2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
23 #include <cstddef> // For offsetof()
24 #include <cstring> // For std::memset, std::memcmp
35 #include "syzygy/tbprobe.h"
41 Key psq[PIECE_NB][SQUARE_NB];
42 Key enpassant[FILE_NB];
43 Key castling[CASTLING_RIGHT_NB];
49 const string PieceToChar(" PNBRQK pnbrqk");
51 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
52 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
54 // min_attacker() is a helper function used by see_ge() to locate the least
55 // valuable attacker for the side to move, remove the attacker we just found
56 // from the bitboards and scan for new X-ray attacks behind it.
59 PieceType min_attacker(const Bitboard* byTypeBB, Square to, Bitboard stmAttackers,
60 Bitboard& occupied, Bitboard& attackers) {
62 Bitboard b = stmAttackers & byTypeBB[Pt];
64 return min_attacker<Pt + 1>(byTypeBB, to, stmAttackers, occupied, attackers);
66 occupied ^= lsb(b); // Remove the attacker from occupied
68 // Add any X-ray attack behind the just removed piece. For instance with
69 // rooks in a8 and a7 attacking a1, after removing a7 we add rook in a8.
70 // Note that new added attackers can be of any color.
71 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
72 attackers |= attacks_bb<BISHOP>(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]);
74 if (Pt == ROOK || Pt == QUEEN)
75 attackers |= attacks_bb<ROOK>(to, occupied) & (byTypeBB[ROOK] | byTypeBB[QUEEN]);
77 // X-ray may add already processed pieces because byTypeBB[] is constant: in
78 // the rook example, now attackers contains _again_ rook in a7, so remove it.
79 attackers &= occupied;
84 PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
85 return KING; // No need to update bitboards: it is the last cycle
91 /// operator<<(Position) returns an ASCII representation of the position
93 std::ostream& operator<<(std::ostream& os, const Position& pos) {
95 os << "\n +---+---+---+---+---+---+---+---+\n";
97 for (Rank r = RANK_8; r >= RANK_1; --r)
99 for (File f = FILE_A; f <= FILE_H; ++f)
100 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
102 os << " |\n +---+---+---+---+---+---+---+---+\n";
105 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
106 << std::setfill('0') << std::setw(16) << pos.key()
107 << std::setfill(' ') << std::dec << "\nCheckers: ";
109 for (Bitboard b = pos.checkers(); b; )
110 os << UCI::square(pop_lsb(&b)) << " ";
112 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
113 && !pos.can_castle(ANY_CASTLING))
117 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
118 Tablebases::ProbeState s1, s2;
119 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
120 int dtz = Tablebases::probe_dtz(p, &s2);
121 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
122 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
129 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
130 // situations. Description of the algorithm in the following paper:
131 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
133 // First and second hash functions for indexing the cuckoo tables
134 inline int H1(Key h) { return h & 0x1fff; }
135 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
137 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
139 Move cuckooMove[8192];
142 /// Position::init() initializes at startup the various arrays used to compute
145 void Position::init() {
149 for (Piece pc : Pieces)
150 for (Square s = SQ_A1; s <= SQ_H8; ++s)
151 Zobrist::psq[pc][s] = rng.rand<Key>();
153 for (File f = FILE_A; f <= FILE_H; ++f)
154 Zobrist::enpassant[f] = rng.rand<Key>();
156 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
158 Zobrist::castling[cr] = 0;
162 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
163 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
167 Zobrist::side = rng.rand<Key>();
168 Zobrist::noPawns = rng.rand<Key>();
170 // Prepare the cuckoo tables
171 std::memset(cuckoo, 0, sizeof(cuckoo));
172 std::memset(cuckooMove, 0, sizeof(cuckooMove));
174 for (Piece pc : Pieces)
175 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
176 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
177 if (PseudoAttacks[type_of(pc)][s1] & s2)
179 Move move = make_move(s1, s2);
180 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
184 std::swap(cuckoo[i], key);
185 std::swap(cuckooMove[i], move);
186 if (move == 0) // Arrived at empty slot ?
188 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
192 assert(count == 3668);
196 /// Position::set() initializes the position object with the given FEN string.
197 /// This function is not very robust - make sure that input FENs are correct,
198 /// this is assumed to be the responsibility of the GUI.
200 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
202 A FEN string defines a particular position using only the ASCII character set.
204 A FEN string contains six fields separated by a space. The fields are:
206 1) Piece placement (from white's perspective). Each rank is described, starting
207 with rank 8 and ending with rank 1. Within each rank, the contents of each
208 square are described from file A through file H. Following the Standard
209 Algebraic Notation (SAN), each piece is identified by a single letter taken
210 from the standard English names. White pieces are designated using upper-case
211 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
212 noted using digits 1 through 8 (the number of blank squares), and "/"
215 2) Active color. "w" means white moves next, "b" means black.
217 3) Castling availability. If neither side can castle, this is "-". Otherwise,
218 this has one or more letters: "K" (White can castle kingside), "Q" (White
219 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
220 can castle queenside).
222 4) En passant target square (in algebraic notation). If there's no en passant
223 target square, this is "-". If a pawn has just made a 2-square move, this
224 is the position "behind" the pawn. This is recorded only if there is a pawn
225 in position to make an en passant capture, and if there really is a pawn
226 that might have advanced two squares.
228 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
229 or capture. This is used to determine if a draw can be claimed under the
232 6) Fullmove number. The number of the full move. It starts at 1, and is
233 incremented after Black's move.
236 unsigned char col, row, token;
239 std::istringstream ss(fenStr);
241 std::memset(this, 0, sizeof(Position));
242 std::memset(si, 0, sizeof(StateInfo));
243 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
248 // 1. Piece placement
249 while ((ss >> token) && !isspace(token))
252 sq += (token - '0') * EAST; // Advance the given number of files
254 else if (token == '/')
257 else if ((idx = PieceToChar.find(token)) != string::npos)
259 put_piece(Piece(idx), sq);
266 sideToMove = (token == 'w' ? WHITE : BLACK);
269 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
270 // Shredder-FEN that uses the letters of the columns on which the rooks began
271 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
272 // if an inner rook is associated with the castling right, the castling tag is
273 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
274 while ((ss >> token) && !isspace(token))
277 Color c = islower(token) ? BLACK : WHITE;
278 Piece rook = make_piece(c, ROOK);
280 token = char(toupper(token));
283 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
285 else if (token == 'Q')
286 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
288 else if (token >= 'A' && token <= 'H')
289 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
294 set_castling_right(c, rsq);
297 // 4. En passant square. Ignore if no pawn capture is possible
298 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
299 && ((ss >> row) && (row == '3' || row == '6')))
301 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
303 if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
304 || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
305 st->epSquare = SQ_NONE;
308 st->epSquare = SQ_NONE;
310 // 5-6. Halfmove clock and fullmove number
311 ss >> std::skipws >> st->rule50 >> gamePly;
313 // Convert from fullmove starting from 1 to gamePly starting from 0,
314 // handle also common incorrect FEN with fullmove = 0.
315 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
317 chess960 = isChess960;
327 /// Position::set_castling_right() is a helper function used to set castling
328 /// rights given the corresponding color and the rook starting square.
330 void Position::set_castling_right(Color c, Square rfrom) {
332 Square kfrom = square<KING>(c);
333 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
334 CastlingRight cr = (c | cs);
336 st->castlingRights |= cr;
337 castlingRightsMask[kfrom] |= cr;
338 castlingRightsMask[rfrom] |= cr;
339 castlingRookSquare[cr] = rfrom;
341 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
342 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
344 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
345 if (s != kfrom && s != rfrom)
346 castlingPath[cr] |= s;
348 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
349 if (s != kfrom && s != rfrom)
350 castlingPath[cr] |= s;
354 /// Position::set_check_info() sets king attacks to detect if a move gives check
356 void Position::set_check_info(StateInfo* si) const {
358 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
359 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
361 Square ksq = square<KING>(~sideToMove);
363 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
364 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
365 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
366 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
367 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
368 si->checkSquares[KING] = 0;
372 /// Position::set_state() computes the hash keys of the position, and other
373 /// data that once computed is updated incrementally as moves are made.
374 /// The function is only used when a new position is set up, and to verify
375 /// the correctness of the StateInfo data when running in debug mode.
377 void Position::set_state(StateInfo* si) const {
379 si->key = si->materialKey = 0;
380 si->pawnKey = Zobrist::noPawns;
381 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
382 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
386 for (Bitboard b = pieces(); b; )
388 Square s = pop_lsb(&b);
389 Piece pc = piece_on(s);
390 si->key ^= Zobrist::psq[pc][s];
393 if (si->epSquare != SQ_NONE)
394 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
396 if (sideToMove == BLACK)
397 si->key ^= Zobrist::side;
399 si->key ^= Zobrist::castling[si->castlingRights];
401 for (Bitboard b = pieces(PAWN); b; )
403 Square s = pop_lsb(&b);
404 si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
407 for (Piece pc : Pieces)
409 if (type_of(pc) != PAWN && type_of(pc) != KING)
410 si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
412 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
413 si->materialKey ^= Zobrist::psq[pc][cnt];
418 /// Position::set() is an overload to initialize the position object with
419 /// the given endgame code string like "KBPKN". It is mainly a helper to
420 /// get the material key out of an endgame code.
422 Position& Position::set(const string& code, Color c, StateInfo* si) {
424 assert(code.length() > 0 && code.length() < 8);
425 assert(code[0] == 'K');
427 string sides[] = { code.substr(code.find('K', 1)), // Weak
428 code.substr(0, code.find('K', 1)) }; // Strong
430 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
432 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
433 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
435 return set(fenStr, false, si, nullptr);
439 /// Position::fen() returns a FEN representation of the position. In case of
440 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
442 const string Position::fen() const {
445 std::ostringstream ss;
447 for (Rank r = RANK_8; r >= RANK_1; --r)
449 for (File f = FILE_A; f <= FILE_H; ++f)
451 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
458 ss << PieceToChar[piece_on(make_square(f, r))];
465 ss << (sideToMove == WHITE ? " w " : " b ");
467 if (can_castle(WHITE_OO))
468 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | KING_SIDE))) : 'K');
470 if (can_castle(WHITE_OOO))
471 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
473 if (can_castle(BLACK_OO))
474 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | KING_SIDE))) : 'k');
476 if (can_castle(BLACK_OOO))
477 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
479 if (!can_castle(ANY_CASTLING))
482 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
483 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
489 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
490 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
491 /// slider if removing that piece from the board would result in a position where
492 /// square 's' is attacked. For example, a king-attack blocking piece can be either
493 /// a pinned or a discovered check piece, according if its color is the opposite
494 /// or the same of the color of the slider.
496 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
498 Bitboard blockers = 0;
501 // Snipers are sliders that attack 's' when a piece is removed
502 Bitboard snipers = ( (PseudoAttacks[ ROOK][s] & pieces(QUEEN, ROOK))
503 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
507 Square sniperSq = pop_lsb(&snipers);
508 Bitboard b = between_bb(s, sniperSq) & pieces();
510 if (b && !more_than_one(b))
513 if (b & pieces(color_of(piece_on(s))))
521 /// Position::attackers_to() computes a bitboard of all pieces which attack a
522 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
524 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
526 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
527 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
528 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
529 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
530 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
531 | (attacks_from<KING>(s) & pieces(KING));
535 /// Position::legal() tests whether a pseudo-legal move is legal
537 bool Position::legal(Move m) const {
541 Color us = sideToMove;
542 Square from = from_sq(m);
544 assert(color_of(moved_piece(m)) == us);
545 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
547 // En passant captures are a tricky special case. Because they are rather
548 // uncommon, we do it simply by testing whether the king is attacked after
550 if (type_of(m) == ENPASSANT)
552 Square ksq = square<KING>(us);
553 Square to = to_sq(m);
554 Square capsq = to - pawn_push(us);
555 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
557 assert(to == ep_square());
558 assert(moved_piece(m) == make_piece(us, PAWN));
559 assert(piece_on(capsq) == make_piece(~us, PAWN));
560 assert(piece_on(to) == NO_PIECE);
562 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
563 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
566 // If the moving piece is a king, check whether the destination
567 // square is attacked by the opponent. Castling moves are checked
568 // for legality during move generation.
569 if (type_of(piece_on(from)) == KING)
570 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
572 // A non-king move is legal if and only if it is not pinned or it
573 // is moving along the ray towards or away from the king.
574 return !(blockers_for_king(us) & from)
575 || aligned(from, to_sq(m), square<KING>(us));
579 /// Position::pseudo_legal() takes a random move and tests whether the move is
580 /// pseudo legal. It is used to validate moves from TT that can be corrupted
581 /// due to SMP concurrent access or hash position key aliasing.
583 bool Position::pseudo_legal(const Move m) const {
585 Color us = sideToMove;
586 Square from = from_sq(m);
587 Square to = to_sq(m);
588 Piece pc = moved_piece(m);
590 // Use a slower but simpler function for uncommon cases
591 if (type_of(m) != NORMAL)
592 return MoveList<LEGAL>(*this).contains(m);
594 // Is not a promotion, so promotion piece must be empty
595 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
598 // If the 'from' square is not occupied by a piece belonging to the side to
599 // move, the move is obviously not legal.
600 if (pc == NO_PIECE || color_of(pc) != us)
603 // The destination square cannot be occupied by a friendly piece
607 // Handle the special case of a pawn move
608 if (type_of(pc) == PAWN)
610 // We have already handled promotion moves, so destination
611 // cannot be on the 8th/1st rank.
612 if (rank_of(to) == relative_rank(us, RANK_8))
615 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
616 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
617 && !( (from + 2 * pawn_push(us) == to) // Not a double push
618 && (rank_of(from) == relative_rank(us, RANK_2))
620 && empty(to - pawn_push(us))))
623 else if (!(attacks_from(type_of(pc), from) & to))
626 // Evasions generator already takes care to avoid some kind of illegal moves
627 // and legal() relies on this. We therefore have to take care that the same
628 // kind of moves are filtered out here.
631 if (type_of(pc) != KING)
633 // Double check? In this case a king move is required
634 if (more_than_one(checkers()))
637 // Our move must be a blocking evasion or a capture of the checking piece
638 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
641 // In case of king moves under check we have to remove king so as to catch
642 // invalid moves like b1a1 when opposite queen is on c1.
643 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
651 /// Position::gives_check() tests whether a pseudo-legal move gives a check
653 bool Position::gives_check(Move m) const {
656 assert(color_of(moved_piece(m)) == sideToMove);
658 Square from = from_sq(m);
659 Square to = to_sq(m);
661 // Is there a direct check?
662 if (st->checkSquares[type_of(piece_on(from))] & to)
665 // Is there a discovered check?
666 if ( (st->blockersForKing[~sideToMove] & from)
667 && !aligned(from, to, square<KING>(~sideToMove)))
676 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
678 // En passant capture with check? We have already handled the case
679 // of direct checks and ordinary discovered check, so the only case we
680 // need to handle is the unusual case of a discovered check through
681 // the captured pawn.
684 Square capsq = make_square(file_of(to), rank_of(from));
685 Bitboard b = (pieces() ^ from ^ capsq) | to;
687 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
688 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
693 Square rfrom = to; // Castling is encoded as 'King captures the rook'
694 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
695 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
697 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
698 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
707 /// Position::do_move() makes a move, and saves all information necessary
708 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
709 /// moves should be filtered out before this function is called.
711 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
714 assert(&newSt != st);
716 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
717 Key k = st->key ^ Zobrist::side;
719 // Copy some fields of the old state to our new StateInfo object except the
720 // ones which are going to be recalculated from scratch anyway and then switch
721 // our state pointer to point to the new (ready to be updated) state.
722 std::memcpy(&newSt, st, offsetof(StateInfo, key));
726 // Increment ply counters. In particular, rule50 will be reset to zero later on
727 // in case of a capture or a pawn move.
732 Color us = sideToMove;
734 Square from = from_sq(m);
735 Square to = to_sq(m);
736 Piece pc = piece_on(from);
737 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
739 assert(color_of(pc) == us);
740 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
741 assert(type_of(captured) != KING);
743 if (type_of(m) == CASTLING)
745 assert(pc == make_piece(us, KING));
746 assert(captured == make_piece(us, ROOK));
749 do_castling<true>(us, from, to, rfrom, rto);
751 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
759 // If the captured piece is a pawn, update pawn hash key, otherwise
760 // update non-pawn material.
761 if (type_of(captured) == PAWN)
763 if (type_of(m) == ENPASSANT)
765 capsq -= pawn_push(us);
767 assert(pc == make_piece(us, PAWN));
768 assert(to == st->epSquare);
769 assert(relative_rank(us, to) == RANK_6);
770 assert(piece_on(to) == NO_PIECE);
771 assert(piece_on(capsq) == make_piece(them, PAWN));
773 board[capsq] = NO_PIECE; // Not done by remove_piece()
776 st->pawnKey ^= Zobrist::psq[captured][capsq];
779 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
781 // Update board and piece lists
782 remove_piece(captured, capsq);
784 // Update material hash key and prefetch access to materialTable
785 k ^= Zobrist::psq[captured][capsq];
786 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
787 prefetch(thisThread->materialTable[st->materialKey]);
789 // Reset rule 50 counter
794 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
796 // Reset en passant square
797 if (st->epSquare != SQ_NONE)
799 k ^= Zobrist::enpassant[file_of(st->epSquare)];
800 st->epSquare = SQ_NONE;
803 // Update castling rights if needed
804 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
806 int cr = castlingRightsMask[from] | castlingRightsMask[to];
807 k ^= Zobrist::castling[st->castlingRights & cr];
808 st->castlingRights &= ~cr;
811 // Move the piece. The tricky Chess960 castling is handled earlier
812 if (type_of(m) != CASTLING)
813 move_piece(pc, from, to);
815 // If the moving piece is a pawn do some special extra work
816 if (type_of(pc) == PAWN)
818 // Set en-passant square if the moved pawn can be captured
819 if ( (int(to) ^ int(from)) == 16
820 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
822 st->epSquare = to - pawn_push(us);
823 k ^= Zobrist::enpassant[file_of(st->epSquare)];
826 else if (type_of(m) == PROMOTION)
828 Piece promotion = make_piece(us, promotion_type(m));
830 assert(relative_rank(us, to) == RANK_8);
831 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
833 remove_piece(pc, to);
834 put_piece(promotion, to);
837 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
838 st->pawnKey ^= Zobrist::psq[pc][to];
839 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
840 ^ Zobrist::psq[pc][pieceCount[pc]];
843 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
846 // Update pawn hash key and prefetch access to pawnsTable
847 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
848 prefetch2(thisThread->pawnsTable[st->pawnKey]);
850 // Reset rule 50 draw counter
855 st->capturedPiece = captured;
857 // Update the key with the final value
860 // Calculate checkers bitboard (if move gives check)
861 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
863 sideToMove = ~sideToMove;
865 // Update king attacks used for fast check detection
872 /// Position::undo_move() unmakes a move. When it returns, the position should
873 /// be restored to exactly the same state as before the move was made.
875 void Position::undo_move(Move m) {
879 sideToMove = ~sideToMove;
881 Color us = sideToMove;
882 Square from = from_sq(m);
883 Square to = to_sq(m);
884 Piece pc = piece_on(to);
886 assert(empty(from) || type_of(m) == CASTLING);
887 assert(type_of(st->capturedPiece) != KING);
889 if (type_of(m) == PROMOTION)
891 assert(relative_rank(us, to) == RANK_8);
892 assert(type_of(pc) == promotion_type(m));
893 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
895 remove_piece(pc, to);
896 pc = make_piece(us, PAWN);
900 if (type_of(m) == CASTLING)
903 do_castling<false>(us, from, to, rfrom, rto);
907 move_piece(pc, to, from); // Put the piece back at the source square
909 if (st->capturedPiece)
913 if (type_of(m) == ENPASSANT)
915 capsq -= pawn_push(us);
917 assert(type_of(pc) == PAWN);
918 assert(to == st->previous->epSquare);
919 assert(relative_rank(us, to) == RANK_6);
920 assert(piece_on(capsq) == NO_PIECE);
921 assert(st->capturedPiece == make_piece(~us, PAWN));
924 put_piece(st->capturedPiece, capsq); // Restore the captured piece
928 // Finally point our state pointer back to the previous state
936 /// Position::do_castling() is a helper used to do/undo a castling move. This
937 /// is a bit tricky in Chess960 where from/to squares can overlap.
939 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
941 bool kingSide = to > from;
942 rfrom = to; // Castling is encoded as "king captures friendly rook"
943 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
944 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
946 // Remove both pieces first since squares could overlap in Chess960
947 remove_piece(make_piece(us, KING), Do ? from : to);
948 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
949 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
950 put_piece(make_piece(us, KING), Do ? to : from);
951 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
955 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
956 /// the side to move without executing any move on the board.
958 void Position::do_null_move(StateInfo& newSt) {
961 assert(&newSt != st);
963 std::memcpy(&newSt, st, sizeof(StateInfo));
967 if (st->epSquare != SQ_NONE)
969 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
970 st->epSquare = SQ_NONE;
973 st->key ^= Zobrist::side;
974 prefetch(TT.first_entry(st->key));
977 st->pliesFromNull = 0;
979 sideToMove = ~sideToMove;
986 void Position::undo_null_move() {
991 sideToMove = ~sideToMove;
995 /// Position::key_after() computes the new hash key after the given move. Needed
996 /// for speculative prefetch. It doesn't recognize special moves like castling,
997 /// en-passant and promotions.
999 Key Position::key_after(Move m) const {
1001 Square from = from_sq(m);
1002 Square to = to_sq(m);
1003 Piece pc = piece_on(from);
1004 Piece captured = piece_on(to);
1005 Key k = st->key ^ Zobrist::side;
1008 k ^= Zobrist::psq[captured][to];
1010 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1014 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1015 /// SEE value of move is greater or equal to the given threshold. We'll use an
1016 /// algorithm similar to alpha-beta pruning with a null window.
1018 bool Position::see_ge(Move m, Value threshold) const {
1022 // Only deal with normal moves, assume others pass a simple see
1023 if (type_of(m) != NORMAL)
1024 return VALUE_ZERO >= threshold;
1026 Bitboard stmAttackers;
1027 Square from = from_sq(m), to = to_sq(m);
1028 PieceType nextVictim = type_of(piece_on(from));
1029 Color us = color_of(piece_on(from));
1030 Color stm = ~us; // First consider opponent's move
1031 Value balance; // Values of the pieces taken by us minus opponent's ones
1033 // The opponent may be able to recapture so this is the best result
1035 balance = PieceValue[MG][piece_on(to)] - threshold;
1037 if (balance < VALUE_ZERO)
1040 // Now assume the worst possible result: that the opponent can
1041 // capture our piece for free.
1042 balance -= PieceValue[MG][nextVictim];
1044 // If it is enough (like in PxQ) then return immediately. Note that
1045 // in case nextVictim == KING we always return here, this is ok
1046 // if the given move is legal.
1047 if (balance >= VALUE_ZERO)
1050 // Find all attackers to the destination square, with the moving piece
1051 // removed, but possibly an X-ray attacker added behind it.
1052 Bitboard occupied = pieces() ^ from ^ to;
1053 Bitboard attackers = attackers_to(to, occupied) & occupied;
1057 stmAttackers = attackers & pieces(stm);
1059 // Don't allow pinned pieces to attack (except the king) as long as
1060 // all pinners are on their original square.
1061 if (!(st->pinners[~stm] & ~occupied))
1062 stmAttackers &= ~st->blockersForKing[stm];
1064 // If stm has no more attackers then give up: stm loses
1068 // Locate and remove the next least valuable attacker, and add to
1069 // the bitboard 'attackers' the possibly X-ray attackers behind it.
1070 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1072 stm = ~stm; // Switch side to move
1074 // Negamax the balance with alpha = balance, beta = balance+1 and
1075 // add nextVictim's value.
1077 // (balance, balance+1) -> (-balance-1, -balance)
1079 assert(balance < VALUE_ZERO);
1081 balance = -balance - 1 - PieceValue[MG][nextVictim];
1083 // If balance is still non-negative after giving away nextVictim then we
1084 // win. The only thing to be careful about it is that we should revert
1085 // stm if we captured with the king when the opponent still has attackers.
1086 if (balance >= VALUE_ZERO)
1088 if (nextVictim == KING && (attackers & pieces(stm)))
1092 assert(nextVictim != KING);
1094 return us != stm; // We break the above loop when stm loses
1098 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1099 /// or by repetition. It does not detect stalemates.
1101 bool Position::is_draw(int ply) const {
1103 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1106 int end = std::min(st->rule50, st->pliesFromNull);
1111 StateInfo* stp = st->previous->previous;
1114 for (int i = 4; i <= end; i += 2)
1116 stp = stp->previous->previous;
1118 // Return a draw score if a position repeats once earlier but strictly
1119 // after the root, or repeats twice before or at the root.
1120 if ( stp->key == st->key
1121 && ++cnt + (ply > i) == 2)
1129 // Position::has_repeated() tests whether there has been at least one repetition
1130 // of positions since the last capture or pawn move.
1132 bool Position::has_repeated() const {
1134 StateInfo* stc = st;
1137 int i = 4, end = std::min(stc->rule50, stc->pliesFromNull);
1142 StateInfo* stp = stc->previous->previous;
1145 stp = stp->previous->previous;
1147 if (stp->key == stc->key)
1153 stc = stc->previous;
1158 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1159 /// or an earlier position has a move that directly reaches the current position.
1161 bool Position::has_game_cycle(int ply) const {
1165 int end = std::min(st->rule50, st->pliesFromNull);
1170 Key originalKey = st->key;
1171 StateInfo* stp = st->previous;
1173 for (int i = 3; i <= end; i += 2)
1175 stp = stp->previous->previous;
1177 Key moveKey = originalKey ^ stp->key;
1178 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1179 || (j = H2(moveKey), cuckoo[j] == moveKey))
1181 Move move = cuckooMove[j];
1182 Square s1 = from_sq(move);
1183 Square s2 = to_sq(move);
1185 if (!(between_bb(s1, s2) & pieces()))
1187 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in the same
1188 // location. We select the legal one by reversing the move variable if necessary.
1190 move = make_move(s2, s1);
1195 // For repetitions before or at the root, require one more
1196 StateInfo* next_stp = stp;
1197 for (int k = i + 2; k <= end; k += 2)
1199 next_stp = next_stp->previous->previous;
1200 if (next_stp->key == stp->key)
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; c <= BLACK; ++c)
1301 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
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");