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) 2015-2016 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"
40 extern Score psq[PIECE_NB][SQUARE_NB];
45 Key psq[PIECE_NB][SQUARE_NB];
46 Key enpassant[FILE_NB];
47 Key castling[CASTLING_RIGHT_NB];
53 const string PieceToChar(" PNBRQK pnbrqk");
55 // min_attacker() is a helper function used by see_ge() to locate the least
56 // valuable attacker for the side to move, remove the attacker we just found
57 // from the bitboards and scan for new X-ray attacks behind it.
60 PieceType min_attacker(const Bitboard* bb, Square to, Bitboard stmAttackers,
61 Bitboard& occupied, Bitboard& attackers) {
63 Bitboard b = stmAttackers & bb[Pt];
65 return min_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
67 occupied ^= b & ~(b - 1);
69 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
70 attackers |= attacks_bb<BISHOP>(to, occupied) & (bb[BISHOP] | bb[QUEEN]);
72 if (Pt == ROOK || Pt == QUEEN)
73 attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
75 attackers &= occupied; // After X-ray that may add already processed pieces
80 PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
81 return KING; // No need to update bitboards: it is the last cycle
87 /// operator<<(Position) returns an ASCII representation of the position
89 std::ostream& operator<<(std::ostream& os, Position& pos) {
91 os << "\n +---+---+---+---+---+---+---+---+\n";
93 for (Rank r = RANK_8; r >= RANK_1; --r)
95 for (File f = FILE_A; f <= FILE_H; ++f)
96 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
98 os << " |\n +---+---+---+---+---+---+---+---+\n";
101 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
102 << std::setfill('0') << std::setw(16) << pos.key()
103 << std::setfill(' ') << std::dec << "\nCheckers: ";
105 for (Bitboard b = pos.checkers(); b; )
106 os << UCI::square(pop_lsb(&b)) << " ";
108 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
109 && !pos.can_castle(ANY_CASTLING))
111 Tablebases::ProbeState s1, s2;
112 Tablebases::WDLScore wdl = Tablebases::probe_wdl(pos, &s1);
113 int dtz = Tablebases::probe_dtz(pos, &s2);
114 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
115 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
122 /// Position::init() initializes at startup the various arrays used to compute
125 void Position::init() {
129 for (Piece pc : Pieces)
130 for (Square s = SQ_A1; s <= SQ_H8; ++s)
131 Zobrist::psq[pc][s] = rng.rand<Key>();
133 for (File f = FILE_A; f <= FILE_H; ++f)
134 Zobrist::enpassant[f] = rng.rand<Key>();
136 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
138 Zobrist::castling[cr] = 0;
142 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
143 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
147 Zobrist::side = rng.rand<Key>();
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. This is recorded only if there is a pawn
180 in position to make an en passant capture, and if there really is a pawn
181 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 += Square(token - '0'); // Advance the given number of files
209 else if (token == '/')
212 else if ((idx = PieceToChar.find(token)) != string::npos)
214 put_piece(Piece(idx), sq);
221 sideToMove = (token == 'w' ? WHITE : BLACK);
224 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
225 // Shredder-FEN that uses the letters of the columns on which the rooks began
226 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
227 // if an inner rook is associated with the castling right, the castling tag is
228 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
229 while ((ss >> token) && !isspace(token))
232 Color c = islower(token) ? BLACK : WHITE;
233 Piece rook = make_piece(c, ROOK);
235 token = char(toupper(token));
238 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
240 else if (token == 'Q')
241 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
243 else if (token >= 'A' && token <= 'H')
244 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
249 set_castling_right(c, rsq);
252 // 4. En passant square. Ignore if no pawn capture is possible
253 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
254 && ((ss >> row) && (row == '3' || row == '6')))
256 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
258 if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
259 || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
260 st->epSquare = SQ_NONE;
263 st->epSquare = SQ_NONE;
265 // 5-6. Halfmove clock and fullmove number
266 ss >> std::skipws >> st->rule50 >> gamePly;
268 // Convert from fullmove starting from 1 to ply starting from 0,
269 // handle also common incorrect FEN with fullmove = 0.
270 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
272 chess960 = isChess960;
282 /// Position::set_castling_right() is a helper function used to set castling
283 /// rights given the corresponding color and the rook starting square.
285 void Position::set_castling_right(Color c, Square rfrom) {
287 Square kfrom = square<KING>(c);
288 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
289 CastlingRight cr = (c | cs);
291 st->castlingRights |= cr;
292 castlingRightsMask[kfrom] |= cr;
293 castlingRightsMask[rfrom] |= cr;
294 castlingRookSquare[cr] = rfrom;
296 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
297 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
299 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
300 if (s != kfrom && s != rfrom)
301 castlingPath[cr] |= s;
303 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
304 if (s != kfrom && s != rfrom)
305 castlingPath[cr] |= s;
309 /// Position::set_check_info() sets king attacks to detect if a move gives check
311 void Position::set_check_info(StateInfo* si) const {
313 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinnersForKing[WHITE]);
314 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinnersForKing[BLACK]);
316 Square ksq = square<KING>(~sideToMove);
318 si->checkSquares[PAWN] = attacks_from<PAWN>(ksq, ~sideToMove);
319 si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
320 si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
321 si->checkSquares[ROOK] = attacks_from<ROOK>(ksq);
322 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
323 si->checkSquares[KING] = 0;
327 /// Position::set_state() computes the hash keys of the position, and other
328 /// data that once computed is updated incrementally as moves are made.
329 /// The function is only used when a new position is set up, and to verify
330 /// the correctness of the StateInfo data when running in debug mode.
332 void Position::set_state(StateInfo* si) const {
334 si->key = si->pawnKey = si->materialKey = 0;
335 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
336 si->psq = SCORE_ZERO;
337 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
341 for (Bitboard b = pieces(); b; )
343 Square s = pop_lsb(&b);
344 Piece pc = piece_on(s);
345 si->key ^= Zobrist::psq[pc][s];
346 si->psq += PSQT::psq[pc][s];
349 if (si->epSquare != SQ_NONE)
350 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
352 if (sideToMove == BLACK)
353 si->key ^= Zobrist::side;
355 si->key ^= Zobrist::castling[si->castlingRights];
357 for (Bitboard b = pieces(PAWN); b; )
359 Square s = pop_lsb(&b);
360 si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
363 for (Piece pc : Pieces)
365 if (type_of(pc) != PAWN && type_of(pc) != KING)
366 si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
368 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
369 si->materialKey ^= Zobrist::psq[pc][cnt];
374 /// Position::set() is an overload to initialize the position object with
375 /// the given endgame code string like "KBPKN". It is manily an helper to
376 /// get the material key out of an endgame code. Position is not playable,
377 /// indeed is even not guaranteed to be legal.
379 Position& Position::set(const string& code, Color c, StateInfo* si) {
381 assert(code.length() > 0 && code.length() < 8);
382 assert(code[0] == 'K');
384 string sides[] = { code.substr(code.find('K', 1)), // Weak
385 code.substr(0, code.find('K', 1)) }; // Strong
387 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
389 string fenStr = sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/8/8/"
390 + sides[1] + char(8 - sides[1].length() + '0') + " w - - 0 10";
392 return set(fenStr, false, si, nullptr);
396 /// Position::fen() returns a FEN representation of the position. In case of
397 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
399 const string Position::fen() const {
402 std::ostringstream ss;
404 for (Rank r = RANK_8; r >= RANK_1; --r)
406 for (File f = FILE_A; f <= FILE_H; ++f)
408 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
415 ss << PieceToChar[piece_on(make_square(f, r))];
422 ss << (sideToMove == WHITE ? " w " : " b ");
424 if (can_castle(WHITE_OO))
425 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | KING_SIDE))) : 'K');
427 if (can_castle(WHITE_OOO))
428 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
430 if (can_castle(BLACK_OO))
431 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | KING_SIDE))) : 'k');
433 if (can_castle(BLACK_OOO))
434 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
436 if (!can_castle(WHITE) && !can_castle(BLACK))
439 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
440 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
446 /// Position::game_phase() calculates the game phase interpolating total non-pawn
447 /// material between endgame and midgame limits.
449 Phase Position::game_phase() const {
451 Value npm = st->nonPawnMaterial[WHITE] + st->nonPawnMaterial[BLACK];
453 npm = std::max(EndgameLimit, std::min(npm, MidgameLimit));
455 return Phase(((npm - EndgameLimit) * PHASE_MIDGAME) / (MidgameLimit - EndgameLimit));
459 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
460 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
461 /// slider if removing that piece from the board would result in a position where
462 /// square 's' is attacked. For example, a king-attack blocking piece can be either
463 /// a pinned or a discovered check piece, according if its color is the opposite
464 /// or the same of the color of the slider.
466 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
471 // Snipers are sliders that attack 's' when a piece is removed
472 Bitboard snipers = ( (PseudoAttacks[ROOK ][s] & pieces(QUEEN, ROOK))
473 | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
477 Square sniperSq = pop_lsb(&snipers);
478 Bitboard b = between_bb(s, sniperSq) & pieces();
480 if (!more_than_one(b))
483 if (b & pieces(color_of(piece_on(s))))
491 /// Position::attackers_to() computes a bitboard of all pieces which attack a
492 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
494 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
496 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
497 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
498 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
499 | (attacks_bb<ROOK >(s, occupied) & pieces(ROOK, QUEEN))
500 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
501 | (attacks_from<KING>(s) & pieces(KING));
505 /// Position::legal() tests whether a pseudo-legal move is legal
507 bool Position::legal(Move m) const {
511 Color us = sideToMove;
512 Square from = from_sq(m);
514 assert(color_of(moved_piece(m)) == us);
515 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
517 // En passant captures are a tricky special case. Because they are rather
518 // uncommon, we do it simply by testing whether the king is attacked after
520 if (type_of(m) == ENPASSANT)
522 Square ksq = square<KING>(us);
523 Square to = to_sq(m);
524 Square capsq = to - pawn_push(us);
525 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
527 assert(to == ep_square());
528 assert(moved_piece(m) == make_piece(us, PAWN));
529 assert(piece_on(capsq) == make_piece(~us, PAWN));
530 assert(piece_on(to) == NO_PIECE);
532 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
533 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
536 // If the moving piece is a king, check whether the destination
537 // square is attacked by the opponent. Castling moves are checked
538 // for legality during move generation.
539 if (type_of(piece_on(from)) == KING)
540 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
542 // A non-king move is legal if and only if it is not pinned or it
543 // is moving along the ray towards or away from the king.
544 return !(pinned_pieces(us) & from)
545 || aligned(from, to_sq(m), square<KING>(us));
549 /// Position::pseudo_legal() takes a random move and tests whether the move is
550 /// pseudo legal. It is used to validate moves from TT that can be corrupted
551 /// due to SMP concurrent access or hash position key aliasing.
553 bool Position::pseudo_legal(const Move m) const {
555 Color us = sideToMove;
556 Square from = from_sq(m);
557 Square to = to_sq(m);
558 Piece pc = moved_piece(m);
560 // Use a slower but simpler function for uncommon cases
561 if (type_of(m) != NORMAL)
562 return MoveList<LEGAL>(*this).contains(m);
564 // Is not a promotion, so promotion piece must be empty
565 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
568 // If the 'from' square is not occupied by a piece belonging to the side to
569 // move, the move is obviously not legal.
570 if (pc == NO_PIECE || color_of(pc) != us)
573 // The destination square cannot be occupied by a friendly piece
577 // Handle the special case of a pawn move
578 if (type_of(pc) == PAWN)
580 // We have already handled promotion moves, so destination
581 // cannot be on the 8th/1st rank.
582 if (rank_of(to) == relative_rank(us, RANK_8))
585 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
586 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
587 && !( (from + 2 * pawn_push(us) == to) // Not a double push
588 && (rank_of(from) == relative_rank(us, RANK_2))
590 && empty(to - pawn_push(us))))
593 else if (!(attacks_from(pc, from) & to))
596 // Evasions generator already takes care to avoid some kind of illegal moves
597 // and legal() relies on this. We therefore have to take care that the same
598 // kind of moves are filtered out here.
601 if (type_of(pc) != KING)
603 // Double check? In this case a king move is required
604 if (more_than_one(checkers()))
607 // Our move must be a blocking evasion or a capture of the checking piece
608 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
611 // In case of king moves under check we have to remove king so as to catch
612 // invalid moves like b1a1 when opposite queen is on c1.
613 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
621 /// Position::gives_check() tests whether a pseudo-legal move gives a check
623 bool Position::gives_check(Move m) const {
626 assert(color_of(moved_piece(m)) == sideToMove);
628 Square from = from_sq(m);
629 Square to = to_sq(m);
631 // Is there a direct check?
632 if (st->checkSquares[type_of(piece_on(from))] & to)
635 // Is there a discovered check?
636 if ( (discovered_check_candidates() & from)
637 && !aligned(from, to, square<KING>(~sideToMove)))
646 return attacks_bb(Piece(promotion_type(m)), to, pieces() ^ from) & square<KING>(~sideToMove);
648 // En passant capture with check? We have already handled the case
649 // of direct checks and ordinary discovered check, so the only case we
650 // need to handle is the unusual case of a discovered check through
651 // the captured pawn.
654 Square capsq = make_square(file_of(to), rank_of(from));
655 Bitboard b = (pieces() ^ from ^ capsq) | to;
657 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
658 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
663 Square rfrom = to; // Castling is encoded as 'King captures the rook'
664 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
665 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
667 return (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
668 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
677 /// Position::do_move() makes a move, and saves all information necessary
678 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
679 /// moves should be filtered out before this function is called.
681 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
684 assert(&newSt != st);
687 Key k = st->key ^ Zobrist::side;
689 // Copy some fields of the old state to our new StateInfo object except the
690 // ones which are going to be recalculated from scratch anyway and then switch
691 // our state pointer to point to the new (ready to be updated) state.
692 std::memcpy(&newSt, st, offsetof(StateInfo, key));
696 // Increment ply counters. In particular, rule50 will be reset to zero later on
697 // in case of a capture or a pawn move.
702 Color us = sideToMove;
704 Square from = from_sq(m);
705 Square to = to_sq(m);
706 Piece pc = piece_on(from);
707 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
709 assert(color_of(pc) == us);
710 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
711 assert(type_of(captured) != KING);
713 if (type_of(m) == CASTLING)
715 assert(pc == make_piece(us, KING));
716 assert(captured == make_piece(us, ROOK));
719 do_castling<true>(us, from, to, rfrom, rto);
721 st->psq += PSQT::psq[captured][rto] - PSQT::psq[captured][rfrom];
722 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
730 // If the captured piece is a pawn, update pawn hash key, otherwise
731 // update non-pawn material.
732 if (type_of(captured) == PAWN)
734 if (type_of(m) == ENPASSANT)
736 capsq -= pawn_push(us);
738 assert(pc == make_piece(us, PAWN));
739 assert(to == st->epSquare);
740 assert(relative_rank(us, to) == RANK_6);
741 assert(piece_on(to) == NO_PIECE);
742 assert(piece_on(capsq) == make_piece(them, PAWN));
744 board[capsq] = NO_PIECE; // Not done by remove_piece()
747 st->pawnKey ^= Zobrist::psq[captured][capsq];
750 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
752 // Update board and piece lists
753 remove_piece(captured, capsq);
755 // Update material hash key and prefetch access to materialTable
756 k ^= Zobrist::psq[captured][capsq];
757 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
758 prefetch(thisThread->materialTable[st->materialKey]);
760 // Update incremental scores
761 st->psq -= PSQT::psq[captured][capsq];
763 // Reset rule 50 counter
768 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
770 // Reset en passant square
771 if (st->epSquare != SQ_NONE)
773 k ^= Zobrist::enpassant[file_of(st->epSquare)];
774 st->epSquare = SQ_NONE;
777 // Update castling rights if needed
778 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
780 int cr = castlingRightsMask[from] | castlingRightsMask[to];
781 k ^= Zobrist::castling[st->castlingRights & cr];
782 st->castlingRights &= ~cr;
785 // Move the piece. The tricky Chess960 castling is handled earlier
786 if (type_of(m) != CASTLING)
787 move_piece(pc, from, to);
789 // If the moving piece is a pawn do some special extra work
790 if (type_of(pc) == PAWN)
792 // Set en-passant square if the moved pawn can be captured
793 if ( (int(to) ^ int(from)) == 16
794 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
796 st->epSquare = (from + to) / 2;
797 k ^= Zobrist::enpassant[file_of(st->epSquare)];
800 else if (type_of(m) == PROMOTION)
802 Piece promotion = make_piece(us, promotion_type(m));
804 assert(relative_rank(us, to) == RANK_8);
805 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
807 remove_piece(pc, to);
808 put_piece(promotion, to);
811 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
812 st->pawnKey ^= Zobrist::psq[pc][to];
813 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
814 ^ Zobrist::psq[pc][pieceCount[pc]];
816 // Update incremental score
817 st->psq += PSQT::psq[promotion][to] - PSQT::psq[pc][to];
820 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
823 // Update pawn hash key and prefetch access to pawnsTable
824 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
825 prefetch(thisThread->pawnsTable[st->pawnKey]);
827 // Reset rule 50 draw counter
831 // Update incremental scores
832 st->psq += PSQT::psq[pc][to] - PSQT::psq[pc][from];
835 st->capturedPiece = captured;
837 // Update the key with the final value
840 // Calculate checkers bitboard (if move gives check)
841 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
843 sideToMove = ~sideToMove;
845 // Update king attacks used for fast check detection
852 /// Position::undo_move() unmakes a move. When it returns, the position should
853 /// be restored to exactly the same state as before the move was made.
855 void Position::undo_move(Move m) {
859 sideToMove = ~sideToMove;
861 Color us = sideToMove;
862 Square from = from_sq(m);
863 Square to = to_sq(m);
864 Piece pc = piece_on(to);
866 assert(empty(from) || type_of(m) == CASTLING);
867 assert(type_of(st->capturedPiece) != KING);
869 if (type_of(m) == PROMOTION)
871 assert(relative_rank(us, to) == RANK_8);
872 assert(type_of(pc) == promotion_type(m));
873 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
875 remove_piece(pc, to);
876 pc = make_piece(us, PAWN);
880 if (type_of(m) == CASTLING)
883 do_castling<false>(us, from, to, rfrom, rto);
887 move_piece(pc, to, from); // Put the piece back at the source square
889 if (st->capturedPiece)
893 if (type_of(m) == ENPASSANT)
895 capsq -= pawn_push(us);
897 assert(type_of(pc) == PAWN);
898 assert(to == st->previous->epSquare);
899 assert(relative_rank(us, to) == RANK_6);
900 assert(piece_on(capsq) == NO_PIECE);
901 assert(st->capturedPiece == make_piece(~us, PAWN));
904 put_piece(st->capturedPiece, capsq); // Restore the captured piece
908 // Finally point our state pointer back to the previous state
916 /// Position::do_castling() is a helper used to do/undo a castling move. This
917 /// is a bit tricky in Chess960 where from/to squares can overlap.
919 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
921 bool kingSide = to > from;
922 rfrom = to; // Castling is encoded as "king captures friendly rook"
923 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
924 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
926 // Remove both pieces first since squares could overlap in Chess960
927 remove_piece(make_piece(us, KING), Do ? from : to);
928 remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
929 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
930 put_piece(make_piece(us, KING), Do ? to : from);
931 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
935 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
936 /// the side to move without executing any move on the board.
938 void Position::do_null_move(StateInfo& newSt) {
941 assert(&newSt != st);
943 std::memcpy(&newSt, st, sizeof(StateInfo));
947 if (st->epSquare != SQ_NONE)
949 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
950 st->epSquare = SQ_NONE;
953 st->key ^= Zobrist::side;
954 prefetch(TT.first_entry(st->key));
957 st->pliesFromNull = 0;
959 sideToMove = ~sideToMove;
966 void Position::undo_null_move() {
971 sideToMove = ~sideToMove;
975 /// Position::key_after() computes the new hash key after the given move. Needed
976 /// for speculative prefetch. It doesn't recognize special moves like castling,
977 /// en-passant and promotions.
979 Key Position::key_after(Move m) const {
981 Square from = from_sq(m);
982 Square to = to_sq(m);
983 Piece pc = piece_on(from);
984 Piece captured = piece_on(to);
985 Key k = st->key ^ Zobrist::side;
988 k ^= Zobrist::psq[captured][to];
990 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
994 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
995 /// SEE value of move is greater or equal to the given value. We'll use an
996 /// algorithm similar to alpha-beta pruning with a null window.
998 bool Position::see_ge(Move m, Value v) const {
1002 // Castling moves are implemented as king capturing the rook so cannot be
1003 // handled correctly. Simply assume the SEE value is VALUE_ZERO that is always
1004 // correct unless in the rare case the rook ends up under attack.
1005 if (type_of(m) == CASTLING)
1006 return VALUE_ZERO >= v;
1008 Square from = from_sq(m), to = to_sq(m);
1009 PieceType nextVictim = type_of(piece_on(from));
1010 Color stm = ~color_of(piece_on(from)); // First consider opponent's move
1011 Value balance; // Values of the pieces taken by us minus opponent's ones
1012 Bitboard occupied, stmAttackers;
1014 if (type_of(m) == ENPASSANT)
1016 occupied = SquareBB[to - pawn_push(~stm)]; // Remove the captured pawn
1017 balance = PieceValue[MG][PAWN];
1021 balance = PieceValue[MG][piece_on(to)];
1028 if (nextVictim == KING)
1031 balance -= PieceValue[MG][nextVictim];
1036 bool relativeStm = true; // True if the opponent is to move
1037 occupied ^= pieces() ^ from ^ to;
1039 // Find all attackers to the destination square, with the moving piece removed,
1040 // but possibly an X-ray attacker added behind it.
1041 Bitboard attackers = attackers_to(to, occupied) & occupied;
1045 stmAttackers = attackers & pieces(stm);
1047 // Don't allow pinned pieces to attack pieces except the king as long all
1048 // pinners are on their original square.
1049 if (!(st->pinnersForKing[stm] & ~occupied))
1050 stmAttackers &= ~st->blockersForKing[stm];
1055 // Locate and remove the next least valuable attacker
1056 nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1058 if (nextVictim == KING)
1059 return relativeStm == bool(attackers & pieces(~stm));
1061 balance += relativeStm ? PieceValue[MG][nextVictim]
1062 : -PieceValue[MG][nextVictim];
1064 relativeStm = !relativeStm;
1066 if (relativeStm == (balance >= v))
1074 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1075 /// or by repetition. It does not detect stalemates.
1077 bool Position::is_draw() const {
1079 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1082 StateInfo* stp = st;
1083 for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
1085 stp = stp->previous->previous;
1087 if (stp->key == st->key)
1088 return true; // Draw at first repetition
1095 /// Position::flip() flips position with the white and black sides reversed. This
1096 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1098 void Position::flip() {
1101 std::stringstream ss(fen());
1103 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1105 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1106 f.insert(0, token + (f.empty() ? " " : "/"));
1109 ss >> token; // Active color
1110 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1112 ss >> token; // Castling availability
1115 std::transform(f.begin(), f.end(), f.begin(),
1116 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1118 ss >> token; // En passant square
1119 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1121 std::getline(ss, token); // Half and full moves
1124 set(f, is_chess960(), st, this_thread());
1126 assert(pos_is_ok());
1130 /// Position::pos_is_ok() performs some consistency checks for the position object.
1131 /// This is meant to be helpful when debugging.
1133 bool Position::pos_is_ok(int* failedStep) const {
1135 const bool Fast = true; // Quick (default) or full check?
1137 enum { Default, King, Bitboards, State, Lists, Castling };
1139 for (int step = Default; step <= (Fast ? Default : Castling); step++)
1144 if (step == Default)
1145 if ( (sideToMove != WHITE && sideToMove != BLACK)
1146 || piece_on(square<KING>(WHITE)) != W_KING
1147 || piece_on(square<KING>(BLACK)) != B_KING
1148 || ( ep_square() != SQ_NONE
1149 && relative_rank(sideToMove, ep_square()) != RANK_6))
1153 if ( std::count(board, board + SQUARE_NB, W_KING) != 1
1154 || std::count(board, board + SQUARE_NB, B_KING) != 1
1155 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1158 if (step == Bitboards)
1160 if ( (pieces(WHITE) & pieces(BLACK))
1161 ||(pieces(WHITE) | pieces(BLACK)) != pieces())
1164 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1165 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1166 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1174 if (std::memcmp(&si, st, sizeof(StateInfo)))
1179 for (Piece pc : Pieces)
1181 if (pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc))))
1184 for (int i = 0; i < pieceCount[pc]; ++i)
1185 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1189 if (step == Castling)
1190 for (Color c = WHITE; c <= BLACK; ++c)
1191 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1193 if (!can_castle(c | s))
1196 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1197 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1198 ||(castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))