2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
21 #include <cstddef> // For offsetof()
22 #include <cstring> // For std::memset, std::memcmp
33 #include "syzygy/tbprobe.h"
39 Key psq[PIECE_NB][SQUARE_NB];
40 Key enpassant[FILE_NB];
41 Key castling[CASTLING_RIGHT_NB];
47 const string PieceToChar(" PNBRQK pnbrqk");
49 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
50 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
54 /// operator<<(Position) returns an ASCII representation of the position
56 std::ostream& operator<<(std::ostream& os, const Position& pos) {
58 os << "\n +---+---+---+---+---+---+---+---+\n";
60 for (Rank r = RANK_8; r >= RANK_1; --r)
62 for (File f = FILE_A; f <= FILE_H; ++f)
63 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
65 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
68 os << " a b c d e f g h\n"
69 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
70 << std::setfill('0') << std::setw(16) << pos.key()
71 << std::setfill(' ') << std::dec << "\nCheckers: ";
73 for (Bitboard b = pos.checkers(); b; )
74 os << UCI::square(pop_lsb(&b)) << " ";
76 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
77 && !pos.can_castle(ANY_CASTLING))
81 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
82 Tablebases::ProbeState s1, s2;
83 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
84 int dtz = Tablebases::probe_dtz(p, &s2);
85 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
86 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
93 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
94 // situations. Description of the algorithm in the following paper:
95 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
97 // First and second hash functions for indexing the cuckoo tables
98 inline int H1(Key h) { return h & 0x1fff; }
99 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
101 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
103 Move cuckooMove[8192];
106 /// Position::init() initializes at startup the various arrays used to compute hash keys
108 void Position::init() {
112 for (Piece pc : Pieces)
113 for (Square s = SQ_A1; s <= SQ_H8; ++s)
114 Zobrist::psq[pc][s] = rng.rand<Key>();
116 for (File f = FILE_A; f <= FILE_H; ++f)
117 Zobrist::enpassant[f] = rng.rand<Key>();
119 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
120 Zobrist::castling[cr] = rng.rand<Key>();
122 Zobrist::side = rng.rand<Key>();
123 Zobrist::noPawns = rng.rand<Key>();
125 // Prepare the cuckoo tables
126 std::memset(cuckoo, 0, sizeof(cuckoo));
127 std::memset(cuckooMove, 0, sizeof(cuckooMove));
129 for (Piece pc : Pieces)
130 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
131 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
132 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
134 Move move = make_move(s1, s2);
135 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
139 std::swap(cuckoo[i], key);
140 std::swap(cuckooMove[i], move);
141 if (move == MOVE_NONE) // Arrived at empty slot?
143 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
147 assert(count == 3668);
151 /// Position::set() initializes the position object with the given FEN string.
152 /// This function is not very robust - make sure that input FENs are correct,
153 /// this is assumed to be the responsibility of the GUI.
155 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
157 A FEN string defines a particular position using only the ASCII character set.
159 A FEN string contains six fields separated by a space. The fields are:
161 1) Piece placement (from white's perspective). Each rank is described, starting
162 with rank 8 and ending with rank 1. Within each rank, the contents of each
163 square are described from file A through file H. Following the Standard
164 Algebraic Notation (SAN), each piece is identified by a single letter taken
165 from the standard English names. White pieces are designated using upper-case
166 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
167 noted using digits 1 through 8 (the number of blank squares), and "/"
170 2) Active color. "w" means white moves next, "b" means black.
172 3) Castling availability. If neither side can castle, this is "-". Otherwise,
173 this has one or more letters: "K" (White can castle kingside), "Q" (White
174 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
175 can castle queenside).
177 4) En passant target square (in algebraic notation). If there's no en passant
178 target square, this is "-". If a pawn has just made a 2-square move, this
179 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
180 if there is a pawn in position to make an en passant capture, and if there really
181 is a pawn that might have advanced two squares.
183 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
184 or capture. This is used to determine if a draw can be claimed under the
187 6) Fullmove number. The number of the full move. It starts at 1, and is
188 incremented after Black's move.
191 unsigned char col, row, token;
194 std::istringstream ss(fenStr);
196 std::memset(this, 0, sizeof(Position));
197 std::memset(si, 0, sizeof(StateInfo));
198 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
203 // 1. Piece placement
204 while ((ss >> token) && !isspace(token))
207 sq += (token - '0') * EAST; // Advance the given number of files
209 else if (token == '/')
212 else if ((idx = PieceToChar.find(token)) != string::npos) {
213 put_piece(Piece(idx), sq);
220 sideToMove = (token == 'w' ? WHITE : BLACK);
223 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
224 // Shredder-FEN that uses the letters of the columns on which the rooks began
225 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
226 // if an inner rook is associated with the castling right, the castling tag is
227 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
228 while ((ss >> token) && !isspace(token))
231 Color c = islower(token) ? BLACK : WHITE;
232 Piece rook = make_piece(c, ROOK);
234 token = char(toupper(token));
237 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
239 else if (token == 'Q')
240 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
242 else if (token >= 'A' && token <= 'H')
243 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
248 set_castling_right(c, rsq);
251 // 4. En passant square.
252 // Ignore if square is invalid or not on side to move relative rank 6.
253 bool enpassant = false;
255 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
256 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
258 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
260 // En passant square will be considered only if
261 // a) side to move have a pawn threatening epSquare
262 // b) there is an enemy pawn in front of epSquare
263 // c) there is no piece on epSquare or behind epSquare
264 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
265 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
266 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
270 st->epSquare = SQ_NONE;
272 // 5-6. Halfmove clock and fullmove number
273 ss >> std::skipws >> st->rule50 >> gamePly;
275 // Convert from fullmove starting from 1 to gamePly starting from 0,
276 // handle also common incorrect FEN with fullmove = 0.
277 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
279 chess960 = isChess960;
289 /// Position::set_castling_right() is a helper function used to set castling
290 /// rights given the corresponding color and the rook starting square.
292 void Position::set_castling_right(Color c, Square rfrom) {
294 Square kfrom = square<KING>(c);
295 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
297 st->castlingRights |= cr;
298 castlingRightsMask[kfrom] |= cr;
299 castlingRightsMask[rfrom] |= cr;
300 castlingRookSquare[cr] = rfrom;
302 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
303 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
305 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
310 /// Position::set_check_info() sets king attacks to detect if a move gives check
312 void Position::set_check_info(StateInfo* si) const {
314 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
315 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
317 Square ksq = square<KING>(~sideToMove);
319 si->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
320 si->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
321 si->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
322 si->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
323 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
324 si->checkSquares[KING] = 0;
328 /// Position::set_state() computes the hash keys of the position, and other
329 /// data that once computed is updated incrementally as moves are made.
330 /// The function is only used when a new position is set up, and to verify
331 /// the correctness of the StateInfo data when running in debug mode.
333 void Position::set_state(StateInfo* si) const {
335 si->key = si->materialKey = 0;
336 si->pawnKey = Zobrist::noPawns;
337 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
338 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
342 for (Bitboard b = pieces(); b; )
344 Square s = pop_lsb(&b);
345 Piece pc = piece_on(s);
346 si->key ^= Zobrist::psq[pc][s];
348 if (type_of(pc) == PAWN)
349 si->pawnKey ^= Zobrist::psq[pc][s];
351 else if (type_of(pc) != KING)
352 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
355 if (si->epSquare != SQ_NONE)
356 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
358 if (sideToMove == BLACK)
359 si->key ^= Zobrist::side;
361 si->key ^= Zobrist::castling[si->castlingRights];
363 for (Piece pc : Pieces)
364 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
365 si->materialKey ^= Zobrist::psq[pc][cnt];
369 /// Position::set() is an overload to initialize the position object with
370 /// the given endgame code string like "KBPKN". It is mainly a helper to
371 /// get the material key out of an endgame code.
373 Position& Position::set(const string& code, Color c, StateInfo* si) {
375 assert(code[0] == 'K');
377 string sides[] = { code.substr(code.find('K', 1)), // Weak
378 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
380 assert(sides[0].length() > 0 && sides[0].length() < 8);
381 assert(sides[1].length() > 0 && sides[1].length() < 8);
383 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
385 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
386 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
388 return set(fenStr, false, si, nullptr);
392 /// Position::fen() returns a FEN representation of the position. In case of
393 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
395 const string Position::fen() const {
398 std::ostringstream ss;
400 for (Rank r = RANK_8; r >= RANK_1; --r)
402 for (File f = FILE_A; f <= FILE_H; ++f)
404 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
411 ss << PieceToChar[piece_on(make_square(f, r))];
418 ss << (sideToMove == WHITE ? " w " : " b ");
420 if (can_castle(WHITE_OO))
421 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
423 if (can_castle(WHITE_OOO))
424 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
426 if (can_castle(BLACK_OO))
427 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
429 if (can_castle(BLACK_OOO))
430 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
432 if (!can_castle(ANY_CASTLING))
435 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
436 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
442 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
443 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
444 /// slider if removing that piece from the board would result in a position where
445 /// square 's' is attacked. For example, a king-attack blocking piece can be either
446 /// a pinned or a discovered check piece, according if its color is the opposite
447 /// or the same of the color of the slider.
449 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
451 Bitboard blockers = 0;
454 // Snipers are sliders that attack 's' when a piece and other snipers are removed
455 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
456 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
457 Bitboard occupancy = pieces() ^ snipers;
461 Square sniperSq = pop_lsb(&snipers);
462 Bitboard b = between_bb(s, sniperSq) & occupancy;
464 if (b && !more_than_one(b))
467 if (b & pieces(color_of(piece_on(s))))
475 /// Position::attackers_to() computes a bitboard of all pieces which attack a
476 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
478 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
480 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
481 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
482 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
483 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
484 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
485 | (attacks_bb<KING>(s) & pieces(KING));
489 /// Position::legal() tests whether a pseudo-legal move is legal
491 bool Position::legal(Move m) const {
495 Color us = sideToMove;
496 Square from = from_sq(m);
497 Square to = to_sq(m);
499 assert(color_of(moved_piece(m)) == us);
500 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
502 // En passant captures are a tricky special case. Because they are rather
503 // uncommon, we do it simply by testing whether the king is attacked after
505 if (type_of(m) == ENPASSANT)
507 Square ksq = square<KING>(us);
508 Square capsq = to - pawn_push(us);
509 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
511 assert(to == ep_square());
512 assert(moved_piece(m) == make_piece(us, PAWN));
513 assert(piece_on(capsq) == make_piece(~us, PAWN));
514 assert(piece_on(to) == NO_PIECE);
516 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
517 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
520 // Castling moves generation does not check if the castling path is clear of
521 // enemy attacks, it is delayed at a later time: now!
522 if (type_of(m) == CASTLING)
524 // After castling, the rook and king final positions are the same in
525 // Chess960 as they would be in standard chess.
526 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
527 Direction step = to > from ? WEST : EAST;
529 for (Square s = to; s != from; s += step)
530 if (attackers_to(s) & pieces(~us))
533 // In case of Chess960, verify that when moving the castling rook we do
534 // not discover some hidden checker.
535 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
537 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
540 // If the moving piece is a king, check whether the destination square is
541 // attacked by the opponent.
542 if (type_of(piece_on(from)) == KING)
543 return !(attackers_to(to) & pieces(~us));
545 // A non-king move is legal if and only if it is not pinned or it
546 // is moving along the ray towards or away from the king.
547 return !(blockers_for_king(us) & from)
548 || aligned(from, to, square<KING>(us));
552 /// Position::pseudo_legal() takes a random move and tests whether the move is
553 /// pseudo legal. It is used to validate moves from TT that can be corrupted
554 /// due to SMP concurrent access or hash position key aliasing.
556 bool Position::pseudo_legal(const Move m) const {
558 Color us = sideToMove;
559 Square from = from_sq(m);
560 Square to = to_sq(m);
561 Piece pc = moved_piece(m);
563 // Use a slower but simpler function for uncommon cases
564 if (type_of(m) != NORMAL)
565 return MoveList<LEGAL>(*this).contains(m);
567 // Is not a promotion, so promotion piece must be empty
568 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
571 // If the 'from' square is not occupied by a piece belonging to the side to
572 // move, the move is obviously not legal.
573 if (pc == NO_PIECE || color_of(pc) != us)
576 // The destination square cannot be occupied by a friendly piece
580 // Handle the special case of a pawn move
581 if (type_of(pc) == PAWN)
583 // We have already handled promotion moves, so destination
584 // cannot be on the 8th/1st rank.
585 if ((Rank8BB | Rank1BB) & to)
588 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
589 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
590 && !( (from + 2 * pawn_push(us) == to) // Not a double push
591 && (relative_rank(us, from) == RANK_2)
593 && empty(to - pawn_push(us))))
596 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
599 // Evasions generator already takes care to avoid some kind of illegal moves
600 // and legal() relies on this. We therefore have to take care that the same
601 // kind of moves are filtered out here.
604 if (type_of(pc) != KING)
606 // Double check? In this case a king move is required
607 if (more_than_one(checkers()))
610 // Our move must be a blocking evasion or a capture of the checking piece
611 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
614 // In case of king moves under check we have to remove king so as to catch
615 // invalid moves like b1a1 when opposite queen is on c1.
616 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
624 /// Position::gives_check() tests whether a pseudo-legal move gives a check
626 bool Position::gives_check(Move m) const {
629 assert(color_of(moved_piece(m)) == sideToMove);
631 Square from = from_sq(m);
632 Square to = to_sq(m);
634 // Is there a direct check?
635 if (check_squares(type_of(piece_on(from))) & to)
638 // Is there a discovered check?
639 if ( (blockers_for_king(~sideToMove) & from)
640 && !aligned(from, to, square<KING>(~sideToMove)))
649 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
651 // En passant capture with check? We have already handled the case
652 // of direct checks and ordinary discovered check, so the only case we
653 // need to handle is the unusual case of a discovered check through
654 // the captured pawn.
657 Square capsq = make_square(file_of(to), rank_of(from));
658 Bitboard b = (pieces() ^ from ^ capsq) | to;
660 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
661 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
666 Square rfrom = to; // Castling is encoded as 'king captures the rook'
667 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
668 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
670 return (attacks_bb<ROOK>(rto) & square<KING>(~sideToMove))
671 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
680 /// Position::do_move() makes a move, and saves all information necessary
681 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
682 /// moves should be filtered out before this function is called.
684 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
687 assert(&newSt != st);
689 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
690 Key k = st->key ^ Zobrist::side;
692 // Copy some fields of the old state to our new StateInfo object except the
693 // ones which are going to be recalculated from scratch anyway and then switch
694 // our state pointer to point to the new (ready to be updated) state.
695 std::memcpy(&newSt, st, offsetof(StateInfo, key));
699 // Increment ply counters. In particular, rule50 will be reset to zero later on
700 // in case of a capture or a pawn move.
706 st->accumulator.computed_accumulation = false;
707 st->accumulator.computed_score = false;
708 auto& dp = st->dirtyPiece;
711 Color us = sideToMove;
713 Square from = from_sq(m);
714 Square to = to_sq(m);
715 Piece pc = piece_on(from);
716 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
718 assert(color_of(pc) == us);
719 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
720 assert(type_of(captured) != KING);
722 if (type_of(m) == CASTLING)
724 assert(pc == make_piece(us, KING));
725 assert(captured == make_piece(us, ROOK));
728 do_castling<true>(us, from, to, rfrom, rto);
730 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
738 // If the captured piece is a pawn, update pawn hash key, otherwise
739 // update non-pawn material.
740 if (type_of(captured) == PAWN)
742 if (type_of(m) == ENPASSANT)
744 capsq -= pawn_push(us);
746 assert(pc == make_piece(us, PAWN));
747 assert(to == st->epSquare);
748 assert(relative_rank(us, to) == RANK_6);
749 assert(piece_on(to) == NO_PIECE);
750 assert(piece_on(capsq) == make_piece(them, PAWN));
753 st->pawnKey ^= Zobrist::psq[captured][capsq];
756 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
760 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
761 dp.piece[1] = captured;
766 // Update board and piece lists
769 if (type_of(m) == ENPASSANT)
770 board[capsq] = NO_PIECE;
772 // Update material hash key and prefetch access to materialTable
773 k ^= Zobrist::psq[captured][capsq];
774 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
775 prefetch(thisThread->materialTable[st->materialKey]);
777 // Reset rule 50 counter
782 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
784 // Reset en passant square
785 if (st->epSquare != SQ_NONE)
787 k ^= Zobrist::enpassant[file_of(st->epSquare)];
788 st->epSquare = SQ_NONE;
791 // Update castling rights if needed
792 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
794 k ^= Zobrist::castling[st->castlingRights];
795 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
796 k ^= Zobrist::castling[st->castlingRights];
799 // Move the piece. The tricky Chess960 castling is handled earlier
800 if (type_of(m) != CASTLING)
809 move_piece(from, to);
812 // If the moving piece is a pawn do some special extra work
813 if (type_of(pc) == PAWN)
815 // Set en-passant square if the moved pawn can be captured
816 if ( (int(to) ^ int(from)) == 16
817 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
819 st->epSquare = to - pawn_push(us);
820 k ^= Zobrist::enpassant[file_of(st->epSquare)];
823 else if (type_of(m) == PROMOTION)
825 Piece promotion = make_piece(us, promotion_type(m));
827 assert(relative_rank(us, to) == RANK_8);
828 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
831 put_piece(promotion, to);
835 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
837 dp.piece[dp.dirty_num] = promotion;
838 dp.from[dp.dirty_num] = SQ_NONE;
839 dp.to[dp.dirty_num] = to;
844 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
845 st->pawnKey ^= Zobrist::psq[pc][to];
846 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
847 ^ Zobrist::psq[pc][pieceCount[pc]];
850 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
853 // Update pawn hash key
854 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
856 // Reset rule 50 draw counter
861 st->capturedPiece = captured;
863 // Update the key with the final value
866 // Calculate checkers bitboard (if move gives check)
867 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
869 sideToMove = ~sideToMove;
871 // Update king attacks used for fast check detection
874 // Calculate the repetition info. It is the ply distance from the previous
875 // occurrence of the same position, negative in the 3-fold case, or zero
876 // if the position was not repeated.
878 int end = std::min(st->rule50, st->pliesFromNull);
881 StateInfo* stp = st->previous->previous;
882 for (int i = 4; i <= end; i += 2)
884 stp = stp->previous->previous;
885 if (stp->key == st->key)
887 st->repetition = stp->repetition ? -i : i;
897 /// Position::undo_move() unmakes a move. When it returns, the position should
898 /// be restored to exactly the same state as before the move was made.
900 void Position::undo_move(Move m) {
904 sideToMove = ~sideToMove;
906 Color us = sideToMove;
907 Square from = from_sq(m);
908 Square to = to_sq(m);
909 Piece pc = piece_on(to);
911 assert(empty(from) || type_of(m) == CASTLING);
912 assert(type_of(st->capturedPiece) != KING);
914 if (type_of(m) == PROMOTION)
916 assert(relative_rank(us, to) == RANK_8);
917 assert(type_of(pc) == promotion_type(m));
918 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
921 pc = make_piece(us, PAWN);
925 if (type_of(m) == CASTLING)
928 do_castling<false>(us, from, to, rfrom, rto);
932 move_piece(to, from); // Put the piece back at the source square
934 if (st->capturedPiece)
938 if (type_of(m) == ENPASSANT)
940 capsq -= pawn_push(us);
942 assert(type_of(pc) == PAWN);
943 assert(to == st->previous->epSquare);
944 assert(relative_rank(us, to) == RANK_6);
945 assert(piece_on(capsq) == NO_PIECE);
946 assert(st->capturedPiece == make_piece(~us, PAWN));
949 put_piece(st->capturedPiece, capsq); // Restore the captured piece
953 // Finally point our state pointer back to the previous state
961 /// Position::do_castling() is a helper used to do/undo a castling move. This
962 /// is a bit tricky in Chess960 where from/to squares can overlap.
964 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
966 bool kingSide = to > from;
967 rfrom = to; // Castling is encoded as "king captures friendly rook"
968 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
969 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
971 if (Do && Eval::useNNUE)
973 auto& dp = st->dirtyPiece;
974 dp.piece[0] = make_piece(us, KING);
977 dp.piece[1] = make_piece(us, ROOK);
983 // Remove both pieces first since squares could overlap in Chess960
984 remove_piece(Do ? from : to);
985 remove_piece(Do ? rfrom : rto);
986 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
987 put_piece(make_piece(us, KING), Do ? to : from);
988 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
992 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
993 /// the side to move without executing any move on the board.
995 void Position::do_null_move(StateInfo& newSt) {
998 assert(&newSt != st);
1002 std::memcpy(&newSt, st, sizeof(StateInfo));
1003 st->accumulator.computed_score = false;
1006 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1008 newSt.previous = st;
1011 if (st->epSquare != SQ_NONE)
1013 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1014 st->epSquare = SQ_NONE;
1017 st->key ^= Zobrist::side;
1018 prefetch(TT.first_entry(st->key));
1021 st->pliesFromNull = 0;
1023 sideToMove = ~sideToMove;
1029 assert(pos_is_ok());
1032 void Position::undo_null_move() {
1034 assert(!checkers());
1037 sideToMove = ~sideToMove;
1041 /// Position::key_after() computes the new hash key after the given move. Needed
1042 /// for speculative prefetch. It doesn't recognize special moves like castling,
1043 /// en-passant and promotions.
1045 Key Position::key_after(Move m) const {
1047 Square from = from_sq(m);
1048 Square to = to_sq(m);
1049 Piece pc = piece_on(from);
1050 Piece captured = piece_on(to);
1051 Key k = st->key ^ Zobrist::side;
1054 k ^= Zobrist::psq[captured][to];
1056 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1060 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1061 /// SEE value of move is greater or equal to the given threshold. We'll use an
1062 /// algorithm similar to alpha-beta pruning with a null window.
1064 bool Position::see_ge(Move m, Value threshold) const {
1068 // Only deal with normal moves, assume others pass a simple see
1069 if (type_of(m) != NORMAL)
1070 return VALUE_ZERO >= threshold;
1072 Square from = from_sq(m), to = to_sq(m);
1074 int swap = PieceValue[MG][piece_on(to)] - threshold;
1078 swap = PieceValue[MG][piece_on(from)] - swap;
1082 Bitboard occupied = pieces() ^ from ^ to;
1083 Color stm = color_of(piece_on(from));
1084 Bitboard attackers = attackers_to(to, occupied);
1085 Bitboard stmAttackers, bb;
1091 attackers &= occupied;
1093 // If stm has no more attackers then give up: stm loses
1094 if (!(stmAttackers = attackers & pieces(stm)))
1097 // Don't allow pinned pieces to attack (except the king) as long as
1098 // there are pinners on their original square.
1099 if (pinners(~stm) & occupied)
1100 stmAttackers &= ~blockers_for_king(stm);
1107 // Locate and remove the next least valuable attacker, and add to
1108 // the bitboard 'attackers' any X-ray attackers behind it.
1109 if ((bb = stmAttackers & pieces(PAWN)))
1111 if ((swap = PawnValueMg - swap) < res)
1114 occupied ^= lsb(bb);
1115 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1118 else if ((bb = stmAttackers & pieces(KNIGHT)))
1120 if ((swap = KnightValueMg - swap) < res)
1123 occupied ^= lsb(bb);
1126 else if ((bb = stmAttackers & pieces(BISHOP)))
1128 if ((swap = BishopValueMg - swap) < res)
1131 occupied ^= lsb(bb);
1132 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1135 else if ((bb = stmAttackers & pieces(ROOK)))
1137 if ((swap = RookValueMg - swap) < res)
1140 occupied ^= lsb(bb);
1141 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1144 else if ((bb = stmAttackers & pieces(QUEEN)))
1146 if ((swap = QueenValueMg - swap) < res)
1149 occupied ^= lsb(bb);
1150 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1151 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1155 // If we "capture" with the king but opponent still has attackers,
1156 // reverse the result.
1157 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1164 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1165 /// or by repetition. It does not detect stalemates.
1167 bool Position::is_draw(int ply) const {
1169 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1172 // Return a draw score if a position repeats once earlier but strictly
1173 // after the root, or repeats twice before or at the root.
1174 return st->repetition && st->repetition < ply;
1178 // Position::has_repeated() tests whether there has been at least one repetition
1179 // of positions since the last capture or pawn move.
1181 bool Position::has_repeated() const {
1183 StateInfo* stc = st;
1184 int end = std::min(st->rule50, st->pliesFromNull);
1187 if (stc->repetition)
1190 stc = stc->previous;
1196 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1197 /// or an earlier position has a move that directly reaches the current position.
1199 bool Position::has_game_cycle(int ply) const {
1203 int end = std::min(st->rule50, st->pliesFromNull);
1208 Key originalKey = st->key;
1209 StateInfo* stp = st->previous;
1211 for (int i = 3; i <= end; i += 2)
1213 stp = stp->previous->previous;
1215 Key moveKey = originalKey ^ stp->key;
1216 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1217 || (j = H2(moveKey), cuckoo[j] == moveKey))
1219 Move move = cuckooMove[j];
1220 Square s1 = from_sq(move);
1221 Square s2 = to_sq(move);
1223 if (!(between_bb(s1, s2) & pieces()))
1228 // For nodes before or at the root, check that the move is a
1229 // repetition rather than a move to the current position.
1230 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1231 // the same location, so we have to select which square to check.
1232 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1235 // For repetitions before or at the root, require one more
1236 if (stp->repetition)
1245 /// Position::flip() flips position with the white and black sides reversed. This
1246 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1248 void Position::flip() {
1251 std::stringstream ss(fen());
1253 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1255 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1256 f.insert(0, token + (f.empty() ? " " : "/"));
1259 ss >> token; // Active color
1260 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1262 ss >> token; // Castling availability
1265 std::transform(f.begin(), f.end(), f.begin(),
1266 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1268 ss >> token; // En passant square
1269 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1271 std::getline(ss, token); // Half and full moves
1274 set(f, is_chess960(), st, this_thread());
1276 assert(pos_is_ok());
1280 /// Position::pos_is_ok() performs some consistency checks for the
1281 /// position object and raises an asserts if something wrong is detected.
1282 /// This is meant to be helpful when debugging.
1284 bool Position::pos_is_ok() const {
1286 constexpr bool Fast = true; // Quick (default) or full check?
1288 if ( (sideToMove != WHITE && sideToMove != BLACK)
1289 || piece_on(square<KING>(WHITE)) != W_KING
1290 || piece_on(square<KING>(BLACK)) != B_KING
1291 || ( ep_square() != SQ_NONE
1292 && relative_rank(sideToMove, ep_square()) != RANK_6))
1293 assert(0 && "pos_is_ok: Default");
1298 if ( pieceCount[W_KING] != 1
1299 || pieceCount[B_KING] != 1
1300 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1301 assert(0 && "pos_is_ok: Kings");
1303 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1304 || pieceCount[W_PAWN] > 8
1305 || pieceCount[B_PAWN] > 8)
1306 assert(0 && "pos_is_ok: Pawns");
1308 if ( (pieces(WHITE) & pieces(BLACK))
1309 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1310 || popcount(pieces(WHITE)) > 16
1311 || popcount(pieces(BLACK)) > 16)
1312 assert(0 && "pos_is_ok: Bitboards");
1314 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1315 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1316 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1317 assert(0 && "pos_is_ok: Bitboards");
1321 if (std::memcmp(&si, st, sizeof(StateInfo)))
1322 assert(0 && "pos_is_ok: State");
1324 for (Piece pc : Pieces)
1326 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1327 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1328 assert(0 && "pos_is_ok: Pieces");
1330 for (int i = 0; i < pieceCount[pc]; ++i)
1331 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1332 assert(0 && "pos_is_ok: Index");
1335 for (Color c : { WHITE, BLACK })
1336 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1338 if (!can_castle(cr))
1341 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1342 || castlingRightsMask[castlingRookSquare[cr]] != cr
1343 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1344 assert(0 && "pos_is_ok: Castling");