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;
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 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
343 & ~(square_bb(kfrom) | rfrom);
347 /// Position::set_check_info() sets king attacks to detect if a move gives check
349 void Position::set_check_info(StateInfo* si) const {
351 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
352 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
354 Square ksq = square<KING>(~sideToMove);
356 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
357 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
358 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
359 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
360 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
361 si->checkSquares[KING] = 0;
365 /// Position::set_state() computes the hash keys of the position, and other
366 /// data that once computed is updated incrementally as moves are made.
367 /// The function is only used when a new position is set up, and to verify
368 /// the correctness of the StateInfo data when running in debug mode.
370 void Position::set_state(StateInfo* si) const {
372 si->key = si->materialKey = 0;
373 si->pawnKey = Zobrist::noPawns;
374 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
375 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
379 for (Bitboard b = pieces(); b; )
381 Square s = pop_lsb(&b);
382 Piece pc = piece_on(s);
383 si->key ^= Zobrist::psq[pc][s];
385 if (type_of(pc) == PAWN)
386 si->pawnKey ^= Zobrist::psq[pc][s];
388 else if (type_of(pc) != KING)
389 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
392 if (si->epSquare != SQ_NONE)
393 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
395 if (sideToMove == BLACK)
396 si->key ^= Zobrist::side;
398 si->key ^= Zobrist::castling[si->castlingRights];
400 for (Piece pc : Pieces)
401 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
402 si->materialKey ^= Zobrist::psq[pc][cnt];
406 /// Position::set() is an overload to initialize the position object with
407 /// the given endgame code string like "KBPKN". It is mainly a helper to
408 /// get the material key out of an endgame code.
410 Position& Position::set(const string& code, Color c, StateInfo* si) {
412 assert(code.length() > 0 && code.length() < 8);
413 assert(code[0] == 'K');
415 string sides[] = { code.substr(code.find('K', 1)), // Weak
416 code.substr(0, code.find('K', 1)) }; // Strong
418 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
420 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
421 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
423 return set(fenStr, false, si, nullptr);
427 /// Position::fen() returns a FEN representation of the position. In case of
428 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
430 const string Position::fen() const {
433 std::ostringstream ss;
435 for (Rank r = RANK_8; r >= RANK_1; --r)
437 for (File f = FILE_A; f <= FILE_H; ++f)
439 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
446 ss << PieceToChar[piece_on(make_square(f, r))];
453 ss << (sideToMove == WHITE ? " w " : " b ");
455 if (can_castle(WHITE_OO))
456 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
458 if (can_castle(WHITE_OOO))
459 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
461 if (can_castle(BLACK_OO))
462 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
464 if (can_castle(BLACK_OOO))
465 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
467 if (!can_castle(ANY_CASTLING))
470 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
471 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
477 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
478 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
479 /// slider if removing that piece from the board would result in a position where
480 /// square 's' is attacked. For example, a king-attack blocking piece can be either
481 /// a pinned or a discovered check piece, according if its color is the opposite
482 /// or the same of the color of the slider.
484 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
486 Bitboard blockers = 0;
489 // Snipers are sliders that attack 's' when a piece and other snipers are removed
490 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
491 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
492 Bitboard occupancy = pieces() ^ snipers;
496 Square sniperSq = pop_lsb(&snipers);
497 Bitboard b = between_bb(s, sniperSq) & occupancy;
499 if (b && !more_than_one(b))
502 if (b & pieces(color_of(piece_on(s))))
510 /// Position::attackers_to() computes a bitboard of all pieces which attack a
511 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
513 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
515 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
516 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
517 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
518 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
519 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
520 | (attacks_from<KING>(s) & pieces(KING));
524 /// Position::legal() tests whether a pseudo-legal move is legal
526 bool Position::legal(Move m) const {
530 Color us = sideToMove;
531 Square from = from_sq(m);
532 Square to = to_sq(m);
534 assert(color_of(moved_piece(m)) == us);
535 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
537 // En passant captures are a tricky special case. Because they are rather
538 // uncommon, we do it simply by testing whether the king is attacked after
540 if (type_of(m) == ENPASSANT)
542 Square ksq = square<KING>(us);
543 Square capsq = to - pawn_push(us);
544 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
546 assert(to == ep_square());
547 assert(moved_piece(m) == make_piece(us, PAWN));
548 assert(piece_on(capsq) == make_piece(~us, PAWN));
549 assert(piece_on(to) == NO_PIECE);
551 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
552 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
555 // Castling moves generation does not check if the castling path is clear of
556 // enemy attacks, it is delayed at a later time: now!
557 if (type_of(m) == CASTLING)
559 // After castling, the rook and king final positions are the same in
560 // Chess960 as they would be in standard chess.
561 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
562 Direction step = to > from ? WEST : EAST;
564 for (Square s = to; s != from; s += step)
565 if (attackers_to(s) & pieces(~us))
568 // In case of Chess960, verify that when moving the castling rook we do
569 // not discover some hidden checker.
570 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
572 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
575 // If the moving piece is a king, check whether the destination square is
576 // attacked by the opponent.
577 if (type_of(piece_on(from)) == KING)
578 return !(attackers_to(to) & pieces(~us));
580 // A non-king move is legal if and only if it is not pinned or it
581 // is moving along the ray towards or away from the king.
582 return !(blockers_for_king(us) & from)
583 || aligned(from, to, square<KING>(us));
587 /// Position::pseudo_legal() takes a random move and tests whether the move is
588 /// pseudo legal. It is used to validate moves from TT that can be corrupted
589 /// due to SMP concurrent access or hash position key aliasing.
591 bool Position::pseudo_legal(const Move m) const {
593 Color us = sideToMove;
594 Square from = from_sq(m);
595 Square to = to_sq(m);
596 Piece pc = moved_piece(m);
598 // Use a slower but simpler function for uncommon cases
599 if (type_of(m) != NORMAL)
600 return MoveList<LEGAL>(*this).contains(m);
602 // Is not a promotion, so promotion piece must be empty
603 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
606 // If the 'from' square is not occupied by a piece belonging to the side to
607 // move, the move is obviously not legal.
608 if (pc == NO_PIECE || color_of(pc) != us)
611 // The destination square cannot be occupied by a friendly piece
615 // Handle the special case of a pawn move
616 if (type_of(pc) == PAWN)
618 // We have already handled promotion moves, so destination
619 // cannot be on the 8th/1st rank.
620 if ((Rank8BB | Rank1BB) & to)
623 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
624 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
625 && !( (from + 2 * pawn_push(us) == to) // Not a double push
626 && (rank_of(from) == relative_rank(us, RANK_2))
628 && empty(to - pawn_push(us))))
631 else if (!(attacks_from(type_of(pc), from) & to))
634 // Evasions generator already takes care to avoid some kind of illegal moves
635 // and legal() relies on this. We therefore have to take care that the same
636 // kind of moves are filtered out here.
639 if (type_of(pc) != KING)
641 // Double check? In this case a king move is required
642 if (more_than_one(checkers()))
645 // Our move must be a blocking evasion or a capture of the checking piece
646 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
649 // In case of king moves under check we have to remove king so as to catch
650 // invalid moves like b1a1 when opposite queen is on c1.
651 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
659 /// Position::gives_check() tests whether a pseudo-legal move gives a check
661 bool Position::gives_check(Move m) const {
664 assert(color_of(moved_piece(m)) == sideToMove);
666 Square from = from_sq(m);
667 Square to = to_sq(m);
669 // Is there a direct check?
670 if (st->checkSquares[type_of(piece_on(from))] & to)
673 // Is there a discovered check?
674 if ( (st->blockersForKing[~sideToMove] & from)
675 && !aligned(from, to, square<KING>(~sideToMove)))
684 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
686 // En passant capture with check? We have already handled the case
687 // of direct checks and ordinary discovered check, so the only case we
688 // need to handle is the unusual case of a discovered check through
689 // the captured pawn.
692 Square capsq = make_square(file_of(to), rank_of(from));
693 Bitboard b = (pieces() ^ from ^ capsq) | to;
695 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
696 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
701 Square rfrom = to; // Castling is encoded as 'King captures the rook'
702 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
703 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
705 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
706 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
715 /// Position::do_move() makes a move, and saves all information necessary
716 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
717 /// moves should be filtered out before this function is called.
719 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
722 assert(&newSt != st);
724 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
725 Key k = st->key ^ Zobrist::side;
727 // Copy some fields of the old state to our new StateInfo object except the
728 // ones which are going to be recalculated from scratch anyway and then switch
729 // our state pointer to point to the new (ready to be updated) state.
730 std::memcpy(&newSt, st, offsetof(StateInfo, key));
734 // Increment ply counters. In particular, rule50 will be reset to zero later on
735 // in case of a capture or a pawn move.
740 Color us = sideToMove;
742 Square from = from_sq(m);
743 Square to = to_sq(m);
744 Piece pc = piece_on(from);
745 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
747 assert(color_of(pc) == us);
748 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
749 assert(type_of(captured) != KING);
751 if (type_of(m) == CASTLING)
753 assert(pc == make_piece(us, KING));
754 assert(captured == make_piece(us, ROOK));
757 do_castling<true>(us, from, to, rfrom, rto);
759 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
767 // If the captured piece is a pawn, update pawn hash key, otherwise
768 // update non-pawn material.
769 if (type_of(captured) == PAWN)
771 if (type_of(m) == ENPASSANT)
773 capsq -= pawn_push(us);
775 assert(pc == make_piece(us, PAWN));
776 assert(to == st->epSquare);
777 assert(relative_rank(us, to) == RANK_6);
778 assert(piece_on(to) == NO_PIECE);
779 assert(piece_on(capsq) == make_piece(them, PAWN));
781 board[capsq] = NO_PIECE; // Not done by remove_piece()
784 st->pawnKey ^= Zobrist::psq[captured][capsq];
787 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
789 // Update board and piece lists
790 remove_piece(captured, capsq);
792 // Update material hash key and prefetch access to materialTable
793 k ^= Zobrist::psq[captured][capsq];
794 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
795 prefetch(thisThread->materialTable[st->materialKey]);
797 // Reset rule 50 counter
802 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
804 // Reset en passant square
805 if (st->epSquare != SQ_NONE)
807 k ^= Zobrist::enpassant[file_of(st->epSquare)];
808 st->epSquare = SQ_NONE;
811 // Update castling rights if needed
812 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
814 int cr = castlingRightsMask[from] | castlingRightsMask[to];
815 k ^= Zobrist::castling[st->castlingRights & cr];
816 st->castlingRights &= ~cr;
819 // Move the piece. The tricky Chess960 castling is handled earlier
820 if (type_of(m) != CASTLING)
821 move_piece(pc, from, to);
823 // If the moving piece is a pawn do some special extra work
824 if (type_of(pc) == PAWN)
826 // Set en-passant square if the moved pawn can be captured
827 if ( (int(to) ^ int(from)) == 16
828 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
830 st->epSquare = to - pawn_push(us);
831 k ^= Zobrist::enpassant[file_of(st->epSquare)];
834 else if (type_of(m) == PROMOTION)
836 Piece promotion = make_piece(us, promotion_type(m));
838 assert(relative_rank(us, to) == RANK_8);
839 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
841 remove_piece(pc, to);
842 put_piece(promotion, to);
845 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
846 st->pawnKey ^= Zobrist::psq[pc][to];
847 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
848 ^ Zobrist::psq[pc][pieceCount[pc]];
851 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
854 // Update pawn hash key and prefetch access to pawnsTable
855 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
857 // Reset rule 50 draw counter
862 st->capturedPiece = captured;
864 // Update the key with the final value
867 // Calculate checkers bitboard (if move gives check)
868 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
870 sideToMove = ~sideToMove;
872 // Update king attacks used for fast check detection
875 // Calculate the repetition info. It is the ply distance from the previous
876 // occurrence of the same position, negative in the 3-fold case, or zero
877 // if the position was not repeated.
879 int end = std::min(st->rule50, st->pliesFromNull);
882 StateInfo* stp = st->previous->previous;
883 for (int i=4; i <= end; i += 2)
885 stp = stp->previous->previous;
886 if (stp->key == st->key)
888 st->repetition = stp->repetition ? -i : i;
898 /// Position::undo_move() unmakes a move. When it returns, the position should
899 /// be restored to exactly the same state as before the move was made.
901 void Position::undo_move(Move m) {
905 sideToMove = ~sideToMove;
907 Color us = sideToMove;
908 Square from = from_sq(m);
909 Square to = to_sq(m);
910 Piece pc = piece_on(to);
912 assert(empty(from) || type_of(m) == CASTLING);
913 assert(type_of(st->capturedPiece) != KING);
915 if (type_of(m) == PROMOTION)
917 assert(relative_rank(us, to) == RANK_8);
918 assert(type_of(pc) == promotion_type(m));
919 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
921 remove_piece(pc, to);
922 pc = make_piece(us, PAWN);
926 if (type_of(m) == CASTLING)
929 do_castling<false>(us, from, to, rfrom, rto);
933 move_piece(pc, to, from); // Put the piece back at the source square
935 if (st->capturedPiece)
939 if (type_of(m) == ENPASSANT)
941 capsq -= pawn_push(us);
943 assert(type_of(pc) == PAWN);
944 assert(to == st->previous->epSquare);
945 assert(relative_rank(us, to) == RANK_6);
946 assert(piece_on(capsq) == NO_PIECE);
947 assert(st->capturedPiece == make_piece(~us, PAWN));
950 put_piece(st->capturedPiece, capsq); // Restore the captured piece
954 // Finally point our state pointer back to the previous state
962 /// Position::do_castling() is a helper used to do/undo a castling move. This
963 /// is a bit tricky in Chess960 where from/to squares can overlap.
965 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
967 bool kingSide = to > from;
968 rfrom = to; // Castling is encoded as "king captures friendly rook"
969 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
970 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
972 // Remove both pieces first since squares could overlap in Chess960
973 remove_piece(make_piece(us, KING), Do ? from : to);
974 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
975 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
976 put_piece(make_piece(us, KING), Do ? to : from);
977 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
981 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
982 /// the side to move without executing any move on the board.
984 void Position::do_null_move(StateInfo& newSt) {
987 assert(&newSt != st);
989 std::memcpy(&newSt, st, sizeof(StateInfo));
993 if (st->epSquare != SQ_NONE)
995 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
996 st->epSquare = SQ_NONE;
999 st->key ^= Zobrist::side;
1000 prefetch(TT.first_entry(st->key));
1003 st->pliesFromNull = 0;
1005 sideToMove = ~sideToMove;
1011 assert(pos_is_ok());
1014 void Position::undo_null_move() {
1016 assert(!checkers());
1019 sideToMove = ~sideToMove;
1023 /// Position::key_after() computes the new hash key after the given move. Needed
1024 /// for speculative prefetch. It doesn't recognize special moves like castling,
1025 /// en-passant and promotions.
1027 Key Position::key_after(Move m) const {
1029 Square from = from_sq(m);
1030 Square to = to_sq(m);
1031 Piece pc = piece_on(from);
1032 Piece captured = piece_on(to);
1033 Key k = st->key ^ Zobrist::side;
1036 k ^= Zobrist::psq[captured][to];
1038 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1042 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1043 /// SEE value of move is greater or equal to the given threshold. We'll use an
1044 /// algorithm similar to alpha-beta pruning with a null window.
1046 bool Position::see_ge(Move m, Value threshold) const {
1050 // Only deal with normal moves, assume others pass a simple see
1051 if (type_of(m) != NORMAL)
1052 return VALUE_ZERO >= threshold;
1054 Bitboard stmAttackers;
1055 Square from = from_sq(m), to = to_sq(m);
1056 PieceType nextVictim = type_of(piece_on(from));
1057 Color us = color_of(piece_on(from));
1058 Color stm = ~us; // First consider opponent's move
1059 Value balance; // Values of the pieces taken by us minus opponent's ones
1061 // The opponent may be able to recapture so this is the best result
1063 balance = PieceValue[MG][piece_on(to)] - threshold;
1065 if (balance < VALUE_ZERO)
1068 // Now assume the worst possible result: that the opponent can
1069 // capture our piece for free.
1070 balance -= PieceValue[MG][nextVictim];
1072 // If it is enough (like in PxQ) then return immediately. Note that
1073 // in case nextVictim == KING we always return here, this is ok
1074 // if the given move is legal.
1075 if (balance >= VALUE_ZERO)
1078 // Find all attackers to the destination square, with the moving piece
1079 // removed, but possibly an X-ray attacker added behind it.
1080 Bitboard occupied = pieces() ^ from ^ to;
1081 Bitboard attackers = attackers_to(to, occupied) & occupied;
1085 stmAttackers = attackers & pieces(stm);
1087 // Don't allow pinned pieces to attack (except the king) as long as
1088 // any pinners are on their original square.
1089 if (st->pinners[~stm] & occupied)
1090 stmAttackers &= ~st->blockersForKing[stm];
1092 // If stm has no more attackers then give up: stm loses
1096 // Locate and remove the next least valuable attacker, and add to
1097 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1098 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1100 stm = ~stm; // Switch side to move
1102 // Negamax the balance with alpha = balance, beta = balance+1 and
1103 // add nextVictim's value.
1105 // (balance, balance+1) -> (-balance-1, -balance)
1107 assert(balance < VALUE_ZERO);
1109 balance = -balance - 1 - PieceValue[MG][nextVictim];
1111 // If balance is still non-negative after giving away nextVictim then we
1112 // win. The only thing to be careful about it is that we should revert
1113 // stm if we captured with the king when the opponent still has attackers.
1114 if (balance >= VALUE_ZERO)
1116 if (nextVictim == KING && (attackers & pieces(stm)))
1120 assert(nextVictim != KING);
1122 return us != stm; // We break the above loop when stm loses
1126 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1127 /// or by repetition. It does not detect stalemates.
1129 bool Position::is_draw(int ply) const {
1131 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1134 // Return a draw score if a position repeats once earlier but strictly
1135 // after the root, or repeats twice before or at the root.
1136 if (st->repetition && st->repetition < ply)
1143 // Position::has_repeated() tests whether there has been at least one repetition
1144 // of positions since the last capture or pawn move.
1146 bool Position::has_repeated() const {
1148 StateInfo* stc = st;
1149 int end = std::min(st->rule50, st->pliesFromNull);
1152 if (stc->repetition)
1155 stc = stc->previous;
1161 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1162 /// or an earlier position has a move that directly reaches the current position.
1164 bool Position::has_game_cycle(int ply) const {
1168 int end = std::min(st->rule50, st->pliesFromNull);
1173 Key originalKey = st->key;
1174 StateInfo* stp = st->previous;
1176 for (int i = 3; i <= end; i += 2)
1178 stp = stp->previous->previous;
1180 Key moveKey = originalKey ^ stp->key;
1181 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1182 || (j = H2(moveKey), cuckoo[j] == moveKey))
1184 Move move = cuckooMove[j];
1185 Square s1 = from_sq(move);
1186 Square s2 = to_sq(move);
1188 if (!(between_bb(s1, s2) & pieces()))
1193 // For nodes before or at the root, check that the move is a
1194 // repetition rather than a move to the current position.
1195 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1196 // the same location, so we have to select which square to check.
1197 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1200 // For repetitions before or at the root, require one more
1201 if (stp->repetition)
1210 /// Position::flip() flips position with the white and black sides reversed. This
1211 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1213 void Position::flip() {
1216 std::stringstream ss(fen());
1218 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1220 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1221 f.insert(0, token + (f.empty() ? " " : "/"));
1224 ss >> token; // Active color
1225 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1227 ss >> token; // Castling availability
1230 std::transform(f.begin(), f.end(), f.begin(),
1231 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1233 ss >> token; // En passant square
1234 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1236 std::getline(ss, token); // Half and full moves
1239 set(f, is_chess960(), st, this_thread());
1241 assert(pos_is_ok());
1245 /// Position::pos_is_ok() performs some consistency checks for the
1246 /// position object and raises an asserts if something wrong is detected.
1247 /// This is meant to be helpful when debugging.
1249 bool Position::pos_is_ok() const {
1251 constexpr bool Fast = true; // Quick (default) or full check?
1253 if ( (sideToMove != WHITE && sideToMove != BLACK)
1254 || piece_on(square<KING>(WHITE)) != W_KING
1255 || piece_on(square<KING>(BLACK)) != B_KING
1256 || ( ep_square() != SQ_NONE
1257 && relative_rank(sideToMove, ep_square()) != RANK_6))
1258 assert(0 && "pos_is_ok: Default");
1263 if ( pieceCount[W_KING] != 1
1264 || pieceCount[B_KING] != 1
1265 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1266 assert(0 && "pos_is_ok: Kings");
1268 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1269 || pieceCount[W_PAWN] > 8
1270 || pieceCount[B_PAWN] > 8)
1271 assert(0 && "pos_is_ok: Pawns");
1273 if ( (pieces(WHITE) & pieces(BLACK))
1274 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1275 || popcount(pieces(WHITE)) > 16
1276 || popcount(pieces(BLACK)) > 16)
1277 assert(0 && "pos_is_ok: Bitboards");
1279 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1280 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1281 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1282 assert(0 && "pos_is_ok: Bitboards");
1286 if (std::memcmp(&si, st, sizeof(StateInfo)))
1287 assert(0 && "pos_is_ok: State");
1289 for (Piece pc : Pieces)
1291 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1292 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1293 assert(0 && "pos_is_ok: Pieces");
1295 for (int i = 0; i < pieceCount[pc]; ++i)
1296 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1297 assert(0 && "pos_is_ok: Index");
1300 for (Color c : { WHITE, BLACK })
1301 for (CastlingSide s : {KING_SIDE, QUEEN_SIDE})
1303 if (!can_castle(c | s))
1306 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1307 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1308 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1309 assert(0 && "pos_is_ok: Castling");