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;
325 /// Position::set_castling_right() is a helper function used to set castling
326 /// rights given the corresponding color and the rook starting square.
328 void Position::set_castling_right(Color c, Square rfrom) {
330 Square kfrom = square<KING>(c);
331 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
332 CastlingRight cr = (c | cs);
334 st->castlingRights |= cr;
335 castlingRightsMask[kfrom] |= cr;
336 castlingRightsMask[rfrom] |= cr;
337 castlingRookSquare[cr] = rfrom;
339 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
340 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
342 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
343 if (s != kfrom && s != rfrom)
344 castlingPath[cr] |= s;
346 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
347 if (s != kfrom && s != rfrom)
348 castlingPath[cr] |= s;
352 /// Position::set_check_info() sets king attacks to detect if a move gives check
354 void Position::set_check_info(StateInfo* si) const {
356 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
357 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
359 Square ksq = square<KING>(~sideToMove);
361 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
362 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
363 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
364 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
365 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
366 si->checkSquares[KING] = 0;
370 /// Position::set_state() computes the hash keys of the position, and other
371 /// data that once computed is updated incrementally as moves are made.
372 /// The function is only used when a new position is set up, and to verify
373 /// the correctness of the StateInfo data when running in debug mode.
375 void Position::set_state(StateInfo* si) const {
377 si->key = si->materialKey = 0;
378 si->pawnKey = Zobrist::noPawns;
379 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
380 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
384 for (Bitboard b = pieces(); b; )
386 Square s = pop_lsb(&b);
387 Piece pc = piece_on(s);
388 si->key ^= Zobrist::psq[pc][s];
391 if (si->epSquare != SQ_NONE)
392 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
394 if (sideToMove == BLACK)
395 si->key ^= Zobrist::side;
397 si->key ^= Zobrist::castling[si->castlingRights];
399 for (Bitboard b = pieces(PAWN); b; )
401 Square s = pop_lsb(&b);
402 si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
405 for (Piece pc : Pieces)
407 if (type_of(pc) != PAWN && type_of(pc) != KING)
408 si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
410 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
411 si->materialKey ^= Zobrist::psq[pc][cnt];
416 /// Position::set() is an overload to initialize the position object with
417 /// the given endgame code string like "KBPKN". It is mainly a helper to
418 /// get the material key out of an endgame code.
420 Position& Position::set(const string& code, Color c, StateInfo* si) {
422 assert(code.length() > 0 && code.length() < 8);
423 assert(code[0] == 'K');
425 string sides[] = { code.substr(code.find('K', 1)), // Weak
426 code.substr(0, code.find('K', 1)) }; // Strong
428 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
430 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
431 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
433 return set(fenStr, false, si, nullptr);
437 /// Position::fen() returns a FEN representation of the position. In case of
438 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
440 const string Position::fen() const {
443 std::ostringstream ss;
445 for (Rank r = RANK_8; r >= RANK_1; --r)
447 for (File f = FILE_A; f <= FILE_H; ++f)
449 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
456 ss << PieceToChar[piece_on(make_square(f, r))];
463 ss << (sideToMove == WHITE ? " w " : " b ");
465 if (can_castle(WHITE_OO))
466 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | KING_SIDE))) : 'K');
468 if (can_castle(WHITE_OOO))
469 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
471 if (can_castle(BLACK_OO))
472 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | KING_SIDE))) : 'k');
474 if (can_castle(BLACK_OOO))
475 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
477 if (!can_castle(WHITE) && !can_castle(BLACK))
480 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
481 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
487 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
488 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
489 /// slider if removing that piece from the board would result in a position where
490 /// square 's' is attacked. For example, a king-attack blocking piece can be either
491 /// a pinned or a discovered check piece, according if its color is the opposite
492 /// or the same of the color of the slider.
494 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
496 Bitboard blockers = 0;
499 // Snipers are sliders that attack 's' when a piece is removed
500 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
501 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
505 Square sniperSq = pop_lsb(&snipers);
506 Bitboard b = between_bb(s, sniperSq) & pieces();
508 if (b && !more_than_one(b))
511 if (b & pieces(color_of(piece_on(s))))
519 /// Position::attackers_to() computes a bitboard of all pieces which attack a
520 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
522 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
524 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
525 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
526 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
527 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
528 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
529 | (attacks_from<KING>(s) & pieces(KING));
533 /// Position::legal() tests whether a pseudo-legal move is legal
535 bool Position::legal(Move m) const {
539 Color us = sideToMove;
540 Square from = from_sq(m);
542 assert(color_of(moved_piece(m)) == us);
543 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
545 // En passant captures are a tricky special case. Because they are rather
546 // uncommon, we do it simply by testing whether the king is attacked after
548 if (type_of(m) == ENPASSANT)
550 Square ksq = square<KING>(us);
551 Square to = to_sq(m);
552 Square capsq = to - pawn_push(us);
553 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
555 assert(to == ep_square());
556 assert(moved_piece(m) == make_piece(us, PAWN));
557 assert(piece_on(capsq) == make_piece(~us, PAWN));
558 assert(piece_on(to) == NO_PIECE);
560 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
561 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
564 // If the moving piece is a king, check whether the destination
565 // square is attacked by the opponent. Castling moves are checked
566 // for legality during move generation.
567 if (type_of(piece_on(from)) == KING)
568 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
570 // A non-king move is legal if and only if it is not pinned or it
571 // is moving along the ray towards or away from the king.
572 return !(blockers_for_king(us) & from)
573 || aligned(from, to_sq(m), square<KING>(us));
577 /// Position::pseudo_legal() takes a random move and tests whether the move is
578 /// pseudo legal. It is used to validate moves from TT that can be corrupted
579 /// due to SMP concurrent access or hash position key aliasing.
581 bool Position::pseudo_legal(const Move m) const {
583 Color us = sideToMove;
584 Square from = from_sq(m);
585 Square to = to_sq(m);
586 Piece pc = moved_piece(m);
588 // Use a slower but simpler function for uncommon cases
589 if (type_of(m) != NORMAL)
590 return MoveList<LEGAL>(*this).contains(m);
592 // Is not a promotion, so promotion piece must be empty
593 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
596 // If the 'from' square is not occupied by a piece belonging to the side to
597 // move, the move is obviously not legal.
598 if (pc == NO_PIECE || color_of(pc) != us)
601 // The destination square cannot be occupied by a friendly piece
605 // Handle the special case of a pawn move
606 if (type_of(pc) == PAWN)
608 // We have already handled promotion moves, so destination
609 // cannot be on the 8th/1st rank.
610 if (rank_of(to) == relative_rank(us, RANK_8))
613 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
614 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
615 && !( (from + 2 * pawn_push(us) == to) // Not a double push
616 && (rank_of(from) == relative_rank(us, RANK_2))
618 && empty(to - pawn_push(us))))
621 else if (!(attacks_from(type_of(pc), from) & to))
624 // Evasions generator already takes care to avoid some kind of illegal moves
625 // and legal() relies on this. We therefore have to take care that the same
626 // kind of moves are filtered out here.
629 if (type_of(pc) != KING)
631 // Double check? In this case a king move is required
632 if (more_than_one(checkers()))
635 // Our move must be a blocking evasion or a capture of the checking piece
636 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
639 // In case of king moves under check we have to remove king so as to catch
640 // invalid moves like b1a1 when opposite queen is on c1.
641 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
649 /// Position::gives_check() tests whether a pseudo-legal move gives a check
651 bool Position::gives_check(Move m) const {
654 assert(color_of(moved_piece(m)) == sideToMove);
656 Square from = from_sq(m);
657 Square to = to_sq(m);
659 // Is there a direct check?
660 if (st->checkSquares[type_of(piece_on(from))] & to)
663 // Is there a discovered check?
664 if ( (st->blockersForKing[~sideToMove] & from)
665 && !aligned(from, to, square<KING>(~sideToMove)))
674 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
676 // En passant capture with check? We have already handled the case
677 // of direct checks and ordinary discovered check, so the only case we
678 // need to handle is the unusual case of a discovered check through
679 // the captured pawn.
682 Square capsq = make_square(file_of(to), rank_of(from));
683 Bitboard b = (pieces() ^ from ^ capsq) | to;
685 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
686 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
691 Square rfrom = to; // Castling is encoded as 'King captures the rook'
692 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
693 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
695 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
696 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
705 /// Position::do_move() makes a move, and saves all information necessary
706 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
707 /// moves should be filtered out before this function is called.
709 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
712 assert(&newSt != st);
714 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
715 Key k = st->key ^ Zobrist::side;
717 // Copy some fields of the old state to our new StateInfo object except the
718 // ones which are going to be recalculated from scratch anyway and then switch
719 // our state pointer to point to the new (ready to be updated) state.
720 std::memcpy(&newSt, st, offsetof(StateInfo, key));
724 // Increment ply counters. In particular, rule50 will be reset to zero later on
725 // in case of a capture or a pawn move.
730 Color us = sideToMove;
732 Square from = from_sq(m);
733 Square to = to_sq(m);
734 Piece pc = piece_on(from);
735 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
737 assert(color_of(pc) == us);
738 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
739 assert(type_of(captured) != KING);
741 if (type_of(m) == CASTLING)
743 assert(pc == make_piece(us, KING));
744 assert(captured == make_piece(us, ROOK));
747 do_castling<true>(us, from, to, rfrom, rto);
749 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
757 // If the captured piece is a pawn, update pawn hash key, otherwise
758 // update non-pawn material.
759 if (type_of(captured) == PAWN)
761 if (type_of(m) == ENPASSANT)
763 capsq -= pawn_push(us);
765 assert(pc == make_piece(us, PAWN));
766 assert(to == st->epSquare);
767 assert(relative_rank(us, to) == RANK_6);
768 assert(piece_on(to) == NO_PIECE);
769 assert(piece_on(capsq) == make_piece(them, PAWN));
771 board[capsq] = NO_PIECE; // Not done by remove_piece()
774 st->pawnKey ^= Zobrist::psq[captured][capsq];
777 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
779 // Update board and piece lists
780 remove_piece(captured, capsq);
782 // Update material hash key and prefetch access to materialTable
783 k ^= Zobrist::psq[captured][capsq];
784 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
785 prefetch(thisThread->materialTable[st->materialKey]);
787 // Reset rule 50 counter
792 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
794 // Reset en passant square
795 if (st->epSquare != SQ_NONE)
797 k ^= Zobrist::enpassant[file_of(st->epSquare)];
798 st->epSquare = SQ_NONE;
801 // Update castling rights if needed
802 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
804 int cr = castlingRightsMask[from] | castlingRightsMask[to];
805 k ^= Zobrist::castling[st->castlingRights & cr];
806 st->castlingRights &= ~cr;
809 // Move the piece. The tricky Chess960 castling is handled earlier
810 if (type_of(m) != CASTLING)
811 move_piece(pc, from, to);
813 // If the moving piece is a pawn do some special extra work
814 if (type_of(pc) == PAWN)
816 // Set en-passant square if the moved pawn can be captured
817 if ( (int(to) ^ int(from)) == 16
818 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
820 st->epSquare = to - pawn_push(us);
821 k ^= Zobrist::enpassant[file_of(st->epSquare)];
824 else if (type_of(m) == PROMOTION)
826 Piece promotion = make_piece(us, promotion_type(m));
828 assert(relative_rank(us, to) == RANK_8);
829 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
831 remove_piece(pc, to);
832 put_piece(promotion, to);
835 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
836 st->pawnKey ^= Zobrist::psq[pc][to];
837 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
838 ^ Zobrist::psq[pc][pieceCount[pc]];
841 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
844 // Update pawn hash key and prefetch access to pawnsTable
845 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
846 prefetch2(thisThread->pawnsTable[st->pawnKey]);
848 // Reset rule 50 draw counter
853 st->capturedPiece = captured;
855 // Update the key with the final value
858 // Calculate checkers bitboard (if move gives check)
859 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
861 sideToMove = ~sideToMove;
863 // Update king attacks used for fast check detection
870 /// Position::undo_move() unmakes a move. When it returns, the position should
871 /// be restored to exactly the same state as before the move was made.
873 void Position::undo_move(Move m) {
877 sideToMove = ~sideToMove;
879 Color us = sideToMove;
880 Square from = from_sq(m);
881 Square to = to_sq(m);
882 Piece pc = piece_on(to);
884 assert(empty(from) || type_of(m) == CASTLING);
885 assert(type_of(st->capturedPiece) != KING);
887 if (type_of(m) == PROMOTION)
889 assert(relative_rank(us, to) == RANK_8);
890 assert(type_of(pc) == promotion_type(m));
891 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
893 remove_piece(pc, to);
894 pc = make_piece(us, PAWN);
898 if (type_of(m) == CASTLING)
901 do_castling<false>(us, from, to, rfrom, rto);
905 move_piece(pc, to, from); // Put the piece back at the source square
907 if (st->capturedPiece)
911 if (type_of(m) == ENPASSANT)
913 capsq -= pawn_push(us);
915 assert(type_of(pc) == PAWN);
916 assert(to == st->previous->epSquare);
917 assert(relative_rank(us, to) == RANK_6);
918 assert(piece_on(capsq) == NO_PIECE);
919 assert(st->capturedPiece == make_piece(~us, PAWN));
922 put_piece(st->capturedPiece, capsq); // Restore the captured piece
926 // Finally point our state pointer back to the previous state
934 /// Position::do_castling() is a helper used to do/undo a castling move. This
935 /// is a bit tricky in Chess960 where from/to squares can overlap.
937 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
939 bool kingSide = to > from;
940 rfrom = to; // Castling is encoded as "king captures friendly rook"
941 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
942 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
944 // Remove both pieces first since squares could overlap in Chess960
945 remove_piece(make_piece(us, KING), Do ? from : to);
946 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
947 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
948 put_piece(make_piece(us, KING), Do ? to : from);
949 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
953 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
954 /// the side to move without executing any move on the board.
956 void Position::do_null_move(StateInfo& newSt) {
959 assert(&newSt != st);
961 std::memcpy(&newSt, st, sizeof(StateInfo));
965 if (st->epSquare != SQ_NONE)
967 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
968 st->epSquare = SQ_NONE;
971 st->key ^= Zobrist::side;
972 prefetch(TT.first_entry(st->key));
975 st->pliesFromNull = 0;
977 sideToMove = ~sideToMove;
984 void Position::undo_null_move() {
989 sideToMove = ~sideToMove;
993 /// Position::key_after() computes the new hash key after the given move. Needed
994 /// for speculative prefetch. It doesn't recognize special moves like castling,
995 /// en-passant and promotions.
997 Key Position::key_after(Move m) const {
999 Square from = from_sq(m);
1000 Square to = to_sq(m);
1001 Piece pc = piece_on(from);
1002 Piece captured = piece_on(to);
1003 Key k = st->key ^ Zobrist::side;
1006 k ^= Zobrist::psq[captured][to];
1008 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1012 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1013 /// SEE value of move is greater or equal to the given threshold. We'll use an
1014 /// algorithm similar to alpha-beta pruning with a null window.
1016 bool Position::see_ge(Move m, Value threshold) const {
1020 // Only deal with normal moves, assume others pass a simple see
1021 if (type_of(m) != NORMAL)
1022 return VALUE_ZERO >= threshold;
1024 Bitboard stmAttackers;
1025 Square from = from_sq(m), to = to_sq(m);
1026 PieceType nextVictim = type_of(piece_on(from));
1027 Color us = color_of(piece_on(from));
1028 Color stm = ~us; // First consider opponent's move
1029 Value balance; // Values of the pieces taken by us minus opponent's ones
1031 // The opponent may be able to recapture so this is the best result
1033 balance = PieceValue[MG][piece_on(to)] - threshold;
1035 if (balance < VALUE_ZERO)
1038 // Now assume the worst possible result: that the opponent can
1039 // capture our piece for free.
1040 balance -= PieceValue[MG][nextVictim];
1042 // If it is enough (like in PxQ) then return immediately. Note that
1043 // in case nextVictim == KING we always return here, this is ok
1044 // if the given move is legal.
1045 if (balance >= VALUE_ZERO)
1048 // Find all attackers to the destination square, with the moving piece
1049 // removed, but possibly an X-ray attacker added behind it.
1050 Bitboard occupied = pieces() ^ from ^ to;
1051 Bitboard attackers = attackers_to(to, occupied) & occupied;
1055 stmAttackers = attackers & pieces(stm);
1057 // Don't allow pinned pieces to attack (except the king) as long as
1058 // all pinners are on their original square.
1059 if (!(st->pinners[~stm] & ~occupied))
1060 stmAttackers &= ~st->blockersForKing[stm];
1062 // If stm has no more attackers then give up: stm loses
1066 // Locate and remove the next least valuable attacker, and add to
1067 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1068 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1070 stm = ~stm; // Switch side to move
1072 // Negamax the balance with alpha = balance, beta = balance+1 and
1073 // add nextVictim's value.
1075 // (balance, balance+1) -> (-balance-1, -balance)
1077 assert(balance < VALUE_ZERO);
1079 balance = -balance - 1 - PieceValue[MG][nextVictim];
1081 // If balance is still non-negative after giving away nextVictim then we
1082 // win. The only thing to be careful about it is that we should revert
1083 // stm if we captured with the king when the opponent still has attackers.
1084 if (balance >= VALUE_ZERO)
1086 if (nextVictim == KING && (attackers & pieces(stm)))
1090 assert(nextVictim != KING);
1092 return us != stm; // We break the above loop when stm loses
1096 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1097 /// or by repetition. It does not detect stalemates.
1099 bool Position::is_draw(int ply) const {
1101 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1104 int end = std::min(st->rule50, st->pliesFromNull);
1109 StateInfo* stp = st->previous->previous;
1112 for (int i = 4; i <= end; i += 2)
1114 stp = stp->previous->previous;
1116 // Return a draw score if a position repeats once earlier but strictly
1117 // after the root, or repeats twice before or at the root.
1118 if ( stp->key == st->key
1119 && ++cnt + (ply > i) == 2)
1127 // Position::has_repeated() tests whether there has been at least one repetition
1128 // of positions since the last capture or pawn move.
1130 bool Position::has_repeated() const {
1132 StateInfo* stc = st;
1135 int i = 4, end = std::min(stc->rule50, stc->pliesFromNull);
1140 StateInfo* stp = stc->previous->previous;
1143 stp = stp->previous->previous;
1145 if (stp->key == stc->key)
1151 stc = stc->previous;
1156 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1157 /// or an earlier position has a move that directly reaches the current position.
1159 bool Position::has_game_cycle(int ply) const {
1163 int end = std::min(st->rule50, st->pliesFromNull);
1168 Key originalKey = st->key;
1169 StateInfo* stp = st->previous;
1171 for (int i = 3; i <= end; i += 2)
1173 stp = stp->previous->previous;
1175 Key moveKey = originalKey ^ stp->key;
1176 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1177 || (j = H2(moveKey), cuckoo[j] == moveKey))
1179 Move move = cuckooMove[j];
1180 Square s1 = from_sq(move);
1181 Square s2 = to_sq(move);
1183 if (!(between_bb(s1, s2) & pieces()))
1185 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in the same
1186 // location. We select the legal one by reversing the move variable if necessary.
1188 move = make_move(s2, s1);
1193 // For repetitions before or at the root, require one more
1194 StateInfo* next_stp = stp;
1195 for (int k = i + 2; k <= end; k += 2)
1197 next_stp = next_stp->previous->previous;
1198 if (next_stp->key == stp->key)
1208 /// Position::flip() flips position with the white and black sides reversed. This
1209 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1211 void Position::flip() {
1214 std::stringstream ss(fen());
1216 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1218 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1219 f.insert(0, token + (f.empty() ? " " : "/"));
1222 ss >> token; // Active color
1223 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1225 ss >> token; // Castling availability
1228 std::transform(f.begin(), f.end(), f.begin(),
1229 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1231 ss >> token; // En passant square
1232 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1234 std::getline(ss, token); // Half and full moves
1237 set(f, is_chess960(), st, this_thread());
1239 assert(pos_is_ok());
1243 /// Position::pos_is_ok() performs some consistency checks for the
1244 /// position object and raises an asserts if something wrong is detected.
1245 /// This is meant to be helpful when debugging.
1247 bool Position::pos_is_ok() const {
1249 constexpr bool Fast = true; // Quick (default) or full check?
1251 if ( (sideToMove != WHITE && sideToMove != BLACK)
1252 || piece_on(square<KING>(WHITE)) != W_KING
1253 || piece_on(square<KING>(BLACK)) != B_KING
1254 || ( ep_square() != SQ_NONE
1255 && relative_rank(sideToMove, ep_square()) != RANK_6))
1256 assert(0 && "pos_is_ok: Default");
1261 if ( pieceCount[W_KING] != 1
1262 || pieceCount[B_KING] != 1
1263 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1264 assert(0 && "pos_is_ok: Kings");
1266 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1267 || pieceCount[W_PAWN] > 8
1268 || pieceCount[B_PAWN] > 8)
1269 assert(0 && "pos_is_ok: Pawns");
1271 if ( (pieces(WHITE) & pieces(BLACK))
1272 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1273 || popcount(pieces(WHITE)) > 16
1274 || popcount(pieces(BLACK)) > 16)
1275 assert(0 && "pos_is_ok: Bitboards");
1277 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1278 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1279 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1280 assert(0 && "pos_is_ok: Bitboards");
1284 if (std::memcmp(&si, st, sizeof(StateInfo)))
1285 assert(0 && "pos_is_ok: State");
1287 for (Piece pc : Pieces)
1289 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1290 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1291 assert(0 && "pos_is_ok: Pieces");
1293 for (int i = 0; i < pieceCount[pc]; ++i)
1294 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1295 assert(0 && "pos_is_ok: Index");
1298 for (Color c = WHITE; c <= BLACK; ++c)
1299 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1301 if (!can_castle(c | s))
1304 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1305 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1306 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1307 assert(0 && "pos_is_ok: Castling");