color c = WHITE; c <= BLACK; ++c)
1302 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1304 if (!can_castle(c | s))
1307 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1308 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1309 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1310 assert(0 && "pos_is_ok: Castling");