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  1. /*
  2.   Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  3.   Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
  4.   Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
  5.   Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
  6.  
  7.   Stockfish is free software: you can redistribute it and/or modify
  8.   it under the terms of the GNU General Public License as published by
  9.   the Free Software Foundation, either version 3 of the License, or
  10.   (at your option) any later version.
  11.  
  12.   Stockfish is distributed in the hope that it will be useful,
  13.   but WITHOUT ANY WARRANTY; without even the implied warranty of
  14.   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15.   GNU General Public License for more details.
  16.  
  17.   You should have received a copy of the GNU General Public License
  18.   along with this program.  If not, see <http://www.gnu.org/licenses/>.
  19. */
  20.  
  21. #include <algorithm>
  22. #include <cassert>
  23. #include <cstddef> // For offsetof()
  24. #include <cstring> // For std::memset, std::memcmp
  25. #include <iomanip>
  26. #include <sstream>
  27.  
  28. #include "bitboard.h"
  29. #include "misc.h"
  30. #include "movegen.h"
  31. #include "position.h"
  32. #include "thread.h"
  33. #include "tt.h"
  34. #include "uci.h"
  35.  
  36. using std::string;
  37.  
  38. namespace PSQT {
  39.   extern Score psq[PIECE_NB][SQUARE_NB];
  40. }
  41.  
  42. namespace Zobrist {
  43.  
  44.   Key psq[PIECE_NB][SQUARE_NB];
  45.   Key enpassant[FILE_NB];
  46.   Key castling[CASTLING_RIGHT_NB];
  47.   Key side;
  48. }
  49.  
  50. namespace {
  51.  
  52. const string PieceToChar(" PNBRQK  pnbrqk");
  53.  
  54. // min_attacker() is a helper function used by see() to locate the least
  55. // valuable attacker for the side to move, remove the attacker we just found
  56. // from the bitboards and scan for new X-ray attacks behind it.
  57.  
  58. template<int Pt>
  59. PieceType min_attacker(const Bitboard* bb, Square to, Bitboard stmAttackers,
  60.                        Bitboard& occupied, Bitboard& attackers) {
  61.  
  62.   Bitboard b = stmAttackers & bb[Pt];
  63.   if (!b)
  64.       return min_attacker<Pt+1>(bb, to, stmAttackers, occupied, attackers);
  65.  
  66.   occupied ^= b & ~(b - 1);
  67.  
  68.   if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
  69.       attackers |= attacks_bb<BISHOP>(to, occupied) & (bb[BISHOP] | bb[QUEEN]);
  70.  
  71.   if (Pt == ROOK || Pt == QUEEN)
  72.       attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
  73.  
  74.   attackers &= occupied; // After X-ray that may add already processed pieces
  75.   return (PieceType)Pt;
  76. }
  77.  
  78. template<>
  79. PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
  80.   return KING; // No need to update bitboards: it is the last cycle
  81. }
  82.  
  83. } // namespace
  84.  
  85.  
  86. /// operator<<(Position) returns an ASCII representation of the position
  87.  
  88. std::ostream& operator<<(std::ostream& os, const Position& pos) {
  89.  
  90.   os << "\n +---+---+---+---+---+---+---+---+\n";
  91.  
  92.   for (Rank r = RANK_8; r >= RANK_1; --r)
  93.   {
  94.       for (File f = FILE_A; f <= FILE_H; ++f)
  95.           os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
  96.  
  97.       os << " |\n +---+---+---+---+---+---+---+---+\n";
  98.   }
  99.  
  100.   os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
  101.      << std::setfill('0') << std::setw(16) << pos.key() << std::dec << "\nCheckers: ";
  102.  
  103.   for (Bitboard b = pos.checkers(); b; )
  104.       os << UCI::square(pop_lsb(&b)) << " ";
  105.  
  106.   return os;
  107. }
  108.  
  109.  
  110. /// Position::init() initializes at startup the various arrays used to compute
  111. /// hash keys.
  112.  
  113. void Position::init() {
  114.  
  115.   PRNG rng(1070372);
  116.  
  117.   for (Piece pc : Pieces)
  118.       for (Square s = SQ_A1; s <= SQ_H8; ++s)
  119.           Zobrist::psq[pc][s] = rng.rand<Key>();
  120.  
  121.   for (File f = FILE_A; f <= FILE_H; ++f)
  122.       Zobrist::enpassant[f] = rng.rand<Key>();
  123.  
  124.   for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
  125.   {
  126.       Zobrist::castling[cr] = 0;
  127.       Bitboard b = cr;
  128.       while (b)
  129.       {
  130.           Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
  131.           Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
  132.       }
  133.   }
  134.  
  135.   Zobrist::side = rng.rand<Key>();
  136. }
  137.  
  138.  
  139. /// Position::set() initializes the position object with the given FEN string.
  140. /// This function is not very robust - make sure that input FENs are correct,
  141. /// this is assumed to be the responsibility of the GUI.
  142.  
  143. Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
  144. /*
  145.    A FEN string defines a particular position using only the ASCII character set.
  146.  
  147.    A FEN string contains six fields separated by a space. The fields are:
  148.  
  149.    1) Piece placement (from white's perspective). Each rank is described, starting
  150.       with rank 8 and ending with rank 1. Within each rank, the contents of each
  151.       square are described from file A through file H. Following the Standard
  152.       Algebraic Notation (SAN), each piece is identified by a single letter taken
  153.       from the standard English names. White pieces are designated using upper-case
  154.       letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
  155.       noted using digits 1 through 8 (the number of blank squares), and "/"
  156.       separates ranks.
  157.  
  158.    2) Active color. "w" means white moves next, "b" means black.
  159.  
  160.    3) Castling availability. If neither side can castle, this is "-". Otherwise,
  161.       this has one or more letters: "K" (White can castle kingside), "Q" (White
  162.       can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
  163.       can castle queenside).
  164.  
  165.    4) En passant target square (in algebraic notation). If there's no en passant
  166.       target square, this is "-". If a pawn has just made a 2-square move, this
  167.       is the position "behind" the pawn. This is recorded regardless of whether
  168.       there is a pawn in position to make an en passant capture.
  169.  
  170.    5) Halfmove clock. This is the number of halfmoves since the last pawn advance
  171.       or capture. This is used to determine if a draw can be claimed under the
  172.       fifty-move rule.
  173.  
  174.    6) Fullmove number. The number of the full move. It starts at 1, and is
  175.       incremented after Black's move.
  176. */
  177.  
  178.   unsigned char col, row, token;
  179.   size_t idx;
  180.   Square sq = SQ_A8;
  181.   std::istringstream ss(fenStr);
  182.  
  183.   std::memset(this, 0, sizeof(Position));
  184.   std::memset(si, 0, sizeof(StateInfo));
  185.   std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
  186.   st = si;
  187.  
  188.   ss >> std::noskipws;
  189.  
  190.   // 1. Piece placement
  191.   while ((ss >> token) && !isspace(token))
  192.   {
  193.       if (isdigit(token))
  194.           sq += Square(token - '0'); // Advance the given number of files
  195.  
  196.       else if (token == '/')
  197.           sq -= Square(16);
  198.  
  199.       else if ((idx = PieceToChar.find(token)) != string::npos)
  200.       {
  201.           put_piece(Piece(idx), sq);
  202.           ++sq;
  203.       }
  204.   }
  205.  
  206.   // 2. Active color
  207.   ss >> token;
  208.   sideToMove = (token == 'w' ? WHITE : BLACK);
  209.   ss >> token;
  210.  
  211.   // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
  212.   // Shredder-FEN that uses the letters of the columns on which the rooks began
  213.   // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
  214.   // if an inner rook is associated with the castling right, the castling tag is
  215.   // replaced by the file letter of the involved rook, as for the Shredder-FEN.
  216.   while ((ss >> token) && !isspace(token))
  217.   {
  218.       Square rsq;
  219.       Color c = islower(token) ? BLACK : WHITE;
  220.       Piece rook = make_piece(c, ROOK);
  221.  
  222.       token = char(toupper(token));
  223.  
  224.       if (token == 'K')
  225.           for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
  226.  
  227.       else if (token == 'Q')
  228.           for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
  229.  
  230.       else if (token >= 'A' && token <= 'H')
  231.           rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
  232.  
  233.       else
  234.           continue;
  235.  
  236.       set_castling_right(c, rsq);
  237.   }
  238.  
  239.   // 4. En passant square. Ignore if no pawn capture is possible
  240.   if (   ((ss >> col) && (col >= 'a' && col <= 'h'))
  241.       && ((ss >> row) && (row == '3' || row == '6')))
  242.   {
  243.       st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
  244.  
  245.       if (!(attackers_to(st->epSquare) & pieces(sideToMove, PAWN)))
  246.           st->epSquare = SQ_NONE;
  247.   }
  248.   else
  249.       st->epSquare = SQ_NONE;
  250.  
  251.   // 5-6. Halfmove clock and fullmove number
  252.   ss >> std::skipws >> st->rule50 >> gamePly;
  253.  
  254.   // Convert from fullmove starting from 1 to ply starting from 0,
  255.   // handle also common incorrect FEN with fullmove = 0.
  256.   gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
  257.  
  258.   chess960 = isChess960;
  259.   thisThread = th;
  260.   set_state(st);
  261.  
  262.   assert(pos_is_ok());
  263.  
  264.   return *this;
  265. }
  266.  
  267.  
  268. /// Position::set_castling_right() is a helper function used to set castling
  269. /// rights given the corresponding color and the rook starting square.
  270.  
  271. void Position::set_castling_right(Color c, Square rfrom) {
  272.  
  273.   Square kfrom = square<KING>(c);
  274.   CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
  275.   CastlingRight cr = (c | cs);
  276.  
  277.   st->castlingRights |= cr;
  278.   castlingRightsMask[kfrom] |= cr;
  279.   castlingRightsMask[rfrom] |= cr;
  280.   castlingRookSquare[cr] = rfrom;
  281.  
  282.   Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
  283.   Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
  284.  
  285.   for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
  286.       if (s != kfrom && s != rfrom)
  287.           castlingPath[cr] |= s;
  288.  
  289.   for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
  290.       if (s != kfrom && s != rfrom)
  291.           castlingPath[cr] |= s;
  292. }
  293.  
  294.  
  295. /// Position::set_check_info() sets king attacks to detect if a move gives check
  296.  
  297. void Position::set_check_info(StateInfo* si) const {
  298.  
  299.   si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinnersForKing[WHITE]);
  300.   si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinnersForKing[BLACK]);
  301.  
  302.   Square ksq = square<KING>(~sideToMove);
  303.  
  304.   si->checkSquares[PAWN]   = attacks_from<PAWN>(ksq, ~sideToMove);
  305.   si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
  306.   si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
  307.   si->checkSquares[ROOK]   = attacks_from<ROOK>(ksq);
  308.   si->checkSquares[QUEEN]  = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
  309.   si->checkSquares[KING]   = 0;
  310. }
  311.  
  312.  
  313. /// Position::set_state() computes the hash keys of the position, and other
  314. /// data that once computed is updated incrementally as moves are made.
  315. /// The function is only used when a new position is set up, and to verify
  316. /// the correctness of the StateInfo data when running in debug mode.
  317.  
  318. void Position::set_state(StateInfo* si) const {
  319.  
  320.   si->key = si->pawnKey = si->materialKey = 0;
  321.   si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
  322.   si->psq = SCORE_ZERO;
  323.   si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
  324.  
  325.   set_check_info(si);
  326.  
  327.   for (Bitboard b = pieces(); b; )
  328.   {
  329.       Square s = pop_lsb(&b);
  330.       Piece pc = piece_on(s);
  331.       si->key ^= Zobrist::psq[pc][s];
  332.       si->psq += PSQT::psq[pc][s];
  333.   }
  334.  
  335.   if (si->epSquare != SQ_NONE)
  336.       si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
  337.  
  338.   if (sideToMove == BLACK)
  339.       si->key ^= Zobrist::side;
  340.  
  341.   si->key ^= Zobrist::castling[si->castlingRights];
  342.  
  343.   for (Bitboard b = pieces(PAWN); b; )
  344.   {
  345.       Square s = pop_lsb(&b);
  346.       si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
  347.   }
  348.  
  349.   for (Piece pc : Pieces)
  350.   {
  351.       if (type_of(pc) != PAWN && type_of(pc) != KING)
  352.           si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
  353.  
  354.       for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
  355.           si->materialKey ^= Zobrist::psq[pc][cnt];
  356.   }
  357. }
  358.  
  359.  
  360. /// Position::fen() returns a FEN representation of the position. In case of
  361. /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
  362.  
  363. const string Position::fen() const {
  364.  
  365.   int emptyCnt;
  366.   std::ostringstream ss;
  367.  
  368.   for (Rank r = RANK_8; r >= RANK_1; --r)
  369.   {
  370.       for (File f = FILE_A; f <= FILE_H; ++f)
  371.       {
  372.           for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
  373.               ++emptyCnt;
  374.  
  375.           if (emptyCnt)
  376.               ss << emptyCnt;
  377.  
  378.           if (f <= FILE_H)
  379.               ss << PieceToChar[piece_on(make_square(f, r))];
  380.       }
  381.  
  382.       if (r > RANK_1)
  383.           ss << '/';
  384.   }
  385.  
  386.   ss << (sideToMove == WHITE ? " w " : " b ");
  387.  
  388.   if (can_castle(WHITE_OO))
  389.       ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE |  KING_SIDE))) : 'K');
  390.  
  391.   if (can_castle(WHITE_OOO))
  392.       ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
  393.  
  394.   if (can_castle(BLACK_OO))
  395.       ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK |  KING_SIDE))) : 'k');
  396.  
  397.   if (can_castle(BLACK_OOO))
  398.       ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
  399.  
  400.   if (!can_castle(WHITE) && !can_castle(BLACK))
  401.       ss << '-';
  402.  
  403.   ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
  404.      << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
  405.  
  406.   return ss.str();
  407. }
  408.  
  409.  
  410. /// Position::game_phase() calculates the game phase interpolating total non-pawn
  411. /// material between endgame and midgame limits.
  412.  
  413. Phase Position::game_phase() const {
  414.  
  415.   Value npm = st->nonPawnMaterial[WHITE] + st->nonPawnMaterial[BLACK];
  416.  
  417.   npm = std::max(EndgameLimit, std::min(npm, MidgameLimit));
  418.  
  419.   return Phase(((npm - EndgameLimit) * PHASE_MIDGAME) / (MidgameLimit - EndgameLimit));
  420. }
  421.  
  422.  
  423. /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
  424. /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
  425. /// slider if removing that piece from the board would result in a position where
  426. /// square 's' is attacked. For example, a king-attack blocking piece can be either
  427. /// a pinned or a discovered check piece, according if its color is the opposite
  428. /// or the same of the color of the slider.
  429.  
  430. Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
  431.  
  432.   Bitboard result = 0;
  433.   pinners = 0;
  434.  
  435.   // Snipers are sliders that attack 's' when a piece is removed
  436.   Bitboard snipers = (  (PseudoAttacks[ROOK  ][s] & pieces(QUEEN, ROOK))
  437.                       | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
  438.  
  439.   while (snipers)
  440.   {
  441.     Square sniperSq = pop_lsb(&snipers);
  442.     Bitboard b = between_bb(s, sniperSq) & pieces();
  443.  
  444.     if (!more_than_one(b))
  445.     {
  446.         result |= b;
  447.         if (b & pieces(color_of(piece_on(s))))
  448.             pinners |= sniperSq;
  449.     }
  450.   }
  451.   return result;
  452. }
  453.  
  454.  
  455. /// Position::attackers_to() computes a bitboard of all pieces which attack a
  456. /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
  457.  
  458. Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
  459.  
  460.   return  (attacks_from<PAWN>(s, BLACK)    & pieces(WHITE, PAWN))
  461.         | (attacks_from<PAWN>(s, WHITE)    & pieces(BLACK, PAWN))
  462.         | (attacks_from<KNIGHT>(s)         & pieces(KNIGHT))
  463.         | (attacks_bb<ROOK  >(s, occupied) & pieces(ROOK,   QUEEN))
  464.         | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
  465.         | (attacks_from<KING>(s)           & pieces(KING));
  466. }
  467.  
  468.  
  469. /// Position::legal() tests whether a pseudo-legal move is legal
  470.  
  471. bool Position::legal(Move m) const {
  472.  
  473.   assert(is_ok(m));
  474.  
  475.   Color us = sideToMove;
  476.   Square from = from_sq(m);
  477.  
  478.   assert(color_of(moved_piece(m)) == us);
  479.   assert(piece_on(square<KING>(us)) == make_piece(us, KING));
  480.  
  481.   // En passant captures are a tricky special case. Because they are rather
  482.   // uncommon, we do it simply by testing whether the king is attacked after
  483.   // the move is made.
  484.   if (type_of(m) == ENPASSANT)
  485.   {
  486.       Square ksq = square<KING>(us);
  487.       Square to = to_sq(m);
  488.       Square capsq = to - pawn_push(us);
  489.       Bitboard occupied = (pieces() ^ from ^ capsq) | to;
  490.  
  491.       assert(to == ep_square());
  492.       assert(moved_piece(m) == make_piece(us, PAWN));
  493.       assert(piece_on(capsq) == make_piece(~us, PAWN));
  494.       assert(piece_on(to) == NO_PIECE);
  495.  
  496.       return   !(attacks_bb<  ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
  497.             && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
  498.   }
  499.  
  500.   // If the moving piece is a king, check whether the destination
  501.   // square is attacked by the opponent. Castling moves are checked
  502.   // for legality during move generation.
  503.   if (type_of(piece_on(from)) == KING)
  504.       return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
  505.  
  506.   // A non-king move is legal if and only if it is not pinned or it
  507.   // is moving along the ray towards or away from the king.
  508.   return   !(pinned_pieces(us) & from)
  509.         ||  aligned(from, to_sq(m), square<KING>(us));
  510. }
  511.  
  512.  
  513. /// Position::pseudo_legal() takes a random move and tests whether the move is
  514. /// pseudo legal. It is used to validate moves from TT that can be corrupted
  515. /// due to SMP concurrent access or hash position key aliasing.
  516.  
  517. bool Position::pseudo_legal(const Move m) const {
  518.  
  519.   Color us = sideToMove;
  520.   Square from = from_sq(m);
  521.   Square to = to_sq(m);
  522.   Piece pc = moved_piece(m);
  523.  
  524.   // Use a slower but simpler function for uncommon cases
  525.   if (type_of(m) != NORMAL)
  526.       return MoveList<LEGAL>(*this).contains(m);
  527.  
  528.   // Is not a promotion, so promotion piece must be empty
  529.   if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
  530.       return false;
  531.  
  532.   // If the 'from' square is not occupied by a piece belonging to the side to
  533.   // move, the move is obviously not legal.
  534.   if (pc == NO_PIECE || color_of(pc) != us)
  535.       return false;
  536.  
  537.   // The destination square cannot be occupied by a friendly piece
  538.   if (pieces(us) & to)
  539.       return false;
  540.  
  541.   // Handle the special case of a pawn move
  542.   if (type_of(pc) == PAWN)
  543.   {
  544.       // We have already handled promotion moves, so destination
  545.       // cannot be on the 8th/1st rank.
  546.       if (rank_of(to) == relative_rank(us, RANK_8))
  547.           return false;
  548.  
  549.       if (   !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
  550.           && !((from + pawn_push(us) == to) && empty(to))       // Not a single push
  551.           && !(   (from + 2 * pawn_push(us) == to)              // Not a double push
  552.                && (rank_of(from) == relative_rank(us, RANK_2))
  553.                && empty(to)
  554.                && empty(to - pawn_push(us))))
  555.           return false;
  556.   }
  557.   else if (!(attacks_from(pc, from) & to))
  558.       return false;
  559.  
  560.   // Evasions generator already takes care to avoid some kind of illegal moves
  561.   // and legal() relies on this. We therefore have to take care that the same
  562.   // kind of moves are filtered out here.
  563.   if (checkers())
  564.   {
  565.       if (type_of(pc) != KING)
  566.       {
  567.           // Double check? In this case a king move is required
  568.           if (more_than_one(checkers()))
  569.               return false;
  570.  
  571.           // Our move must be a blocking evasion or a capture of the checking piece
  572.           if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
  573.               return false;
  574.       }
  575.       // In case of king moves under check we have to remove king so as to catch
  576.       // invalid moves like b1a1 when opposite queen is on c1.
  577.       else if (attackers_to(to, pieces() ^ from) & pieces(~us))
  578.           return false;
  579.   }
  580.  
  581.   return true;
  582. }
  583.  
  584.  
  585. /// Position::gives_check() tests whether a pseudo-legal move gives a check
  586.  
  587. bool Position::gives_check(Move m) const {
  588.  
  589.   assert(is_ok(m));
  590.   assert(color_of(moved_piece(m)) == sideToMove);
  591.  
  592.   Square from = from_sq(m);
  593.   Square to = to_sq(m);
  594.  
  595.   // Is there a direct check?
  596.   if (st->checkSquares[type_of(piece_on(from))] & to)
  597.       return true;
  598.  
  599.   // Is there a discovered check?
  600.   if (   (discovered_check_candidates() & from)
  601.       && !aligned(from, to, square<KING>(~sideToMove)))
  602.       return true;
  603.  
  604.   switch (type_of(m))
  605.   {
  606.   case NORMAL:
  607.       return false;
  608.  
  609.   case PROMOTION:
  610.       return attacks_bb(Piece(promotion_type(m)), to, pieces() ^ from) & square<KING>(~sideToMove);
  611.  
  612.   // En passant capture with check? We have already handled the case
  613.   // of direct checks and ordinary discovered check, so the only case we
  614.   // need to handle is the unusual case of a discovered check through
  615.   // the captured pawn.
  616.   case ENPASSANT:
  617.   {
  618.       Square capsq = make_square(file_of(to), rank_of(from));
  619.       Bitboard b = (pieces() ^ from ^ capsq) | to;
  620.  
  621.       return  (attacks_bb<  ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
  622.             | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
  623.   }
  624.   case CASTLING:
  625.   {
  626.       Square kfrom = from;
  627.       Square rfrom = to; // Castling is encoded as 'King captures the rook'
  628.       Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
  629.       Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
  630.  
  631.       return   (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
  632.             && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
  633.   }
  634.   default:
  635.       assert(false);
  636.       return false;
  637.   }
  638. }
  639.  
  640.  
  641. /// Position::do_move() makes a move, and saves all information necessary
  642. /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
  643. /// moves should be filtered out before this function is called.
  644.  
  645. void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
  646.  
  647.   assert(is_ok(m));
  648.   assert(&newSt != st);
  649.  
  650.   ++nodes;
  651.   Key k = st->key ^ Zobrist::side;
  652.  
  653.   // Copy some fields of the old state to our new StateInfo object except the
  654.   // ones which are going to be recalculated from scratch anyway and then switch
  655.   // our state pointer to point to the new (ready to be updated) state.
  656.   std::memcpy(&newSt, st, offsetof(StateInfo, key));
  657.   newSt.previous = st;
  658.   st = &newSt;
  659.  
  660.   // Increment ply counters. In particular, rule50 will be reset to zero later on
  661.   // in case of a capture or a pawn move.
  662.   ++gamePly;
  663.   ++st->rule50;
  664.   ++st->pliesFromNull;
  665.  
  666.   Color us = sideToMove;
  667.   Color them = ~us;
  668.   Square from = from_sq(m);
  669.   Square to = to_sq(m);
  670.   Piece pc = piece_on(from);
  671.   Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
  672.  
  673.   assert(color_of(pc) == us);
  674.   assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
  675.   assert(type_of(captured) != KING);
  676.  
  677.   if (type_of(m) == CASTLING)
  678.   {
  679.       assert(pc == make_piece(us, KING));
  680.       assert(captured == make_piece(us, ROOK));
  681.  
  682.       Square rfrom, rto;
  683.       do_castling<true>(us, from, to, rfrom, rto);
  684.  
  685.       st->psq += PSQT::psq[captured][rto] - PSQT::psq[captured][rfrom];
  686.       k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
  687.       captured = NO_PIECE;
  688.   }
  689.  
  690.   if (captured)
  691.   {
  692.       Square capsq = to;
  693.  
  694.       // If the captured piece is a pawn, update pawn hash key, otherwise
  695.       // update non-pawn material.
  696.       if (type_of(captured) == PAWN)
  697.       {
  698.           if (type_of(m) == ENPASSANT)
  699.           {
  700.               capsq -= pawn_push(us);
  701.  
  702.               assert(pc == make_piece(us, PAWN));
  703.               assert(to == st->epSquare);
  704.               assert(relative_rank(us, to) == RANK_6);
  705.               assert(piece_on(to) == NO_PIECE);
  706.               assert(piece_on(capsq) == make_piece(them, PAWN));
  707.  
  708.               board[capsq] = NO_PIECE; // Not done by remove_piece()
  709.           }
  710.  
  711.           st->pawnKey ^= Zobrist::psq[captured][capsq];
  712.       }
  713.       else
  714.           st->nonPawnMaterial[them] -= PieceValue[MG][captured];
  715.  
  716.       // Update board and piece lists
  717.       remove_piece(captured, capsq);
  718.  
  719.       // Update material hash key and prefetch access to materialTable
  720.       k ^= Zobrist::psq[captured][capsq];
  721.       st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
  722.       prefetch(thisThread->materialTable[st->materialKey]);
  723.  
  724.       // Update incremental scores
  725.       st->psq -= PSQT::psq[captured][capsq];
  726.  
  727.       // Reset rule 50 counter
  728.       st->rule50 = 0;
  729.   }
  730.  
  731.   // Update hash key
  732.   k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
  733.  
  734.   // Reset en passant square
  735.   if (st->epSquare != SQ_NONE)
  736.   {
  737.       k ^= Zobrist::enpassant[file_of(st->epSquare)];
  738.       st->epSquare = SQ_NONE;
  739.   }
  740.  
  741.   // Update castling rights if needed
  742.   if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
  743.   {
  744.       int cr = castlingRightsMask[from] | castlingRightsMask[to];
  745.       k ^= Zobrist::castling[st->castlingRights & cr];
  746.       st->castlingRights &= ~cr;
  747.   }
  748.  
  749.   // Move the piece. The tricky Chess960 castling is handled earlier
  750.   if (type_of(m) != CASTLING)
  751.       move_piece(pc, from, to);
  752.  
  753.   // If the moving piece is a pawn do some special extra work
  754.   if (type_of(pc) == PAWN)
  755.   {
  756.       // Set en-passant square if the moved pawn can be captured
  757.       if (   (int(to) ^ int(from)) == 16
  758.           && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
  759.       {
  760.           st->epSquare = (from + to) / 2;
  761.           k ^= Zobrist::enpassant[file_of(st->epSquare)];
  762.       }
  763.  
  764.       else if (type_of(m) == PROMOTION)
  765.       {
  766.           Piece promotion = make_piece(us, promotion_type(m));
  767.  
  768.           assert(relative_rank(us, to) == RANK_8);
  769.           assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
  770.  
  771.           remove_piece(pc, to);
  772.           put_piece(promotion, to);
  773.  
  774.           // Update hash keys
  775.           k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
  776.           st->pawnKey ^= Zobrist::psq[pc][to];
  777.           st->materialKey ^=  Zobrist::psq[promotion][pieceCount[promotion]-1]
  778.                             ^ Zobrist::psq[pc][pieceCount[pc]];
  779.  
  780.           // Update incremental score
  781.           st->psq += PSQT::psq[promotion][to] - PSQT::psq[pc][to];
  782.  
  783.           // Update material
  784.           st->nonPawnMaterial[us] += PieceValue[MG][promotion];
  785.       }
  786.  
  787.       // Update pawn hash key and prefetch access to pawnsTable
  788.       st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
  789.       prefetch(thisThread->pawnsTable[st->pawnKey]);
  790.  
  791.       // Reset rule 50 draw counter
  792.       st->rule50 = 0;
  793.   }
  794.  
  795.   // Update incremental scores
  796.   st->psq += PSQT::psq[pc][to] - PSQT::psq[pc][from];
  797.  
  798.   // Set capture piece
  799.   st->capturedPiece = captured;
  800.  
  801.   // Update the key with the final value
  802.   st->key = k;
  803.  
  804.   // Calculate checkers bitboard (if move gives check)
  805.   st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
  806.  
  807.   sideToMove = ~sideToMove;
  808.  
  809.   // Update king attacks used for fast check detection
  810.   set_check_info(st);
  811.  
  812.   assert(pos_is_ok());
  813. }
  814.  
  815.  
  816. /// Position::undo_move() unmakes a move. When it returns, the position should
  817. /// be restored to exactly the same state as before the move was made.
  818.  
  819. void Position::undo_move(Move m) {
  820.  
  821.   assert(is_ok(m));
  822.  
  823.   sideToMove = ~sideToMove;
  824.  
  825.   Color us = sideToMove;
  826.   Square from = from_sq(m);
  827.   Square to = to_sq(m);
  828.   Piece pc = piece_on(to);
  829.  
  830.   assert(empty(from) || type_of(m) == CASTLING);
  831.   assert(type_of(st->capturedPiece) != KING);
  832.  
  833.   if (type_of(m) == PROMOTION)
  834.   {
  835.       assert(relative_rank(us, to) == RANK_8);
  836.       assert(type_of(pc) == promotion_type(m));
  837.       assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
  838.  
  839.       remove_piece(pc, to);
  840.       pc = make_piece(us, PAWN);
  841.       put_piece(pc, to);
  842.   }
  843.  
  844.   if (type_of(m) == CASTLING)
  845.   {
  846.       Square rfrom, rto;
  847.       do_castling<false>(us, from, to, rfrom, rto);
  848.   }
  849.   else
  850.   {
  851.       move_piece(pc, to, from); // Put the piece back at the source square
  852.  
  853.       if (st->capturedPiece)
  854.       {
  855.           Square capsq = to;
  856.  
  857.           if (type_of(m) == ENPASSANT)
  858.           {
  859.               capsq -= pawn_push(us);
  860.  
  861.               assert(type_of(pc) == PAWN);
  862.               assert(to == st->previous->epSquare);
  863.               assert(relative_rank(us, to) == RANK_6);
  864.               assert(piece_on(capsq) == NO_PIECE);
  865.               assert(st->capturedPiece == make_piece(~us, PAWN));
  866.           }
  867.  
  868.           put_piece(st->capturedPiece, capsq); // Restore the captured piece
  869.       }
  870.   }
  871.  
  872.   // Finally point our state pointer back to the previous state
  873.   st = st->previous;
  874.   --gamePly;
  875.  
  876.   assert(pos_is_ok());
  877. }
  878.  
  879.  
  880. /// Position::do_castling() is a helper used to do/undo a castling move. This
  881. /// is a bit tricky in Chess960 where from/to squares can overlap.
  882. template<bool Do>
  883. void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
  884.  
  885.   bool kingSide = to > from;
  886.   rfrom = to; // Castling is encoded as "king captures friendly rook"
  887.   rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
  888.   to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
  889.  
  890.   // Remove both pieces first since squares could overlap in Chess960
  891.   remove_piece(make_piece(us, KING), Do ? from : to);
  892.   remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
  893.   board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
  894.   put_piece(make_piece(us, KING), Do ? to : from);
  895.   put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
  896. }
  897.  
  898.  
  899. /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
  900. /// the side to move without executing any move on the board.
  901.  
  902. void Position::do_null_move(StateInfo& newSt) {
  903.  
  904.   assert(!checkers());
  905.   assert(&newSt != st);
  906.  
  907.   std::memcpy(&newSt, st, sizeof(StateInfo));
  908.   newSt.previous = st;
  909.   st = &newSt;
  910.  
  911.   if (st->epSquare != SQ_NONE)
  912.   {
  913.       st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
  914.       st->epSquare = SQ_NONE;
  915.   }
  916.  
  917.   st->key ^= Zobrist::side;
  918.   prefetch(TT.first_entry(st->key));
  919.  
  920.   ++st->rule50;
  921.   st->pliesFromNull = 0;
  922.  
  923.   sideToMove = ~sideToMove;
  924.  
  925.   set_check_info(st);
  926.  
  927.   assert(pos_is_ok());
  928. }
  929.  
  930. void Position::undo_null_move() {
  931.  
  932.   assert(!checkers());
  933.  
  934.   st = st->previous;
  935.   sideToMove = ~sideToMove;
  936. }
  937.  
  938.  
  939. /// Position::key_after() computes the new hash key after the given move. Needed
  940. /// for speculative prefetch. It doesn't recognize special moves like castling,
  941. /// en-passant and promotions.
  942.  
  943. Key Position::key_after(Move m) const {
  944.  
  945.   Square from = from_sq(m);
  946.   Square to = to_sq(m);
  947.   Piece pc = piece_on(from);
  948.   Piece captured = piece_on(to);
  949.   Key k = st->key ^ Zobrist::side;
  950.  
  951.   if (captured)
  952.       k ^= Zobrist::psq[captured][to];
  953.  
  954.   return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
  955. }
  956.  
  957.  
  958. /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
  959. /// SEE value of move is greater or equal to the given value. We'll use an
  960. /// algorithm similar to alpha-beta pruning with a null window.
  961.  
  962. bool Position::see_ge(Move m, Value v) const {
  963.  
  964.   assert(is_ok(m));
  965.  
  966.   // Castling moves are implemented as king capturing the rook so cannot be
  967.   // handled correctly. Simply assume the SEE value is VALUE_ZERO that is always
  968.   // correct unless in the rare case the rook ends up under attack.
  969.   if (type_of(m) == CASTLING)
  970.       return VALUE_ZERO >= v;
  971.  
  972.   Square from = from_sq(m), to = to_sq(m);
  973.   PieceType nextVictim = type_of(piece_on(from));
  974.   Color stm = ~color_of(piece_on(from)); // First consider opponent's move
  975.   Value balance; // Values of the pieces taken by us minus opponent's ones
  976.   Bitboard occupied, stmAttackers;
  977.  
  978.   if (type_of(m) == ENPASSANT)
  979.   {
  980.       occupied = SquareBB[to - pawn_push(~stm)]; // Remove the captured pawn
  981.       balance = PieceValue[MG][PAWN];
  982.   }
  983.   else
  984.   {
  985.       balance = PieceValue[MG][piece_on(to)];
  986.       occupied = 0;
  987.   }
  988.  
  989.   if (balance < v)
  990.       return false;
  991.  
  992.   if (nextVictim == KING)
  993.       return true;
  994.  
  995.   balance -= PieceValue[MG][nextVictim];
  996.  
  997.   if (balance >= v)
  998.       return true;
  999.  
  1000.   bool relativeStm = true; // True if the opponent is to move
  1001.   occupied ^= pieces() ^ from ^ to;
  1002.  
  1003.   // Find all attackers to the destination square, with the moving piece removed,
  1004.   // but possibly an X-ray attacker added behind it.
  1005.   Bitboard attackers = attackers_to(to, occupied) & occupied;
  1006.  
  1007.   while (true)
  1008.   {
  1009.       stmAttackers = attackers & pieces(stm);
  1010.  
  1011.       // Don't allow pinned pieces to attack pieces except the king as long all
  1012.       // pinners are on their original square.
  1013.       if (!(st->pinnersForKing[stm] & ~occupied))
  1014.           stmAttackers &= ~st->blockersForKing[stm];
  1015.  
  1016.       if (!stmAttackers)
  1017.           return relativeStm;
  1018.  
  1019.       // Locate and remove the next least valuable attacker
  1020.       nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
  1021.  
  1022.       if (nextVictim == KING)
  1023.           return relativeStm == bool(attackers & pieces(~stm));
  1024.  
  1025.       balance += relativeStm ?  PieceValue[MG][nextVictim]
  1026.                              : -PieceValue[MG][nextVictim];
  1027.  
  1028.       relativeStm = !relativeStm;
  1029.  
  1030.       if (relativeStm == (balance >= v))
  1031.           return relativeStm;
  1032.  
  1033.       stm = ~stm;
  1034.   }
  1035. }
  1036.  
  1037.  
  1038. /// Position::is_draw() tests whether the position is drawn by 50-move rule
  1039. /// or by repetition. It does not detect stalemates.
  1040.  
  1041. bool Position::is_draw() const {
  1042.  
  1043.   if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
  1044.       return true;
  1045.  
  1046.   StateInfo* stp = st;
  1047.   for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
  1048.   {
  1049.       stp = stp->previous->previous;
  1050.  
  1051.       if (stp->key == st->key)
  1052.           return true; // Draw at first repetition
  1053.   }
  1054.  
  1055.   return false;
  1056. }
  1057.  
  1058.  
  1059. /// Position::flip() flips position with the white and black sides reversed. This
  1060. /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
  1061.  
  1062. void Position::flip() {
  1063.  
  1064.   string f, token;
  1065.   std::stringstream ss(fen());
  1066.  
  1067.   for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
  1068.   {
  1069.       std::getline(ss, token, r > RANK_1 ? '/' : ' ');
  1070.       f.insert(0, token + (f.empty() ? " " : "/"));
  1071.   }
  1072.  
  1073.   ss >> token; // Active color
  1074.   f += (token == "w" ? "B " : "W "); // Will be lowercased later
  1075.  
  1076.   ss >> token; // Castling availability
  1077.   f += token + " ";
  1078.  
  1079.   std::transform(f.begin(), f.end(), f.begin(),
  1080.                  [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
  1081.  
  1082.   ss >> token; // En passant square
  1083.   f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
  1084.  
  1085.   std::getline(ss, token); // Half and full moves
  1086.   f += token;
  1087.  
  1088.   set(f, is_chess960(), st, this_thread());
  1089.  
  1090.   assert(pos_is_ok());
  1091. }
  1092.  
  1093.  
  1094. /// Position::pos_is_ok() performs some consistency checks for the position object.
  1095. /// This is meant to be helpful when debugging.
  1096.  
  1097. bool Position::pos_is_ok(int* failedStep) const {
  1098.  
  1099.   const bool Fast = true; // Quick (default) or full check?
  1100.  
  1101.   enum { Default, King, Bitboards, State, Lists, Castling };
  1102.  
  1103.   for (int step = Default; step <= (Fast ? Default : Castling); step++)
  1104.   {
  1105.       if (failedStep)
  1106.           *failedStep = step;
  1107.  
  1108.       if (step == Default)
  1109.           if (   (sideToMove != WHITE && sideToMove != BLACK)
  1110.               || piece_on(square<KING>(WHITE)) != W_KING
  1111.               || piece_on(square<KING>(BLACK)) != B_KING
  1112.               || (   ep_square() != SQ_NONE
  1113.                   && relative_rank(sideToMove, ep_square()) != RANK_6))
  1114.               return false;
  1115.  
  1116.       if (step == King)
  1117.           if (   std::count(board, board + SQUARE_NB, W_KING) != 1
  1118.               || std::count(board, board + SQUARE_NB, B_KING) != 1
  1119.               || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
  1120.               return false;
  1121.  
  1122.       if (step == Bitboards)
  1123.       {
  1124.           if (  (pieces(WHITE) & pieces(BLACK))
  1125.               ||(pieces(WHITE) | pieces(BLACK)) != pieces())
  1126.               return false;
  1127.  
  1128.           for (PieceType p1 = PAWN; p1 <= KING; ++p1)
  1129.               for (PieceType p2 = PAWN; p2 <= KING; ++p2)
  1130.                   if (p1 != p2 && (pieces(p1) & pieces(p2)))
  1131.                       return false;
  1132.       }
  1133.  
  1134.       if (step == State)
  1135.       {
  1136.           StateInfo si = *st;
  1137.           set_state(&si);
  1138.           if (std::memcmp(&si, st, sizeof(StateInfo)))
  1139.               return false;
  1140.       }
  1141.  
  1142.       if (step == Lists)
  1143.           for (Piece pc : Pieces)
  1144.           {
  1145.               if (pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc))))
  1146.                   return false;
  1147.  
  1148.               for (int i = 0; i < pieceCount[pc]; ++i)
  1149.                   if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
  1150.                       return false;
  1151.           }
  1152.  
  1153.       if (step == Castling)
  1154.           for (Color c = WHITE; c <= BLACK; ++c)
  1155.               for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
  1156.               {
  1157.                   if (!can_castle(c | s))
  1158.                       continue;
  1159.  
  1160.                   if (   piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
  1161.                       || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
  1162.                       ||(castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
  1163.                       return false;
  1164.               }
  1165.   }
  1166.  
  1167.   return true;
  1168. }
  1169.