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 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 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
345 & ~(square_bb(kfrom) | rfrom);
349 /// Position::set_check_info() sets king attacks to detect if a move gives check
351 void Position::set_check_info(StateInfo* si) const {
353 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
354 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
356 Square ksq = square<KING>(~sideToMove);
358 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
359 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
360 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
361 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
362 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
363 si->checkSquares[KING] = 0;
367 /// Position::set_state() computes the hash keys of the position, and other
368 /// data that once computed is updated incrementally as moves are made.
369 /// The function is only used when a new position is set up, and to verify
370 /// the correctness of the StateInfo data when running in debug mode.
372 void Position::set_state(StateInfo* si) const {
374 si->key = si->materialKey = 0;
375 si->pawnKey = Zobrist::noPawns;
376 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
377 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
381 for (Bitboard b = pieces(); b; )
383 Square s = pop_lsb(&b);
384 Piece pc = piece_on(s);
385 si->key ^= Zobrist::psq[pc][s];
387 if (type_of(pc) == PAWN)
388 si->pawnKey ^= Zobrist::psq[pc][s];
390 else if (type_of(pc) != KING)
391 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
394 if (si->epSquare != SQ_NONE)
395 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
397 if (sideToMove == BLACK)
398 si->key ^= Zobrist::side;
400 si->key ^= Zobrist::castling[si->castlingRights];
402 for (Piece pc : Pieces)
403 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
404 si->materialKey ^= Zobrist::psq[pc][cnt];
408 /// Position::set() is an overload to initialize the position object with
409 /// the given endgame code string like "KBPKN". It is mainly a helper to
410 /// get the material key out of an endgame code.
412 Position& Position::set(const string& code, Color c, StateInfo* si) {
414 assert(code.length() > 0 && code.length() < 8);
415 assert(code[0] == 'K');
417 string sides[] = { code.substr(code.find('K', 1)), // Weak
418 code.substr(0, code.find('K', 1)) }; // Strong
420 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
422 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
423 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
425 return set(fenStr, false, si, nullptr);
429 /// Position::fen() returns a FEN representation of the position. In case of
430 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
432 const string Position::fen() const {
435 std::ostringstream ss;
437 for (Rank r = RANK_8; r >= RANK_1; --r)
439 for (File f = FILE_A; f <= FILE_H; ++f)
441 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
448 ss << PieceToChar[piece_on(make_square(f, r))];
455 ss << (sideToMove == WHITE ? " w " : " b ");
457 if (can_castle(WHITE_OO))
458 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
460 if (can_castle(WHITE_OOO))
461 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
463 if (can_castle(BLACK_OO))
464 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
466 if (can_castle(BLACK_OOO))
467 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
469 if (!can_castle(ANY_CASTLING))
472 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
473 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
479 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
480 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
481 /// slider if removing that piece from the board would result in a position where
482 /// square 's' is attacked. For example, a king-attack blocking piece can be either
483 /// a pinned or a discovered check piece, according if its color is the opposite
484 /// or the same of the color of the slider.
486 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
488 Bitboard blockers = 0;
491 // Snipers are sliders that attack 's' when a piece and other snipers are removed
492 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
493 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
494 Bitboard occupancy = pieces() ^ snipers;
498 Square sniperSq = pop_lsb(&snipers);
499 Bitboard b = between_bb(s, sniperSq) & occupancy;
501 if (b && !more_than_one(b))
504 if (b & pieces(color_of(piece_on(s))))
512 /// Position::attackers_to() computes a bitboard of all pieces which attack a
513 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
515 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
517 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
518 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
519 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
520 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
521 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
522 | (attacks_from<KING>(s) & pieces(KING));
526 /// Position::legal() tests whether a pseudo-legal move is legal
528 bool Position::legal(Move m) const {
532 Color us = sideToMove;
533 Square from = from_sq(m);
534 Square to = to_sq(m);
536 assert(color_of(moved_piece(m)) == us);
537 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
539 // En passant captures are a tricky special case. Because they are rather
540 // uncommon, we do it simply by testing whether the king is attacked after
542 if (type_of(m) == ENPASSANT)
544 Square ksq = square<KING>(us);
545 Square capsq = to - pawn_push(us);
546 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
548 assert(to == ep_square());
549 assert(moved_piece(m) == make_piece(us, PAWN));
550 assert(piece_on(capsq) == make_piece(~us, PAWN));
551 assert(piece_on(to) == NO_PIECE);
553 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
554 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
557 // Castling moves generation does not check if the castling path is clear of
558 // enemy attacks, it is delayed at a later time: now!
559 if (type_of(m) == CASTLING)
561 // After castling, the rook and king final positions are the same in
562 // Chess960 as they would be in standard chess.
563 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
564 Direction step = to > from ? WEST : EAST;
566 for (Square s = to; s != from; s += step)
567 if (attackers_to(s) & pieces(~us))
570 // In case of Chess960, verify that when moving the castling rook we do
571 // not discover some hidden checker.
572 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
574 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
577 // If the moving piece is a king, check whether the destination square is
578 // attacked by the opponent.
579 if (type_of(piece_on(from)) == KING)
580 return !(attackers_to(to) & pieces(~us));
582 // A non-king move is legal if and only if it is not pinned or it
583 // is moving along the ray towards or away from the king.
584 return !(blockers_for_king(us) & from)
585 || aligned(from, to, square<KING>(us));
589 /// Position::pseudo_legal() takes a random move and tests whether the move is
590 /// pseudo legal. It is used to validate moves from TT that can be corrupted
591 /// due to SMP concurrent access or hash position key aliasing.
593 bool Position::pseudo_legal(const Move m) const {
595 Color us = sideToMove;
596 Square from = from_sq(m);
597 Square to = to_sq(m);
598 Piece pc = moved_piece(m);
600 // Use a slower but simpler function for uncommon cases
601 if (type_of(m) != NORMAL)
602 return MoveList<LEGAL>(*this).contains(m);
604 // Is not a promotion, so promotion piece must be empty
605 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
608 // If the 'from' square is not occupied by a piece belonging to the side to
609 // move, the move is obviously not legal.
610 if (pc == NO_PIECE || color_of(pc) != us)
613 // The destination square cannot be occupied by a friendly piece
617 // Handle the special case of a pawn move
618 if (type_of(pc) == PAWN)
620 // We have already handled promotion moves, so destination
621 // cannot be on the 8th/1st rank.
622 if ((Rank8BB | Rank1BB) & to)
625 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
626 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
627 && !( (from + 2 * pawn_push(us) == to) // Not a double push
628 && (rank_of(from) == relative_rank(us, RANK_2))
630 && empty(to - pawn_push(us))))
633 else if (!(attacks_from(type_of(pc), from) & to))
636 // Evasions generator already takes care to avoid some kind of illegal moves
637 // and legal() relies on this. We therefore have to take care that the same
638 // kind of moves are filtered out here.
641 if (type_of(pc) != KING)
643 // Double check? In this case a king move is required
644 if (more_than_one(checkers()))
647 // Our move must be a blocking evasion or a capture of the checking piece
648 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
651 // In case of king moves under check we have to remove king so as to catch
652 // invalid moves like b1a1 when opposite queen is on c1.
653 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
661 /// Position::gives_check() tests whether a pseudo-legal move gives a check
663 bool Position::gives_check(Move m) const {
666 assert(color_of(moved_piece(m)) == sideToMove);
668 Square from = from_sq(m);
669 Square to = to_sq(m);
671 // Is there a direct check?
672 if (st->checkSquares[type_of(piece_on(from))] & to)
675 // Is there a discovered check?
676 if ( (st->blockersForKing[~sideToMove] & from)
677 && !aligned(from, to, square<KING>(~sideToMove)))
686 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
688 // En passant capture with check? We have already handled the case
689 // of direct checks and ordinary discovered check, so the only case we
690 // need to handle is the unusual case of a discovered check through
691 // the captured pawn.
694 Square capsq = make_square(file_of(to), rank_of(from));
695 Bitboard b = (pieces() ^ from ^ capsq) | to;
697 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
698 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
703 Square rfrom = to; // Castling is encoded as 'King captures the rook'
704 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
705 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
707 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
708 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
717 /// Position::do_move() makes a move, and saves all information necessary
718 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
719 /// moves should be filtered out before this function is called.
721 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
724 assert(&newSt != st);
726 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
727 Key k = st->key ^ Zobrist::side;
729 // Copy some fields of the old state to our new StateInfo object except the
730 // ones which are going to be recalculated from scratch anyway and then switch
731 // our state pointer to point to the new (ready to be updated) state.
732 std::memcpy(&newSt, st, offsetof(StateInfo, key));
736 // Increment ply counters. In particular, rule50 will be reset to zero later on
737 // in case of a capture or a pawn move.
742 Color us = sideToMove;
744 Square from = from_sq(m);
745 Square to = to_sq(m);
746 Piece pc = piece_on(from);
747 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
749 assert(color_of(pc) == us);
750 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
751 assert(type_of(captured) != KING);
753 if (type_of(m) == CASTLING)
755 assert(pc == make_piece(us, KING));
756 assert(captured == make_piece(us, ROOK));
759 do_castling<true>(us, from, to, rfrom, rto);
761 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
769 // If the captured piece is a pawn, update pawn hash key, otherwise
770 // update non-pawn material.
771 if (type_of(captured) == PAWN)
773 if (type_of(m) == ENPASSANT)
775 capsq -= pawn_push(us);
777 assert(pc == make_piece(us, PAWN));
778 assert(to == st->epSquare);
779 assert(relative_rank(us, to) == RANK_6);
780 assert(piece_on(to) == NO_PIECE);
781 assert(piece_on(capsq) == make_piece(them, PAWN));
783 board[capsq] = NO_PIECE; // Not done by remove_piece()
786 st->pawnKey ^= Zobrist::psq[captured][capsq];
789 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
791 // Update board and piece lists
792 remove_piece(captured, capsq);
794 // Update material hash key and prefetch access to materialTable
795 k ^= Zobrist::psq[captured][capsq];
796 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
797 prefetch(thisThread->materialTable[st->materialKey]);
799 // Reset rule 50 counter
804 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
806 // Reset en passant square
807 if (st->epSquare != SQ_NONE)
809 k ^= Zobrist::enpassant[file_of(st->epSquare)];
810 st->epSquare = SQ_NONE;
813 // Update castling rights if needed
814 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
816 int cr = castlingRightsMask[from] | castlingRightsMask[to];
817 k ^= Zobrist::castling[st->castlingRights & cr];
818 st->castlingRights &= ~cr;
821 // Move the piece. The tricky Chess960 castling is handled earlier
822 if (type_of(m) != CASTLING)
823 move_piece(pc, from, to);
825 // If the moving piece is a pawn do some special extra work
826 if (type_of(pc) == PAWN)
828 // Set en-passant square if the moved pawn can be captured
829 if ( (int(to) ^ int(from)) == 16
830 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
832 st->epSquare = to - pawn_push(us);
833 k ^= Zobrist::enpassant[file_of(st->epSquare)];
836 else if (type_of(m) == PROMOTION)
838 Piece promotion = make_piece(us, promotion_type(m));
840 assert(relative_rank(us, to) == RANK_8);
841 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
843 remove_piece(pc, to);
844 put_piece(promotion, to);
847 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
848 st->pawnKey ^= Zobrist::psq[pc][to];
849 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
850 ^ Zobrist::psq[pc][pieceCount[pc]];
853 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
856 // Update pawn hash key and prefetch access to pawnsTable
857 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
859 // Reset rule 50 draw counter
864 st->capturedPiece = captured;
866 // Update the key with the final value
869 // Calculate checkers bitboard (if move gives check)
870 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
872 sideToMove = ~sideToMove;
874 // Update king attacks used for fast check detection
877 // Calculate the repetition info. It is the ply distance from the previous
878 // occurrence of the same position, negative in the 3-fold case, or zero
879 // if the position was not repeated.
881 int end = std::min(st->rule50, st->pliesFromNull);
884 StateInfo* stp = st->previous->previous;
885 for (int i=4; i <= end; i += 2)
887 stp = stp->previous->previous;
888 if (stp->key == st->key)
890 st->repetition = stp->repetition ? -i : i;
900 /// Position::undo_move() unmakes a move. When it returns, the position should
901 /// be restored to exactly the same state as before the move was made.
903 void Position::undo_move(Move m) {
907 sideToMove = ~sideToMove;
909 Color us = sideToMove;
910 Square from = from_sq(m);
911 Square to = to_sq(m);
912 Piece pc = piece_on(to);
914 assert(empty(from) || type_of(m) == CASTLING);
915 assert(type_of(st->capturedPiece) != KING);
917 if (type_of(m) == PROMOTION)
919 assert(relative_rank(us, to) == RANK_8);
920 assert(type_of(pc) == promotion_type(m));
921 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
923 remove_piece(pc, to);
924 pc = make_piece(us, PAWN);
928 if (type_of(m) == CASTLING)
931 do_castling<false>(us, from, to, rfrom, rto);
935 move_piece(pc, to, from); // Put the piece back at the source square
937 if (st->capturedPiece)
941 if (type_of(m) == ENPASSANT)
943 capsq -= pawn_push(us);
945 assert(type_of(pc) == PAWN);
946 assert(to == st->previous->epSquare);
947 assert(relative_rank(us, to) == RANK_6);
948 assert(piece_on(capsq) == NO_PIECE);
949 assert(st->capturedPiece == make_piece(~us, PAWN));
952 put_piece(st->capturedPiece, capsq); // Restore the captured piece
956 // Finally point our state pointer back to the previous state
964 /// Position::do_castling() is a helper used to do/undo a castling move. This
965 /// is a bit tricky in Chess960 where from/to squares can overlap.
967 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
969 bool kingSide = to > from;
970 rfrom = to; // Castling is encoded as "king captures friendly rook"
971 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
972 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
974 // Remove both pieces first since squares could overlap in Chess960
975 remove_piece(make_piece(us, KING), Do ? from : to);
976 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
977 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
978 put_piece(make_piece(us, KING), Do ? to : from);
979 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
983 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
984 /// the side to move without executing any move on the board.
986 void Position::do_null_move(StateInfo& newSt) {
989 assert(&newSt != st);
991 std::memcpy(&newSt, st, sizeof(StateInfo));
995 if (st->epSquare != SQ_NONE)
997 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
998 st->epSquare = SQ_NONE;
1001 st->key ^= Zobrist::side;
1002 prefetch(TT.first_entry(st->key));
1005 st->pliesFromNull = 0;
1007 sideToMove = ~sideToMove;
1013 assert(pos_is_ok());
1016 void Position::undo_null_move() {
1018 assert(!checkers());
1021 sideToMove = ~sideToMove;
1025 /// Position::key_after() computes the new hash key after the given move. Needed
1026 /// for speculative prefetch. It doesn't recognize special moves like castling,
1027 /// en-passant and promotions.
1029 Key Position::key_after(Move m) const {
1031 Square from = from_sq(m);
1032 Square to = to_sq(m);
1033 Piece pc = piece_on(from);
1034 Piece captured = piece_on(to);
1035 Key k = st->key ^ Zobrist::side;
1038 k ^= Zobrist::psq[captured][to];
1040 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1044 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1045 /// SEE value of move is greater or equal to the given threshold. We'll use an
1046 /// algorithm similar to alpha-beta pruning with a null window.
1048 bool Position::see_ge(Move m, Value threshold) const {
1052 // Only deal with normal moves, assume others pass a simple see
1053 if (type_of(m) != NORMAL)
1054 return VALUE_ZERO >= threshold;
1056 Bitboard stmAttackers;
1057 Square from = from_sq(m), to = to_sq(m);
1058 PieceType nextVictim = type_of(piece_on(from));
1059 Color us = color_of(piece_on(from));
1060 Color stm = ~us; // First consider opponent's move
1061 Value balance; // Values of the pieces taken by us minus opponent's ones
1063 // The opponent may be able to recapture so this is the best result
1065 balance = PieceValue[MG][piece_on(to)] - threshold;
1067 if (balance < VALUE_ZERO)
1070 // Now assume the worst possible result: that the opponent can
1071 // capture our piece for free.
1072 balance -= PieceValue[MG][nextVictim];
1074 // If it is enough (like in PxQ) then return immediately. Note that
1075 // in case nextVictim == KING we always return here, this is ok
1076 // if the given move is legal.
1077 if (balance >= VALUE_ZERO)
1080 // Find all attackers to the destination square, with the moving piece
1081 // removed, but possibly an X-ray attacker added behind it.
1082 Bitboard occupied = pieces() ^ from ^ to;
1083 Bitboard attackers = attackers_to(to, occupied) & occupied;
1087 stmAttackers = attackers & pieces(stm);
1089 // Don't allow pinned pieces to attack (except the king) as long as
1090 // any pinners are on their original square.
1091 if (st->pinners[~stm] & occupied)
1092 stmAttackers &= ~st->blockersForKing[stm];
1094 // If stm has no more attackers then give up: stm loses
1098 // Locate and remove the next least valuable attacker, and add to
1099 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1100 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1102 stm = ~stm; // Switch side to move
1104 // Negamax the balance with alpha = balance, beta = balance+1 and
1105 // add nextVictim's value.
1107 // (balance, balance+1) -> (-balance-1, -balance)
1109 assert(balance < VALUE_ZERO);
1111 balance = -balance - 1 - PieceValue[MG][nextVictim];
1113 // If balance is still non-negative after giving away nextVictim then we
1114 // win. The only thing to be careful about it is that we should revert
1115 // stm if we captured with the king when the opponent still has attackers.
1116 if (balance >= VALUE_ZERO)
1118 if (nextVictim == KING && (attackers & pieces(stm)))
1122 assert(nextVictim != KING);
1124 return us != stm; // We break the above loop when stm loses
1128 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1129 /// or by repetition. It does not detect stalemates.
1131 bool Position::is_draw(int ply) const {
1133 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1136 // Return a draw score if a position repeats once earlier but strictly
1137 // after the root, or repeats twice before or at the root.
1138 if (st->repetition && st->repetition < ply)
1145 // Position::has_repeated() tests whether there has been at least one repetition
1146 // of positions since the last capture or pawn move.
1148 bool Position::has_repeated() const {
1150 StateInfo* stc = st;
1151 int end = std::min(st->rule50, st->pliesFromNull);
1154 if (stc->repetition)
1157 stc = stc->previous;
1163 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1164 /// or an earlier position has a move that directly reaches the current position.
1166 bool Position::has_game_cycle(int ply) const {
1170 int end = std::min(st->rule50, st->pliesFromNull);
1175 Key originalKey = st->key;
1176 StateInfo* stp = st->previous;
1178 for (int i = 3; i <= end; i += 2)
1180 stp = stp->previous->previous;
1182 Key moveKey = originalKey ^ stp->key;
1183 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1184 || (j = H2(moveKey), cuckoo[j] == moveKey))
1186 Move move = cuckooMove[j];
1187 Square s1 = from_sq(move);
1188 Square s2 = to_sq(move);
1190 if (!(between_bb(s1, s2) & pieces()))
1195 // For nodes before or at the root, check that the move is a
1196 // repetition rather than a move to the current position.
1197 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1198 // the same location, so we have to select which square to check.
1199 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1202 // For repetitions before or at the root, require one more
1203 if (stp->repetition)
1212 /// Position::flip() flips position with the white and black sides reversed. This
1213 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1215 void Position::flip() {
1218 std::stringstream ss(fen());
1220 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1222 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1223 f.insert(0, token + (f.empty() ? " " : "/"));
1226 ss >> token; // Active color
1227 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1229 ss >> token; // Castling availability
1232 std::transform(f.begin(), f.end(), f.begin(),
1233 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1235 ss >> token; // En passant square
1236 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1238 std::getline(ss, token); // Half and full moves
1241 set(f, is_chess960(), st, this_thread());
1243 assert(pos_is_ok());
1247 /// Position::pos_is_ok() performs some consistency checks for the
1248 /// position object and raises an asserts if something wrong is detected.
1249 /// This is meant to be helpful when debugging.
1251 bool Position::pos_is_ok() const {
1253 constexpr bool Fast = true; // Quick (default) or full check?
1255 if ( (sideToMove != WHITE && sideToMove != BLACK)
1256 || piece_on(square<KING>(WHITE)) != W_KING
1257 || piece_on(square<KING>(BLACK)) != B_KING
1258 || ( ep_square() != SQ_NONE
1259 && relative_rank(sideToMove, ep_square()) != RANK_6))
1260 assert(0 && "pos_is_ok: Default");
1265 if ( pieceCount[W_KING] != 1
1266 || pieceCount[B_KING] != 1
1267 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1268 assert(0 && "pos_is_ok: Kings");
1270 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1271 || pieceCount[W_PAWN] > 8
1272 || pieceCount[B_PAWN] > 8)
1273 assert(0 && "pos_is_ok: Pawns");
1275 if ( (pieces(WHITE) & pieces(BLACK))
1276 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1277 || popcount(pieces(WHITE)) > 16
1278 || popcount(pieces(BLACK)) > 16)
1279 assert(0 && "pos_is_ok: Bitboards");
1281 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1282 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1283 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1284 assert(0 && "pos_is_ok: Bitboards");
1288 if (std::memcmp(&si, st, sizeof(StateInfo)))
1289 assert(0 && "pos_is_ok: State");
1291 for (Piece pc : Pieces)
1293 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1294 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1295 assert(0 && "pos_is_ok: Pieces");
1297 for (int i = 0; i < pieceCount[pc]; ++i)
1298 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1299 assert(0 && "pos_is_ok: Index");
1302 for (Color c = WHITE; c <= BLACK; ++c)
1303 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1305 if (!can_castle(c | s))
1308 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1309 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1310 || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
1311 assert(0 && "pos_is_ok: Castling");