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))
80 ASSERT_ALIGNED(&st, Eval::NNUE::kCacheLineSize);
83 p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
84 Tablebases::ProbeState s1, s2;
85 Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
86 int dtz = Tablebases::probe_dtz(p, &s2);
87 os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
88 << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
95 // Marcel van Kervinck's cuckoo algorithm for fast detection of "upcoming repetition"
96 // situations. Description of the algorithm in the following paper:
97 // https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
99 // First and second hash functions for indexing the cuckoo tables
100 inline int H1(Key h) { return h & 0x1fff; }
101 inline int H2(Key h) { return (h >> 16) & 0x1fff; }
103 // Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
105 Move cuckooMove[8192];
108 /// Position::init() initializes at startup the various arrays used to compute hash keys
110 void Position::init() {
114 for (Piece pc : Pieces)
115 for (Square s = SQ_A1; s <= SQ_H8; ++s)
116 Zobrist::psq[pc][s] = rng.rand<Key>();
118 for (File f = FILE_A; f <= FILE_H; ++f)
119 Zobrist::enpassant[f] = rng.rand<Key>();
121 for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
122 Zobrist::castling[cr] = rng.rand<Key>();
124 Zobrist::side = rng.rand<Key>();
125 Zobrist::noPawns = rng.rand<Key>();
127 // Prepare the cuckoo tables
128 std::memset(cuckoo, 0, sizeof(cuckoo));
129 std::memset(cuckooMove, 0, sizeof(cuckooMove));
131 for (Piece pc : Pieces)
132 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
133 for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
134 if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
136 Move move = make_move(s1, s2);
137 Key key = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
141 std::swap(cuckoo[i], key);
142 std::swap(cuckooMove[i], move);
143 if (move == MOVE_NONE) // Arrived at empty slot?
145 i = (i == H1(key)) ? H2(key) : H1(key); // Push victim to alternative slot
149 assert(count == 3668);
153 /// Position::set() initializes the position object with the given FEN string.
154 /// This function is not very robust - make sure that input FENs are correct,
155 /// this is assumed to be the responsibility of the GUI.
157 Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
159 A FEN string defines a particular position using only the ASCII character set.
161 A FEN string contains six fields separated by a space. The fields are:
163 1) Piece placement (from white's perspective). Each rank is described, starting
164 with rank 8 and ending with rank 1. Within each rank, the contents of each
165 square are described from file A through file H. Following the Standard
166 Algebraic Notation (SAN), each piece is identified by a single letter taken
167 from the standard English names. White pieces are designated using upper-case
168 letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
169 noted using digits 1 through 8 (the number of blank squares), and "/"
172 2) Active color. "w" means white moves next, "b" means black.
174 3) Castling availability. If neither side can castle, this is "-". Otherwise,
175 this has one or more letters: "K" (White can castle kingside), "Q" (White
176 can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
177 can castle queenside).
179 4) En passant target square (in algebraic notation). If there's no en passant
180 target square, this is "-". If a pawn has just made a 2-square move, this
181 is the position "behind" the pawn. Following X-FEN standard, this is recorded only
182 if there is a pawn in position to make an en passant capture, and if there really
183 is a pawn that might have advanced two squares.
185 5) Halfmove clock. This is the number of halfmoves since the last pawn advance
186 or capture. This is used to determine if a draw can be claimed under the
189 6) Fullmove number. The number of the full move. It starts at 1, and is
190 incremented after Black's move.
193 unsigned char col, row, token;
196 std::istringstream ss(fenStr);
198 std::memset(this, 0, sizeof(Position));
199 std::memset(si, 0, sizeof(StateInfo));
200 std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
205 // 1. Piece placement
206 while ((ss >> token) && !isspace(token))
209 sq += (token - '0') * EAST; // Advance the given number of files
211 else if (token == '/')
214 else if ((idx = PieceToChar.find(token)) != string::npos) {
215 put_piece(Piece(idx), sq);
222 sideToMove = (token == 'w' ? WHITE : BLACK);
225 // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
226 // Shredder-FEN that uses the letters of the columns on which the rooks began
227 // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
228 // if an inner rook is associated with the castling right, the castling tag is
229 // replaced by the file letter of the involved rook, as for the Shredder-FEN.
230 while ((ss >> token) && !isspace(token))
233 Color c = islower(token) ? BLACK : WHITE;
234 Piece rook = make_piece(c, ROOK);
236 token = char(toupper(token));
239 for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
241 else if (token == 'Q')
242 for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
244 else if (token >= 'A' && token <= 'H')
245 rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
250 set_castling_right(c, rsq);
253 // 4. En passant square.
254 // Ignore if square is invalid or not on side to move relative rank 6.
255 bool enpassant = false;
257 if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
258 && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
260 st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
262 // En passant square will be considered only if
263 // a) side to move have a pawn threatening epSquare
264 // b) there is an enemy pawn in front of epSquare
265 // c) there is no piece on epSquare or behind epSquare
266 enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
267 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
268 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
272 st->epSquare = SQ_NONE;
274 // 5-6. Halfmove clock and fullmove number
275 ss >> std::skipws >> st->rule50 >> gamePly;
277 // Convert from fullmove starting from 1 to gamePly starting from 0,
278 // handle also common incorrect FEN with fullmove = 0.
279 gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
281 chess960 = isChess960;
284 st->accumulator.state[WHITE] = Eval::NNUE::INIT;
285 st->accumulator.state[BLACK] = Eval::NNUE::INIT;
293 /// Position::set_castling_right() is a helper function used to set castling
294 /// rights given the corresponding color and the rook starting square.
296 void Position::set_castling_right(Color c, Square rfrom) {
298 Square kfrom = square<KING>(c);
299 CastlingRights cr = c & (kfrom < rfrom ? KING_SIDE: QUEEN_SIDE);
301 st->castlingRights |= cr;
302 castlingRightsMask[kfrom] |= cr;
303 castlingRightsMask[rfrom] |= cr;
304 castlingRookSquare[cr] = rfrom;
306 Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
307 Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);
309 castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto) | rto | kto)
314 /// Position::set_check_info() sets king attacks to detect if a move gives check
316 void Position::set_check_info(StateInfo* si) const {
318 si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
319 si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
321 Square ksq = square<KING>(~sideToMove);
323 si->checkSquares[PAWN] = pawn_attacks_bb(~sideToMove, ksq);
324 si->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
325 si->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
326 si->checkSquares[ROOK] = attacks_bb<ROOK>(ksq, pieces());
327 si->checkSquares[QUEEN] = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
328 si->checkSquares[KING] = 0;
332 /// Position::set_state() computes the hash keys of the position, and other
333 /// data that once computed is updated incrementally as moves are made.
334 /// The function is only used when a new position is set up, and to verify
335 /// the correctness of the StateInfo data when running in debug mode.
337 void Position::set_state(StateInfo* si) const {
339 si->key = si->materialKey = 0;
340 si->pawnKey = Zobrist::noPawns;
341 si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
342 si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
346 for (Bitboard b = pieces(); b; )
348 Square s = pop_lsb(&b);
349 Piece pc = piece_on(s);
350 si->key ^= Zobrist::psq[pc][s];
352 if (type_of(pc) == PAWN)
353 si->pawnKey ^= Zobrist::psq[pc][s];
355 else if (type_of(pc) != KING)
356 si->nonPawnMaterial[color_of(pc)] += PieceValue[MG][pc];
359 if (si->epSquare != SQ_NONE)
360 si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
362 if (sideToMove == BLACK)
363 si->key ^= Zobrist::side;
365 si->key ^= Zobrist::castling[si->castlingRights];
367 for (Piece pc : Pieces)
368 for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
369 si->materialKey ^= Zobrist::psq[pc][cnt];
373 /// Position::set() is an overload to initialize the position object with
374 /// the given endgame code string like "KBPKN". It is mainly a helper to
375 /// get the material key out of an endgame code.
377 Position& Position::set(const string& code, Color c, StateInfo* si) {
379 assert(code[0] == 'K');
381 string sides[] = { code.substr(code.find('K', 1)), // Weak
382 code.substr(0, std::min(code.find('v'), code.find('K', 1))) }; // Strong
384 assert(sides[0].length() > 0 && sides[0].length() < 8);
385 assert(sides[1].length() > 0 && sides[1].length() < 8);
387 std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
389 string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
390 + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
392 return set(fenStr, false, si, nullptr);
396 /// Position::fen() returns a FEN representation of the position. In case of
397 /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
399 const string Position::fen() const {
402 std::ostringstream ss;
404 for (Rank r = RANK_8; r >= RANK_1; --r)
406 for (File f = FILE_A; f <= FILE_H; ++f)
408 for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
415 ss << PieceToChar[piece_on(make_square(f, r))];
422 ss << (sideToMove == WHITE ? " w " : " b ");
424 if (can_castle(WHITE_OO))
425 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OO ))) : 'K');
427 if (can_castle(WHITE_OOO))
428 ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');
430 if (can_castle(BLACK_OO))
431 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO ))) : 'k');
433 if (can_castle(BLACK_OOO))
434 ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');
436 if (!can_castle(ANY_CASTLING))
439 ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
440 << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
446 /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
447 /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
448 /// slider if removing that piece from the board would result in a position where
449 /// square 's' is attacked. For example, a king-attack blocking piece can be either
450 /// a pinned or a discovered check piece, according if its color is the opposite
451 /// or the same of the color of the slider.
453 Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
455 Bitboard blockers = 0;
458 // Snipers are sliders that attack 's' when a piece and other snipers are removed
459 Bitboard snipers = ( (attacks_bb< ROOK>(s) & pieces(QUEEN, ROOK))
460 | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
461 Bitboard occupancy = pieces() ^ snipers;
465 Square sniperSq = pop_lsb(&snipers);
466 Bitboard b = between_bb(s, sniperSq) & occupancy;
468 if (b && !more_than_one(b))
471 if (b & pieces(color_of(piece_on(s))))
479 /// Position::attackers_to() computes a bitboard of all pieces which attack a
480 /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
482 Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
484 return (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
485 | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
486 | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
487 | (attacks_bb< ROOK>(s, occupied) & pieces( ROOK, QUEEN))
488 | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
489 | (attacks_bb<KING>(s) & pieces(KING));
493 /// Position::legal() tests whether a pseudo-legal move is legal
495 bool Position::legal(Move m) const {
499 Color us = sideToMove;
500 Square from = from_sq(m);
501 Square to = to_sq(m);
503 assert(color_of(moved_piece(m)) == us);
504 assert(piece_on(square<KING>(us)) == make_piece(us, KING));
506 // En passant captures are a tricky special case. Because they are rather
507 // uncommon, we do it simply by testing whether the king is attacked after
509 if (type_of(m) == ENPASSANT)
511 Square ksq = square<KING>(us);
512 Square capsq = to - pawn_push(us);
513 Bitboard occupied = (pieces() ^ from ^ capsq) | to;
515 assert(to == ep_square());
516 assert(moved_piece(m) == make_piece(us, PAWN));
517 assert(piece_on(capsq) == make_piece(~us, PAWN));
518 assert(piece_on(to) == NO_PIECE);
520 return !(attacks_bb< ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
521 && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
524 // Castling moves generation does not check if the castling path is clear of
525 // enemy attacks, it is delayed at a later time: now!
526 if (type_of(m) == CASTLING)
528 // After castling, the rook and king final positions are the same in
529 // Chess960 as they would be in standard chess.
530 to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
531 Direction step = to > from ? WEST : EAST;
533 for (Square s = to; s != from; s += step)
534 if (attackers_to(s) & pieces(~us))
537 // In case of Chess960, verify that when moving the castling rook we do
538 // not discover some hidden checker.
539 // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
541 || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
544 // If the moving piece is a king, check whether the destination square is
545 // attacked by the opponent.
546 if (type_of(piece_on(from)) == KING)
547 return !(attackers_to(to) & pieces(~us));
549 // A non-king move is legal if and only if it is not pinned or it
550 // is moving along the ray towards or away from the king.
551 return !(blockers_for_king(us) & from)
552 || aligned(from, to, square<KING>(us));
556 /// Position::pseudo_legal() takes a random move and tests whether the move is
557 /// pseudo legal. It is used to validate moves from TT that can be corrupted
558 /// due to SMP concurrent access or hash position key aliasing.
560 bool Position::pseudo_legal(const Move m) const {
562 Color us = sideToMove;
563 Square from = from_sq(m);
564 Square to = to_sq(m);
565 Piece pc = moved_piece(m);
567 // Use a slower but simpler function for uncommon cases
568 if (type_of(m) != NORMAL)
569 return MoveList<LEGAL>(*this).contains(m);
571 // Is not a promotion, so promotion piece must be empty
572 if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
575 // If the 'from' square is not occupied by a piece belonging to the side to
576 // move, the move is obviously not legal.
577 if (pc == NO_PIECE || color_of(pc) != us)
580 // The destination square cannot be occupied by a friendly piece
584 // Handle the special case of a pawn move
585 if (type_of(pc) == PAWN)
587 // We have already handled promotion moves, so destination
588 // cannot be on the 8th/1st rank.
589 if ((Rank8BB | Rank1BB) & to)
592 if ( !(pawn_attacks_bb(us, from) & pieces(~us) & to) // Not a capture
593 && !((from + pawn_push(us) == to) && empty(to)) // Not a single push
594 && !( (from + 2 * pawn_push(us) == to) // Not a double push
595 && (relative_rank(us, from) == RANK_2)
597 && empty(to - pawn_push(us))))
600 else if (!(attacks_bb(type_of(pc), from, pieces()) & to))
603 // Evasions generator already takes care to avoid some kind of illegal moves
604 // and legal() relies on this. We therefore have to take care that the same
605 // kind of moves are filtered out here.
608 if (type_of(pc) != KING)
610 // Double check? In this case a king move is required
611 if (more_than_one(checkers()))
614 // Our move must be a blocking evasion or a capture of the checking piece
615 if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
618 // In case of king moves under check we have to remove king so as to catch
619 // invalid moves like b1a1 when opposite queen is on c1.
620 else if (attackers_to(to, pieces() ^ from) & pieces(~us))
628 /// Position::gives_check() tests whether a pseudo-legal move gives a check
630 bool Position::gives_check(Move m) const {
633 assert(color_of(moved_piece(m)) == sideToMove);
635 Square from = from_sq(m);
636 Square to = to_sq(m);
638 // Is there a direct check?
639 if (check_squares(type_of(piece_on(from))) & to)
642 // Is there a discovered check?
643 if ( (blockers_for_king(~sideToMove) & from)
644 && !aligned(from, to, square<KING>(~sideToMove)))
653 return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
655 // En passant capture with check? We have already handled the case
656 // of direct checks and ordinary discovered check, so the only case we
657 // need to handle is the unusual case of a discovered check through
658 // the captured pawn.
661 Square capsq = make_square(file_of(to), rank_of(from));
662 Bitboard b = (pieces() ^ from ^ capsq) | to;
664 return (attacks_bb< ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
665 | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
670 Square rfrom = to; // Castling is encoded as 'king captures the rook'
671 Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
672 Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
674 return (attacks_bb<ROOK>(rto) & square<KING>(~sideToMove))
675 && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
684 /// Position::do_move() makes a move, and saves all information necessary
685 /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
686 /// moves should be filtered out before this function is called.
688 void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
691 assert(&newSt != st);
693 thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
694 Key k = st->key ^ Zobrist::side;
696 // Copy some fields of the old state to our new StateInfo object except the
697 // ones which are going to be recalculated from scratch anyway and then switch
698 // our state pointer to point to the new (ready to be updated) state.
699 std::memcpy(&newSt, st, offsetof(StateInfo, key));
703 // Increment ply counters. In particular, rule50 will be reset to zero later on
704 // in case of a capture or a pawn move.
710 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
711 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
712 auto& dp = st->dirtyPiece;
715 Color us = sideToMove;
717 Square from = from_sq(m);
718 Square to = to_sq(m);
719 Piece pc = piece_on(from);
720 Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
722 assert(color_of(pc) == us);
723 assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
724 assert(type_of(captured) != KING);
726 if (type_of(m) == CASTLING)
728 assert(pc == make_piece(us, KING));
729 assert(captured == make_piece(us, ROOK));
732 do_castling<true>(us, from, to, rfrom, rto);
734 k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
742 // If the captured piece is a pawn, update pawn hash key, otherwise
743 // update non-pawn material.
744 if (type_of(captured) == PAWN)
746 if (type_of(m) == ENPASSANT)
748 capsq -= pawn_push(us);
750 assert(pc == make_piece(us, PAWN));
751 assert(to == st->epSquare);
752 assert(relative_rank(us, to) == RANK_6);
753 assert(piece_on(to) == NO_PIECE);
754 assert(piece_on(capsq) == make_piece(them, PAWN));
757 st->pawnKey ^= Zobrist::psq[captured][capsq];
760 st->nonPawnMaterial[them] -= PieceValue[MG][captured];
764 dp.dirty_num = 2; // 1 piece moved, 1 piece captured
765 dp.piece[1] = captured;
770 // Update board and piece lists
773 if (type_of(m) == ENPASSANT)
774 board[capsq] = NO_PIECE;
776 // Update material hash key and prefetch access to materialTable
777 k ^= Zobrist::psq[captured][capsq];
778 st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
779 prefetch(thisThread->materialTable[st->materialKey]);
781 // Reset rule 50 counter
786 k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
788 // Reset en passant square
789 if (st->epSquare != SQ_NONE)
791 k ^= Zobrist::enpassant[file_of(st->epSquare)];
792 st->epSquare = SQ_NONE;
795 // Update castling rights if needed
796 if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
798 k ^= Zobrist::castling[st->castlingRights];
799 st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
800 k ^= Zobrist::castling[st->castlingRights];
803 // Move the piece. The tricky Chess960 castling is handled earlier
804 if (type_of(m) != CASTLING)
813 move_piece(from, to);
816 // If the moving piece is a pawn do some special extra work
817 if (type_of(pc) == PAWN)
819 // Set en-passant square if the moved pawn can be captured
820 if ( (int(to) ^ int(from)) == 16
821 && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
823 st->epSquare = to - pawn_push(us);
824 k ^= Zobrist::enpassant[file_of(st->epSquare)];
827 else if (type_of(m) == PROMOTION)
829 Piece promotion = make_piece(us, promotion_type(m));
831 assert(relative_rank(us, to) == RANK_8);
832 assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
835 put_piece(promotion, to);
839 // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
841 dp.piece[dp.dirty_num] = promotion;
842 dp.from[dp.dirty_num] = SQ_NONE;
843 dp.to[dp.dirty_num] = to;
848 k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
849 st->pawnKey ^= Zobrist::psq[pc][to];
850 st->materialKey ^= Zobrist::psq[promotion][pieceCount[promotion]-1]
851 ^ Zobrist::psq[pc][pieceCount[pc]];
854 st->nonPawnMaterial[us] += PieceValue[MG][promotion];
857 // Update pawn hash key
858 st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
860 // Reset rule 50 draw counter
865 st->capturedPiece = captured;
867 // Update the key with the final value
870 // Calculate checkers bitboard (if move gives check)
871 st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
873 sideToMove = ~sideToMove;
875 // Update king attacks used for fast check detection
878 // Calculate the repetition info. It is the ply distance from the previous
879 // occurrence of the same position, negative in the 3-fold case, or zero
880 // if the position was not repeated.
882 int end = std::min(st->rule50, st->pliesFromNull);
885 StateInfo* stp = st->previous->previous;
886 for (int i = 4; i <= end; i += 2)
888 stp = stp->previous->previous;
889 if (stp->key == st->key)
891 st->repetition = stp->repetition ? -i : i;
901 /// Position::undo_move() unmakes a move. When it returns, the position should
902 /// be restored to exactly the same state as before the move was made.
904 void Position::undo_move(Move m) {
908 sideToMove = ~sideToMove;
910 Color us = sideToMove;
911 Square from = from_sq(m);
912 Square to = to_sq(m);
913 Piece pc = piece_on(to);
915 assert(empty(from) || type_of(m) == CASTLING);
916 assert(type_of(st->capturedPiece) != KING);
918 if (type_of(m) == PROMOTION)
920 assert(relative_rank(us, to) == RANK_8);
921 assert(type_of(pc) == promotion_type(m));
922 assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
925 pc = make_piece(us, PAWN);
929 if (type_of(m) == CASTLING)
932 do_castling<false>(us, from, to, rfrom, rto);
936 move_piece(to, from); // Put the piece back at the source square
938 if (st->capturedPiece)
942 if (type_of(m) == ENPASSANT)
944 capsq -= pawn_push(us);
946 assert(type_of(pc) == PAWN);
947 assert(to == st->previous->epSquare);
948 assert(relative_rank(us, to) == RANK_6);
949 assert(piece_on(capsq) == NO_PIECE);
950 assert(st->capturedPiece == make_piece(~us, PAWN));
953 put_piece(st->capturedPiece, capsq); // Restore the captured piece
957 // Finally point our state pointer back to the previous state
965 /// Position::do_castling() is a helper used to do/undo a castling move. This
966 /// is a bit tricky in Chess960 where from/to squares can overlap.
968 void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
970 bool kingSide = to > from;
971 rfrom = to; // Castling is encoded as "king captures friendly rook"
972 rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
973 to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
975 if (Do && Eval::useNNUE)
977 auto& dp = st->dirtyPiece;
978 dp.piece[0] = make_piece(us, KING);
981 dp.piece[1] = make_piece(us, ROOK);
987 // Remove both pieces first since squares could overlap in Chess960
988 remove_piece(Do ? from : to);
989 remove_piece(Do ? rfrom : rto);
990 board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do this for us
991 put_piece(make_piece(us, KING), Do ? to : from);
992 put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
996 /// Position::do(undo)_null_move() is used to do(undo) a "null move": it flips
997 /// the side to move without executing any move on the board.
999 void Position::do_null_move(StateInfo& newSt) {
1001 assert(!checkers());
1002 assert(&newSt != st);
1004 std::memcpy(&newSt, st, offsetof(StateInfo, accumulator));
1006 newSt.previous = st;
1009 st->dirtyPiece.dirty_num = 0;
1010 st->dirtyPiece.piece[0] = NO_PIECE; // Avoid checks in UpdateAccumulator()
1011 st->accumulator.state[WHITE] = Eval::NNUE::EMPTY;
1012 st->accumulator.state[BLACK] = Eval::NNUE::EMPTY;
1014 if (st->epSquare != SQ_NONE)
1016 st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
1017 st->epSquare = SQ_NONE;
1020 st->key ^= Zobrist::side;
1021 prefetch(TT.first_entry(st->key));
1024 st->pliesFromNull = 0;
1026 sideToMove = ~sideToMove;
1032 assert(pos_is_ok());
1035 void Position::undo_null_move() {
1037 assert(!checkers());
1040 sideToMove = ~sideToMove;
1044 /// Position::key_after() computes the new hash key after the given move. Needed
1045 /// for speculative prefetch. It doesn't recognize special moves like castling,
1046 /// en-passant and promotions.
1048 Key Position::key_after(Move m) const {
1050 Square from = from_sq(m);
1051 Square to = to_sq(m);
1052 Piece pc = piece_on(from);
1053 Piece captured = piece_on(to);
1054 Key k = st->key ^ Zobrist::side;
1057 k ^= Zobrist::psq[captured][to];
1059 return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
1063 /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
1064 /// SEE value of move is greater or equal to the given threshold. We'll use an
1065 /// algorithm similar to alpha-beta pruning with a null window.
1067 bool Position::see_ge(Move m, Value threshold) const {
1071 // Only deal with normal moves, assume others pass a simple see
1072 if (type_of(m) != NORMAL)
1073 return VALUE_ZERO >= threshold;
1075 Square from = from_sq(m), to = to_sq(m);
1077 int swap = PieceValue[MG][piece_on(to)] - threshold;
1081 swap = PieceValue[MG][piece_on(from)] - swap;
1085 Bitboard occupied = pieces() ^ from ^ to;
1086 Color stm = color_of(piece_on(from));
1087 Bitboard attackers = attackers_to(to, occupied);
1088 Bitboard stmAttackers, bb;
1094 attackers &= occupied;
1096 // If stm has no more attackers then give up: stm loses
1097 if (!(stmAttackers = attackers & pieces(stm)))
1100 // Don't allow pinned pieces to attack (except the king) as long as
1101 // there are pinners on their original square.
1102 if (pinners(~stm) & occupied)
1103 stmAttackers &= ~blockers_for_king(stm);
1110 // Locate and remove the next least valuable attacker, and add to
1111 // the bitboard 'attackers' any X-ray attackers behind it.
1112 if ((bb = stmAttackers & pieces(PAWN)))
1114 if ((swap = PawnValueMg - swap) < res)
1117 occupied ^= lsb(bb);
1118 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1121 else if ((bb = stmAttackers & pieces(KNIGHT)))
1123 if ((swap = KnightValueMg - swap) < res)
1126 occupied ^= lsb(bb);
1129 else if ((bb = stmAttackers & pieces(BISHOP)))
1131 if ((swap = BishopValueMg - swap) < res)
1134 occupied ^= lsb(bb);
1135 attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
1138 else if ((bb = stmAttackers & pieces(ROOK)))
1140 if ((swap = RookValueMg - swap) < res)
1143 occupied ^= lsb(bb);
1144 attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
1147 else if ((bb = stmAttackers & pieces(QUEEN)))
1149 if ((swap = QueenValueMg - swap) < res)
1152 occupied ^= lsb(bb);
1153 attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
1154 | (attacks_bb<ROOK >(to, occupied) & pieces(ROOK , QUEEN));
1158 // If we "capture" with the king but opponent still has attackers,
1159 // reverse the result.
1160 return (attackers & ~pieces(stm)) ? res ^ 1 : res;
1167 /// Position::is_draw() tests whether the position is drawn by 50-move rule
1168 /// or by repetition. It does not detect stalemates.
1170 bool Position::is_draw(int ply) const {
1172 if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
1175 // Return a draw score if a position repeats once earlier but strictly
1176 // after the root, or repeats twice before or at the root.
1177 return st->repetition && st->repetition < ply;
1181 // Position::has_repeated() tests whether there has been at least one repetition
1182 // of positions since the last capture or pawn move.
1184 bool Position::has_repeated() const {
1186 StateInfo* stc = st;
1187 int end = std::min(st->rule50, st->pliesFromNull);
1190 if (stc->repetition)
1193 stc = stc->previous;
1199 /// Position::has_game_cycle() tests if the position has a move which draws by repetition,
1200 /// or an earlier position has a move that directly reaches the current position.
1202 bool Position::has_game_cycle(int ply) const {
1206 int end = std::min(st->rule50, st->pliesFromNull);
1211 Key originalKey = st->key;
1212 StateInfo* stp = st->previous;
1214 for (int i = 3; i <= end; i += 2)
1216 stp = stp->previous->previous;
1218 Key moveKey = originalKey ^ stp->key;
1219 if ( (j = H1(moveKey), cuckoo[j] == moveKey)
1220 || (j = H2(moveKey), cuckoo[j] == moveKey))
1222 Move move = cuckooMove[j];
1223 Square s1 = from_sq(move);
1224 Square s2 = to_sq(move);
1226 if (!(between_bb(s1, s2) & pieces()))
1231 // For nodes before or at the root, check that the move is a
1232 // repetition rather than a move to the current position.
1233 // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
1234 // the same location, so we have to select which square to check.
1235 if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
1238 // For repetitions before or at the root, require one more
1239 if (stp->repetition)
1248 /// Position::flip() flips position with the white and black sides reversed. This
1249 /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
1251 void Position::flip() {
1254 std::stringstream ss(fen());
1256 for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
1258 std::getline(ss, token, r > RANK_1 ? '/' : ' ');
1259 f.insert(0, token + (f.empty() ? " " : "/"));
1262 ss >> token; // Active color
1263 f += (token == "w" ? "B " : "W "); // Will be lowercased later
1265 ss >> token; // Castling availability
1268 std::transform(f.begin(), f.end(), f.begin(),
1269 [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
1271 ss >> token; // En passant square
1272 f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
1274 std::getline(ss, token); // Half and full moves
1277 set(f, is_chess960(), st, this_thread());
1279 assert(pos_is_ok());
1283 /// Position::pos_is_ok() performs some consistency checks for the
1284 /// position object and raises an asserts if something wrong is detected.
1285 /// This is meant to be helpful when debugging.
1287 bool Position::pos_is_ok() const {
1289 constexpr bool Fast = true; // Quick (default) or full check?
1291 if ( (sideToMove != WHITE && sideToMove != BLACK)
1292 || piece_on(square<KING>(WHITE)) != W_KING
1293 || piece_on(square<KING>(BLACK)) != B_KING
1294 || ( ep_square() != SQ_NONE
1295 && relative_rank(sideToMove, ep_square()) != RANK_6))
1296 assert(0 && "pos_is_ok: Default");
1301 if ( pieceCount[W_KING] != 1
1302 || pieceCount[B_KING] != 1
1303 || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
1304 assert(0 && "pos_is_ok: Kings");
1306 if ( (pieces(PAWN) & (Rank1BB | Rank8BB))
1307 || pieceCount[W_PAWN] > 8
1308 || pieceCount[B_PAWN] > 8)
1309 assert(0 && "pos_is_ok: Pawns");
1311 if ( (pieces(WHITE) & pieces(BLACK))
1312 || (pieces(WHITE) | pieces(BLACK)) != pieces()
1313 || popcount(pieces(WHITE)) > 16
1314 || popcount(pieces(BLACK)) > 16)
1315 assert(0 && "pos_is_ok: Bitboards");
1317 for (PieceType p1 = PAWN; p1 <= KING; ++p1)
1318 for (PieceType p2 = PAWN; p2 <= KING; ++p2)
1319 if (p1 != p2 && (pieces(p1) & pieces(p2)))
1320 assert(0 && "pos_is_ok: Bitboards");
1323 ASSERT_ALIGNED(&si, Eval::NNUE::kCacheLineSize);
1326 if (std::memcmp(&si, st, sizeof(StateInfo)))
1327 assert(0 && "pos_is_ok: State");
1329 for (Piece pc : Pieces)
1331 if ( pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
1332 || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
1333 assert(0 && "pos_is_ok: Pieces");
1335 for (int i = 0; i < pieceCount[pc]; ++i)
1336 if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
1337 assert(0 && "pos_is_ok: Index");
1340 for (Color c : { WHITE, BLACK })
1341 for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
1343 if (!can_castle(cr))
1346 if ( piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
1347 || castlingRightsMask[castlingRookSquare[cr]] != cr
1348 || (castlingRightsMask[square<KING>(c)] & cr) != cr)
1349 assert(0 && "pos_is_ok: Castling");