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/>.
37 static const string PieceToChar(" PNBRQK pnbrqk");
41 Value PieceValue[PHASE_NB][PIECE_NB] = {
42 { VALUE_ZERO, PawnValueMg, KnightValueMg, BishopValueMg, RookValueMg, QueenValueMg },
43 { VALUE_ZERO, PawnValueEg, KnightValueEg, BishopValueEg, RookValueEg, QueenValueEg } };
45 static Score psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
49 Key psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];
50 Key enpassant[FILE_NB];
51 Key castling[CASTLING_RIGHT_NB];
56 Key Position::exclusion_key() const { return st->key ^ Zobrist::exclusion;}
60 // min_attacker() is a helper function used by see() to locate the least
61 // valuable attacker for the side to move, remove the attacker we just found
62 // from the bitboards and scan for new X-ray attacks behind it.
64 template<int Pt> FORCE_INLINE
65 PieceType min_attacker(const Bitboard* bb, const Square& to, const Bitboard& stmAttackers,
66 Bitboard& occupied, Bitboard& attackers) {
68 Bitboard b = stmAttackers & bb[Pt];
70 return min_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
72 occupied ^= b & ~(b - 1);
74 if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
75 attackers |= attacks_bb<BISHOP>(to, occupied) & (bb[BISHOP] | bb[QUEEN]);
77 if (Pt == ROOK || Pt == QUEEN)
78 attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
80 attackers &= occupied; // After X-ray that may add already processed pieces
84 template<> FORCE_INLINE
85 PieceType min_attacker<KING>(const Bitboard*, const Square&, const Bitboard&, Bitboard&, Bitboard&) {
86 return KING; // No need to update bitboards: it is the last cycle
94 CheckInfo::CheckInfo(const Position& pos) {
96 Color them = ~pos.side_to_move();
97 ksq = pos.king_square(them);
99 pinned = pos.pinned_pieces(pos.side_to_move());
100 dcCandidates = pos.discovered_check_candidates();
102 checkSq[PAWN] = pos.attacks_from<PAWN>(ksq, them);
103 checkSq[KNIGHT] = pos.attacks_from<KNIGHT>(ksq);
104 checkSq[BISHOP] = pos.attacks_from<BISHOP>(ksq);
105 checkSq[ROOK] = pos.attacks_from<ROOK>(ksq);
106 checkSq[QUEEN] = checkSq[BISHOP] | checkSq[ROOK];
111 /// Position::init() initializes at startup the various arrays used to compute
112 /// hash keys and the piece square tables. The latter is a two-step operation:
113 /// Firstly, the white halves of the tables are copied from PSQT[] tables.
114 /// Secondly, the black halves of the tables are initialized by flipping and
115 /// changing the sign of the white scores.
117 void Position::init() {
121 for (Color c = WHITE; c <= BLACK; ++c)
122 for (PieceType pt = PAWN; pt <= KING; ++pt)
123 for (Square s = SQ_A1; s <= SQ_H8; ++s)
124 Zobrist::psq[c][pt][s] = rk.rand<Key>();
126 for (File f = FILE_A; f <= FILE_H; ++f)
127 Zobrist::enpassant[f] = rk.rand<Key>();
129 for (int cf = NO_CASTLING; cf <= ANY_CASTLING; ++cf)
134 Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
135 Zobrist::castling[cf] ^= k ? k : rk.rand<Key>();
139 Zobrist::side = rk.rand<Key>();
140 Zobrist::exclusion = rk.rand<Key>();
142 for (PieceType pt = PAWN; pt <= KING; ++pt)
144 PieceValue[MG][make_piece(BLACK, pt)] = PieceValue[MG][pt];
145 PieceValue[EG][make_piece(BLACK, pt)] = PieceValue[EG][pt];
147 Score v = make_score(PieceValue[MG][pt], PieceValue[EG][pt]);
149 for (Square s = SQ_A1; s <= SQ_H8; ++s)
151 psq[WHITE][pt][ s] = (v + PSQT[pt][s]);
152 psq[BLACK][pt][~s] = -(v + PSQT[pt][s]);
158 /// Position::operator=() creates a copy of 'pos'. We want the new born Position
159 /// object to not depend on any external data so we detach state pointer from
162 Position& Position::operator=(const Position& pos) {
164 std::memcpy(this, &pos, sizeof(Position));
175 /// Position::clear() erases the position object to a pristine state, with an
176 /// empty board, white to move, and no castling rights.
178 void Position::clear() {
180 std::memset(this, 0, sizeof(Position));
181 startState.epSquare = SQ_NONE;
184 for (int i = 0; i < PIECE_TYPE_NB; ++i)
185 for (int j = 0; j < 16; ++j)
186 pieceList[WHITE][i][j] = pieceList[BLACK][i][j] = SQ_NONE;
190 /// Position::set() initializes the position object with the given FEN string.
191 /// This function is not very robust - make sure that input FENs are correct,
192 /// this is assumed to be the responsibility of the GUI.
194 void Position::set(const string& fenStr, bool isChess960, Thread* th) {
196 A FEN string defines a particular position using only the ASCII character set.
198 A FEN string contains six fields separated by a space. The fields are:
200 1) Piece placement (from white's perspective). Each rank is described, starting
201 with rank 8 and ending with rank 1. Within each rank, the contents of each
202 square are described from file A through file H. Following the Standard
203 Algebraic Notation (SAN), each piece is identified by a single letter taken
204 from the standard English names. White pieces are designated using upper-case
205 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
206 noted using digits 1 through 8 (the number of blank squares), and "/"
209 2) Active color. "w" means white moves next, "b" means black.
211 3) Castling availability. If neither side can castle, this is "-". Otherwise,
212 this has one or more letters: "K" (White can castle kingside), "Q" (White
213 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
214 can castle queenside).
216 4) En passant target square (in algebraic notation). If there's no en passant
217 target square, this is "-". If a pawn has just made a 2-square move, this
218 is the position "behind" the pawn. This is recorded regardless of whether
219 there is a pawn in position to make an en passant capture.
221 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
222 or capture. This is used to determine if a draw can be claimed under the
225 6) Fullmove number. The number of the full move. It starts at 1, and is
226 incremented after Black's move.
229 unsigned char col, row, token;
232 std::istringstream ss(fenStr);
237 // 1. Piece placement
238 while ((ss >> token) && !isspace(token))
241 sq += Square(token - '0'); // Advance the given number of files
243 else if (token == '/')
246 else if ((idx = PieceToChar.find(token)) != string::npos)
248 put_piece(sq, color_of(Piece(idx)), type_of(Piece(idx)));
255 sideToMove = (token == 'w' ? WHITE : BLACK);
258 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
259 // Shredder-FEN that uses the letters of the columns on which the rooks began
260 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
261 // if an inner rook is associated with the castling right, the castling tag is
262 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
263 while ((ss >> token) && !isspace(token))
266 Color c = islower(token) ? BLACK : WHITE;
268 token = char(toupper(token));
271 for (rsq = relative_square(c, SQ_H1); type_of(piece_on(rsq)) != ROOK; --rsq) {}
273 else if (token == 'Q')
274 for (rsq = relative_square(c, SQ_A1); type_of(piece_on(rsq)) != ROOK; ++rsq) {}
276 else if (token >= 'A' && token <= 'H')
277 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
282 set_castling_right(c, rsq);
285 // 4. En passant square. Ignore if no pawn capture is possible
286 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
287 && ((ss >> row) && (row == '3' || row == '6')))
289 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
291 if (!(attackers_to(st->epSquare) & pieces(sideToMove, PAWN)))
292 st->epSquare = SQ_NONE;
295 // 5-6. Halfmove clock and fullmove number
296 ss >> std::skipws >> st->rule50 >> gamePly;
298 // Convert from fullmove starting from 1 to ply starting from 0,
299 // handle also common incorrect FEN with fullmove = 0.
300 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
302 chess960 = isChess960;
310 /// Position::set_castling_right() is a helper function used to set castling
311 /// rights given the corresponding color and the rook starting square.
313 void Position::set_castling_right(Color c, Square rfrom) {
315 Square kfrom = king_square(c);
316 CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
317 CastlingRight cr = (c | cs);
319 st->castlingRights |= cr;
320 castlingRightsMask[kfrom] |= cr;
321 castlingRightsMask[rfrom] |= cr;
322 castlingRookSquare[cr] = rfrom;
324 Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
325 Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
327 for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
328 if (s != kfrom && s != rfrom)
329 castlingPath[cr] |= s;
331 for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
332 if (s != kfrom && s != rfrom)
333 castlingPath[cr] |= s;
337 /// Position::set_state() computes the hash keys of the position, and other
338 /// data that once computed is updated incrementally as moves are made.
339 /// The function is only used when a new position is set up, and to verify
340 /// the correctness of the StateInfo data when running in debug mode.
342 void Position::set_state(StateInfo* si) const {
344 si->key = si->pawnKey = si->materialKey = 0;
345 si->npMaterial[WHITE] = si->npMaterial[BLACK] = VALUE_ZERO;
346 si->psq = SCORE_ZERO;
348 si->checkersBB = attackers_to(king_square(sideToMove)) & pieces(~sideToMove);
350 for (Bitboard b = pieces(); b; )
352 Square s = pop_lsb(&b);
353 Piece pc = piece_on(s);
354 si->key ^= Zobrist::psq[color_of(pc)][type_of(pc)][s];
355 si->psq += psq[color_of(pc)][type_of(pc)][s];
358 if (ep_square() != SQ_NONE)
359 si->key ^= Zobrist::enpassant[file_of(ep_square())];
361 if (sideToMove == BLACK)
362 si->key ^= Zobrist::side;
364 si->key ^= Zobrist::castling[st->castlingRights];
366 for (Bitboard b = pieces(PAWN); b; )
368 Square s = pop_lsb(&b);
369 si->pawnKey ^= Zobrist::psq[color_of(piece_on(s))][PAWN][s];
372 for (Color c = WHITE; c <= BLACK; ++c)
373 for (PieceType pt = PAWN; pt <= KING; ++pt)
374 for (int cnt = 0; cnt < pieceCount[c][pt]; ++cnt)
375 si->materialKey ^= Zobrist::psq[c][pt][cnt];
377 for (Color c = WHITE; c <= BLACK; ++c)
378 for (PieceType pt = KNIGHT; pt <= QUEEN; ++pt)
379 si->npMaterial[c] += pieceCount[c][pt] * PieceValue[MG][pt];
383 /// Position::fen() returns a FEN representation of the position. In case of
384 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
386 const string Position::fen() const {
389 std::ostringstream ss;
391 for (Rank r = RANK_8; r >= RANK_1; --r)
393 for (File f = FILE_A; f <= FILE_H; ++f)
395 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
402 ss << PieceToChar[piece_on(make_square(f, r))];
409 ss << (sideToMove == WHITE ? " w " : " b ");
411 if (can_castle(WHITE_OO))
412 ss << (chess960 ? to_char(file_of(castling_rook_square(WHITE | KING_SIDE)), false) : 'K');
414 if (can_castle(WHITE_OOO))
415 ss << (chess960 ? to_char(file_of(castling_rook_square(WHITE | QUEEN_SIDE)), false) : 'Q');
417 if (can_castle(BLACK_OO))
418 ss << (chess960 ? to_char(file_of(castling_rook_square(BLACK | KING_SIDE)), true) : 'k');
420 if (can_castle(BLACK_OOO))
421 ss << (chess960 ? to_char(file_of(castling_rook_square(BLACK | QUEEN_SIDE)), true) : 'q');
423 if (!can_castle(WHITE) && !can_castle(BLACK))
426 ss << (ep_square() == SQ_NONE ? " - " : " " + to_string(ep_square()) + " ")
427 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
433 /// Position::pretty() returns an ASCII representation of the position
435 const string Position::pretty() const {
437 std::ostringstream ss;
439 ss << "\n +---+---+---+---+---+---+---+---+\n";
441 for (Rank r = RANK_8; r >= RANK_1; --r)
443 for (File f = FILE_A; f <= FILE_H; ++f)
444 ss << " | " << PieceToChar[piece_on(make_square(f, r))];
446 ss << " |\n +---+---+---+---+---+---+---+---+\n";
449 ss << "\nFen: " << fen() << "\nKey: " << std::hex << std::uppercase
450 << std::setfill('0') << std::setw(16) << st->key << "\nCheckers: ";
452 for (Bitboard b = checkers(); b; )
453 ss << to_string(pop_lsb(&b)) << " ";
459 /// Position::game_phase() calculates the game phase interpolating total non-pawn
460 /// material between endgame and midgame limits.
462 Phase Position::game_phase() const {
464 Value npm = st->npMaterial[WHITE] + st->npMaterial[BLACK];
466 npm = std::max(EndgameLimit, std::min(npm, MidgameLimit));
468 return Phase(((npm - EndgameLimit) * 128) / (MidgameLimit - EndgameLimit));
472 /// Position::check_blockers() returns a bitboard of all the pieces with color
473 /// 'c' that are blocking check on the king with color 'kingColor'. A piece
474 /// blocks a check if removing that piece from the board would result in a
475 /// position where the king is in check. A check blocking piece can be either a
476 /// pinned or a discovered check piece, according if its color 'c' is the same
477 /// or the opposite of 'kingColor'.
479 Bitboard Position::check_blockers(Color c, Color kingColor) const {
481 Bitboard b, pinners, result = 0;
482 Square ksq = king_square(kingColor);
484 // Pinners are sliders that give check when a pinned piece is removed
485 pinners = ( (pieces( ROOK, QUEEN) & PseudoAttacks[ROOK ][ksq])
486 | (pieces(BISHOP, QUEEN) & PseudoAttacks[BISHOP][ksq])) & pieces(~kingColor);
490 b = between_bb(ksq, pop_lsb(&pinners)) & pieces();
492 if (!more_than_one(b))
493 result |= b & pieces(c);
499 /// Position::attackers_to() computes a bitboard of all pieces which attack a
500 /// given square. Slider attacks use the occ bitboard to indicate occupancy.
502 Bitboard Position::attackers_to(Square s, Bitboard occ) const {
504 return (attacks_from<PAWN>(s, BLACK) & pieces(WHITE, PAWN))
505 | (attacks_from<PAWN>(s, WHITE) & pieces(BLACK, PAWN))
506 | (attacks_from<KNIGHT>(s) & pieces(KNIGHT))
507 | (attacks_bb<ROOK>(s, occ) & pieces(ROOK, QUEEN))
508 | (attacks_bb<BISHOP>(s, occ) & pieces(BISHOP, QUEEN))
509 | (attacks_from<KING>(s) & pieces(KING));
513 /// Position::legal() tests whether a pseudo-legal move is legal
515 bool Position::legal(Move m, Bitboard pinned) const {
518 assert(pinned == pinned_pieces(sideToMove));
520 Color us = sideToMove;
521 Square from = from_sq(m);
523 assert(color_of(moved_piece(m)) == us);
524 assert(piece_on(king_square(us)) == make_piece(us, KING));
526 // En passant captures are a tricky special case. Because they are rather
527 // uncommon, we do it simply by testing whether the king is attacked after
529 if (type_of(m) == ENPASSANT)
531 Square ksq = king_square(us);
532 Square to = to_sq(m);
533 Square capsq = to - pawn_push(us);
534 Bitboard occ = (pieces() ^ from ^ capsq) | to;
536 assert(to == ep_square());
537 assert(moved_piece(m) == make_piece(us, PAWN));
538 assert(piece_on(capsq) == make_piece(~us, PAWN));
539 assert(piece_on(to) == NO_PIECE);
541 return !(attacks_bb< ROOK>(ksq, occ) & pieces(~us, QUEEN, ROOK))
542 && !(attacks_bb<BISHOP>(ksq, occ) & pieces(~us, QUEEN, BISHOP));
545 // If the moving piece is a king, check whether the destination
546 // square is attacked by the opponent. Castling moves are checked
547 // for legality during move generation.
548 if (type_of(piece_on(from)) == KING)
549 return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
551 // A non-king move is legal if and only if it is not pinned or it
552 // is moving along the ray towards or away from the king.
555 || aligned(from, to_sq(m), king_square(us));
559 /// Position::pseudo_legal() takes a random move and tests whether the move is
560 /// pseudo legal. It is used to validate moves from TT that can be corrupted
561 /// due to SMP concurrent access or hash position key aliasing.
563 bool Position::pseudo_legal(const Move m) const {
565 Color us = sideToMove;
566 Square from = from_sq(m);
567 Square to = to_sq(m);
568 Piece pc = moved_piece(m);
570 // Use a slower but simpler function for uncommon cases
571 if (type_of(m) != NORMAL)
572 return MoveList<LEGAL>(*this).contains(m);
574 // Is not a promotion, so promotion piece must be empty
575 if (promotion_type(m) - 2 != NO_PIECE_TYPE)
578 // If the 'from' square is not occupied by a piece belonging to the side to
579 // move, the move is obviously not legal.
580 if (pc == NO_PIECE || color_of(pc) != us)
583 // The destination square cannot be occupied by a friendly piece
587 // Handle the special case of a pawn move
588 if (type_of(pc) == PAWN)
590 // We have already handled promotion moves, so destination
591 // cannot be on the 8th/1st rank.
592 if (rank_of(to) == relative_rank(us, RANK_8))
595 if ( !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
597 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
599 && !( (from + 2 * pawn_push(us) == to) // Not a double push
600 && (rank_of(from) == relative_rank(us, RANK_2))
602 && empty(to - pawn_push(us))))
605 else if (!(attacks_from(pc, from) & to))
608 // Evasions generator already takes care to avoid some kind of illegal moves
609 // and legal() relies on this. We therefore have to take care that the same
610 // kind of moves are filtered out here.
613 if (type_of(pc) != KING)
615 // Double check? In this case a king move is required
616 if (more_than_one(checkers()))
619 // Our move must be a blocking evasion or a capture of the checking piece
620 if (!((between_bb(lsb(checkers()), king_square(us)) | checkers()) & to))
623 // In case of king moves under check we have to remove king so as to catch
624 // invalid moves like b1a1 when opposite queen is on c1.
625 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
633 /// Position::gives_check() tests whether a pseudo-legal move gives a check
635 bool Position::gives_check(Move m, const CheckInfo& ci) const {
638 assert(ci.dcCandidates == discovered_check_candidates());
639 assert(color_of(moved_piece(m)) == sideToMove);
641 Square from = from_sq(m);
642 Square to = to_sq(m);
643 PieceType pt = type_of(piece_on(from));
645 // Is there a direct check?
646 if (ci.checkSq[pt] & to)
649 // Is there a discovered check?
650 if ( unlikely(ci.dcCandidates)
651 && (ci.dcCandidates & from)
652 && !aligned(from, to, ci.ksq))
661 return attacks_bb(Piece(promotion_type(m)), to, pieces() ^ from) & ci.ksq;
663 // En passant capture with check? We have already handled the case
664 // of direct checks and ordinary discovered check, so the only case we
665 // need to handle is the unusual case of a discovered check through
666 // the captured pawn.
669 Square capsq = make_square(file_of(to), rank_of(from));
670 Bitboard b = (pieces() ^ from ^ capsq) | to;
672 return (attacks_bb< ROOK>(ci.ksq, b) & pieces(sideToMove, QUEEN, ROOK))
673 | (attacks_bb<BISHOP>(ci.ksq, b) & pieces(sideToMove, QUEEN, BISHOP));
678 Square rfrom = to; // Castling is encoded as 'King captures the rook'
679 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
680 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
682 return (PseudoAttacks[ROOK][rto] & ci.ksq)
683 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & ci.ksq);
692 /// Position::do_move() makes a move, and saves all information necessary
693 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
694 /// moves should be filtered out before this function is called.
696 void Position::do_move(Move m, StateInfo& newSt) {
699 do_move(m, newSt, ci, gives_check(m, ci));
702 void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool moveIsCheck) {
705 assert(&newSt != st);
710 // Copy some fields of the old state to our new StateInfo object except the
711 // ones which are going to be recalculated from scratch anyway and then switch
712 // our state pointer to point to the new (ready to be updated) state.
713 std::memcpy(&newSt, st, StateCopySize64 * sizeof(uint64_t));
718 // Update side to move
721 // Increment ply counters. In particular, rule50 will be reset to zero later on
722 // in case of a capture or a pawn move.
727 Color us = sideToMove;
729 Square from = from_sq(m);
730 Square to = to_sq(m);
731 Piece pc = piece_on(from);
732 PieceType pt = type_of(pc);
733 PieceType captured = type_of(m) == ENPASSANT ? PAWN : type_of(piece_on(to));
735 assert(color_of(pc) == us);
736 assert(piece_on(to) == NO_PIECE || color_of(piece_on(to)) == them || type_of(m) == CASTLING);
737 assert(captured != KING);
739 if (type_of(m) == CASTLING)
741 assert(pc == make_piece(us, KING));
744 do_castling<true>(from, to, rfrom, rto);
746 captured = NO_PIECE_TYPE;
747 st->psq += psq[us][ROOK][rto] - psq[us][ROOK][rfrom];
748 k ^= Zobrist::psq[us][ROOK][rfrom] ^ Zobrist::psq[us][ROOK][rto];
755 // If the captured piece is a pawn, update pawn hash key, otherwise
756 // update non-pawn material.
757 if (captured == PAWN)
759 if (type_of(m) == ENPASSANT)
761 capsq += pawn_push(them);
764 assert(to == st->epSquare);
765 assert(relative_rank(us, to) == RANK_6);
766 assert(piece_on(to) == NO_PIECE);
767 assert(piece_on(capsq) == make_piece(them, PAWN));
769 board[capsq] = NO_PIECE;
772 st->pawnKey ^= Zobrist::psq[them][PAWN][capsq];
775 st->npMaterial[them] -= PieceValue[MG][captured];
777 // Update board and piece lists
778 remove_piece(capsq, them, captured);
780 // Update material hash key and prefetch access to materialTable
781 k ^= Zobrist::psq[them][captured][capsq];
782 st->materialKey ^= Zobrist::psq[them][captured][pieceCount[them][captured]];
783 prefetch((char*)thisThread->materialTable[st->materialKey]);
785 // Update incremental scores
786 st->psq -= psq[them][captured][capsq];
788 // Reset rule 50 counter
793 k ^= Zobrist::psq[us][pt][from] ^ Zobrist::psq[us][pt][to];
795 // Reset en passant square
796 if (st->epSquare != SQ_NONE)
798 k ^= Zobrist::enpassant[file_of(st->epSquare)];
799 st->epSquare = SQ_NONE;
802 // Update castling rights if needed
803 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
805 int cr = castlingRightsMask[from] | castlingRightsMask[to];
806 k ^= Zobrist::castling[st->castlingRights & cr];
807 st->castlingRights &= ~cr;
810 // Prefetch TT access as soon as we know the new hash key
811 prefetch((char*)TT.first_entry(k));
813 // Move the piece. The tricky Chess960 castling is handled earlier
814 if (type_of(m) != CASTLING)
815 move_piece(from, to, us, pt);
817 // If the moving piece is a pawn do some special extra work
820 // Set en-passant square if the moved pawn can be captured
821 if ( (int(to) ^ int(from)) == 16
822 && (attacks_from<PAWN>(from + pawn_push(us), us) & pieces(them, PAWN)))
824 st->epSquare = Square((from + to) / 2);
825 k ^= Zobrist::enpassant[file_of(st->epSquare)];
828 else if (type_of(m) == PROMOTION)
830 PieceType promotion = promotion_type(m);
832 assert(relative_rank(us, to) == RANK_8);
833 assert(promotion >= KNIGHT && promotion <= QUEEN);
835 remove_piece(to, us, PAWN);
836 put_piece(to, us, promotion);
839 k ^= Zobrist::psq[us][PAWN][to] ^ Zobrist::psq[us][promotion][to];
840 st->pawnKey ^= Zobrist::psq[us][PAWN][to];
841 st->materialKey ^= Zobrist::psq[us][promotion][pieceCount[us][promotion]-1]
842 ^ Zobrist::psq[us][PAWN][pieceCount[us][PAWN]];
844 // Update incremental score
845 st->psq += psq[us][promotion][to] - psq[us][PAWN][to];
848 st->npMaterial[us] += PieceValue[MG][promotion];
851 // Update pawn hash key and prefetch access to pawnsTable
852 st->pawnKey ^= Zobrist::psq[us][PAWN][from] ^ Zobrist::psq[us][PAWN][to];
853 prefetch((char*)thisThread->pawnsTable[st->pawnKey]);
855 // Reset rule 50 draw counter
859 // Update incremental scores
860 st->psq += psq[us][pt][to] - psq[us][pt][from];
863 st->capturedType = captured;
865 // Update the key with the final value
868 // Update checkers bitboard: piece must be already moved due to attacks_from()
873 if (type_of(m) != NORMAL)
874 st->checkersBB = attackers_to(king_square(them)) & pieces(us);
878 if (ci.checkSq[pt] & to)
879 st->checkersBB |= to;
882 if (unlikely(ci.dcCandidates) && (ci.dcCandidates & from))
885 st->checkersBB |= attacks_from<ROOK>(king_square(them)) & pieces(us, QUEEN, ROOK);
888 st->checkersBB |= attacks_from<BISHOP>(king_square(them)) & pieces(us, QUEEN, BISHOP);
893 sideToMove = ~sideToMove;
899 /// Position::undo_move() unmakes a move. When it returns, the position should
900 /// be restored to exactly the same state as before the move was made.
902 void Position::undo_move(Move m) {
906 sideToMove = ~sideToMove;
908 Color us = sideToMove;
909 Square from = from_sq(m);
910 Square to = to_sq(m);
911 PieceType pt = type_of(piece_on(to));
913 assert(empty(from) || type_of(m) == CASTLING);
914 assert(st->capturedType != KING);
916 if (type_of(m) == PROMOTION)
918 assert(pt == promotion_type(m));
919 assert(relative_rank(us, to) == RANK_8);
920 assert(promotion_type(m) >= KNIGHT && promotion_type(m) <= QUEEN);
922 remove_piece(to, us, promotion_type(m));
923 put_piece(to, us, PAWN);
927 if (type_of(m) == CASTLING)
930 do_castling<false>(from, to, rfrom, rto);
934 move_piece(to, from, us, pt); // Put the piece back at the source square
936 if (st->capturedType)
940 if (type_of(m) == ENPASSANT)
942 capsq -= pawn_push(us);
945 assert(to == st->previous->epSquare);
946 assert(relative_rank(us, to) == RANK_6);
947 assert(piece_on(capsq) == NO_PIECE);
950 put_piece(capsq, ~us, st->capturedType); // Restore the captured piece
954 // Finally point our state pointer back to the previous state
962 /// Position::do_castling() is a helper used to do/undo a castling move. This
963 /// is a bit tricky, especially in Chess960.
965 void Position::do_castling(Square from, Square& to, Square& rfrom, Square& rto) {
967 bool kingSide = to > from;
968 rfrom = to; // Castling is encoded as "king captures friendly rook"
969 rto = relative_square(sideToMove, kingSide ? SQ_F1 : SQ_D1);
970 to = relative_square(sideToMove, kingSide ? SQ_G1 : SQ_C1);
972 // Remove both pieces first since squares could overlap in Chess960
973 remove_piece(Do ? from : to, sideToMove, KING);
974 remove_piece(Do ? rfrom : rto, sideToMove, ROOK);
975 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
976 put_piece(Do ? to : from, sideToMove, KING);
977 put_piece(Do ? rto : rfrom, sideToMove, ROOK);
981 /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
982 /// the side to move without executing any move on the board.
984 void Position::do_null_move(StateInfo& newSt) {
988 std::memcpy(&newSt, st, sizeof(StateInfo)); // Fully copy here
993 if (st->epSquare != SQ_NONE)
995 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
996 st->epSquare = SQ_NONE;
999 st->key ^= Zobrist::side;
1000 prefetch((char*)TT.first_entry(st->key));
1003 st->pliesFromNull = 0;
1005 sideToMove = ~sideToMove;
1007 assert(pos_is_ok());
1010 void Position::undo_null_move() {
1012 assert(!checkers());
1015 sideToMove = ~sideToMove;
1018 // Position::hash_after_move() updates the hash key needed for the speculative prefetch.
1019 // It doesn't recognize special moves like castling, en-passant and promotions.
1020 Key Position::hash_after_move(Move m) const {
1022 int from = from_sq(m);
1024 Piece p = board[from];
1025 Piece capP = board[to];
1026 Key ret = st->key ^ Zobrist::side;
1027 if (capP != NO_PIECE)
1028 ret ^= Zobrist::psq[color_of(capP)][type_of(capP)][to];
1029 ret ^= Zobrist::psq[color_of(p)][type_of(p)][to];
1030 ret ^= Zobrist::psq[color_of(p)][type_of(p)][from];
1034 /// Position::see() is a static exchange evaluator: It tries to estimate the
1035 /// material gain or loss resulting from a move.
1037 Value Position::see_sign(Move m) const {
1041 // Early return if SEE cannot be negative because captured piece value
1042 // is not less then capturing one. Note that king moves always return
1043 // here because king midgame value is set to 0.
1044 if (PieceValue[MG][moved_piece(m)] <= PieceValue[MG][piece_on(to_sq(m))])
1045 return VALUE_KNOWN_WIN;
1050 Value Position::see(Move m) const {
1053 Bitboard occupied, attackers, stmAttackers;
1063 swapList[0] = PieceValue[MG][piece_on(to)];
1064 stm = color_of(piece_on(from));
1065 occupied = pieces() ^ from;
1067 // Castling moves are implemented as king capturing the rook so cannot be
1068 // handled correctly. Simply return 0 that is always the correct value
1069 // unless in the rare case the rook ends up under attack.
1070 if (type_of(m) == CASTLING)
1073 if (type_of(m) == ENPASSANT)
1075 occupied ^= to - pawn_push(stm); // Remove the captured pawn
1076 swapList[0] = PieceValue[MG][PAWN];
1079 // Find all attackers to the destination square, with the moving piece
1080 // removed, but possibly an X-ray attacker added behind it.
1081 attackers = attackers_to(to, occupied) & occupied;
1083 // If the opponent has no attackers we are finished
1085 stmAttackers = attackers & pieces(stm);
1089 // The destination square is defended, which makes things rather more
1090 // difficult to compute. We proceed by building up a "swap list" containing
1091 // the material gain or loss at each stop in a sequence of captures to the
1092 // destination square, where the sides alternately capture, and always
1093 // capture with the least valuable piece. After each capture, we look for
1094 // new X-ray attacks from behind the capturing piece.
1095 captured = type_of(piece_on(from));
1098 assert(slIndex < 32);
1100 // Add the new entry to the swap list
1101 swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][captured];
1103 // Locate and remove the next least valuable attacker
1104 captured = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
1106 // Stop before processing a king capture
1107 if (captured == KING)
1109 if (stmAttackers == attackers)
1116 stmAttackers = attackers & pieces(stm);
1119 } while (stmAttackers);
1121 // Having built the swap list, we negamax through it to find the best
1122 // achievable score from the point of view of the side to move.
1124 swapList[slIndex - 1] = std::min(-swapList[slIndex], swapList[slIndex - 1]);
1130 /// Position::is_draw() tests whether the position is drawn by material, 50 moves
1131 /// rule or repetition. It does not detect stalemates.
1133 bool Position::is_draw() const {
1135 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1138 StateInfo* stp = st;
1139 for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
1141 stp = stp->previous->previous;
1143 if (stp->key == st->key)
1144 return true; // Draw at first repetition
1151 /// Position::flip() flips position with the white and black sides reversed. This
1152 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1154 static char toggle_case(char c) {
1155 return char(islower(c) ? toupper(c) : tolower(c));
1158 void Position::flip() {
1161 std::stringstream ss(fen());
1163 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1165 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1166 f.insert(0, token + (f.empty() ? " " : "/"));
1169 ss >> token; // Active color
1170 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1172 ss >> token; // Castling availability
1175 std::transform(f.begin(), f.end(), f.begin(), toggle_case);
1177 ss >> token; // En passant square
1178 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1180 std::getline(ss, token); // Half and full moves
1183 set(f, is_chess960(), this_thread());
1185 assert(pos_is_ok());
1189 /// Position::pos_is_ok() performs some consistency checks for the position object.
1190 /// This is meant to be helpful when debugging.
1192 bool Position::pos_is_ok(int* step) const {
1194 // Which parts of the position should be verified?
1195 const bool all = false;
1197 const bool testBitboards = all || false;
1198 const bool testState = all || false;
1199 const bool testKingCount = all || false;
1200 const bool testKingCapture = all || false;
1201 const bool testPieceCounts = all || false;
1202 const bool testPieceList = all || false;
1203 const bool testCastlingSquares = all || false;
1208 if ( (sideToMove != WHITE && sideToMove != BLACK)
1209 || piece_on(king_square(WHITE)) != W_KING
1210 || piece_on(king_square(BLACK)) != B_KING
1211 || ( ep_square() != SQ_NONE
1212 && relative_rank(sideToMove, ep_square()) != RANK_6))
1215 if (step && ++*step, testBitboards)
1217 // The intersection of the white and black pieces must be empty
1218 if (pieces(WHITE) & pieces(BLACK))
1221 // The union of the white and black pieces must be equal to all
1223 if ((pieces(WHITE) | pieces(BLACK)) != pieces())
1226 // Separate piece type bitboards must have empty intersections
1227 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1228 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1229 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1233 if (step && ++*step, testState)
1237 if ( st->key != si.key
1238 || st->pawnKey != si.pawnKey
1239 || st->materialKey != si.materialKey
1240 || st->npMaterial[WHITE] != si.npMaterial[WHITE]
1241 || st->npMaterial[BLACK] != si.npMaterial[BLACK]
1242 || st->psq != si.psq
1243 || st->checkersBB != si.checkersBB)
1247 if (step && ++*step, testKingCount)
1248 if ( std::count(board, board + SQUARE_NB, W_KING) != 1
1249 || std::count(board, board + SQUARE_NB, B_KING) != 1)
1252 if (step && ++*step, testKingCapture)
1253 if (attackers_to(king_square(~sideToMove)) & pieces(sideToMove))
1256 if (step && ++*step, testPieceCounts)
1257 for (Color c = WHITE; c <= BLACK; ++c)
1258 for (PieceType pt = PAWN; pt <= KING; ++pt)
1259 if (pieceCount[c][pt] != popcount<Full>(pieces(c, pt)))
1262 if (step && ++*step, testPieceList)
1263 for (Color c = WHITE; c <= BLACK; ++c)
1264 for (PieceType pt = PAWN; pt <= KING; ++pt)
1265 for (int i = 0; i < pieceCount[c][pt]; ++i)
1266 if ( board[pieceList[c][pt][i]] != make_piece(c, pt)
1267 || index[pieceList[c][pt][i]] != i)
1270 if (step && ++*step, testCastlingSquares)
1271 for (Color c = WHITE; c <= BLACK; ++c)
1272 for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
1274 if (!can_castle(c | s))
1277 if ( (castlingRightsMask[king_square(c)] & (c | s)) != (c | s)
1278 || piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
1279 || castlingRightsMask[castlingRookSquare[c | s]] != (c | s))