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 == MOVE_NONE) // 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_OO ))) : 'K');
470 if (can_castle(WHITE_OOO))
471 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
473 if (can_castle(BLACK_OO))
474 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
476 if (can_castle(BLACK_OOO))
477 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
479 if (!can_castle(ANY_CASTLING))
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);
543 Square to = to_sq(m);
545 assert(color_of(moved_piece(m)) == us);
546 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
548 // En passant captures are a tricky special case. Because they are rather
549 // uncommon, we do it simply by testing whether the king is attacked after
551 if (type_of(m) == ENPASSANT)
553 Square ksq = square<KING>(us);
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 // Castling moves generation does not check if the castling path is clear of
567 // enemy attacks, it is delayed at a later time: now!
568 if (type_of(m) == CASTLING)
570 // After castling, the rook and king final positions are the same in
571 // Chess960 as they would be in standard chess.
572 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
573 Direction step = to > from ? WEST : EAST;
575 for (Square s = to; s != from; s += step)
576 if (attackers_to(s) & pieces(~us))
579 // In case of Chess960, verify that when moving the castling rook we do
580 // not discover some hidden checker.
581 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
583 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
586 // If the moving piece is a king, check whether the destination square is
587 // attacked by the opponent.
588 if (type_of(piece_on(from)) == KING)
589 return !(attackers_to(to) & pieces(~us));
591 // A non-king move is legal if and only if it is not pinned or it
592 // is moving along the ray towards or away from the king.
593 return !(blockers_for_king(us) & from)
594 || aligned(from, to, square<KING>(us));
598 /// Position::pseudo_legal() takes a random move and tests whether the move is
599 /// pseudo legal. It is used to validate moves from TT that can be corrupted
600 /// due to SMP concurrent access or hash position key aliasing.
602 bool Position::pseudo_legal(const Move m) const {
604 Color us = sideToMove;
605 Square from = from_sq(m);
606 Square to = to_sq(m);
607 Piece pc = moved_piece(m);
609 // Use a slower but simpler function for uncommon cases
610 if (type_of(m) != NORMAL)
611 return MoveList<LEGAL>(*this).contains(m);
613 // Is not a promotion, so promotion piece must be empty
614 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
617 // If the 'from' square is not occupied by a piece belonging to the side to
618 // move, the move is obviously not legal.
619 if (pc == NO_PIECE || color_of(pc) != us)
622 // The destination square cannot be occupied by a friendly piece
626 // Handle the special case of a pawn move
627 if (type_of(pc) == PAWN)
629 // We have already handled promotion moves, so destination
630 // cannot be on the 8th/1st rank.
631 if (rank_of(to) == relative_rank(us, RANK_8))
634 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
635 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
636 && !( (from + 2 * pawn_push(us) == to) // Not a double push
637 && (rank_of(from) == relative_rank(us, RANK_2))
639 && empty(to - pawn_push(us))))
642 else if (!(attacks_from(type_of(pc), from) & to))
645 // Evasions generator already takes care to avoid some kind of illegal moves
646 // and legal() relies on this. We therefore have to take care that the same
647 // kind of moves are filtered out here.
650 if (type_of(pc) != KING)
652 // Double check? In this case a king move is required
653 if (more_than_one(checkers()))
656 // Our move must be a blocking evasion or a capture of the checking piece
657 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
660 // In case of king moves under check we have to remove king so as to catch
661 // invalid moves like b1a1 when opposite queen is on c1.
662 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
670 /// Position::gives_check() tests whether a pseudo-legal move gives a check
672 bool Position::gives_check(Move m) const {
675 assert(color_of(moved_piece(m)) == sideToMove);
677 Square from = from_sq(m);
678 Square to = to_sq(m);
680 // Is there a direct check?
681 if (st->checkSquares[type_of(piece_on(from))] & to)
684 // Is there a discovered check?
685 if ( (st->blockersForKing[~sideToMove] & from)
686 && !aligned(from, to, square<KING>(~sideToMove)))
695 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
697 // En passant capture with check? We have already handled the case
698 // of direct checks and ordinary discovered check, so the only case we
699 // need to handle is the unusual case of a discovered check through
700 // the captured pawn.
703 Square capsq = make_square(file_of(to), rank_of(from));
704 Bitboard b = (pieces() ^ from ^ capsq) | to;
706 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
707 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
712 Square rfrom = to; // Castling is encoded as 'King captures the rook'
713 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
714 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
716 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
717 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
726 /// Position::do_move() makes a move, and saves all information necessary
727 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
728 /// moves should be filtered out before this function is called.
730 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
733 assert(&newSt != st);
735 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
736 Key k = st->key ^ Zobrist::side;
738 // Copy some fields of the old state to our new StateInfo object except the
739 // ones which are going to be recalculated from scratch anyway and then switch
740 // our state pointer to point to the new (ready to be updated) state.
741 std::memcpy(&newSt, st, offsetof(StateInfo, key));
745 // Increment ply counters. In particular, rule50 will be reset to zero later on
746 // in case of a capture or a pawn move.
751 Color us = sideToMove;
753 Square from = from_sq(m);
754 Square to = to_sq(m);
755 Piece pc = piece_on(from);
756 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
758 assert(color_of(pc) == us);
759 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
760 assert(type_of(captured) != KING);
762 if (type_of(m) == CASTLING)
764 assert(pc == make_piece(us, KING));
765 assert(captured == make_piece(us, ROOK));
768 do_castling<true>(us, from, to, rfrom, rto);
770 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
778 // If the captured piece is a pawn, update pawn hash key, otherwise
779 // update non-pawn material.
780 if (type_of(captured) == PAWN)
782 if (type_of(m) == ENPASSANT)
784 capsq -= pawn_push(us);
786 assert(pc == make_piece(us, PAWN));
787 assert(to == st->epSquare);
788 assert(relative_rank(us, to) == RANK_6);
789 assert(piece_on(to) == NO_PIECE);
790 assert(piece_on(capsq) == make_piece(them, PAWN));
792 board[capsq] = NO_PIECE; // Not done by remove_piece()
795 st->pawnKey ^= Zobrist::psq[captured][capsq];
798 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
800 // Update board and piece lists
801 remove_piece(captured, capsq);
803 // Update material hash key and prefetch access to materialTable
804 k ^= Zobrist::psq[captured][capsq];
805 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
806 prefetch(thisThread->materialTable[st->materialKey]);
808 // Reset rule 50 counter
813 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
815 // Reset en passant square
816 if (st->epSquare != SQ_NONE)
818 k ^= Zobrist::enpassant[file_of(st->epSquare)];
819 st->epSquare = SQ_NONE;
822 // Update castling rights if needed
823 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
825 int cr = castlingRightsMask[from] | castlingRightsMask[to];
826 k ^= Zobrist::castling[st->castlingRights & cr];
827 st->castlingRights &= ~cr;
830 // Move the piece. The tricky Chess960 castling is handled earlier
831 if (type_of(m) != CASTLING)
832 move_piece(pc, from, to);
834 // If the moving piece is a pawn do some special extra work
835 if (type_of(pc) == PAWN)
837 // Set en-passant square if the moved pawn can be captured
838 if ( (int(to) ^ int(from)) == 16
839 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
841 st->epSquare = to - pawn_push(us);
842 k ^= Zobrist::enpassant[file_of(st->epSquare)];
845 else if (type_of(m) == PROMOTION)
847 Piece promotion = make_piece(us, promotion_type(m));
849 assert(relative_rank(us, to) == RANK_8);
850 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
852 remove_piece(pc, to);
853 put_piece(promotion, to);
856 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
857 st->pawnKey ^= Zobrist::psq[pc][to];
858 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
859 ^ Zobrist::psq[pc][pieceCount[pc]];
862 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
865 // Update pawn hash key and prefetch access to pawnsTable
866 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
867 prefetch2(thisThread->pawnsTable[st->pawnKey]);
869 // Reset rule 50 draw counter
874 st->capturedPiece = captured;
876 // Update the key with the final value
879 // Calculate checkers bitboard (if move gives check)
880 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
882 sideToMove = ~sideToMove;
884 // Update king attacks used for fast check detection
891 /// Position::undo_move() unmakes a move. When it returns, the position should
892 /// be restored to exactly the same state as before the move was made.
894 void Position::undo_move(Move m) {
898 sideToMove = ~sideToMove;
900 Color us = sideToMove;
901 Square from = from_sq(m);
902 Square to = to_sq(m);
903 Piece pc = piece_on(to);
905 assert(empty(from) || type_of(m) == CASTLING);
906 assert(type_of(st->capturedPiece) != KING);
908 if (type_of(m) == PROMOTION)
910 assert(relative_rank(us, to) == RANK_8);
911 assert(type_of(pc) == promotion_type(m));
912 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
914 remove_piece(pc, to);
915 pc = make_piece(us, PAWN);
919 if (type_of(m) == CASTLING)
922 do_castling<false>(us, from, to, rfrom, rto);
926 move_piece(pc, to, from); // Put the piece back at the source square
928 if (st->capturedPiece)
932 if (type_of(m) == ENPASSANT)
934 capsq -= pawn_push(us);
936 assert(type_of(pc) == PAWN);
937 assert(to == st->previous->epSquare);
938 assert(relative_rank(us, to) == RANK_6);
939 assert(piece_on(capsq) == NO_PIECE);
940 assert(st->capturedPiece == make_piece(~us, PAWN));
943 put_piece(st->capturedPiece, capsq); // Restore the captured piece
947 // Finally point our state pointer back to the previous state
955 /// Position::do_castling() is a helper used to do/undo a castling move. This
956 /// is a bit tricky in Chess960 where from/to squares can overlap.
958 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
960 bool kingSide = to > from;
961 rfrom = to; // Castling is encoded as "king captures friendly rook"
962 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
963 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
965 // Remove both pieces first since squares could overlap in Chess960
966 remove_piece(make_piece(us, KING), Do ? from : to);
967 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
968 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
969 put_piece(make_piece(us, KING), Do ? to : from);
970 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
974 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
975 /// the side to move without executing any move on the board.
977 void Position::do_null_move(StateInfo& newSt) {
980 assert(&newSt != st);
982 std::memcpy(&newSt, st, sizeof(StateInfo));
986 if (st->epSquare != SQ_NONE)
988 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
989 st->epSquare = SQ_NONE;
992 st->key ^= Zobrist::side;
993 prefetch(TT.first_entry(st->key));
996 st->pliesFromNull = 0;
998 sideToMove = ~sideToMove;
1002 assert(pos_is_ok());
1005 void Position::undo_null_move() {
1007 assert(!checkers());
1010 sideToMove = ~sideToMove;
1014 /// Position::key_after() computes the new hash key after the given move. Needed
1015 /// for speculative prefetch. It doesn't recognize special moves like castling,
1016 /// en-passant and promotions.
1018 Key Position::key_after(Move m) const {
1020 Square from = from_sq(m);
1021 Square to = to_sq(m);
1022 Piece pc = piece_on(from);
1023 Piece captured = piece_on(to);
1024 Key k = st->key ^ Zobrist::side;
1027 k ^= Zobrist::psq[captured][to];
1029 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1033 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1034 /// SEE value of move is greater or equal to the given threshold. We'll use an
1035 /// algorithm similar to alpha-beta pruning with a null window.
1037 bool Position::see_ge(Move m, Value threshold) const {
1041 // Only deal with normal moves, assume others pass a simple see
1042 if (type_of(m) != NORMAL)
1043 return VALUE_ZERO >= threshold;
1045 Bitboard stmAttackers;
1046 Square from = from_sq(m), to = to_sq(m);
1047 PieceType nextVictim = type_of(piece_on(from));
1048 Color us = color_of(piece_on(from));
1049 Color stm = ~us; // First consider opponent's move
1050 Value balance; // Values of the pieces taken by us minus opponent's ones
1052 // The opponent may be able to recapture so this is the best result
1054 balance = PieceValue[MG][piece_on(to)] - threshold;
1056 if (balance < VALUE_ZERO)
1059 // Now assume the worst possible result: that the opponent can
1060 // capture our piece for free.
1061 balance -= PieceValue[MG][nextVictim];
1063 // If it is enough (like in PxQ) then return immediately. Note that
1064 // in case nextVictim == KING we always return here, this is ok
1065 // if the given move is legal.
1066 if (balance >= VALUE_ZERO)
1069 // Find all attackers to the destination square, with the moving piece
1070 // removed, but possibly an X-ray attacker added behind it.
1071 Bitboard occupied = pieces() ^ from ^ to;
1072 Bitboard attackers = attackers_to(to, occupied) & occupied;
1076 stmAttackers = attackers & pieces(stm);
1078 // Don't allow pinned pieces to attack (except the king) as long as
1079 // all pinners are on their original square.
1080 if (!(st->pinners[~stm] & ~occupied))
1081 stmAttackers &= ~st->blockersForKing[stm];
1083 // If stm has no more attackers then give up: stm loses
1087 // Locate and remove the next least valuable attacker, and add to
1088 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1089 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1091 stm = ~stm; // Switch side to move
1093 // Negamax the balance with alpha = balance, beta = balance+1 and
1094 // add nextVictim's value.
1096 // (balance, balance+1) -> (-balance-1, -balance)
1098 assert(balance < VALUE_ZERO);
1100 balance = -balance - 1 - PieceValue[MG][nextVictim];
1102 // If balance is still non-negative after giving away nextVictim then we
1103 // win. The only thing to be careful about it is that we should revert
1104 // stm if we captured with the king when the opponent still has attackers.
1105 if (balance >= VALUE_ZERO)
1107 if (nextVictim == KING && (attackers & pieces(stm)))
1111 assert(nextVictim != KING);
1113 return us != stm; // We break the above loop when stm loses
1117 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1118 /// or by repetition. It does not detect stalemates.
1120 bool Position::is_draw(int ply) const {
1122 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1125 int end = std::min(st->rule50, st->pliesFromNull);
1130 StateInfo* stp = st->previous->previous;
1133 for (int i = 4; i <= end; i += 2)
1135 stp = stp->previous->previous;
1137 // Return a draw score if a position repeats once earlier but strictly
1138 // after the root, or repeats twice before or at the root.
1139 if ( stp->key == st->key
1140 && ++cnt + (ply > i) == 2)
1148 // Position::has_repeated() tests whether there has been at least one repetition
1149 // of positions since the last capture or pawn move.
1151 bool Position::has_repeated() const {
1153 StateInfo* stc = st;
1156 int i = 4, end = std::min(stc->rule50, stc->pliesFromNull);
1161 StateInfo* stp = stc->previous->previous;
1164 stp = stp->previous->previous;
1166 if (stp->key == stc->key)
1172 stc = stc->previous;
1177 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1178 /// or an earlier position has a move that directly reaches the current position.
1180 bool Position::has_game_cycle(int ply) const {
1184 int end = std::min(st->rule50, st->pliesFromNull);
1189 Key originalKey = st->key;
1190 StateInfo* stp = st->previous;
1192 for (int i = 3; i <= end; i += 2)
1194 stp = stp->previous->previous;
1196 Key moveKey = originalKey ^ stp->key;
1197 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1198 || (j = H2(moveKey), cuckoo[j] == moveKey))
1200 Move move = cuckooMove[j];
1201 Square s1 = from_sq(move);
1202 Square s2 = to_sq(move);
1204 if (!(between_bb(s1, s2) & pieces()))
1206 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in the same
1207 // location. We select the legal one by reversing the move variable if necessary.
1209 move = make_move(s2, s1);
1214 // For repetitions before or at the root, require one more
1215 StateInfo* next_stp = stp;
1216 for (int k = i + 2; k <= end; k += 2)
1218 next_stp = next_stp->previous->previous;
1219 if (next_stp->key == stp->key)
1229 /// Position::flip() flips position with the white and black sides reversed. This
1230 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1232 void Position::flip() {
1235 std::stringstream ss(fen());
1237 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1239 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1240 f.insert(0, token + (f.empty() ? " " : "/"));
1243 ss >> token; // Active color
1244 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1246 ss >> token; // Castling availability
1249 std::transform(f.begin(), f.end(), f.begin(),
1250 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1252 ss >> token; // En passant square
1253 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1255 std::getline(ss, token); // Half and full moves
1258 set(f, is_chess960(), st, this_thread());
1260 assert(pos_is_ok());
1264 /// Position::pos_is_ok() performs some consistency checks for the
1265 /// position object and raises an asserts if something wrong is detected.
1266 /// This is meant to be helpful when debugging.
1268 bool Position::pos_is_ok() const {
1270 constexpr bool Fast = true; // Quick (default) or full check?
1272 if ( (sideToMove != WHITE && sideToMove != BLACK)
1273 || piece_on(square<KING>(WHITE)) != W_KING
1274 || piece_on(square<KING>(BLACK)) != B_KING
1275 || ( ep_square() != SQ_NONE
1276 && relative_rank(sideToMove, ep_square()) != RANK_6))
1277 assert(0 && "pos_is_ok: Default");
1282 if ( pieceCount[W_KING] != 1
1283 || pieceCount[B_KING] != 1
1284 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1285 assert(0 && "pos_is_ok: Kings");
1287 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1288 || pieceCount[W_PAWN] > 8
1289 || pieceCount[B_PAWN] > 8)
1290 assert(0 && "pos_is_ok: Pawns");
1292 if ( (pieces(WHITE) & pieces(BLACK))
1293 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1294 || popcount(pieces(WHITE)) > 16
1295 || popcount(pieces(BLACK)) > 16)
1296 assert(0 && "pos_is_ok: Bitboards");
1298 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1299 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1300 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1301 assert(0 && "pos_is_ok: Bitboards");
1305 if (std::memcmp(&si, st, sizeof(StateInfo)))
1306 assert(0 && "pos_is_ok: State");
1308 for (Piece pc : Pieces)
1310 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1311 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1312 assert(0 && "pos_is_ok: Pieces");
1314 for (int i = 0; i < pieceCount[pc]; ++i)
1315 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1316 assert(0 && "pos_is_ok: Index");
1319 for (Color c = WHITE; c <= BLACK; ++c)
1320 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1322 if (!can_castle(c | s))
1325 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1326 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1327 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1328 assert(0 && "pos_is_ok: Castling");