2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
21 #include <cstddef> // For offsetof()
22 #include <cstring> // For std::memset, std::memcmp
33 #include "syzygy/tbprobe.h"
39 Key psq[PIECE_NB][SQUARE_NB];
40 Key enpassant[FILE_NB];
41 Key castling[CASTLING_RIGHT_NB];
47 const string PieceToChar(" PNBRQK pnbrqk");
49 constexpr Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
50 B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
54 /// operator<<(Position) returns an ASCII representation of the position
56 std::ostream& operator<<(std::ostream& os, const Position& pos) {
58 os << "\n +---+---+---+---+---+---+---+---+\n";
60 for (Rank r = RANK_8; r >= RANK_1; --r)
62 for (File f = FILE_A; f <= FILE_H; ++f)
63 os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
65 os << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
68 os << " a b c d e f g h\n"
69 << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
70 << std::setfill('0') << std::setw(16) << pos.key()
71 << std::setfill(' ') << std::dec << "\nCheckers: ";
73 for (Bitboard b = pos.checkers(); b; )
74 os << UCI::square(pop_lsb(&b)) << " ";
76 if ( int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
77 && !pos.can_castle(ANY_CASTLING))
81 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
82 Tablebases::ProbeState s1, s2;
83 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
84 int dtz = Tablebases::probe_dtz(p, &s2);
85 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
86 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
93 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
94 // situations. Description of the algorithm in the following paper:
95 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
97 // First and second hash functions for indexing the cuckoo tables
98 inline int H1(Key h) { return h & 0x1fff; }
99 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
101 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
103 Move cuckooMove[8192];
106 /// Position::init() initializes at startup the various arrays used to compute hash keys
108 void Position::init() {
112 for (Piece pc : Pieces)
113 for (Square s = SQ_A1; s <= SQ_H8; ++s)
114 Zobrist::psq[pc][s] = rng.rand<Key>();
116 for (File f = FILE_A; f <= FILE_H; ++f)
117 Zobrist::enpassant[f] = rng.rand<Key>();
119 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
120 Zobrist::castling[cr] = rng.rand<Key>();
122 Zobrist::side = rng.rand<Key>();
123 Zobrist::noPawns = rng.rand<Key>();
125 // Prepare the cuckoo tables
126 std::memset(cuckoo, 0, sizeof(cuckoo));
127 std::memset(cuckooMove, 0, sizeof(cuckooMove));
129 for (Piece pc : Pieces)
130 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
131 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
132 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
134 Move move = make_move(s1, s2);
135 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
139 std::swap(cuckoo[i], key);
140 std::swap(cuckooMove[i], move);
141 if (move == MOVE_NONE) // Arrived at empty slot?
143 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
147 assert(count == 3668);
151 /// Position::set() initializes the position object with the given FEN string.
152 /// This function is not very robust - make sure that input FENs are correct,
153 /// this is assumed to be the responsibility of the GUI.
155 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
157 A FEN string defines a particular position using only the ASCII character set.
159 A FEN string contains six fields separated by a space. The fields are:
161 1) Piece placement (from white's perspective). Each rank is described, starting
162 with rank 8 and ending with rank 1. Within each rank, the contents of each
163 square are described from file A through file H. Following the Standard
164 Algebraic Notation (SAN), each piece is identified by a single letter taken
165 from the standard English names. White pieces are designated using upper-case
166 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
167 noted using digits 1 through 8 (the number of blank squares), and "/"
170 2) Active color. "w" means white moves next, "b" means black.
172 3) Castling availability. If neither side can castle, this is "-". Otherwise,
173 this has one or more letters: "K" (White can castle kingside), "Q" (White
174 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
175 can castle queenside).
177 4) En passant target square (in algebraic notation). If there's no en passant
178 target square, this is "-". If a pawn has just made a 2-square move, this
179 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
180 if there is a pawn in position to make an en passant capture, and if there really
181 is a pawn that might have advanced two squares.
183 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
184 or capture. This is used to determine if a draw can be claimed under the
187 6) Fullmove number. The number of the full move. It starts at 1, and is
188 incremented after Black's move.
191 unsigned char col, row, token;
194 std::istringstream ss(fenStr);
196 std::memset(this, 0, sizeof(Position));
197 std::memset(si, 0, sizeof(StateInfo));
198 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
203 // 1. Piece placement
204 while ((ss >> token) && !isspace(token))
207 sq += (token - '0') * EAST; // Advance the given number of files
209 else if (token == '/')
212 else if ((idx = PieceToChar.find(token)) != string::npos) {
213 put_piece(Piece(idx), sq);
220 sideToMove = (token == 'w' ? WHITE : BLACK);
223 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
224 // Shredder-FEN that uses the letters of the columns on which the rooks began
225 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
226 // if an inner rook is associated with the castling right, the castling tag is
227 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
228 while ((ss >> token) && !isspace(token))
231 Color c = islower(token) ? BLACK : WHITE;
232 Piece rook = make_piece(c, ROOK);
234 token = char(toupper(token));
237 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
239 else if (token == 'Q')
240 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
242 else if (token >= 'A' && token <= 'H')
243 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
248 set_castling_right(c, rsq);
251 // 4. En passant square.
252 // Ignore if square is invalid or not on side to move relative rank 6.
253 bool enpassant = false;
255 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
256 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
258 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
260 // En passant square will be considered only if
261 // a) side to move have a pawn threatening epSquare
262 // b) there is an enemy pawn in front of epSquare
263 // c) there is no piece on epSquare or behind epSquare
264 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
265 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
266 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
270 st->epSquare = SQ_NONE;
272 // 5-6. Halfmove clock and fullmove number
273 ss >> std::skipws >> st->rule50 >> gamePly;
275 // Convert from fullmove starting from 1 to gamePly starting from 0,
276 // handle also common incorrect FEN with fullmove = 0.
277 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
279 chess960 = isChess960;
287 /// Position::set_castling_right() is a helper function used to set castling
288 /// rights given the corresponding color and the rook starting square.
290 void Position::set_castling_right(Color c, Square rfrom) {
292 Square kfrom = square<KING>(c);
293 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
295 st->castlingRights |= cr;
296 castlingRightsMask[kfrom] |= cr;
297 castlingRightsMask[rfrom] |= cr;
298 castlingRookSquare[cr] = rfrom;
300 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
301 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
303 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
308 /// Position::set_check_info() sets king attacks to detect if a move gives check
310 void Position::set_check_info(StateInfo* si) const {
312 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
313 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
315 Square ksq = square<KING>(~sideToMove);
317 si->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
318 si->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
319 si->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
320 si->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
321 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
322 si->checkSquares[KING] = 0;
326 /// Position::set_state() computes the hash keys of the position, and other
327 /// data that once computed is updated incrementally as moves are made.
328 /// The function is only used when a new position is set up, and to verify
329 /// the correctness of the StateInfo data when running in debug mode.
331 void Position::set_state(StateInfo* si) const {
333 si->key = si->materialKey = 0;
334 si->pawnKey = Zobrist::noPawns;
335 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
336 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
340 for (Bitboard b = pieces(); b; )
342 Square s = pop_lsb(&b);
343 Piece pc = piece_on(s);
344 si->key ^= Zobrist::psq[pc][s];
346 if (type_of(pc) == PAWN)
347 si->pawnKey ^= Zobrist::psq[pc][s];
349 else if (type_of(pc) != KING)
350 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
353 if (si->epSquare != SQ_NONE)
354 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
356 if (sideToMove == BLACK)
357 si->key ^= Zobrist::side;
359 si->key ^= Zobrist::castling[si->castlingRights];
361 for (Piece pc : Pieces)
362 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
363 si->materialKey ^= Zobrist::psq[pc][cnt];
367 /// Position::set() is an overload to initialize the position object with
368 /// the given endgame code string like "KBPKN". It is mainly a helper to
369 /// get the material key out of an endgame code.
371 Position& Position::set(const string& code, Color c, StateInfo* si) {
373 assert(code[0] == 'K');
375 string sides[] = { code.substr(code.find('K', 1)), // Weak
376 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
378 assert(sides[0].length() > 0 && sides[0].length() < 8);
379 assert(sides[1].length() > 0 && sides[1].length() < 8);
381 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
383 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
384 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
386 return set(fenStr, false, si, nullptr);
390 /// Position::fen() returns a FEN representation of the position. In case of
391 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
393 const string Position::fen() const {
396 std::ostringstream ss;
398 for (Rank r = RANK_8; r >= RANK_1; --r)
400 for (File f = FILE_A; f <= FILE_H; ++f)
402 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
409 ss << PieceToChar[piece_on(make_square(f, r))];
416 ss << (sideToMove == WHITE ? " w " : " b ");
418 if (can_castle(WHITE_OO))
419 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
421 if (can_castle(WHITE_OOO))
422 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
424 if (can_castle(BLACK_OO))
425 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
427 if (can_castle(BLACK_OOO))
428 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
430 if (!can_castle(ANY_CASTLING))
433 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
434 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
440 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
441 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
442 /// slider if removing that piece from the board would result in a position where
443 /// square 's' is attacked. For example, a king-attack blocking piece can be either
444 /// a pinned or a discovered check piece, according if its color is the opposite
445 /// or the same of the color of the slider.
447 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
449 Bitboard blockers = 0;
452 // Snipers are sliders that attack 's' when a piece and other snipers are removed
453 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
454 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
455 Bitboard occupancy = pieces() ^ snipers;
459 Square sniperSq = pop_lsb(&snipers);
460 Bitboard b = between_bb(s, sniperSq) & occupancy;
462 if (b && !more_than_one(b))
465 if (b & pieces(color_of(piece_on(s))))
473 /// Position::attackers_to() computes a bitboard of all pieces which attack a
474 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
476 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
478 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
479 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
480 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
481 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
482 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
483 | (attacks_bb<KING>(s) & pieces(KING));
487 /// Position::legal() tests whether a pseudo-legal move is legal
489 bool Position::legal(Move m) const {
493 Color us = sideToMove;
494 Square from = from_sq(m);
495 Square to = to_sq(m);
497 assert(color_of(moved_piece(m)) == us);
498 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
500 // En passant captures are a tricky special case. Because they are rather
501 // uncommon, we do it simply by testing whether the king is attacked after
503 if (type_of(m) == ENPASSANT)
505 Square ksq = square<KING>(us);
506 Square capsq = to - pawn_push(us);
507 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
509 assert(to == ep_square());
510 assert(moved_piece(m) == make_piece(us, PAWN));
511 assert(piece_on(capsq) == make_piece(~us, PAWN));
512 assert(piece_on(to) == NO_PIECE);
514 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
515 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
518 // Castling moves generation does not check if the castling path is clear of
519 // enemy attacks, it is delayed at a later time: now!
520 if (type_of(m) == CASTLING)
522 // After castling, the rook and king final positions are the same in
523 // Chess960 as they would be in standard chess.
524 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
525 Direction step = to > from ? WEST : EAST;
527 for (Square s = to; s != from; s += step)
528 if (attackers_to(s) & pieces(~us))
531 // In case of Chess960, verify that when moving the castling rook we do
532 // not discover some hidden checker.
533 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
535 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
538 // If the moving piece is a king, check whether the destination square is
539 // attacked by the opponent.
540 if (type_of(piece_on(from)) == KING)
541 return !(attackers_to(to) & pieces(~us));
543 // A non-king move is legal if and only if it is not pinned or it
544 // is moving along the ray towards or away from the king.
545 return !(blockers_for_king(us) & from)
546 || aligned(from, to, square<KING>(us));
550 /// Position::pseudo_legal() takes a random move and tests whether the move is
551 /// pseudo legal. It is used to validate moves from TT that can be corrupted
552 /// due to SMP concurrent access or hash position key aliasing.
554 bool Position::pseudo_legal(const Move m) const {
556 Color us = sideToMove;
557 Square from = from_sq(m);
558 Square to = to_sq(m);
559 Piece pc = moved_piece(m);
561 // Use a slower but simpler function for uncommon cases
562 if (type_of(m) != NORMAL)
563 return MoveList<LEGAL>(*this).contains(m);
565 // Is not a promotion, so promotion piece must be empty
566 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
569 // If the 'from' square is not occupied by a piece belonging to the side to
570 // move, the move is obviously not legal.
571 if (pc == NO_PIECE || color_of(pc) != us)
574 // The destination square cannot be occupied by a friendly piece
578 // Handle the special case of a pawn move
579 if (type_of(pc) == PAWN)
581 // We have already handled promotion moves, so destination
582 // cannot be on the 8th/1st rank.
583 if ((Rank8BB | Rank1BB) & to)
586 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
587 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
588 && !( (from + 2 * pawn_push(us) == to) // Not a double push
589 && (relative_rank(us, from) == RANK_2)
591 && empty(to - pawn_push(us))))
594 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
597 // Evasions generator already takes care to avoid some kind of illegal moves
598 // and legal() relies on this. We therefore have to take care that the same
599 // kind of moves are filtered out here.
602 if (type_of(pc) != KING)
604 // Double check? In this case a king move is required
605 if (more_than_one(checkers()))
608 // Our move must be a blocking evasion or a capture of the checking piece
609 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
612 // In case of king moves under check we have to remove king so as to catch
613 // invalid moves like b1a1 when opposite queen is on c1.
614 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
622 /// Position::gives_check() tests whether a pseudo-legal move gives a check
624 bool Position::gives_check(Move m) const {
627 assert(color_of(moved_piece(m)) == sideToMove);
629 Square from = from_sq(m);
630 Square to = to_sq(m);
632 // Is there a direct check?
633 if (check_squares(type_of(piece_on(from))) & to)
636 // Is there a discovered check?
637 if ( (blockers_for_king(~sideToMove) & from)
638 && !aligned(from, to, square<KING>(~sideToMove)))
647 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
649 // En passant capture with check? We have already handled the case
650 // of direct checks and ordinary discovered check, so the only case we
651 // need to handle is the unusual case of a discovered check through
652 // the captured pawn.
655 Square capsq = make_square(file_of(to), rank_of(from));
656 Bitboard b = (pieces() ^ from ^ capsq) | to;
658 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
659 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
664 Square rfrom = to; // Castling is encoded as 'king captures the rook'
665 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
666 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
668 return (attacks_bb<ROOK>(rto) & square<KING>(~sideToMove))
669 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
678 /// Position::do_move() makes a move, and saves all information necessary
679 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
680 /// moves should be filtered out before this function is called.
682 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
685 assert(&newSt != st);
687 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
688 Key k = st->key ^ Zobrist::side;
690 // Copy some fields of the old state to our new StateInfo object except the
691 // ones which are going to be recalculated from scratch anyway and then switch
692 // our state pointer to point to the new (ready to be updated) state.
693 std::memcpy(&newSt, st, offsetof(StateInfo, key));
697 // Increment ply counters. In particular, rule50 will be reset to zero later on
698 // in case of a capture or a pawn move.
704 st->accumulator.computed_accumulation = false;
705 auto& dp = st->dirtyPiece;
708 Color us = sideToMove;
710 Square from = from_sq(m);
711 Square to = to_sq(m);
712 Piece pc = piece_on(from);
713 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
715 assert(color_of(pc) == us);
716 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
717 assert(type_of(captured) != KING);
719 if (type_of(m) == CASTLING)
721 assert(pc == make_piece(us, KING));
722 assert(captured == make_piece(us, ROOK));
725 do_castling<true>(us, from, to, rfrom, rto);
727 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
735 // If the captured piece is a pawn, update pawn hash key, otherwise
736 // update non-pawn material.
737 if (type_of(captured) == PAWN)
739 if (type_of(m) == ENPASSANT)
741 capsq -= pawn_push(us);
743 assert(pc == make_piece(us, PAWN));
744 assert(to == st->epSquare);
745 assert(relative_rank(us, to) == RANK_6);
746 assert(piece_on(to) == NO_PIECE);
747 assert(piece_on(capsq) == make_piece(them, PAWN));
750 st->pawnKey ^= Zobrist::psq[captured][capsq];
753 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
757 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
758 dp.piece[1] = captured;
763 // Update board and piece lists
766 if (type_of(m) == ENPASSANT)
767 board[capsq] = NO_PIECE;
769 // Update material hash key and prefetch access to materialTable
770 k ^= Zobrist::psq[captured][capsq];
771 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
772 prefetch(thisThread->materialTable[st->materialKey]);
774 // Reset rule 50 counter
779 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][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 k ^= Zobrist::castling[st->castlingRights];
792 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
793 k ^= Zobrist::castling[st->castlingRights];
796 // Move the piece. The tricky Chess960 castling is handled earlier
797 if (type_of(m) != CASTLING)
806 move_piece(from, to);
809 // If the moving piece is a pawn do some special extra work
810 if (type_of(pc) == PAWN)
812 // Set en-passant square if the moved pawn can be captured
813 if ( (int(to) ^ int(from)) == 16
814 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
816 st->epSquare = to - pawn_push(us);
817 k ^= Zobrist::enpassant[file_of(st->epSquare)];
820 else if (type_of(m) == PROMOTION)
822 Piece promotion = make_piece(us, promotion_type(m));
824 assert(relative_rank(us, to) == RANK_8);
825 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
828 put_piece(promotion, to);
832 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
834 dp.piece[dp.dirty_num] = promotion;
835 dp.from[dp.dirty_num] = SQ_NONE;
836 dp.to[dp.dirty_num] = to;
841 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
842 st->pawnKey ^= Zobrist::psq[pc][to];
843 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
844 ^ Zobrist::psq[pc][pieceCount[pc]];
847 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
850 // Update pawn hash key
851 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
853 // Reset rule 50 draw counter
858 st->capturedPiece = captured;
860 // Update the key with the final value
863 // Calculate checkers bitboard (if move gives check)
864 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
866 sideToMove = ~sideToMove;
868 // Update king attacks used for fast check detection
871 // Calculate the repetition info. It is the ply distance from the previous
872 // occurrence of the same position, negative in the 3-fold case, or zero
873 // if the position was not repeated.
875 int end = std::min(st->rule50, st->pliesFromNull);
878 StateInfo* stp = st->previous->previous;
879 for (int i = 4; i <= end; i += 2)
881 stp = stp->previous->previous;
882 if (stp->key == st->key)
884 st->repetition = stp->repetition ? -i : i;
894 /// Position::undo_move() unmakes a move. When it returns, the position should
895 /// be restored to exactly the same state as before the move was made.
897 void Position::undo_move(Move m) {
901 sideToMove = ~sideToMove;
903 Color us = sideToMove;
904 Square from = from_sq(m);
905 Square to = to_sq(m);
906 Piece pc = piece_on(to);
908 assert(empty(from) || type_of(m) == CASTLING);
909 assert(type_of(st->capturedPiece) != KING);
911 if (type_of(m) == PROMOTION)
913 assert(relative_rank(us, to) == RANK_8);
914 assert(type_of(pc) == promotion_type(m));
915 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
918 pc = make_piece(us, PAWN);
922 if (type_of(m) == CASTLING)
925 do_castling<false>(us, from, to, rfrom, rto);
929 move_piece(to, from); // Put the piece back at the source square
931 if (st->capturedPiece)
935 if (type_of(m) == ENPASSANT)
937 capsq -= pawn_push(us);
939 assert(type_of(pc) == PAWN);
940 assert(to == st->previous->epSquare);
941 assert(relative_rank(us, to) == RANK_6);
942 assert(piece_on(capsq) == NO_PIECE);
943 assert(st->capturedPiece == make_piece(~us, PAWN));
946 put_piece(st->capturedPiece, capsq); // Restore the captured piece
950 // Finally point our state pointer back to the previous state
958 /// Position::do_castling() is a helper used to do/undo a castling move. This
959 /// is a bit tricky in Chess960 where from/to squares can overlap.
961 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
963 bool kingSide = to > from;
964 rfrom = to; // Castling is encoded as "king captures friendly rook"
965 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
966 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
968 if (Do && Eval::useNNUE)
970 auto& dp = st->dirtyPiece;
971 dp.piece[0] = make_piece(us, KING);
974 dp.piece[1] = make_piece(us, ROOK);
980 // Remove both pieces first since squares could overlap in Chess960
981 remove_piece(Do ? from : to);
982 remove_piece(Do ? rfrom : rto);
983 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
984 put_piece(make_piece(us, KING), Do ? to : from);
985 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
989 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
990 /// the side to move without executing any move on the board.
992 void Position::do_null_move(StateInfo& newSt) {
995 assert(&newSt != st);
999 std::memcpy(&newSt, st, sizeof(StateInfo));
1002 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1004 newSt.previous = st;
1007 if (st->epSquare != SQ_NONE)
1009 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1010 st->epSquare = SQ_NONE;
1013 st->key ^= Zobrist::side;
1014 prefetch(TT.first_entry(st->key));
1017 st->pliesFromNull = 0;
1019 sideToMove = ~sideToMove;
1025 assert(pos_is_ok());
1028 void Position::undo_null_move() {
1030 assert(!checkers());
1033 sideToMove = ~sideToMove;
1037 /// Position::key_after() computes the new hash key after the given move. Needed
1038 /// for speculative prefetch. It doesn't recognize special moves like castling,
1039 /// en-passant and promotions.
1041 Key Position::key_after(Move m) const {
1043 Square from = from_sq(m);
1044 Square to = to_sq(m);
1045 Piece pc = piece_on(from);
1046 Piece captured = piece_on(to);
1047 Key k = st->key ^ Zobrist::side;
1050 k ^= Zobrist::psq[captured][to];
1052 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1056 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1057 /// SEE value of move is greater or equal to the given threshold. We'll use an
1058 /// algorithm similar to alpha-beta pruning with a null window.
1060 bool Position::see_ge(Move m, Value threshold) const {
1064 // Only deal with normal moves, assume others pass a simple see
1065 if (type_of(m) != NORMAL)
1066 return VALUE_ZERO >= threshold;
1068 Square from = from_sq(m), to = to_sq(m);
1070 int swap = PieceValue[MG][piece_on(to)] - threshold;
1074 swap = PieceValue[MG][piece_on(from)] - swap;
1078 Bitboard occupied = pieces() ^ from ^ to;
1079 Color stm = color_of(piece_on(from));
1080 Bitboard attackers = attackers_to(to, occupied);
1081 Bitboard stmAttackers, bb;
1087 attackers &= occupied;
1089 // If stm has no more attackers then give up: stm loses
1090 if (!(stmAttackers = attackers & pieces(stm)))
1093 // Don't allow pinned pieces to attack (except the king) as long as
1094 // there are pinners on their original square.
1095 if (pinners(~stm) & occupied)
1096 stmAttackers &= ~blockers_for_king(stm);
1103 // Locate and remove the next least valuable attacker, and add to
1104 // the bitboard 'attackers' any X-ray attackers behind it.
1105 if ((bb = stmAttackers & pieces(PAWN)))
1107 if ((swap = PawnValueMg - swap) < res)
1110 occupied ^= lsb(bb);
1111 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1114 else if ((bb = stmAttackers & pieces(KNIGHT)))
1116 if ((swap = KnightValueMg - swap) < res)
1119 occupied ^= lsb(bb);
1122 else if ((bb = stmAttackers & pieces(BISHOP)))
1124 if ((swap = BishopValueMg - swap) < res)
1127 occupied ^= lsb(bb);
1128 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1131 else if ((bb = stmAttackers & pieces(ROOK)))
1133 if ((swap = RookValueMg - swap) < res)
1136 occupied ^= lsb(bb);
1137 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1140 else if ((bb = stmAttackers & pieces(QUEEN)))
1142 if ((swap = QueenValueMg - swap) < res)
1145 occupied ^= lsb(bb);
1146 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1147 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1151 // If we "capture" with the king but opponent still has attackers,
1152 // reverse the result.
1153 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1160 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1161 /// or by repetition. It does not detect stalemates.
1163 bool Position::is_draw(int ply) const {
1165 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1168 // Return a draw score if a position repeats once earlier but strictly
1169 // after the root, or repeats twice before or at the root.
1170 return st->repetition && st->repetition < ply;
1174 // Position::has_repeated() tests whether there has been at least one repetition
1175 // of positions since the last capture or pawn move.
1177 bool Position::has_repeated() const {
1179 StateInfo* stc = st;
1180 int end = std::min(st->rule50, st->pliesFromNull);
1183 if (stc->repetition)
1186 stc = stc->previous;
1192 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1193 /// or an earlier position has a move that directly reaches the current position.
1195 bool Position::has_game_cycle(int ply) const {
1199 int end = std::min(st->rule50, st->pliesFromNull);
1204 Key originalKey = st->key;
1205 StateInfo* stp = st->previous;
1207 for (int i = 3; i <= end; i += 2)
1209 stp = stp->previous->previous;
1211 Key moveKey = originalKey ^ stp->key;
1212 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1213 || (j = H2(moveKey), cuckoo[j] == moveKey))
1215 Move move = cuckooMove[j];
1216 Square s1 = from_sq(move);
1217 Square s2 = to_sq(move);
1219 if (!(between_bb(s1, s2) & pieces()))
1224 // For nodes before or at the root, check that the move is a
1225 // repetition rather than a move to the current position.
1226 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1227 // the same location, so we have to select which square to check.
1228 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1231 // For repetitions before or at the root, require one more
1232 if (stp->repetition)
1241 /// Position::flip() flips position with the white and black sides reversed. This
1242 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1244 void Position::flip() {
1247 std::stringstream ss(fen());
1249 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1251 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1252 f.insert(0, token + (f.empty() ? " " : "/"));
1255 ss >> token; // Active color
1256 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1258 ss >> token; // Castling availability
1261 std::transform(f.begin(), f.end(), f.begin(),
1262 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1264 ss >> token; // En passant square
1265 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1267 std::getline(ss, token); // Half and full moves
1270 set(f, is_chess960(), st, this_thread());
1272 assert(pos_is_ok());
1276 /// Position::pos_is_ok() performs some consistency checks for the
1277 /// position object and raises an asserts if something wrong is detected.
1278 /// This is meant to be helpful when debugging.
1280 bool Position::pos_is_ok() const {
1282 constexpr bool Fast = true; // Quick (default) or full check?
1284 if ( (sideToMove != WHITE && sideToMove != BLACK)
1285 || piece_on(square<KING>(WHITE)) != W_KING
1286 || piece_on(square<KING>(BLACK)) != B_KING
1287 || ( ep_square() != SQ_NONE
1288 && relative_rank(sideToMove, ep_square()) != RANK_6))
1289 assert(0 && "pos_is_ok: Default");
1294 if ( pieceCount[W_KING] != 1
1295 || pieceCount[B_KING] != 1
1296 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1297 assert(0 && "pos_is_ok: Kings");
1299 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1300 || pieceCount[W_PAWN] > 8
1301 || pieceCount[B_PAWN] > 8)
1302 assert(0 && "pos_is_ok: Pawns");
1304 if ( (pieces(WHITE) & pieces(BLACK))
1305 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1306 || popcount(pieces(WHITE)) > 16
1307 || popcount(pieces(BLACK)) > 16)
1308 assert(0 && "pos_is_ok: Bitboards");
1310 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1311 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1312 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1313 assert(0 && "pos_is_ok: Bitboards");
1317 if (std::memcmp(&si, st, sizeof(StateInfo)))
1318 assert(0 && "pos_is_ok: State");
1320 for (Piece pc : Pieces)
1322 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1323 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1324 assert(0 && "pos_is_ok: Pieces");
1326 for (int i = 0; i < pieceCount[pc]; ++i)
1327 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1328 assert(0 && "pos_is_ok: Index");
1331 for (Color c : { WHITE, BLACK })
1332 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1334 if (!can_castle(cr))
1337 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1338 || castlingRightsMask[castlingRookSquare[cr]] != cr
1339 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1340 assert(0 && "pos_is_ok: Castling");