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
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/>.
22 #include <cstring> // For std::memset, std::memcmp
37 Value PieceValue[PHASE_NB][PIECE_NB] = {
38 { VALUE_ZERO, PawnValueMg, KnightValueMg, BishopValueMg, RookValueMg, QueenValueMg },
39 { VALUE_ZERO, PawnValueEg, KnightValueEg, BishopValueEg, RookValueEg, QueenValueEg } };
43 Key psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
44 Key enpassant[FILE_NB];
45 Key castling[CASTLING_RIGHT_NB];
50 Key Position::exclusion_key() const { return st->key ^ Zobrist::exclusion; }
54 const string PieceToChar(" PNBRQK pnbrqk");
55 Score psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
57 // min_attacker() is a helper function used by see() to locate the least
58 // valuable attacker for the side to move, remove the attacker we just found
59 // from the bitboards and scan for new X-ray attacks behind it.
61 template<int Pt> FORCE_INLINE
62 PieceType min_attacker(const Bitboard* bb, const Square& to, const Bitboard& stmAttackers,
63 Bitboard& occupied, Bitboard& attackers) {
65 Bitboard b = stmAttackers & bb[Pt];
67 return min_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
69 occupied ^= b & ~(b - 1);
71 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
72 attackers |= attacks_bb<BISHOP>(to, occupied) & (bb[BISHOP] | bb[QUEEN]);
74 if (Pt == ROOK || Pt == QUEEN)
75 attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
77 attackers &= occupied; // After X-ray that may add already processed pieces
81 template<> FORCE_INLINE
82 PieceType min_attacker<KING>(const Bitboard*, const Square&, const Bitboard&, Bitboard&, Bitboard&) {
83 return KING; // No need to update bitboards: it is the last cycle
91 CheckInfo::CheckInfo(const Position& pos) {
93 Color them = ~pos.side_to_move();
94 ksq = pos.king_square(them);
96 pinned = pos.pinned_pieces(pos.side_to_move());
97 dcCandidates = pos.discovered_check_candidates();
99 checkSq[PAWN] = pos.attacks_from<PAWN>(ksq, them);
100 checkSq[KNIGHT] = pos.attacks_from<KNIGHT>(ksq);
101 checkSq[BISHOP] = pos.attacks_from<BISHOP>(ksq);
102 checkSq[ROOK] = pos.attacks_from<ROOK>(ksq);
103 checkSq[QUEEN] = checkSq[BISHOP] | checkSq[ROOK];
108 /// operator<<(Position) returns an ASCII representation of the position
110 std::ostream& operator<<(std::ostream& os, const Position& pos) {
112 os << "\n +---+---+---+---+---+---+---+---+\n";
114 for (Rank r = RANK_8; r >= RANK_1; --r)
116 for (File f = FILE_A; f <= FILE_H; ++f)
117 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
119 os << " |\n +---+---+---+---+---+---+---+---+\n";
122 os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
123 << std::setfill('0') << std::setw(16) << pos.st->key << std::dec << "\nCheckers: ";
125 for (Bitboard b = pos.checkers(); b; )
126 os << UCI::square(pop_lsb(&b)) << " ";
132 /// Position::init() initializes at startup the various arrays used to compute
133 /// hash keys and the piece square tables. The latter is a two-step operation:
134 /// Firstly, the white halves of the tables are copied from PSQT[] tables.
135 /// Secondly, the black halves of the tables are initialized by flipping and
136 /// changing the sign of the white scores.
138 void Position::init() {
142 for (Color c = WHITE; c <= BLACK; ++c)
143 for (PieceType pt = PAWN; pt <= KING; ++pt)
144 for (Square s = SQ_A1; s <= SQ_H8; ++s)
145 Zobrist::psq[c][pt][s] = rng.rand<Key>();
147 for (File f = FILE_A; f <= FILE_H; ++f)
148 Zobrist::enpassant[f] = rng.rand<Key>();
150 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
155 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
156 Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
160 Zobrist::side = rng.rand<Key>();
161 Zobrist::exclusion = rng.rand<Key>();
163 for (PieceType pt = PAWN; pt <= KING; ++pt)
165 PieceValue[MG][make_piece(BLACK, pt)] = PieceValue[MG][pt];
166 PieceValue[EG][make_piece(BLACK, pt)] = PieceValue[EG][pt];
168 Score v = make_score(PieceValue[MG][pt], PieceValue[EG][pt]);
170 for (Square s = SQ_A1; s <= SQ_H8; ++s)
172 psq[WHITE][pt][ s] = (v + PSQT[pt][s]);
173 psq[BLACK][pt][~s] = -(v + PSQT[pt][s]);
179 /// Position::operator=() creates a copy of 'pos' but detaching the state pointer
180 /// from the source to be self-consistent and not depending on any external data.
182 Position& Position::operator=(const Position& pos) {
184 std::memcpy(this, &pos, sizeof(Position));
185 std::memcpy(&startState, st, sizeof(StateInfo));
195 /// Position::clear() erases the position object to a pristine state, with an
196 /// empty board, white to move, and no castling rights.
198 void Position::clear() {
200 std::memset(this, 0, sizeof(Position));
201 startState.epSquare = SQ_NONE;
204 for (int i = 0; i < PIECE_TYPE_NB; ++i)
205 for (int j = 0; j < 16; ++j)
206 pieceList[WHITE][i][j] = pieceList[BLACK][i][j] = SQ_NONE;
210 /// Position::set() initializes the position object with the given FEN string.
211 /// This function is not very robust - make sure that input FENs are correct,
212 /// this is assumed to be the responsibility of the GUI.
214 void Position::set(const string& fenStr, bool isChess960, Thread* th) {
216 A FEN string defines a particular position using only the ASCII character set.
218 A FEN string contains six fields separated by a space. The fields are:
220 1) Piece placement (from white's perspective). Each rank is described, starting
221 with rank 8 and ending with rank 1. Within each rank, the contents of each
222 square are described from file A through file H. Following the Standard
223 Algebraic Notation (SAN), each piece is identified by a single letter taken
224 from the standard English names. White pieces are designated using upper-case
225 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
226 noted using digits 1 through 8 (the number of blank squares), and "/"
229 2) Active color. "w" means white moves next, "b" means black.
231 3) Castling availability. If neither side can castle, this is "-". Otherwise,
232 this has one or more letters: "K" (White can castle kingside), "Q" (White
233 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
234 can castle queenside).
236 4) En passant target square (in algebraic notation). If there's no en passant
237 target square, this is "-". If a pawn has just made a 2-square move, this
238 is the position "behind" the pawn. This is recorded regardless of whether
239 there is a pawn in position to make an en passant capture.
241 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
242 or capture. This is used to determine if a draw can be claimed under the
245 6) Fullmove number. The number of the full move. It starts at 1, and is
246 incremented after Black's move.
249 unsigned char col, row, token;
252 std::istringstream ss(fenStr);
257 // 1. Piece placement
258 while ((ss >> token) && !isspace(token))
261 sq += Square(token - '0'); // Advance the given number of files
263 else if (token == '/')
266 else if ((idx = PieceToChar.find(token)) != string::npos)
268 put_piece(color_of(Piece(idx)), type_of(Piece(idx)), sq);
275 sideToMove = (token == 'w' ? WHITE : BLACK);
278 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
279 // Shredder-FEN that uses the letters of the columns on which the rooks began
280 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
281 // if an inner rook is associated with the castling right, the castling tag is
282 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
283 while ((ss >> token) && !isspace(token))
286 Color c = islower(token) ? BLACK : WHITE;
288 token = char(toupper(token));
291 for (rsq = relative_square(c, SQ_H1); type_of(piece_on(rsq)) != ROOK; --rsq) {}
293 else if (token == 'Q')
294 for (rsq = relative_square(c, SQ_A1); type_of(piece_on(rsq)) != ROOK; ++rsq) {}
296 else if (token >= 'A' && token <= 'H')
297 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
302 set_castling_right(c, rsq);
305 // 4. En passant square. Ignore if no pawn capture is possible
306 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
307 && ((ss >> row) && (row == '3' || row == '6')))
309 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
311 if (!(attackers_to(st->epSquare) & pieces(sideToMove, PAWN)))
312 st->epSquare = SQ_NONE;
315 // 5-6. Halfmove clock and fullmove number
316 ss >> std::skipws >> st->rule50 >> gamePly;
318 // Convert from fullmove starting from 1 to ply starting from 0,
319 // handle also common incorrect FEN with fullmove = 0.
320 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
322 chess960 = isChess960;
330 /// Position::set_castling_right() is a helper function used to set castling
331 /// rights given the corresponding color and the rook starting square.
333 void Position::set_castling_right(Color c, Square rfrom) {
335 Square kfrom = king_square(c);
336 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
337 CastlingRight cr = (c | cs);
339 st->castlingRights |= cr;
340 castlingRightsMask[kfrom] |= cr;
341 castlingRightsMask[rfrom] |= cr;
342 castlingRookSquare[cr] = rfrom;
344 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
345 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
347 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
348 if (s != kfrom && s != rfrom)
349 castlingPath[cr] |= s;
351 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
352 if (s != kfrom && s != rfrom)
353 castlingPath[cr] |= s;
357 /// Position::set_state() computes the hash keys of the position, and other
358 /// data that once computed is updated incrementally as moves are made.
359 /// The function is only used when a new position is set up, and to verify
360 /// the correctness of the StateInfo data when running in debug mode.
362 void Position::set_state(StateInfo* si) const {
364 si->key = si->pawnKey = si->materialKey = 0;
365 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
366 si->psq = SCORE_ZERO;
368 si->checkersBB = attackers_to(king_square(sideToMove)) & pieces(~sideToMove);
370 for (Bitboard b = pieces(); b; )
372 Square s = pop_lsb(&b);
373 Piece pc = piece_on(s);
374 si->key ^= Zobrist::psq[color_of(pc)][type_of(pc)][s];
375 si->psq += psq[color_of(pc)][type_of(pc)][s];
378 if (si->epSquare != SQ_NONE)
379 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
381 if (sideToMove == BLACK)
382 si->key ^= Zobrist::side;
384 si->key ^= Zobrist::castling[si->castlingRights];
386 for (Bitboard b = pieces(PAWN); b; )
388 Square s = pop_lsb(&b);
389 si->pawnKey ^= Zobrist::psq[color_of(piece_on(s))][PAWN][s];
392 for (Color c = WHITE; c <= BLACK; ++c)
393 for (PieceType pt = PAWN; pt <= KING; ++pt)
394 for (int cnt = 0; cnt < pieceCount[c][pt]; ++cnt)
395 si->materialKey ^= Zobrist::psq[c][pt][cnt];
397 for (Color c = WHITE; c <= BLACK; ++c)
398 for (PieceType pt = KNIGHT; pt <= QUEEN; ++pt)
399 si->nonPawnMaterial[c] += pieceCount[c][pt] * PieceValue[MG][pt];
403 /// Position::fen() returns a FEN representation of the position. In case of
404 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
406 const string Position::fen() const {
409 std::ostringstream ss;
411 for (Rank r = RANK_8; r >= RANK_1; --r)
413 for (File f = FILE_A; f <= FILE_H; ++f)
415 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
422 ss << PieceToChar[piece_on(make_square(f, r))];
429 ss << (sideToMove == WHITE ? " w " : " b ");
431 if (can_castle(WHITE_OO))
432 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | KING_SIDE))) : 'K');
434 if (can_castle(WHITE_OOO))
435 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
437 if (can_castle(BLACK_OO))
438 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | KING_SIDE))) : 'k');
440 if (can_castle(BLACK_OOO))
441 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
443 if (!can_castle(WHITE) && !can_castle(BLACK))
446 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
447 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
453 /// Position::game_phase() calculates the game phase interpolating total non-pawn
454 /// material between endgame and midgame limits.
456 Phase Position::game_phase() const {
458 Value npm = st->nonPawnMaterial[WHITE] + st->nonPawnMaterial[BLACK];
460 npm = std::max(EndgameLimit, std::min(npm, MidgameLimit));
462 return Phase(((npm - EndgameLimit) * PHASE_MIDGAME) / (MidgameLimit - EndgameLimit));
466 /// Position::check_blockers() returns a bitboard of all the pieces with color
467 /// 'c' that are blocking check on the king with color 'kingColor'. A piece
468 /// blocks a check if removing that piece from the board would result in a
469 /// position where the king is in check. A check blocking piece can be either a
470 /// pinned or a discovered check piece, according if its color 'c' is the same
471 /// or the opposite of 'kingColor'.
473 Bitboard Position::check_blockers(Color c, Color kingColor) const {
475 Bitboard b, pinners, result = 0;
476 Square ksq = king_square(kingColor);
478 // Pinners are sliders that give check when a pinned piece is removed
479 pinners = ( (pieces( ROOK, QUEEN) & PseudoAttacks[ROOK ][ksq])
480 | (pieces(BISHOP, QUEEN) & PseudoAttacks[BISHOP][ksq])) & pieces(~kingColor);
484 b = between_bb(ksq, pop_lsb(&pinners)) & pieces();
486 if (!more_than_one(b))
487 result |= b & pieces(c);
493 /// Position::attackers_to() computes a bitboard of all pieces which attack a
494 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
496 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
498 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
499 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
500 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
501 | (attacks_bb<ROOK >(s, occupied) & pieces(ROOK, QUEEN))
502 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
503 | (attacks_from<KING>(s) & pieces(KING));
507 /// Position::legal() tests whether a pseudo-legal move is legal
509 bool Position::legal(Move m, Bitboard pinned) const {
512 assert(pinned == pinned_pieces(sideToMove));
514 Color us = sideToMove;
515 Square from = from_sq(m);
517 assert(color_of(moved_piece(m)) == us);
518 assert(piece_on(king_square(us)) == make_piece(us, KING));
520 // En passant captures are a tricky special case. Because they are rather
521 // uncommon, we do it simply by testing whether the king is attacked after
523 if (type_of(m) == ENPASSANT)
525 Square ksq = king_square(us);
526 Square to = to_sq(m);
527 Square capsq = to - pawn_push(us);
528 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
530 assert(to == ep_square());
531 assert(moved_piece(m) == make_piece(us, PAWN));
532 assert(piece_on(capsq) == make_piece(~us, PAWN));
533 assert(piece_on(to) == NO_PIECE);
535 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
536 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
539 // If the moving piece is a king, check whether the destination
540 // square is attacked by the opponent. Castling moves are checked
541 // for legality during move generation.
542 if (type_of(piece_on(from)) == KING)
543 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & 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.
549 || aligned(from, to_sq(m), king_square(us));
553 /// Position::pseudo_legal() takes a random move and tests whether the move is
554 /// pseudo legal. It is used to validate moves from TT that can be corrupted
555 /// due to SMP concurrent access or hash position key aliasing.
557 bool Position::pseudo_legal(const Move m) const {
559 Color us = sideToMove;
560 Square from = from_sq(m);
561 Square to = to_sq(m);
562 Piece pc = moved_piece(m);
564 // Use a slower but simpler function for uncommon cases
565 if (type_of(m) != NORMAL)
566 return MoveList<LEGAL>(*this).contains(m);
568 // Is not a promotion, so promotion piece must be empty
569 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
572 // If the 'from' square is not occupied by a piece belonging to the side to
573 // move, the move is obviously not legal.
574 if (pc == NO_PIECE || color_of(pc) != us)
577 // The destination square cannot be occupied by a friendly piece
581 // Handle the special case of a pawn move
582 if (type_of(pc) == PAWN)
584 // We have already handled promotion moves, so destination
585 // cannot be on the 8th/1st rank.
586 if (rank_of(to) == relative_rank(us, RANK_8))
589 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
590 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
591 && !( (from + 2 * pawn_push(us) == to) // Not a double push
592 && (rank_of(from) == relative_rank(us, RANK_2))
594 && empty(to - pawn_push(us))))
597 else if (!(attacks_from(pc, from) & to))
600 // Evasions generator already takes care to avoid some kind of illegal moves
601 // and legal() relies on this. We therefore have to take care that the same
602 // kind of moves are filtered out here.
605 if (type_of(pc) != KING)
607 // Double check? In this case a king move is required
608 if (more_than_one(checkers()))
611 // Our move must be a blocking evasion or a capture of the checking piece
612 if (!((between_bb(lsb(checkers()), king_square(us)) | checkers()) & to))
615 // In case of king moves under check we have to remove king so as to catch
616 // invalid moves like b1a1 when opposite queen is on c1.
617 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
625 /// Position::gives_check() tests whether a pseudo-legal move gives a check
627 bool Position::gives_check(Move m, const CheckInfo& ci) const {
630 assert(ci.dcCandidates == discovered_check_candidates());
631 assert(color_of(moved_piece(m)) == sideToMove);
633 Square from = from_sq(m);
634 Square to = to_sq(m);
636 // Is there a direct check?
637 if (ci.checkSq[type_of(piece_on(from))] & to)
640 // Is there a discovered check?
642 && (ci.dcCandidates & from)
643 && !aligned(from, to, ci.ksq))
652 return attacks_bb(Piece(promotion_type(m)), to, pieces() ^ from) & ci.ksq;
654 // En passant capture with check? We have already handled the case
655 // of direct checks and ordinary discovered check, so the only case we
656 // need to handle is the unusual case of a discovered check through
657 // the captured pawn.
660 Square capsq = make_square(file_of(to), rank_of(from));
661 Bitboard b = (pieces() ^ from ^ capsq) | to;
663 return (attacks_bb< ROOK>(ci.ksq, b) & pieces(sideToMove, QUEEN, ROOK))
664 | (attacks_bb<BISHOP>(ci.ksq, b) & pieces(sideToMove, QUEEN, BISHOP));
669 Square rfrom = to; // Castling is encoded as 'King captures the rook'
670 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
671 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
673 return (PseudoAttacks[ROOK][rto] & ci.ksq)
674 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & ci.ksq);
683 /// Position::do_move() makes a move, and saves all information necessary
684 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
685 /// moves should be filtered out before this function is called.
687 void Position::do_move(Move m, StateInfo& newSt) {
690 do_move(m, newSt, ci, gives_check(m, ci));
693 void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool givesCheck) {
696 assert(&newSt != st);
699 Key k = st->key ^ Zobrist::side;
701 // Copy some fields of the old state to our new StateInfo object except the
702 // ones which are going to be recalculated from scratch anyway and then switch
703 // our state pointer to point to the new (ready to be updated) state.
704 std::memcpy(&newSt, st, offsetof(StateInfo, key));
708 // Increment ply counters. In particular, rule50 will be reset to zero later on
709 // in case of a capture or a pawn move.
714 Color us = sideToMove;
716 Square from = from_sq(m);
717 Square to = to_sq(m);
718 PieceType pt = type_of(piece_on(from));
719 PieceType captured = type_of(m) == ENPASSANT ? PAWN : type_of(piece_on(to));
721 assert(color_of(piece_on(from)) == us);
722 assert(piece_on(to) == NO_PIECE || color_of(piece_on(to)) == (type_of(m) != CASTLING ? them : us));
723 assert(captured != KING);
725 if (type_of(m) == CASTLING)
730 do_castling<true>(us, from, to, rfrom, rto);
732 captured = NO_PIECE_TYPE;
733 st->psq += psq[us][ROOK][rto] - psq[us][ROOK][rfrom];
734 k ^= Zobrist::psq[us][ROOK][rfrom] ^ Zobrist::psq[us][ROOK][rto];
741 // If the captured piece is a pawn, update pawn hash key, otherwise
742 // update non-pawn material.
743 if (captured == PAWN)
745 if (type_of(m) == ENPASSANT)
747 capsq -= pawn_push(us);
750 assert(to == st->epSquare);
751 assert(relative_rank(us, to) == RANK_6);
752 assert(piece_on(to) == NO_PIECE);
753 assert(piece_on(capsq) == make_piece(them, PAWN));
755 board[capsq] = NO_PIECE; // Not done by remove_piece()
758 st->pawnKey ^= Zobrist::psq[them][PAWN][capsq];
761 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
763 // Update board and piece lists
764 remove_piece(them, captured, capsq);
766 // Update material hash key and prefetch access to materialTable
767 k ^= Zobrist::psq[them][captured][capsq];
768 st->materialKey ^= Zobrist::psq[them][captured][pieceCount[them][captured]];
769 prefetch(thisThread->materialTable[st->materialKey]);
771 // Update incremental scores
772 st->psq -= psq[them][captured][capsq];
774 // Reset rule 50 counter
779 k ^= Zobrist::psq[us][pt][from] ^ Zobrist::psq[us][pt][to];
781 // Reset en passant square
782 if (st->epSquare != SQ_NONE)
784 k ^= Zobrist::enpassant[file_of(st->epSquare)];
785 st->epSquare = SQ_NONE;
788 // Update castling rights if needed
789 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
791 int cr = castlingRightsMask[from] | castlingRightsMask[to];
792 k ^= Zobrist::castling[st->castlingRights & cr];
793 st->castlingRights &= ~cr;
796 // Move the piece. The tricky Chess960 castling is handled earlier
797 if (type_of(m) != CASTLING)
798 move_piece(us, pt, from, to);
800 // If the moving piece is a pawn do some special extra work
803 // Set en-passant square if the moved pawn can be captured
804 if ( (int(to) ^ int(from)) == 16
805 && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
807 st->epSquare = (from + to) / 2;
808 k ^= Zobrist::enpassant[file_of(st->epSquare)];
811 else if (type_of(m) == PROMOTION)
813 PieceType promotion = promotion_type(m);
815 assert(relative_rank(us, to) == RANK_8);
816 assert(promotion >= KNIGHT && promotion <= QUEEN);
818 remove_piece(us, PAWN, to);
819 put_piece(us, promotion, to);
822 k ^= Zobrist::psq[us][PAWN][to] ^ Zobrist::psq[us][promotion][to];
823 st->pawnKey ^= Zobrist::psq[us][PAWN][to];
824 st->materialKey ^= Zobrist::psq[us][promotion][pieceCount[us][promotion]-1]
825 ^ Zobrist::psq[us][PAWN][pieceCount[us][PAWN]];
827 // Update incremental score
828 st->psq += psq[us][promotion][to] - psq[us][PAWN][to];
831 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
834 // Update pawn hash key and prefetch access to pawnsTable
835 st->pawnKey ^= Zobrist::psq[us][PAWN][from] ^ Zobrist::psq[us][PAWN][to];
836 prefetch(thisThread->pawnsTable[st->pawnKey]);
838 // Reset rule 50 draw counter
842 // Update incremental scores
843 st->psq += psq[us][pt][to] - psq[us][pt][from];
846 st->capturedType = captured;
848 // Update the key with the final value
851 // Update checkers bitboard: piece must be already moved due to attacks_from()
856 if (type_of(m) != NORMAL)
857 st->checkersBB = attackers_to(king_square(them)) & pieces(us);
861 if (ci.checkSq[pt] & to)
862 st->checkersBB |= to;
865 if (ci.dcCandidates && (ci.dcCandidates & from))
870 st->checkersBB |= attacks_from<ROOK>(king_square(them)) & pieces(us, QUEEN, ROOK);
873 st->checkersBB |= attacks_from<BISHOP>(king_square(them)) & pieces(us, QUEEN, BISHOP);
878 sideToMove = ~sideToMove;
884 /// Position::undo_move() unmakes a move. When it returns, the position should
885 /// be restored to exactly the same state as before the move was made.
887 void Position::undo_move(Move m) {
891 sideToMove = ~sideToMove;
893 Color us = sideToMove;
894 Square from = from_sq(m);
895 Square to = to_sq(m);
896 PieceType pt = type_of(piece_on(to));
898 assert(empty(from) || type_of(m) == CASTLING);
899 assert(st->capturedType != KING);
901 if (type_of(m) == PROMOTION)
903 assert(relative_rank(us, to) == RANK_8);
904 assert(pt == promotion_type(m));
905 assert(pt >= KNIGHT && pt <= QUEEN);
907 remove_piece(us, pt, to);
908 put_piece(us, PAWN, to);
912 if (type_of(m) == CASTLING)
915 do_castling<false>(us, from, to, rfrom, rto);
919 move_piece(us, pt, to, from); // Put the piece back at the source square
921 if (st->capturedType)
925 if (type_of(m) == ENPASSANT)
927 capsq -= pawn_push(us);
930 assert(to == st->previous->epSquare);
931 assert(relative_rank(us, to) == RANK_6);
932 assert(piece_on(capsq) == NO_PIECE);
933 assert(st->capturedType == PAWN);
936 put_piece(~us, st->capturedType, capsq); // Restore the captured piece
940 // Finally point our state pointer back to the previous state
948 /// Position::do_castling() is a helper used to do/undo a castling move. This
949 /// is a bit tricky, especially in Chess960.
951 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
953 bool kingSide = to > from;
954 rfrom = to; // Castling is encoded as "king captures friendly rook"
955 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
956 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
958 // Remove both pieces first since squares could overlap in Chess960
959 remove_piece(us, KING, Do ? from : to);
960 remove_piece(us, ROOK, Do ? rfrom : rto);
961 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
962 put_piece(us, KING, Do ? to : from);
963 put_piece(us, ROOK, Do ? rto : rfrom);
967 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
968 /// the side to move without executing any move on the board.
970 void Position::do_null_move(StateInfo& newSt) {
973 assert(&newSt != st);
975 std::memcpy(&newSt, st, sizeof(StateInfo));
979 if (st->epSquare != SQ_NONE)
981 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
982 st->epSquare = SQ_NONE;
985 st->key ^= Zobrist::side;
986 prefetch(TT.first_entry(st->key));
989 st->pliesFromNull = 0;
991 sideToMove = ~sideToMove;
996 void Position::undo_null_move() {
1001 sideToMove = ~sideToMove;
1005 /// Position::key_after() computes the new hash key after the given move. Needed
1006 /// for speculative prefetch. It doesn't recognize special moves like castling,
1007 /// en-passant and promotions.
1009 Key Position::key_after(Move m) const {
1011 Color us = sideToMove;
1012 Square from = from_sq(m);
1013 Square to = to_sq(m);
1014 PieceType pt = type_of(piece_on(from));
1015 PieceType captured = type_of(piece_on(to));
1016 Key k = st->key ^ Zobrist::side;
1019 k ^= Zobrist::psq[~us][captured][to];
1021 return k ^ Zobrist::psq[us][pt][to] ^ Zobrist::psq[us][pt][from];
1025 /// Position::see() is a static exchange evaluator: It tries to estimate the
1026 /// material gain or loss resulting from a move.
1028 Value Position::see_sign(Move m) const {
1032 // Early return if SEE cannot be negative because captured piece value
1033 // is not less then capturing one. Note that king moves always return
1034 // here because king midgame value is set to 0.
1035 if (PieceValue[MG][moved_piece(m)] <= PieceValue[MG][piece_on(to_sq(m))])
1036 return VALUE_KNOWN_WIN;
1041 Value Position::see(Move m) const {
1044 Bitboard occupied, attackers, stmAttackers;
1054 swapList[0] = PieceValue[MG][piece_on(to)];
1055 stm = color_of(piece_on(from));
1056 occupied = pieces() ^ from;
1058 // Castling moves are implemented as king capturing the rook so cannot
1059 // be handled correctly. Simply return VALUE_ZERO that is always correct
1060 // unless in the rare case the rook ends up under attack.
1061 if (type_of(m) == CASTLING)
1064 if (type_of(m) == ENPASSANT)
1066 occupied ^= to - pawn_push(stm); // Remove the captured pawn
1067 swapList[0] = PieceValue[MG][PAWN];
1070 // Find all attackers to the destination square, with the moving piece
1071 // removed, but possibly an X-ray attacker added behind it.
1072 attackers = attackers_to(to, occupied) & occupied;
1074 // If the opponent has no attackers we are finished
1076 stmAttackers = attackers & pieces(stm);
1080 // The destination square is defended, which makes things rather more
1081 // difficult to compute. We proceed by building up a "swap list" containing
1082 // the material gain or loss at each stop in a sequence of captures to the
1083 // destination square, where the sides alternately capture, and always
1084 // capture with the least valuable piece. After each capture, we look for
1085 // new X-ray attacks from behind the capturing piece.
1086 captured = type_of(piece_on(from));
1089 assert(slIndex < 32);
1091 // Add the new entry to the swap list
1092 swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][captured];
1094 // Locate and remove the next least valuable attacker
1095 captured = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1097 stmAttackers = attackers & pieces(stm);
1100 } while (stmAttackers && (captured != KING || (--slIndex, false))); // Stop before a king capture
1102 // Having built the swap list, we negamax through it to find the best
1103 // achievable score from the point of view of the side to move.
1105 swapList[slIndex - 1] = std::min(-swapList[slIndex], swapList[slIndex - 1]);
1111 /// Position::is_draw() tests whether the position is drawn by material, 50 moves
1112 /// rule or repetition. It does not detect stalemates.
1114 bool Position::is_draw() const {
1116 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1119 StateInfo* stp = st;
1120 for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
1122 stp = stp->previous->previous;
1124 if (stp->key == st->key)
1125 return true; // Draw at first repetition
1132 /// Position::flip() flips position with the white and black sides reversed. This
1133 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1135 void Position::flip() {
1138 std::stringstream ss(fen());
1140 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1142 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1143 f.insert(0, token + (f.empty() ? " " : "/"));
1146 ss >> token; // Active color
1147 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1149 ss >> token; // Castling availability
1152 std::transform(f.begin(), f.end(), f.begin(),
1153 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1155 ss >> token; // En passant square
1156 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1158 std::getline(ss, token); // Half and full moves
1161 set(f, is_chess960(), this_thread());
1163 assert(pos_is_ok());
1167 /// Position::pos_is_ok() performs some consistency checks for the position object.
1168 /// This is meant to be helpful when debugging.
1170 bool Position::pos_is_ok(int* failedStep) const {
1172 const bool Fast = true; // Quick (default) or full check?
1174 enum { Default, King, Bitboards, State, Lists, Castling };
1176 for (int step = Default; step <= (Fast ? Default : Castling); step++)
1181 if (step == Default)
1182 if ( (sideToMove != WHITE && sideToMove != BLACK)
1183 || piece_on(king_square(WHITE)) != W_KING
1184 || piece_on(king_square(BLACK)) != B_KING
1185 || ( ep_square() != SQ_NONE
1186 && relative_rank(sideToMove, ep_square()) != RANK_6))
1190 if ( std::count(board, board + SQUARE_NB, W_KING) != 1
1191 || std::count(board, board + SQUARE_NB, B_KING) != 1
1192 || attackers_to(king_square(~sideToMove)) & pieces(sideToMove))
1195 if (step == Bitboards)
1197 if ( (pieces(WHITE) & pieces(BLACK))
1198 ||(pieces(WHITE) | pieces(BLACK)) != pieces())
1201 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1202 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1203 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1211 if (std::memcmp(&si, st, sizeof(StateInfo)))
1216 for (Color c = WHITE; c <= BLACK; ++c)
1217 for (PieceType pt = PAWN; pt <= KING; ++pt)
1219 if (pieceCount[c][pt] != popcount<Full>(pieces(c, pt)))
1222 for (int i = 0; i < pieceCount[c][pt]; ++i)
1223 if ( board[pieceList[c][pt][i]] != make_piece(c, pt)
1224 || index[pieceList[c][pt][i]] != i)
1228 if (step == Castling)
1229 for (Color c = WHITE; c <= BLACK; ++c)
1230 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1232 if (!can_castle(c | s))
1235 if ( piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1236 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
1237 ||(castlingRightsMask[king_square(c)] & (c | s)) != (c | s))