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.
58 template<PieceType Pt>
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<PieceType(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 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
335 st->castlingRights |= cr;
336 castlingRightsMask[kfrom] |= cr;
337 castlingRightsMask[rfrom] |= cr;
338 castlingRookSquare[cr] = rfrom;
340 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
341 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
343 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
344 & ~(square_bb(kfrom) | rfrom);
348 /// Position::set_check_info() sets king attacks to detect if a move gives check
350 void Position::set_check_info(StateInfo* si) const {
352 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
353 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
355 Square ksq = square<KING>(~sideToMove);
357 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
358 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
359 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
360 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
361 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
362 si->checkSquares[KING] = 0;
366 /// Position::set_state() computes the hash keys of the position, and other
367 /// data that once computed is updated incrementally as moves are made.
368 /// The function is only used when a new position is set up, and to verify
369 /// the correctness of the StateInfo data when running in debug mode.
371 void Position::set_state(StateInfo* si) const {
373 si->key = si->materialKey = 0;
374 si->pawnKey = Zobrist::noPawns;
375 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
376 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
380 for (Bitboard b = pieces(); b; )
382 Square s = pop_lsb(&b);
383 Piece pc = piece_on(s);
384 si->key ^= Zobrist::psq[pc][s];
386 if (type_of(pc) == PAWN)
387 si->pawnKey ^= Zobrist::psq[pc][s];
389 else if (type_of(pc) != KING)
390 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
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 (Piece pc : Pieces)
402 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
403 si->materialKey ^= Zobrist::psq[pc][cnt];
407 /// Position::set() is an overload to initialize the position object with
408 /// the given endgame code string like "KBPKN". It is mainly a helper to
409 /// get the material key out of an endgame code.
411 Position& Position::set(const string& code, Color c, StateInfo* si) {
413 assert(code.length() > 0 && code.length() < 8);
414 assert(code[0] == 'K');
416 string sides[] = { code.substr(code.find('K', 1)), // Weak
417 code.substr(0, code.find('K', 1)) }; // Strong
419 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
421 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
422 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
424 return set(fenStr, false, si, nullptr);
428 /// Position::fen() returns a FEN representation of the position. In case of
429 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
431 const string Position::fen() const {
434 std::ostringstream ss;
436 for (Rank r = RANK_8; r >= RANK_1; --r)
438 for (File f = FILE_A; f <= FILE_H; ++f)
440 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
447 ss << PieceToChar[piece_on(make_square(f, r))];
454 ss << (sideToMove == WHITE ? " w " : " b ");
456 if (can_castle(WHITE_OO))
457 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
459 if (can_castle(WHITE_OOO))
460 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
462 if (can_castle(BLACK_OO))
463 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
465 if (can_castle(BLACK_OOO))
466 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
468 if (!can_castle(ANY_CASTLING))
471 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
472 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
478 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
479 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
480 /// slider if removing that piece from the board would result in a position where
481 /// square 's' is attacked. For example, a king-attack blocking piece can be either
482 /// a pinned or a discovered check piece, according if its color is the opposite
483 /// or the same of the color of the slider.
485 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
487 Bitboard blockers = 0;
490 // Snipers are sliders that attack 's' when a piece and other snipers are removed
491 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
492 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
493 Bitboard occupancy = pieces() ^ snipers;
497 Square sniperSq = pop_lsb(&snipers);
498 Bitboard b = between_bb(s, sniperSq) & occupancy;
500 if (b && !more_than_one(b))
503 if (b & pieces(color_of(piece_on(s))))
511 /// Position::attackers_to() computes a bitboard of all pieces which attack a
512 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
514 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
516 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
517 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
518 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
519 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
520 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
521 | (attacks_from<KING>(s) & pieces(KING));
525 /// Position::legal() tests whether a pseudo-legal move is legal
527 bool Position::legal(Move m) const {
531 Color us = sideToMove;
532 Square from = from_sq(m);
533 Square to = to_sq(m);
535 assert(color_of(moved_piece(m)) == us);
536 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
538 // En passant captures are a tricky special case. Because they are rather
539 // uncommon, we do it simply by testing whether the king is attacked after
541 if (type_of(m) == ENPASSANT)
543 Square ksq = square<KING>(us);
544 Square capsq = to - pawn_push(us);
545 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
547 assert(to == ep_square());
548 assert(moved_piece(m) == make_piece(us, PAWN));
549 assert(piece_on(capsq) == make_piece(~us, PAWN));
550 assert(piece_on(to) == NO_PIECE);
552 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
553 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
556 // Castling moves generation does not check if the castling path is clear of
557 // enemy attacks, it is delayed at a later time: now!
558 if (type_of(m) == CASTLING)
560 // After castling, the rook and king final positions are the same in
561 // Chess960 as they would be in standard chess.
562 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
563 Direction step = to > from ? WEST : EAST;
565 for (Square s = to; s != from; s += step)
566 if (attackers_to(s) & pieces(~us))
569 // In case of Chess960, verify that when moving the castling rook we do
570 // not discover some hidden checker.
571 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
573 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
576 // If the moving piece is a king, check whether the destination square is
577 // attacked by the opponent.
578 if (type_of(piece_on(from)) == KING)
579 return !(attackers_to(to) & pieces(~us));
581 // A non-king move is legal if and only if it is not pinned or it
582 // is moving along the ray towards or away from the king.
583 return !(blockers_for_king(us) & from)
584 || aligned(from, to, square<KING>(us));
588 /// Position::pseudo_legal() takes a random move and tests whether the move is
589 /// pseudo legal. It is used to validate moves from TT that can be corrupted
590 /// due to SMP concurrent access or hash position key aliasing.
592 bool Position::pseudo_legal(const Move m) const {
594 Color us = sideToMove;
595 Square from = from_sq(m);
596 Square to = to_sq(m);
597 Piece pc = moved_piece(m);
599 // Use a slower but simpler function for uncommon cases
600 if (type_of(m) != NORMAL)
601 return MoveList<LEGAL>(*this).contains(m);
603 // Is not a promotion, so promotion piece must be empty
604 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
607 // If the 'from' square is not occupied by a piece belonging to the side to
608 // move, the move is obviously not legal.
609 if (pc == NO_PIECE || color_of(pc) != us)
612 // The destination square cannot be occupied by a friendly piece
616 // Handle the special case of a pawn move
617 if (type_of(pc) == PAWN)
619 // We have already handled promotion moves, so destination
620 // cannot be on the 8th/1st rank.
621 if ((Rank8BB | Rank1BB) & to)
624 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
625 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
626 && !( (from + 2 * pawn_push(us) == to) // Not a double push
627 && (rank_of(from) == relative_rank(us, RANK_2))
629 && empty(to - pawn_push(us))))
632 else if (!(attacks_from(type_of(pc), from) & to))
635 // Evasions generator already takes care to avoid some kind of illegal moves
636 // and legal() relies on this. We therefore have to take care that the same
637 // kind of moves are filtered out here.
640 if (type_of(pc) != KING)
642 // Double check? In this case a king move is required
643 if (more_than_one(checkers()))
646 // Our move must be a blocking evasion or a capture of the checking piece
647 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
650 // In case of king moves under check we have to remove king so as to catch
651 // invalid moves like b1a1 when opposite queen is on c1.
652 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
660 /// Position::gives_check() tests whether a pseudo-legal move gives a check
662 bool Position::gives_check(Move m) const {
665 assert(color_of(moved_piece(m)) == sideToMove);
667 Square from = from_sq(m);
668 Square to = to_sq(m);
670 // Is there a direct check?
671 if (st->checkSquares[type_of(piece_on(from))] & to)
674 // Is there a discovered check?
675 if ( (st->blockersForKing[~sideToMove] & from)
676 && !aligned(from, to, square<KING>(~sideToMove)))
685 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
687 // En passant capture with check? We have already handled the case
688 // of direct checks and ordinary discovered check, so the only case we
689 // need to handle is the unusual case of a discovered check through
690 // the captured pawn.
693 Square capsq = make_square(file_of(to), rank_of(from));
694 Bitboard b = (pieces() ^ from ^ capsq) | to;
696 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
697 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
702 Square rfrom = to; // Castling is encoded as 'King captures the rook'
703 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
704 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
706 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
707 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
716 /// Position::do_move() makes a move, and saves all information necessary
717 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
718 /// moves should be filtered out before this function is called.
720 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
723 assert(&newSt != st);
725 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
726 Key k = st->key ^ Zobrist::side;
728 // Copy some fields of the old state to our new StateInfo object except the
729 // ones which are going to be recalculated from scratch anyway and then switch
730 // our state pointer to point to the new (ready to be updated) state.
731 std::memcpy(&newSt, st, offsetof(StateInfo, key));
735 // Increment ply counters. In particular, rule50 will be reset to zero later on
736 // in case of a capture or a pawn move.
741 Color us = sideToMove;
743 Square from = from_sq(m);
744 Square to = to_sq(m);
745 Piece pc = piece_on(from);
746 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
748 assert(color_of(pc) == us);
749 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
750 assert(type_of(captured) != KING);
752 if (type_of(m) == CASTLING)
754 assert(pc == make_piece(us, KING));
755 assert(captured == make_piece(us, ROOK));
758 do_castling<true>(us, from, to, rfrom, rto);
760 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
768 // If the captured piece is a pawn, update pawn hash key, otherwise
769 // update non-pawn material.
770 if (type_of(captured) == PAWN)
772 if (type_of(m) == ENPASSANT)
774 capsq -= pawn_push(us);
776 assert(pc == make_piece(us, PAWN));
777 assert(to == st->epSquare);
778 assert(relative_rank(us, to) == RANK_6);
779 assert(piece_on(to) == NO_PIECE);
780 assert(piece_on(capsq) == make_piece(them, PAWN));
782 board[capsq] = NO_PIECE; // Not done by remove_piece()
785 st->pawnKey ^= Zobrist::psq[captured][capsq];
788 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
790 // Update board and piece lists
791 remove_piece(captured, capsq);
793 // Update material hash key and prefetch access to materialTable
794 k ^= Zobrist::psq[captured][capsq];
795 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
796 prefetch(thisThread->materialTable[st->materialKey]);
798 // Reset rule 50 counter
803 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
805 // Reset en passant square
806 if (st->epSquare != SQ_NONE)
808 k ^= Zobrist::enpassant[file_of(st->epSquare)];
809 st->epSquare = SQ_NONE;
812 // Update castling rights if needed
813 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
815 int cr = castlingRightsMask[from] | castlingRightsMask[to];
816 k ^= Zobrist::castling[st->castlingRights & cr];
817 st->castlingRights &= ~cr;
820 // Move the piece. The tricky Chess960 castling is handled earlier
821 if (type_of(m) != CASTLING)
822 move_piece(pc, from, to);
824 // If the moving piece is a pawn do some special extra work
825 if (type_of(pc) == PAWN)
827 // Set en-passant square if the moved pawn can be captured
828 if ( (int(to) ^ int(from)) == 16
829 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
831 st->epSquare = to - pawn_push(us);
832 k ^= Zobrist::enpassant[file_of(st->epSquare)];
835 else if (type_of(m) == PROMOTION)
837 Piece promotion = make_piece(us, promotion_type(m));
839 assert(relative_rank(us, to) == RANK_8);
840 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
842 remove_piece(pc, to);
843 put_piece(promotion, to);
846 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
847 st->pawnKey ^= Zobrist::psq[pc][to];
848 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
849 ^ Zobrist::psq[pc][pieceCount[pc]];
852 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
855 // Update pawn hash key and prefetch access to pawnsTable
856 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
858 // Reset rule 50 draw counter
863 st->capturedPiece = captured;
865 // Update the key with the final value
868 // Calculate checkers bitboard (if move gives check)
869 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
871 sideToMove = ~sideToMove;
873 // Update king attacks used for fast check detection
876 // Calculate the repetition info. It is the ply distance from the previous
877 // occurrence of the same position, negative in the 3-fold case, or zero
878 // if the position was not repeated.
880 int end = std::min(st->rule50, st->pliesFromNull);
883 StateInfo* stp = st->previous->previous;
884 for (int i = 4; i <= end; i += 2)
886 stp = stp->previous->previous;
887 if (stp->key == st->key)
889 st->repetition = stp->repetition ? -i : i;
899 /// Position::undo_move() unmakes a move. When it returns, the position should
900 /// be restored to exactly the same state as before the move was made.
902 void Position::undo_move(Move m) {
906 sideToMove = ~sideToMove;
908 Color us = sideToMove;
909 Square from = from_sq(m);
910 Square to = to_sq(m);
911 Piece pc = piece_on(to);
913 assert(empty(from) || type_of(m) == CASTLING);
914 assert(type_of(st->capturedPiece) != KING);
916 if (type_of(m) == PROMOTION)
918 assert(relative_rank(us, to) == RANK_8);
919 assert(type_of(pc) == promotion_type(m));
920 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
922 remove_piece(pc, to);
923 pc = make_piece(us, PAWN);
927 if (type_of(m) == CASTLING)
930 do_castling<false>(us, from, to, rfrom, rto);
934 move_piece(pc, to, from); // Put the piece back at the source square
936 if (st->capturedPiece)
940 if (type_of(m) == ENPASSANT)
942 capsq -= pawn_push(us);
944 assert(type_of(pc) == PAWN);
945 assert(to == st->previous->epSquare);
946 assert(relative_rank(us, to) == RANK_6);
947 assert(piece_on(capsq) == NO_PIECE);
948 assert(st->capturedPiece == make_piece(~us, PAWN));
951 put_piece(st->capturedPiece, capsq); // Restore the captured piece
955 // Finally point our state pointer back to the previous state
963 /// Position::do_castling() is a helper used to do/undo a castling move. This
964 /// is a bit tricky in Chess960 where from/to squares can overlap.
966 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
968 bool kingSide = to > from;
969 rfrom = to; // Castling is encoded as "king captures friendly rook"
970 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
971 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
973 // Remove both pieces first since squares could overlap in Chess960
974 remove_piece(make_piece(us, KING), Do ? from : to);
975 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
976 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
977 put_piece(make_piece(us, KING), Do ? to : from);
978 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
982 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
983 /// the side to move without executing any move on the board.
985 void Position::do_null_move(StateInfo& newSt) {
988 assert(&newSt != st);
990 std::memcpy(&newSt, st, sizeof(StateInfo));
994 if (st->epSquare != SQ_NONE)
996 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
997 st->epSquare = SQ_NONE;
1000 st->key ^= Zobrist::side;
1001 prefetch(TT.first_entry(st->key));
1004 st->pliesFromNull = 0;
1006 sideToMove = ~sideToMove;
1012 assert(pos_is_ok());
1015 void Position::undo_null_move() {
1017 assert(!checkers());
1020 sideToMove = ~sideToMove;
1024 /// Position::key_after() computes the new hash key after the given move. Needed
1025 /// for speculative prefetch. It doesn't recognize special moves like castling,
1026 /// en-passant and promotions.
1028 Key Position::key_after(Move m) const {
1030 Square from = from_sq(m);
1031 Square to = to_sq(m);
1032 Piece pc = piece_on(from);
1033 Piece captured = piece_on(to);
1034 Key k = st->key ^ Zobrist::side;
1037 k ^= Zobrist::psq[captured][to];
1039 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1043 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1044 /// SEE value of move is greater or equal to the given threshold. We'll use an
1045 /// algorithm similar to alpha-beta pruning with a null window.
1047 bool Position::see_ge(Move m, Value threshold) const {
1051 // Only deal with normal moves, assume others pass a simple see
1052 if (type_of(m) != NORMAL)
1053 return VALUE_ZERO >= threshold;
1055 Bitboard stmAttackers;
1056 Square from = from_sq(m), to = to_sq(m);
1057 PieceType nextVictim = type_of(piece_on(from));
1058 Color us = color_of(piece_on(from));
1059 Color stm = ~us; // First consider opponent's move
1060 Value balance; // Values of the pieces taken by us minus opponent's ones
1062 // The opponent may be able to recapture so this is the best result
1064 balance = PieceValue[MG][piece_on(to)] - threshold;
1066 if (balance < VALUE_ZERO)
1069 // Now assume the worst possible result: that the opponent can
1070 // capture our piece for free.
1071 balance -= PieceValue[MG][nextVictim];
1073 // If it is enough (like in PxQ) then return immediately. Note that
1074 // in case nextVictim == KING we always return here, this is ok
1075 // if the given move is legal.
1076 if (balance >= VALUE_ZERO)
1079 // Find all attackers to the destination square, with the moving piece
1080 // removed, but possibly an X-ray attacker added behind it.
1081 Bitboard occupied = pieces() ^ from ^ to;
1082 Bitboard attackers = attackers_to(to, occupied) & occupied;
1086 stmAttackers = attackers & pieces(stm);
1088 // Don't allow pinned pieces to attack (except the king) as long as
1089 // any pinners are on their original square.
1090 if (st->pinners[~stm] & occupied)
1091 stmAttackers &= ~st->blockersForKing[stm];
1093 // If stm has no more attackers then give up: stm loses
1097 // Locate and remove the next least valuable attacker, and add to
1098 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1099 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1101 stm = ~stm; // Switch side to move
1103 // Negamax the balance with alpha = balance, beta = balance+1 and
1104 // add nextVictim's value.
1106 // (balance, balance+1) -> (-balance-1, -balance)
1108 assert(balance < VALUE_ZERO);
1110 balance = -balance - 1 - PieceValue[MG][nextVictim];
1112 // If balance is still non-negative after giving away nextVictim then we
1113 // win. The only thing to be careful about it is that we should revert
1114 // stm if we captured with the king when the opponent still has attackers.
1115 if (balance >= VALUE_ZERO)
1117 if (nextVictim == KING && (attackers & pieces(stm)))
1121 assert(nextVictim != KING);
1123 return us != stm; // We break the above loop when stm loses
1127 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1128 /// or by repetition. It does not detect stalemates.
1130 bool Position::is_draw(int ply) const {
1132 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1135 // Return a draw score if a position repeats once earlier but strictly
1136 // after the root, or repeats twice before or at the root.
1137 if (st->repetition && st->repetition < ply)
1144 // Position::has_repeated() tests whether there has been at least one repetition
1145 // of positions since the last capture or pawn move.
1147 bool Position::has_repeated() const {
1149 StateInfo* stc = st;
1150 int end = std::min(st->rule50, st->pliesFromNull);
1153 if (stc->repetition)
1156 stc = stc->previous;
1162 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1163 /// or an earlier position has a move that directly reaches the current position.
1165 bool Position::has_game_cycle(int ply) const {
1169 int end = std::min(st->rule50, st->pliesFromNull);
1174 Key originalKey = st->key;
1175 StateInfo* stp = st->previous;
1177 for (int i = 3; i <= end; i += 2)
1179 stp = stp->previous->previous;
1181 Key moveKey = originalKey ^ stp->key;
1182 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1183 || (j = H2(moveKey), cuckoo[j] == moveKey))
1185 Move move = cuckooMove[j];
1186 Square s1 = from_sq(move);
1187 Square s2 = to_sq(move);
1189 if (!(between_bb(s1, s2) & pieces()))
1194 // For nodes before or at the root, check that the move is a
1195 // repetition rather than a move to the current position.
1196 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1197 // the same location, so we have to select which square to check.
1198 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1201 // For repetitions before or at the root, require one more
1202 if (stp->repetition)
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, BLACK })
1302 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1304 if (!can_castle(cr))
1307 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1308 || castlingRightsMask[castlingRookSquare[cr]] != (cr)
1309 || (castlingRightsMask[square<KING>(c)] & (cr)) != (cr))
1310 assert(0 && "pos_is_ok: Castling");