2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2014 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
36 static const string PieceToChar(" PNBRQK pnbrqk");
40 Value PieceValue[PHASE_NB][PIECE_NB] = {
41 { VALUE_ZERO, PawnValueMg, KnightValueMg, BishopValueMg, RookValueMg, QueenValueMg },
42 { VALUE_ZERO, PawnValueEg, KnightValueEg, BishopValueEg, RookValueEg, QueenValueEg } };
44 static Score psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
48 Key psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
49 Key enpassant[FILE_NB];
50 Key castling[CASTLING_RIGHT_NB];
55 Key Position::exclusion_key() const { return st->key ^ Zobrist::exclusion;}
59 // min_attacker() is a helper function used by see() to locate the least
60 // valuable attacker for the side to move, remove the attacker we just found
61 // from the bitboards and scan for new X-ray attacks behind it.
63 template<int Pt> FORCE_INLINE
64 PieceType min_attacker(const Bitboard* bb, const Square& to, const Bitboard& stmAttackers,
65 Bitboard& occupied, Bitboard& attackers) {
67 Bitboard b = stmAttackers & bb[Pt];
69 return min_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
71 occupied ^= b & ~(b - 1);
73 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
74 attackers |= attacks_bb<BISHOP>(to, occupied) & (bb[BISHOP] | bb[QUEEN]);
76 if (Pt == ROOK || Pt == QUEEN)
77 attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
79 attackers &= occupied; // After X-ray that may add already processed pieces
83 template<> FORCE_INLINE
84 PieceType min_attacker<KING>(const Bitboard*, const Square&, const Bitboard&, Bitboard&, Bitboard&) {
85 return KING; // No need to update bitboards: it is the last cycle
93 CheckInfo::CheckInfo(const Position& pos) {
95 Color them = ~pos.side_to_move();
96 ksq = pos.king_square(them);
98 pinned = pos.pinned_pieces(pos.side_to_move());
99 dcCandidates = pos.discovered_check_candidates();
101 checkSq[PAWN] = pos.attacks_from<PAWN>(ksq, them);
102 checkSq[KNIGHT] = pos.attacks_from<KNIGHT>(ksq);
103 checkSq[BISHOP] = pos.attacks_from<BISHOP>(ksq);
104 checkSq[ROOK] = pos.attacks_from<ROOK>(ksq);
105 checkSq[QUEEN] = checkSq[BISHOP] | checkSq[ROOK];
110 /// Position::init() initializes at startup the various arrays used to compute
111 /// hash keys and the piece square tables. The latter is a two-step operation:
112 /// Firstly, the white halves of the tables are copied from PSQT[] tables.
113 /// Secondly, the black halves of the tables are initialized by flipping and
114 /// changing the sign of the white scores.
116 void Position::init() {
120 for (Color c = WHITE; c <= BLACK; ++c)
121 for (PieceType pt = PAWN; pt <= KING; ++pt)
122 for (Square s = SQ_A1; s <= SQ_H8; ++s)
123 Zobrist::psq[c][pt][s] = rk.rand<Key>();
125 for (File f = FILE_A; f <= FILE_H; ++f)
126 Zobrist::enpassant[f] = rk.rand<Key>();
128 for (int cf = NO_CASTLING; cf <= ANY_CASTLING; ++cf)
133 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
134 Zobrist::castling[cf] ^= k ? k : rk.rand<Key>();
138 Zobrist::side = rk.rand<Key>();
139 Zobrist::exclusion = rk.rand<Key>();
141 for (PieceType pt = PAWN; pt <= KING; ++pt)
143 PieceValue[MG][make_piece(BLACK, pt)] = PieceValue[MG][pt];
144 PieceValue[EG][make_piece(BLACK, pt)] = PieceValue[EG][pt];
146 Score v = make_score(PieceValue[MG][pt], PieceValue[EG][pt]);
148 for (Square s = SQ_A1; s <= SQ_H8; ++s)
150 psq[WHITE][pt][ s] = (v + PSQT[pt][s]);
151 psq[BLACK][pt][~s] = -(v + PSQT[pt][s]);
157 /// Position::operator=() creates a copy of 'pos'. We want the new born Position
158 /// object to not depend on any external data so we detach state pointer from
161 Position& Position::operator=(const Position& pos) {
163 std::memcpy(this, &pos, sizeof(Position));
174 /// Position::clear() erases the position object to a pristine state, with an
175 /// empty board, white to move, and no castling rights.
177 void Position::clear() {
179 std::memset(this, 0, sizeof(Position));
180 startState.epSquare = SQ_NONE;
183 for (int i = 0; i < PIECE_TYPE_NB; ++i)
184 for (int j = 0; j < 16; ++j)
185 pieceList[WHITE][i][j] = pieceList[BLACK][i][j] = SQ_NONE;
189 /// Position::set() initializes the position object with the given FEN string.
190 /// This function is not very robust - make sure that input FENs are correct,
191 /// this is assumed to be the responsibility of the GUI.
193 void Position::set(const string& fenStr, bool isChess960, Thread* th) {
195 A FEN string defines a particular position using only the ASCII character set.
197 A FEN string contains six fields separated by a space. The fields are:
199 1) Piece placement (from white's perspective). Each rank is described, starting
200 with rank 8 and ending with rank 1. Within each rank, the contents of each
201 square are described from file A through file H. Following the Standard
202 Algebraic Notation (SAN), each piece is identified by a single letter taken
203 from the standard English names. White pieces are designated using upper-case
204 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
205 noted using digits 1 through 8 (the number of blank squares), and "/"
208 2) Active color. "w" means white moves next, "b" means black.
210 3) Castling availability. If neither side can castle, this is "-". Otherwise,
211 this has one or more letters: "K" (White can castle kingside), "Q" (White
212 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
213 can castle queenside).
215 4) En passant target square (in algebraic notation). If there's no en passant
216 target square, this is "-". If a pawn has just made a 2-square move, this
217 is the position "behind" the pawn. This is recorded regardless of whether
218 there is a pawn in position to make an en passant capture.
220 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
221 or capture. This is used to determine if a draw can be claimed under the
224 6) Fullmove number. The number of the full move. It starts at 1, and is
225 incremented after Black's move.
228 unsigned char col, row, token;
231 std::istringstream ss(fenStr);
236 // 1. Piece placement
237 while ((ss >> token) && !isspace(token))
240 sq += Square(token - '0'); // Advance the given number of files
242 else if (token == '/')
245 else if ((idx = PieceToChar.find(token)) != string::npos)
247 put_piece(sq, color_of(Piece(idx)), type_of(Piece(idx)));
254 sideToMove = (token == 'w' ? WHITE : BLACK);
257 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
258 // Shredder-FEN that uses the letters of the columns on which the rooks began
259 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
260 // if an inner rook is associated with the castling right, the castling tag is
261 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
262 while ((ss >> token) && !isspace(token))
265 Color c = islower(token) ? BLACK : WHITE;
267 token = char(toupper(token));
270 for (rsq = relative_square(c, SQ_H1); type_of(piece_on(rsq)) != ROOK; --rsq) {}
272 else if (token == 'Q')
273 for (rsq = relative_square(c, SQ_A1); type_of(piece_on(rsq)) != ROOK; ++rsq) {}
275 else if (token >= 'A' && token <= 'H')
276 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
281 set_castling_right(c, rsq);
284 // 4. En passant square. Ignore if no pawn capture is possible
285 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
286 && ((ss >> row) && (row == '3' || row == '6')))
288 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
290 if (!(attackers_to(st->epSquare) & pieces(sideToMove, PAWN)))
291 st->epSquare = SQ_NONE;
294 // 5-6. Halfmove clock and fullmove number
295 ss >> std::skipws >> st->rule50 >> gamePly;
297 // Convert from fullmove starting from 1 to ply starting from 0,
298 // handle also common incorrect FEN with fullmove = 0.
299 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
301 chess960 = isChess960;
309 /// Position::set_castling_right() is a helper function used to set castling
310 /// rights given the corresponding color and the rook starting square.
312 void Position::set_castling_right(Color c, Square rfrom) {
314 Square kfrom = king_square(c);
315 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
316 CastlingRight cr = (c | cs);
318 st->castlingRights |= cr;
319 castlingRightsMask[kfrom] |= cr;
320 castlingRightsMask[rfrom] |= cr;
321 castlingRookSquare[cr] = rfrom;
323 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
324 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
326 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
327 if (s != kfrom && s != rfrom)
328 castlingPath[cr] |= s;
330 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
331 if (s != kfrom && s != rfrom)
332 castlingPath[cr] |= s;
336 /// Position::set_state() computes the hash keys of the position, and other
337 /// data that once computed is updated incrementally as moves are made.
338 /// The function is only used when a new position is set up, and to verify
339 /// the correctness of the StateInfo data when running in debug mode.
341 void Position::set_state(StateInfo* si) const {
343 si->key = si->pawnKey = si->materialKey = 0;
344 si->npMaterial[WHITE] = si->npMaterial[BLACK] = VALUE_ZERO;
345 si->psq = SCORE_ZERO;
347 si->checkersBB = attackers_to(king_square(sideToMove)) & pieces(~sideToMove);
349 for (Bitboard b = pieces(); b; )
351 Square s = pop_lsb(&b);
352 Piece pc = piece_on(s);
353 si->key ^= Zobrist::psq[color_of(pc)][type_of(pc)][s];
354 si->psq += psq[color_of(pc)][type_of(pc)][s];
357 if (ep_square() != SQ_NONE)
358 si->key ^= Zobrist::enpassant[file_of(ep_square())];
360 if (sideToMove == BLACK)
361 si->key ^= Zobrist::side;
363 si->key ^= Zobrist::castling[st->castlingRights];
365 for (Bitboard b = pieces(PAWN); b; )
367 Square s = pop_lsb(&b);
368 si->pawnKey ^= Zobrist::psq[color_of(piece_on(s))][PAWN][s];
371 for (Color c = WHITE; c <= BLACK; ++c)
372 for (PieceType pt = PAWN; pt <= KING; ++pt)
373 for (int cnt = 0; cnt < pieceCount[c][pt]; ++cnt)
374 si->materialKey ^= Zobrist::psq[c][pt][cnt];
376 for (Color c = WHITE; c <= BLACK; ++c)
377 for (PieceType pt = KNIGHT; pt <= QUEEN; ++pt)
378 si->npMaterial[c] += pieceCount[c][pt] * PieceValue[MG][pt];
382 /// Position::fen() returns a FEN representation of the position. In case of
383 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
385 const string Position::fen() const {
388 std::ostringstream ss;
390 for (Rank r = RANK_8; r >= RANK_1; --r)
392 for (File f = FILE_A; f <= FILE_H; ++f)
394 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
401 ss << PieceToChar[piece_on(make_square(f, r))];
408 ss << (sideToMove == WHITE ? " w " : " b ");
410 if (can_castle(WHITE_OO))
411 ss << (chess960 ? to_char(file_of(castling_rook_square(WHITE | KING_SIDE)), false) : 'K');
413 if (can_castle(WHITE_OOO))
414 ss << (chess960 ? to_char(file_of(castling_rook_square(WHITE | QUEEN_SIDE)), false) : 'Q');
416 if (can_castle(BLACK_OO))
417 ss << (chess960 ? to_char(file_of(castling_rook_square(BLACK | KING_SIDE)), true) : 'k');
419 if (can_castle(BLACK_OOO))
420 ss << (chess960 ? to_char(file_of(castling_rook_square(BLACK | QUEEN_SIDE)), true) : 'q');
422 if (!can_castle(WHITE) && !can_castle(BLACK))
425 ss << (ep_square() == SQ_NONE ? " - " : " " + to_string(ep_square()) + " ")
426 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
432 /// Position::pretty() returns an ASCII representation of the position
434 const string Position::pretty() const {
436 std::ostringstream ss;
438 ss << "\n +---+---+---+---+---+---+---+---+\n";
440 for (Rank r = RANK_8; r >= RANK_1; --r)
442 for (File f = FILE_A; f <= FILE_H; ++f)
443 ss << " | " << PieceToChar[piece_on(make_square(f, r))];
445 ss << " |\n +---+---+---+---+---+---+---+---+\n";
448 ss << "\nFen: " << fen() << "\nKey: " << std::hex << std::uppercase
449 << std::setfill('0') << std::setw(16) << st->key << "\nCheckers: ";
451 for (Bitboard b = checkers(); b; )
452 ss << to_string(pop_lsb(&b)) << " ";
458 /// Position::game_phase() calculates the game phase interpolating total non-pawn
459 /// material between endgame and midgame limits.
461 Phase Position::game_phase() const {
463 Value npm = st->npMaterial[WHITE] + st->npMaterial[BLACK];
465 npm = std::max(EndgameLimit, std::min(npm, MidgameLimit));
467 return Phase(((npm - EndgameLimit) * 128) / (MidgameLimit - EndgameLimit));
471 /// Position::check_blockers() returns a bitboard of all the pieces with color
472 /// 'c' that are blocking check on the king with color 'kingColor'. A piece
473 /// blocks a check if removing that piece from the board would result in a
474 /// position where the king is in check. A check blocking piece can be either a
475 /// pinned or a discovered check piece, according if its color 'c' is the same
476 /// or the opposite of 'kingColor'.
478 Bitboard Position::check_blockers(Color c, Color kingColor) const {
480 Bitboard b, pinners, result = 0;
481 Square ksq = king_square(kingColor);
483 // Pinners are sliders that give check when a pinned piece is removed
484 pinners = ( (pieces( ROOK, QUEEN) & PseudoAttacks[ROOK ][ksq])
485 | (pieces(BISHOP, QUEEN) & PseudoAttacks[BISHOP][ksq])) & pieces(~kingColor);
489 b = between_bb(ksq, pop_lsb(&pinners)) & pieces();
491 if (!more_than_one(b))
492 result |= b & pieces(c);
498 /// Position::attackers_to() computes a bitboard of all pieces which attack a
499 /// given square. Slider attacks use the occ bitboard to indicate occupancy.
501 Bitboard Position::attackers_to(Square s, Bitboard occ) const {
503 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
504 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
505 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
506 | (attacks_bb<ROOK>(s, occ) & pieces(ROOK, QUEEN))
507 | (attacks_bb<BISHOP>(s, occ) & pieces(BISHOP, QUEEN))
508 | (attacks_from<KING>(s) & pieces(KING));
512 /// Position::legal() tests whether a pseudo-legal move is legal
514 bool Position::legal(Move m, Bitboard pinned) const {
517 assert(pinned == pinned_pieces(sideToMove));
519 Color us = sideToMove;
520 Square from = from_sq(m);
522 assert(color_of(moved_piece(m)) == us);
523 assert(piece_on(king_square(us)) == make_piece(us, KING));
525 // En passant captures are a tricky special case. Because they are rather
526 // uncommon, we do it simply by testing whether the king is attacked after
528 if (type_of(m) == ENPASSANT)
530 Square ksq = king_square(us);
531 Square to = to_sq(m);
532 Square capsq = to - pawn_push(us);
533 Bitboard occ = (pieces() ^ from ^ capsq) | to;
535 assert(to == ep_square());
536 assert(moved_piece(m) == make_piece(us, PAWN));
537 assert(piece_on(capsq) == make_piece(~us, PAWN));
538 assert(piece_on(to) == NO_PIECE);
540 return !(attacks_bb< ROOK>(ksq, occ) & pieces(~us, QUEEN, ROOK))
541 && !(attacks_bb<BISHOP>(ksq, occ) & pieces(~us, QUEEN, BISHOP));
544 // If the moving piece is a king, check whether the destination
545 // square is attacked by the opponent. Castling moves are checked
546 // for legality during move generation.
547 if (type_of(piece_on(from)) == KING)
548 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
550 // A non-king move is legal if and only if it is not pinned or it
551 // is moving along the ray towards or away from the king.
554 || aligned(from, to_sq(m), king_square(us));
558 /// Position::pseudo_legal() takes a random move and tests whether the move is
559 /// pseudo legal. It is used to validate moves from TT that can be corrupted
560 /// due to SMP concurrent access or hash position key aliasing.
562 bool Position::pseudo_legal(const Move m) const {
564 Color us = sideToMove;
565 Square from = from_sq(m);
566 Square to = to_sq(m);
567 Piece pc = moved_piece(m);
569 // Use a slower but simpler function for uncommon cases
570 if (type_of(m) != NORMAL)
571 return MoveList<LEGAL>(*this).contains(m);
573 // Is not a promotion, so promotion piece must be empty
574 if (promotion_type(m) - 2 != NO_PIECE_TYPE)
577 // If the 'from' square is not occupied by a piece belonging to the side to
578 // move, the move is obviously not legal.
579 if (pc == NO_PIECE || color_of(pc) != us)
582 // The destination square cannot be occupied by a friendly piece
586 // Handle the special case of a pawn move
587 if (type_of(pc) == PAWN)
589 // We have already handled promotion moves, so destination
590 // cannot be on the 8th/1st rank.
591 if (rank_of(to) == relative_rank(us, RANK_8))
594 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
596 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
598 && !( (from + 2 * pawn_push(us) == to) // Not a double push
599 && (rank_of(from) == relative_rank(us, RANK_2))
601 && empty(to - pawn_push(us))))
604 else if (!(attacks_from(pc, from) & to))
607 // Evasions generator already takes care to avoid some kind of illegal moves
608 // and legal() relies on this. We therefore have to take care that the same
609 // kind of moves are filtered out here.
612 if (type_of(pc) != KING)
614 // Double check? In this case a king move is required
615 if (more_than_one(checkers()))
618 // Our move must be a blocking evasion or a capture of the checking piece
619 if (!((between_bb(lsb(checkers()), king_square(us)) | checkers()) & to))
622 // In case of king moves under check we have to remove king so as to catch
623 // invalid moves like b1a1 when opposite queen is on c1.
624 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
632 /// Position::gives_check() tests whether a pseudo-legal move gives a check
634 bool Position::gives_check(Move m, const CheckInfo& ci) const {
637 assert(ci.dcCandidates == discovered_check_candidates());
638 assert(color_of(moved_piece(m)) == sideToMove);
640 Square from = from_sq(m);
641 Square to = to_sq(m);
642 PieceType pt = type_of(piece_on(from));
644 // Is there a direct check?
645 if (ci.checkSq[pt] & to)
648 // Is there a discovered check?
649 if ( unlikely(ci.dcCandidates)
650 && (ci.dcCandidates & from)
651 && !aligned(from, to, ci.ksq))
660 return attacks_bb(Piece(promotion_type(m)), to, pieces() ^ from) & ci.ksq;
662 // En passant capture with check? We have already handled the case
663 // of direct checks and ordinary discovered check, so the only case we
664 // need to handle is the unusual case of a discovered check through
665 // the captured pawn.
668 Square capsq = make_square(file_of(to), rank_of(from));
669 Bitboard b = (pieces() ^ from ^ capsq) | to;
671 return (attacks_bb< ROOK>(ci.ksq, b) & pieces(sideToMove, QUEEN, ROOK))
672 | (attacks_bb<BISHOP>(ci.ksq, b) & pieces(sideToMove, QUEEN, BISHOP));
677 Square rfrom = to; // Castling is encoded as 'King captures the rook'
678 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
679 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
681 return (PseudoAttacks[ROOK][rto] & ci.ksq)
682 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & ci.ksq);
691 /// Position::do_move() makes a move, and saves all information necessary
692 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
693 /// moves should be filtered out before this function is called.
695 void Position::do_move(Move m, StateInfo& newSt) {
698 do_move(m, newSt, ci, gives_check(m, ci));
701 void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool moveIsCheck) {
704 assert(&newSt != st);
709 // Copy some fields of the old state to our new StateInfo object except the
710 // ones which are going to be recalculated from scratch anyway and then switch
711 // our state pointer to point to the new (ready to be updated) state.
712 std::memcpy(&newSt, st, StateCopySize64 * sizeof(uint64_t));
717 // Update side to move
720 // Increment ply counters. In particular, rule50 will be reset to zero later on
721 // in case of a capture or a pawn move.
726 Color us = sideToMove;
728 Square from = from_sq(m);
729 Square to = to_sq(m);
730 Piece pc = piece_on(from);
731 PieceType pt = type_of(pc);
732 PieceType captured = type_of(m) == ENPASSANT ? PAWN : type_of(piece_on(to));
734 assert(color_of(pc) == us);
735 assert(piece_on(to) == NO_PIECE || color_of(piece_on(to)) == them || type_of(m) == CASTLING);
736 assert(captured != KING);
738 if (type_of(m) == CASTLING)
740 assert(pc == make_piece(us, KING));
743 do_castling<true>(from, to, rfrom, rto);
745 captured = NO_PIECE_TYPE;
746 st->psq += psq[us][ROOK][rto] - psq[us][ROOK][rfrom];
747 k ^= Zobrist::psq[us][ROOK][rfrom] ^ Zobrist::psq[us][ROOK][rto];
754 // If the captured piece is a pawn, update pawn hash key, otherwise
755 // update non-pawn material.
756 if (captured == PAWN)
758 if (type_of(m) == ENPASSANT)
760 capsq += pawn_push(them);
763 assert(to == st->epSquare);
764 assert(relative_rank(us, to) == RANK_6);
765 assert(piece_on(to) == NO_PIECE);
766 assert(piece_on(capsq) == make_piece(them, PAWN));
768 board[capsq] = NO_PIECE;
771 st->pawnKey ^= Zobrist::psq[them][PAWN][capsq];
774 st->npMaterial[them] -= PieceValue[MG][captured];
776 // Update board and piece lists
777 remove_piece(capsq, them, captured);
779 // Update material hash key and prefetch access to materialTable
780 k ^= Zobrist::psq[them][captured][capsq];
781 st->materialKey ^= Zobrist::psq[them][captured][pieceCount[them][captured]];
782 prefetch((char*)thisThread->materialTable[st->materialKey]);
784 // Update incremental scores
785 st->psq -= psq[them][captured][capsq];
787 // Reset rule 50 counter
792 k ^= Zobrist::psq[us][pt][from] ^ Zobrist::psq[us][pt][to];
794 // Reset en passant square
795 if (st->epSquare != SQ_NONE)
797 k ^= Zobrist::enpassant[file_of(st->epSquare)];
798 st->epSquare = SQ_NONE;
801 // Update castling rights if needed
802 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
804 int cr = castlingRightsMask[from] | castlingRightsMask[to];
805 k ^= Zobrist::castling[st->castlingRights & cr];
806 st->castlingRights &= ~cr;
809 // Prefetch TT access as soon as we know the new hash key
810 prefetch((char*)TT.first_entry(k));
812 // Move the piece. The tricky Chess960 castling is handled earlier
813 if (type_of(m) != CASTLING)
814 move_piece(from, to, us, pt);
816 // If the moving piece is a pawn do some special extra work
819 // Set en-passant square if the moved pawn can be captured
820 if ( (int(to) ^ int(from)) == 16
821 && (attacks_from<PAWN>(from + pawn_push(us), us) & pieces(them, PAWN)))
823 st->epSquare = Square((from + to) / 2);
824 k ^= Zobrist::enpassant[file_of(st->epSquare)];
827 else if (type_of(m) == PROMOTION)
829 PieceType promotion = promotion_type(m);
831 assert(relative_rank(us, to) == RANK_8);
832 assert(promotion >= KNIGHT && promotion <= QUEEN);
834 remove_piece(to, us, PAWN);
835 put_piece(to, us, promotion);
838 k ^= Zobrist::psq[us][PAWN][to] ^ Zobrist::psq[us][promotion][to];
839 st->pawnKey ^= Zobrist::psq[us][PAWN][to];
840 st->materialKey ^= Zobrist::psq[us][promotion][pieceCount[us][promotion]-1]
841 ^ Zobrist::psq[us][PAWN][pieceCount[us][PAWN]];
843 // Update incremental score
844 st->psq += psq[us][promotion][to] - psq[us][PAWN][to];
847 st->npMaterial[us] += PieceValue[MG][promotion];
850 // Update pawn hash key and prefetch access to pawnsTable
851 st->pawnKey ^= Zobrist::psq[us][PAWN][from] ^ Zobrist::psq[us][PAWN][to];
852 prefetch((char*)thisThread->pawnsTable[st->pawnKey]);
854 // Reset rule 50 draw counter
858 // Update incremental scores
859 st->psq += psq[us][pt][to] - psq[us][pt][from];
862 st->capturedType = captured;
864 // Update the key with the final value
867 // Update checkers bitboard: piece must be already moved due to attacks_from()
872 if (type_of(m) != NORMAL)
873 st->checkersBB = attackers_to(king_square(them)) & pieces(us);
877 if (ci.checkSq[pt] & to)
878 st->checkersBB |= to;
881 if (unlikely(ci.dcCandidates) && (ci.dcCandidates & from))
884 st->checkersBB |= attacks_from<ROOK>(king_square(them)) & pieces(us, QUEEN, ROOK);
887 st->checkersBB |= attacks_from<BISHOP>(king_square(them)) & pieces(us, QUEEN, BISHOP);
892 sideToMove = ~sideToMove;
898 /// Position::undo_move() unmakes a move. When it returns, the position should
899 /// be restored to exactly the same state as before the move was made.
901 void Position::undo_move(Move m) {
905 sideToMove = ~sideToMove;
907 Color us = sideToMove;
908 Square from = from_sq(m);
909 Square to = to_sq(m);
910 PieceType pt = type_of(piece_on(to));
912 assert(empty(from) || type_of(m) == CASTLING);
913 assert(st->capturedType != KING);
915 if (type_of(m) == PROMOTION)
917 assert(pt == promotion_type(m));
918 assert(relative_rank(us, to) == RANK_8);
919 assert(promotion_type(m) >= KNIGHT && promotion_type(m) <= QUEEN);
921 remove_piece(to, us, promotion_type(m));
922 put_piece(to, us, PAWN);
926 if (type_of(m) == CASTLING)
929 do_castling<false>(from, to, rfrom, rto);
933 move_piece(to, from, us, pt); // Put the piece back at the source square
935 if (st->capturedType)
939 if (type_of(m) == ENPASSANT)
941 capsq -= pawn_push(us);
944 assert(to == st->previous->epSquare);
945 assert(relative_rank(us, to) == RANK_6);
946 assert(piece_on(capsq) == NO_PIECE);
949 put_piece(capsq, ~us, st->capturedType); // 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, especially in Chess960.
964 void Position::do_castling(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(sideToMove, kingSide ? SQ_F1 : SQ_D1);
969 to = relative_square(sideToMove, kingSide ? SQ_G1 : SQ_C1);
971 // Remove both pieces first since squares could overlap in Chess960
972 remove_piece(Do ? from : to, sideToMove, KING);
973 remove_piece(Do ? rfrom : rto, sideToMove, ROOK);
974 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
975 put_piece(Do ? to : from, sideToMove, KING);
976 put_piece(Do ? rto : rfrom, sideToMove, ROOK);
980 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
981 /// the side to move without executing any move on the board.
983 void Position::do_null_move(StateInfo& newSt) {
987 std::memcpy(&newSt, st, sizeof(StateInfo)); // Fully copy here
992 if (st->epSquare != SQ_NONE)
994 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
995 st->epSquare = SQ_NONE;
998 st->key ^= Zobrist::side;
999 prefetch((char*)TT.first_entry(st->key));
1002 st->pliesFromNull = 0;
1004 sideToMove = ~sideToMove;
1006 assert(pos_is_ok());
1009 void Position::undo_null_move() {
1011 assert(!checkers());
1014 sideToMove = ~sideToMove;
1018 /// Position::see() is a static exchange evaluator: It tries to estimate the
1019 /// material gain or loss resulting from a move.
1021 Value Position::see_sign(Move m) const {
1025 // Early return if SEE cannot be negative because captured piece value
1026 // is not less then capturing one. Note that king moves always return
1027 // here because king midgame value is set to 0.
1028 if (PieceValue[MG][moved_piece(m)] <= PieceValue[MG][piece_on(to_sq(m))])
1029 return VALUE_KNOWN_WIN;
1034 Value Position::see(Move m) const {
1037 Bitboard occupied, attackers, stmAttackers;
1047 swapList[0] = PieceValue[MG][piece_on(to)];
1048 stm = color_of(piece_on(from));
1049 occupied = pieces() ^ from;
1051 // Castling moves are implemented as king capturing the rook so cannot be
1052 // handled correctly. Simply return 0 that is always the correct value
1053 // unless in the rare case the rook ends up under attack.
1054 if (type_of(m) == CASTLING)
1057 if (type_of(m) == ENPASSANT)
1059 occupied ^= to - pawn_push(stm); // Remove the captured pawn
1060 swapList[0] = PieceValue[MG][PAWN];
1063 // Find all attackers to the destination square, with the moving piece
1064 // removed, but possibly an X-ray attacker added behind it.
1065 attackers = attackers_to(to, occupied) & occupied;
1067 // If the opponent has no attackers we are finished
1069 stmAttackers = attackers & pieces(stm);
1073 // The destination square is defended, which makes things rather more
1074 // difficult to compute. We proceed by building up a "swap list" containing
1075 // the material gain or loss at each stop in a sequence of captures to the
1076 // destination square, where the sides alternately capture, and always
1077 // capture with the least valuable piece. After each capture, we look for
1078 // new X-ray attacks from behind the capturing piece.
1079 captured = type_of(piece_on(from));
1082 assert(slIndex < 32);
1084 // Add the new entry to the swap list
1085 swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][captured];
1087 // Locate and remove the next least valuable attacker
1088 captured = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1090 // Stop before processing a king capture
1091 if (captured == KING)
1093 if (stmAttackers == attackers)
1100 stmAttackers = attackers & pieces(stm);
1103 } while (stmAttackers);
1105 // Having built the swap list, we negamax through it to find the best
1106 // achievable score from the point of view of the side to move.
1108 swapList[slIndex - 1] = std::min(-swapList[slIndex], swapList[slIndex - 1]);
1114 /// Position::is_draw() tests whether the position is drawn by material, 50 moves
1115 /// rule or repetition. It does not detect stalemates.
1117 bool Position::is_draw() const {
1119 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1122 StateInfo* stp = st;
1123 for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
1125 stp = stp->previous->previous;
1127 if (stp->key == st->key)
1128 return true; // Draw at first repetition
1135 /// Position::flip() flips position with the white and black sides reversed. This
1136 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1138 static char toggle_case(char c) {
1139 return char(islower(c) ? toupper(c) : tolower(c));
1142 void Position::flip() {
1145 std::stringstream ss(fen());
1147 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1149 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1150 f.insert(0, token + (f.empty() ? " " : "/"));
1153 ss >> token; // Active color
1154 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1156 ss >> token; // Castling availability
1159 std::transform(f.begin(), f.end(), f.begin(), toggle_case);
1161 ss >> token; // En passant square
1162 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1164 std::getline(ss, token); // Half and full moves
1167 set(f, is_chess960(), this_thread());
1169 assert(pos_is_ok());
1173 /// Position::pos_is_ok() performs some consistency checks for the position object.
1174 /// This is meant to be helpful when debugging.
1176 bool Position::pos_is_ok(int* step) const {
1178 // Which parts of the position should be verified?
1179 const bool all = false;
1181 const bool testBitboards = all || false;
1182 const bool testState = all || false;
1183 const bool testKingCount = all || false;
1184 const bool testKingCapture = all || false;
1185 const bool testPieceCounts = all || false;
1186 const bool testPieceList = all || false;
1187 const bool testCastlingSquares = all || false;
1192 if ( (sideToMove != WHITE && sideToMove != BLACK)
1193 || piece_on(king_square(WHITE)) != W_KING
1194 || piece_on(king_square(BLACK)) != B_KING
1195 || ( ep_square() != SQ_NONE
1196 && relative_rank(sideToMove, ep_square()) != RANK_6))
1199 if (step && ++*step, testBitboards)
1201 // The intersection of the white and black pieces must be empty
1202 if (pieces(WHITE) & pieces(BLACK))
1205 // The union of the white and black pieces must be equal to all
1207 if ((pieces(WHITE) | pieces(BLACK)) != pieces())
1210 // Separate piece type bitboards must have empty intersections
1211 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1212 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1213 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1217 if (step && ++*step, testState)
1221 if ( st->key != si.key
1222 || st->pawnKey != si.pawnKey
1223 || st->materialKey != si.materialKey
1224 || st->npMaterial[WHITE] != si.npMaterial[WHITE]
1225 || st->npMaterial[BLACK] != si.npMaterial[BLACK]
1226 || st->psq != si.psq
1227 || st->checkersBB != si.checkersBB)
1231 if (step && ++*step, testKingCount)
1232 if ( std::count(board, board + SQUARE_NB, W_KING) != 1
1233 || std::count(board, board + SQUARE_NB, B_KING) != 1)
1236 if (step && ++*step, testKingCapture)
1237 if (attackers_to(king_square(~sideToMove)) & pieces(sideToMove))
1240 if (step && ++*step, testPieceCounts)
1241 for (Color c = WHITE; c <= BLACK; ++c)
1242 for (PieceType pt = PAWN; pt <= KING; ++pt)
1243 if (pieceCount[c][pt] != popcount<Full>(pieces(c, pt)))
1246 if (step && ++*step, testPieceList)
1247 for (Color c = WHITE; c <= BLACK; ++c)
1248 for (PieceType pt = PAWN; pt <= KING; ++pt)
1249 for (int i = 0; i < pieceCount[c][pt]; ++i)
1250 if ( board[pieceList[c][pt][i]] != make_piece(c, pt)
1251 || index[pieceList[c][pt][i]] != i)
1254 if (step && ++*step, testCastlingSquares)
1255 for (Color c = WHITE; c <= BLACK; ++c)
1256 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1258 if (!can_castle(c | s))
1261 if ( (castlingRightsMask[king_square(c)] & (c | s)) != (c | s)
1262 || piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1263 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s))