text: /*
    Stockfish, a UCI chess playing engine derived from Glaurung 2.1
    Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
    Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
    Copyright (C) 2015-2018 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
  
    Stockfish is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
  
    Stockfish is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
  
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
  
  #include <algorithm>
  #include <cassert>
  #include <cstddef> // For offsetof()
  #include <cstring> // For std::memset, std::memcmp
  #include <iomanip>
  #include <sstream>
  
  #include "bitboard.h"
  #include "misc.h"
  #include "movegen.h"
  #include "position.h"
  #include "thread.h"
  #include "tt.h"
  #include "uci.h"
  #include "syzygy/tbprobe.h"
  
  using std::string;
  
  namespace PSQT {
    extern Score psq[PIECE_NB][SQUARE_NB];
  }
  
  namespace Zobrist {
  
    Key psq[PIECE_NB][SQUARE_NB];
    Key enpassant[FILE_NB];
    Key castling[CASTLING_RIGHT_NB];
    Key side, noPawns;
  }
  
  namespace {
  
  const string PieceToChar(" PNBRQK  pnbrqk");
  
  const Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING,
                           B_PAWN, B_KNIGHT, B_BISHOP, B_ROOK, B_QUEEN, B_KING };
  
  // min_attacker() is a helper function used by see_ge() to locate the least
  // valuable attacker for the side to move, remove the attacker we just found
  // from the bitboards and scan for new X-ray attacks behind it.
  
  template<int Pt>
  PieceType min_attacker(const Bitboard* byTypeBB, Square to, Bitboard stmAttackers,
                         Bitboard& occupied, Bitboard& attackers) {
  
    Bitboard b = stmAttackers & byTypeBB[Pt];
    if (!b)
        return min_attacker<Pt + 1>(byTypeBB, to, stmAttackers, occupied, attackers);
  
    occupied ^= lsb(b); // Remove the attacker from occupied
  
    // Add any X-ray attack behind the just removed piece. For instance with
    // rooks in a8 and a7 attacking a1, after removing a7 we add rook in a8.
    // Note that new added attackers can be of any color.
    if (Pt == PAWN || Pt == BISHOP || Pt == QUEEN)
        attackers |= attacks_bb<BISHOP>(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]);
  
    if (Pt == ROOK || Pt == QUEEN)
        attackers |= attacks_bb<ROOK>(to, occupied) & (byTypeBB[ROOK] | byTypeBB[QUEEN]);
  
    // X-ray may add already processed pieces because byTypeBB[] is constant: in
    // the rook example, now attackers contains _again_ rook in a7, so remove it.
    attackers &= occupied;
    return (PieceType)Pt;
  }
  
  template<>
  PieceType min_attacker<KING>(const Bitboard*, Square, Bitboard, Bitboard&, Bitboard&) {
    return KING; // No need to update bitboards: it is the last cycle
  }
  
  } // namespace
  
  
  /// operator<<(Position) returns an ASCII representation of the position
  
  std::ostream& operator<<(std::ostream& os, const Position& pos) {
  
    os << "\n +---+---+---+---+---+---+---+---+\n";
  
    for (Rank r = RANK_8; r >= RANK_1; --r)
    {
        for (File f = FILE_A; f <= FILE_H; ++f)
            os << " | " << PieceToChar[pos.piece_on(make_square(f, r))];
  
        os << " |\n +---+---+---+---+---+---+---+---+\n";
    }
  
    os << "\nFen: " << pos.fen() << "\nKey: " << std::hex << std::uppercase
       << std::setfill('0') << std::setw(16) << pos.key()
       << std::setfill(' ') << std::dec << "\nCheckers: ";
  
    for (Bitboard b = pos.checkers(); b; )
        os << UCI::square(pop_lsb(&b)) << " ";
  
    if (    int(Tablebases::MaxCardinality) >= popcount(pos.pieces())
        && !pos.can_castle(ANY_CASTLING))
    {
        StateInfo st;
        Position p;
        p.set(pos.fen(), pos.is_chess960(), &st, pos.this_thread());
        Tablebases::ProbeState s1, s2;
        Tablebases::WDLScore wdl = Tablebases::probe_wdl(p, &s1);
        int dtz = Tablebases::probe_dtz(p, &s2);
        os << "\nTablebases WDL: " << std::setw(4) << wdl << " (" << s1 << ")"
           << "\nTablebases DTZ: " << std::setw(4) << dtz << " (" << s2 << ")";
    }
  
    return os;
  }
  
  
  /// Position::init() initializes at startup the various arrays used to compute
  /// hash keys.
  
  void Position::init() {
  
    PRNG rng(1070372);
  
    for (Piece pc : Pieces)
        for (Square s = SQ_A1; s <= SQ_H8; ++s)
            Zobrist::psq[pc][s] = rng.rand<Key>();
  
    for (File f = FILE_A; f <= FILE_H; ++f)
        Zobrist::enpassant[f] = rng.rand<Key>();
  
    for (int cr = NO_CASTLING; cr <= ANY_CASTLING; ++cr)
    {
        Zobrist::castling[cr] = 0;
        Bitboard b = cr;
        while (b)
        {
            Key k = Zobrist::castling[1ULL << pop_lsb(&b)];
            Zobrist::castling[cr] ^= k ? k : rng.rand<Key>();
        }
    }
  
    Zobrist::side = rng.rand<Key>();
    Zobrist::noPawns = rng.rand<Key>();
  }
  
  
  /// Position::set() initializes the position object with the given FEN string.
  /// This function is not very robust - make sure that input FENs are correct,
  /// this is assumed to be the responsibility of the GUI.
  
  Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Thread* th) {
  /*
     A FEN string defines a particular position using only the ASCII character set.
  
     A FEN string contains six fields separated by a space. The fields are:
  
     1) Piece placement (from white's perspective). Each rank is described, starting
        with rank 8 and ending with rank 1. Within each rank, the contents of each
        square are described from file A through file H. Following the Standard
        Algebraic Notation (SAN), each piece is identified by a single letter taken
        from the standard English names. White pieces are designated using upper-case
        letters ("PNBRQK") whilst Black uses lowercase ("pnbrqk"). Blank squares are
        noted using digits 1 through 8 (the number of blank squares), and "/"
        separates ranks.
  
     2) Active color. "w" means white moves next, "b" means black.
  
     3) Castling availability. If neither side can castle, this is "-". Otherwise,
        this has one or more letters: "K" (White can castle kingside), "Q" (White
        can castle queenside), "k" (Black can castle kingside), and/or "q" (Black
        can castle queenside).
  
     4) En passant target square (in algebraic notation). If there's no en passant
        target square, this is "-". If a pawn has just made a 2-square move, this
        is the position "behind" the pawn. This is recorded only if there is a pawn
        in position to make an en passant capture, and if there really is a pawn
        that might have advanced two squares.
  
     5) Halfmove clock. This is the number of halfmoves since the last pawn advance
        or capture. This is used to determine if a draw can be claimed under the
        fifty-move rule.
  
     6) Fullmove number. The number of the full move. It starts at 1, and is
        incremented after Black's move.
  */
  
    unsigned char col, row, token;
    size_t idx;
    Square sq = SQ_A8;
    std::istringstream ss(fenStr);
  
    std::memset(this, 0, sizeof(Position));
    std::memset(si, 0, sizeof(StateInfo));
    std::fill_n(&pieceList[0][0], sizeof(pieceList) / sizeof(Square), SQ_NONE);
    st = si;
  
    ss >> std::noskipws;
  
    // 1. Piece placement
    while ((ss >> token) && !isspace(token))
    {
        if (isdigit(token))
            sq += (token - '0') * EAST; // Advance the given number of files
  
        else if (token == '/')
            sq += 2 * SOUTH;
  
        else if ((idx = PieceToChar.find(token)) != string::npos)
        {
            put_piece(Piece(idx), sq);
            ++sq;
        }
    }
  
    // 2. Active color
    ss >> token;
    sideToMove = (token == 'w' ? WHITE : BLACK);
    ss >> token;
  
    // 3. Castling availability. Compatible with 3 standards: Normal FEN standard,
    // Shredder-FEN that uses the letters of the columns on which the rooks began
    // the game instead of KQkq and also X-FEN standard that, in case of Chess960,
    // if an inner rook is associated with the castling right, the castling tag is
    // replaced by the file letter of the involved rook, as for the Shredder-FEN.
    while ((ss >> token) && !isspace(token))
    {
        Square rsq;
        Color c = islower(token) ? BLACK : WHITE;
        Piece rook = make_piece(c, ROOK);
  
        token = char(toupper(token));
  
        if (token == 'K')
            for (rsq = relative_square(c, SQ_H1); piece_on(rsq) != rook; --rsq) {}
  
        else if (token == 'Q')
            for (rsq = relative_square(c, SQ_A1); piece_on(rsq) != rook; ++rsq) {}
  
        else if (token >= 'A' && token <= 'H')
            rsq = make_square(File(token - 'A'), relative_rank(c, RANK_1));
  
        else
            continue;
  
        set_castling_right(c, rsq);
    }
  
    // 4. En passant square. Ignore if no pawn capture is possible
    if (   ((ss >> col) && (col >= 'a' && col <= 'h'))
        && ((ss >> row) && (row == '3' || row == '6')))
    {
        st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
  
        if (   !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
            || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
            st->epSquare = SQ_NONE;
    }
    else
        st->epSquare = SQ_NONE;
  
    // 5-6. Halfmove clock and fullmove number
    ss >> std::skipws >> st->rule50 >> gamePly;
  
    // Convert from fullmove starting from 1 to gamePly starting from 0,
    // handle also common incorrect FEN with fullmove = 0.
    gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
  
    chess960 = isChess960;
    thisThread = th;
    set_state(st);
  
    assert(pos_is_ok());
  
    return *this;
  }
  
  
  /// Position::set_castling_right() is a helper function used to set castling
  /// rights given the corresponding color and the rook starting square.
  
  void Position::set_castling_right(Color c, Square rfrom) {
  
    Square kfrom = square<KING>(c);
    CastlingSide cs = kfrom < rfrom ? KING_SIDE : QUEEN_SIDE;
    CastlingRight cr = (c | cs);
  
    st->castlingRights |= cr;
    castlingRightsMask[kfrom] |= cr;
    castlingRightsMask[rfrom] |= cr;
    castlingRookSquare[cr] = rfrom;
  
    Square kto = relative_square(c, cs == KING_SIDE ? SQ_G1 : SQ_C1);
    Square rto = relative_square(c, cs == KING_SIDE ? SQ_F1 : SQ_D1);
  
    for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); ++s)
        if (s != kfrom && s != rfrom)
            castlingPath[cr] |= s;
  
    for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); ++s)
        if (s != kfrom && s != rfrom)
            castlingPath[cr] |= s;
  }
  
  
  /// Position::set_check_info() sets king attacks to detect if a move gives check
  
  void Position::set_check_info(StateInfo* si) const {
  
    si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinnersForKing[WHITE]);
    si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinnersForKing[BLACK]);
  
    Square ksq = square<KING>(~sideToMove);
  
    si->checkSquares[PAWN]   = attacks_from<PAWN>(ksq, ~sideToMove);
    si->checkSquares[KNIGHT] = attacks_from<KNIGHT>(ksq);
    si->checkSquares[BISHOP] = attacks_from<BISHOP>(ksq);
    si->checkSquares[ROOK]   = attacks_from<ROOK>(ksq);
    si->checkSquares[QUEEN]  = si->checkSquares[BISHOP] | si->checkSquares[ROOK];
    si->checkSquares[KING]   = 0;
  }
  
  
  /// Position::set_state() computes the hash keys of the position, and other
  /// data that once computed is updated incrementally as moves are made.
  /// The function is only used when a new position is set up, and to verify
  /// the correctness of the StateInfo data when running in debug mode.
  
  void Position::set_state(StateInfo* si) const {
  
    si->key = si->materialKey = 0;
    si->pawnKey = Zobrist::noPawns;
    si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
    si->psq = SCORE_ZERO;
    si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
  
    set_check_info(si);
  
    for (Bitboard b = pieces(); b; )
    {
        Square s = pop_lsb(&b);
        Piece pc = piece_on(s);
        si->key ^= Zobrist::psq[pc][s];
        si->psq += PSQT::psq[pc][s];
    }
  
    if (si->epSquare != SQ_NONE)
        si->key ^= Zobrist::enpassant[file_of(si->epSquare)];
  
    if (sideToMove == BLACK)
        si->key ^= Zobrist::side;
  
    si->key ^= Zobrist::castling[si->castlingRights];
  
    for (Bitboard b = pieces(PAWN); b; )
    {
        Square s = pop_lsb(&b);
        si->pawnKey ^= Zobrist::psq[piece_on(s)][s];
    }
  
    for (Piece pc : Pieces)
    {
        if (type_of(pc) != PAWN && type_of(pc) != KING)
            si->nonPawnMaterial[color_of(pc)] += pieceCount[pc] * PieceValue[MG][pc];
  
        for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
            si->materialKey ^= Zobrist::psq[pc][cnt];
    }
  }
  
  
  /// Position::set() is an overload to initialize the position object with
  /// the given endgame code string like "KBPKN". It is mainly a helper to
  /// get the material key out of an endgame code.
  
  Position& Position::set(const string& code, Color c, StateInfo* si) {
  
    assert(code.length() > 0 && code.length() < 8);
    assert(code[0] == 'K');
  
    string sides[] = { code.substr(code.find('K', 1)),      // Weak
                       code.substr(0, code.find('K', 1)) }; // Strong
  
    std::transform(sides[c].begin(), sides[c].end(), sides[c].begin(), tolower);
  
    string fenStr = "8/" + sides[0] + char(8 - sides[0].length() + '0') + "/8/8/8/8/"
                         + sides[1] + char(8 - sides[1].length() + '0') + "/8 w - - 0 10";
  
    return set(fenStr, false, si, nullptr);
  }
  
  
  /// Position::fen() returns a FEN representation of the position. In case of
  /// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
  
  const string Position::fen() const {
  
    int emptyCnt;
    std::ostringstream ss;
  
    for (Rank r = RANK_8; r >= RANK_1; --r)
    {
        for (File f = FILE_A; f <= FILE_H; ++f)
        {
            for (emptyCnt = 0; f <= FILE_H && empty(make_square(f, r)); ++f)
                ++emptyCnt;
  
            if (emptyCnt)
                ss << emptyCnt;
  
            if (f <= FILE_H)
                ss << PieceToChar[piece_on(make_square(f, r))];
        }
  
        if (r > RANK_1)
            ss << '/';
    }
  
    ss << (sideToMove == WHITE ? " w " : " b ");
  
    if (can_castle(WHITE_OO))
        ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE |  KING_SIDE))) : 'K');
  
    if (can_castle(WHITE_OOO))
        ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE | QUEEN_SIDE))) : 'Q');
  
    if (can_castle(BLACK_OO))
        ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK |  KING_SIDE))) : 'k');
  
    if (can_castle(BLACK_OOO))
        ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK | QUEEN_SIDE))) : 'q');
  
    if (!can_castle(WHITE) && !can_castle(BLACK))
        ss << '-';
  
    ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ")
       << st->rule50 << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;
  
    return ss.str();
  }
  
  
  /// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
  /// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
  /// slider if removing that piece from the board would result in a position where
  /// square 's' is attacked. For example, a king-attack blocking piece can be either
  /// a pinned or a discovered check piece, according if its color is the opposite
  /// or the same of the color of the slider.
  
  Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
  
    Bitboard blockers = 0;
    pinners = 0;
  
    // Snipers are sliders that attack 's' when a piece is removed
    Bitboard snipers = (  (PseudoAttacks[  ROOK][s] & pieces(QUEEN, ROOK))
                        | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
  
    while (snipers)
    {
      Square sniperSq = pop_lsb(&snipers);
      Bitboard b = between_bb(s, sniperSq) & pieces();
  
      if (b && !more_than_one(b))
      {
          blockers |= b;
          if (b & pieces(color_of(piece_on(s))))
              pinners |= sniperSq;
      }
    }
    return blockers;
  }
  
  
  /// Position::attackers_to() computes a bitboard of all pieces which attack a
  /// given square. Slider attacks use the occupied bitboard to indicate occupancy.
  
  Bitboard Position::attackers_to(Square s, Bitboard occupied) const {
  
    return  (attacks_from<PAWN>(s, BLACK)    & pieces(WHITE, PAWN))
          | (attacks_from<PAWN>(s, WHITE)    & pieces(BLACK, PAWN))
          | (attacks_from<KNIGHT>(s)         & pieces(KNIGHT))
          | (attacks_bb<  ROOK>(s, occupied) & pieces(  ROOK, QUEEN))
          | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
          | (attacks_from<KING>(s)           & pieces(KING));
  }
  
  
  /// Position::legal() tests whether a pseudo-legal move is legal
  
  bool Position::legal(Move m) const {
  
    assert(is_ok(m));
  
    Color us = sideToMove;
    Square from = from_sq(m);
  
    assert(color_of(moved_piece(m)) == us);
    assert(piece_on(square<KING>(us)) == make_piece(us, KING));
  
    // En passant captures are a tricky special case. Because they are rather
    // uncommon, we do it simply by testing whether the king is attacked after
    // the move is made.
    if (type_of(m) == ENPASSANT)
    {
        Square ksq = square<KING>(us);
        Square to = to_sq(m);
        Square capsq = to - pawn_push(us);
        Bitboard occupied = (pieces() ^ from ^ capsq) | to;
  
        assert(to == ep_square());
        assert(moved_piece(m) == make_piece(us, PAWN));
        assert(piece_on(capsq) == make_piece(~us, PAWN));
        assert(piece_on(to) == NO_PIECE);
  
        return   !(attacks_bb<  ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
              && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
    }
  
    // If the moving piece is a king, check whether the destination
    // square is attacked by the opponent. Castling moves are checked
    // for legality during move generation.
    if (type_of(piece_on(from)) == KING)
        return type_of(m) == CASTLING || !(attackers_to(to_sq(m)) & pieces(~us));
  
    // A non-king move is legal if and only if it is not pinned or it
    // is moving along the ray towards or away from the king.
    return   !(blockers_for_king(us) & from)
          ||  aligned(from, to_sq(m), square<KING>(us));
  }
  
  
  /// Position::pseudo_legal() takes a random move and tests whether the move is
  /// pseudo legal. It is used to validate moves from TT that can be corrupted
  /// due to SMP concurrent access or hash position key aliasing.
  
  bool Position::pseudo_legal(const Move m) const {
  
    Color us = sideToMove;
    Square from = from_sq(m);
    Square to = to_sq(m);
    Piece pc = moved_piece(m);
  
    // Use a slower but simpler function for uncommon cases
    if (type_of(m) != NORMAL)
        return MoveList<LEGAL>(*this).contains(m);
  
    // Is not a promotion, so promotion piece must be empty
    if (promotion_type(m) - KNIGHT != NO_PIECE_TYPE)
        return false;
  
    // If the 'from' square is not occupied by a piece belonging to the side to
    // move, the move is obviously not legal.
    if (pc == NO_PIECE || color_of(pc) != us)
        return false;
  
    // The destination square cannot be occupied by a friendly piece
    if (pieces(us) & to)
        return false;
  
    // Handle the special case of a pawn move
    if (type_of(pc) == PAWN)
    {
        // We have already handled promotion moves, so destination
        // cannot be on the 8th/1st rank.
        if (rank_of(to) == relative_rank(us, RANK_8))
            return false;
  
        if (   !(attacks_from<PAWN>(from, us) & pieces(~us) & to) // Not a capture
            && !((from + pawn_push(us) == to) && empty(to))       // Not a single push
            && !(   (from + 2 * pawn_push(us) == to)              // Not a double push
                 && (rank_of(from) == relative_rank(us, RANK_2))
                 && empty(to)
                 && empty(to - pawn_push(us))))
            return false;
    }
    else if (!(attacks_from(type_of(pc), from) & to))
        return false;
  
    // Evasions generator already takes care to avoid some kind of illegal moves
    // and legal() relies on this. We therefore have to take care that the same
    // kind of moves are filtered out here.
    if (checkers())
    {
        if (type_of(pc) != KING)
        {
            // Double check? In this case a king move is required
            if (more_than_one(checkers()))
                return false;
  
            // Our move must be a blocking evasion or a capture of the checking piece
            if (!((between_bb(lsb(checkers()), square<KING>(us)) | checkers()) & to))
                return false;
        }
        // In case of king moves under check we have to remove king so as to catch
        // invalid moves like b1a1 when opposite queen is on c1.
        else if (attackers_to(to, pieces() ^ from) & pieces(~us))
            return false;
    }
  
    return true;
  }
  
  
  /// Position::gives_check() tests whether a pseudo-legal move gives a check
  
  bool Position::gives_check(Move m) const {
  
    assert(is_ok(m));
    assert(color_of(moved_piece(m)) == sideToMove);
  
    Square from = from_sq(m);
    Square to = to_sq(m);
  
    // Is there a direct check?
    if (st->checkSquares[type_of(piece_on(from))] & to)
        return true;
  
    // Is there a discovered check?
    if (   (st->blockersForKing[~sideToMove] & from)
        && !aligned(from, to, square<KING>(~sideToMove)))
        return true;
  
    switch (type_of(m))
    {
    case NORMAL:
        return false;
  
    case PROMOTION:
        return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
  
    // En passant capture with check? We have already handled the case
    // of direct checks and ordinary discovered check, so the only case we
    // need to handle is the unusual case of a discovered check through
    // the captured pawn.
    case ENPASSANT:
    {
        Square capsq = make_square(file_of(to), rank_of(from));
        Bitboard b = (pieces() ^ from ^ capsq) | to;
  
        return  (attacks_bb<  ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
              | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
    }
    case CASTLING:
    {
        Square kfrom = from;
        Square rfrom = to; // Castling is encoded as 'King captures the rook'
        Square kto = relative_square(sideToMove, rfrom > kfrom ? SQ_G1 : SQ_C1);
        Square rto = relative_square(sideToMove, rfrom > kfrom ? SQ_F1 : SQ_D1);
  
        return   (PseudoAttacks[ROOK][rto] & square<KING>(~sideToMove))
              && (attacks_bb<ROOK>(rto, (pieces() ^ kfrom ^ rfrom) | rto | kto) & square<KING>(~sideToMove));
    }
    default:
        assert(false);
        return false;
    }
  }
  
  
  /// Position::do_move() makes a move, and saves all information necessary
  /// to a StateInfo object. The move is assumed to be legal. Pseudo-legal
  /// moves should be filtered out before this function is called.
  
  void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
  
    assert(is_ok(m));
    assert(&newSt != st);
  
    thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
    Key k = st->key ^ Zobrist::side;
  
    // Copy some fields of the old state to our new StateInfo object except the
    // ones which are going to be recalculated from scratch anyway and then switch
    // our state pointer to point to the new (ready to be updated) state.
    std::memcpy(&newSt, st, offsetof(StateInfo, key));
    newSt.previous = st;
    st = &newSt;
  
    // Increment ply counters. In particular, rule50 will be reset to zero later on
    // in case of a capture or a pawn move.
    ++gamePly;
    ++st->rule50;
    ++st->pliesFromNull;
  
    Color us = sideToMove;
    Color them = ~us;
    Square from = from_sq(m);
    Square to = to_sq(m);
    Piece pc = piece_on(from);
    Piece captured = type_of(m) == ENPASSANT ? make_piece(them, PAWN) : piece_on(to);
  
    assert(color_of(pc) == us);
    assert(captured == NO_PIECE || color_of(captured) == (type_of(m) != CASTLING ? them : us));
    assert(type_of(captured) != KING);
  
    if (type_of(m) == CASTLING)
    {
        assert(pc == make_piece(us, KING));
        assert(captured == make_piece(us, ROOK));
  
        Square rfrom, rto;
        do_castling<true>(us, from, to, rfrom, rto);
  
        st->psq += PSQT::psq[captured][rto] - PSQT::psq[captured][rfrom];
        k ^= Zobrist::psq[captured][rfrom] ^ Zobrist::psq[captured][rto];
        captured = NO_PIECE;
    }
  
    if (captured)
    {
        Square capsq = to;
  
        // If the captured piece is a pawn, update pawn hash key, otherwise
        // update non-pawn material.
        if (type_of(captured) == PAWN)
        {
            if (type_of(m) == ENPASSANT)
            {
                capsq -= pawn_push(us);
  
                assert(pc == make_piece(us, PAWN));
                assert(to == st->epSquare);
                assert(relative_rank(us, to) == RANK_6);
                assert(piece_on(to) == NO_PIECE);
                assert(piece_on(capsq) == make_piece(them, PAWN));
  
                board[capsq] = NO_PIECE; // Not done by remove_piece()
            }
  
            st->pawnKey ^= Zobrist::psq[captured][capsq];
        }
        else
            st->nonPawnMaterial[them] -= PieceValue[MG][captured];
  
        // Update board and piece lists
        remove_piece(captured, capsq);
  
        // Update material hash key and prefetch access to materialTable
        k ^= Zobrist::psq[captured][capsq];
        st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];
        prefetch(thisThread->materialTable[st->materialKey]);
  
        // Update incremental scores
        st->psq -= PSQT::psq[captured][capsq];
  
        // Reset rule 50 counter
        st->rule50 = 0;
    }
  
    // Update hash key
    k ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
  
    // Reset en passant square
    if (st->epSquare != SQ_NONE)
    {
        k ^= Zobrist::enpassant[file_of(st->epSquare)];
        st->epSquare = SQ_NONE;
    }
  
    // Update castling rights if needed
    if (st->castlingRights && (castlingRightsMask[from] | castlingRightsMask[to]))
    {
        int cr = castlingRightsMask[from] | castlingRightsMask[to];
        k ^= Zobrist::castling[st->castlingRights & cr];
        st->castlingRights &= ~cr;
    }
  
    // Move the piece. The tricky Chess960 castling is handled earlier
    if (type_of(m) != CASTLING)
        move_piece(pc, from, to);
  
    // If the moving piece is a pawn do some special extra work
    if (type_of(pc) == PAWN)
    {
        // Set en-passant square if the moved pawn can be captured
        if (   (int(to) ^ int(from)) == 16
            && (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
        {
            st->epSquare = to - pawn_push(us);
            k ^= Zobrist::enpassant[file_of(st->epSquare)];
        }
  
        else if (type_of(m) == PROMOTION)
        {
            Piece promotion = make_piece(us, promotion_type(m));
  
            assert(relative_rank(us, to) == RANK_8);
            assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
  
            remove_piece(pc, to);
            put_piece(promotion, to);
  
            // Update hash keys
            k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[promotion][to];
            st->pawnKey ^= Zobrist::psq[pc][to];
            st->materialKey ^=  Zobrist::psq[promotion][pieceCount[promotion]-1]
                              ^ Zobrist::psq[pc][pieceCount[pc]];
  
            // Update incremental score
            st->psq += PSQT::psq[promotion][to] - PSQT::psq[pc][to];
  
            // Update material
            st->nonPawnMaterial[us] += PieceValue[MG][promotion];
        }
  
        // Update pawn hash key and prefetch access to pawnsTable
        st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
        prefetch2(thisThread->pawnsTable[st->pawnKey]);
  
        // Reset rule 50 draw counter
        st->rule50 = 0;
    }
  
    // Update incremental scores
    st->psq += PSQT::psq[pc][to] - PSQT::psq[pc][from];
  
    // Set capture piece
    st->capturedPiece = captured;
  
    // Update the key with the final value
    st->key = k;
  
    // Calculate checkers bitboard (if move gives check)
    st->checkersBB = givesCheck ? attackers_to(square<KING>(them)) & pieces(us) : 0;
  
    sideToMove = ~sideToMove;
  
    // Update king attacks used for fast check detection
    set_check_info(st);
  
    assert(pos_is_ok());
  }
  
  
  /// Position::undo_move() unmakes a move. When it returns, the position should
  /// be restored to exactly the same state as before the move was made.
  
  void Position::undo_move(Move m) {
  
    assert(is_ok(m));
  
    sideToMove = ~sideToMove;
  
    Color us = sideToMove;
    Square from = from_sq(m);
    Square to = to_sq(m);
    Piece pc = piece_on(to);
  
    assert(empty(from) || type_of(m) == CASTLING);
    assert(type_of(st->capturedPiece) != KING);
  
    if (type_of(m) == PROMOTION)
    {
        assert(relative_rank(us, to) == RANK_8);
        assert(type_of(pc) == promotion_type(m));
        assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);
  
        remove_piece(pc, to);
        pc = make_piece(us, PAWN);
        put_piece(pc, to);
    }
  
    if (type_of(m) == CASTLING)
    {
        Square rfrom, rto;
        do_castling<false>(us, from, to, rfrom, rto);
    }
    else
    {
        move_piece(pc, to, from); // Put the piece back at the source square
  
        if (st->capturedPiece)
        {
            Square capsq = to;
  
            if (type_of(m) == ENPASSANT)
            {
                capsq -= pawn_push(us);
  
                assert(type_of(pc) == PAWN);
                assert(to == st->previous->epSquare);
                assert(relative_rank(us, to) == RANK_6);
                assert(piece_on(capsq) == NO_PIECE);
                assert(st->capturedPiece == make_piece(~us, PAWN));
            }
  
            put_piece(st->capturedPiece, capsq); // Restore the captured piece
        }
    }
  
    // Finally point our state pointer back to the previous state
    st = st->previous;
    --gamePly;
  
    assert(pos_is_ok());
  }
  
  
  /// Position::do_castling() is a helper used to do/undo a castling move. This
  /// is a bit tricky in Chess960 where from/to squares can overlap.
  template<bool Do>
  void Position::do_castling(Color us, Square from, Square& to, Square& rfrom, Square& rto) {
  
    bool kingSide = to > from;
    rfrom = to; // Castling is encoded as "king captures friendly rook"
    rto = relative_square(us, kingSide ? SQ_F1 : SQ_D1);
    to = relative_square(us, kingSide ? SQ_G1 : SQ_C1);
  
    // Remove both pieces first since squares could overlap in Chess960
    remove_piece(make_piece(us, KING), Do ? from : to);
    remove_piece(make_piece(us, ROOK), Do ? rfrom : rto);
    board[Do ? from : to] = board[Do ? rfrom : rto] = NO_PIECE; // Since remove_piece doesn't do it for us
    put_piece(make_piece(us, KING), Do ? to : from);
    put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
  }
  
  
  /// Position::do(undo)_null_move() is used to do(undo) a "null move": It flips
  /// the side to move without executing any move on the board.
  
  void Position::do_null_move(StateInfo& newSt) {
  
    assert(!checkers());
    assert(&newSt != st);
  
    std::memcpy(&newSt, st, sizeof(StateInfo));
    newSt.previous = st;
    st = &newSt;
  
    if (st->epSquare != SQ_NONE)
    {
        st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
        st->epSquare = SQ_NONE;
    }
  
    st->key ^= Zobrist::side;
    prefetch(TT.first_entry(st->key));
  
    ++st->rule50;
    st->pliesFromNull = 0;
  
    sideToMove = ~sideToMove;
  
    set_check_info(st);
  
    assert(pos_is_ok());
  }
  
  void Position::undo_null_move() {
  
    assert(!checkers());
  
    st = st->previous;
    sideToMove = ~sideToMove;
  }
  
  
  /// Position::key_after() computes the new hash key after the given move. Needed
  /// for speculative prefetch. It doesn't recognize special moves like castling,
  /// en-passant and promotions.
  
  Key Position::key_after(Move m) const {
  
    Square from = from_sq(m);
    Square to = to_sq(m);
    Piece pc = piece_on(from);
    Piece captured = piece_on(to);
    Key k = st->key ^ Zobrist::side;
  
    if (captured)
        k ^= Zobrist::psq[captured][to];
  
    return k ^ Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];
  }
  
  
  /// Position::see_ge (Static Exchange Evaluation Greater or Equal) tests if the
  /// SEE value of move is greater or equal to the given threshold. We'll use an
  /// algorithm similar to alpha-beta pruning with a null window.
  
  bool Position::see_ge(Move m, Value threshold) const {
  
    assert(is_ok(m));
  
    // Only deal with normal moves, assume others pass a simple see
    if (type_of(m) != NORMAL)
        return VALUE_ZERO >= threshold;
  
    Bitboard stmAttackers;
    Square from = from_sq(m), to = to_sq(m);
    PieceType nextVictim = type_of(piece_on(from));
    Color us = color_of(piece_on(from));
    Color stm = ~us; // First consider opponent's move
    Value balance;   // Values of the pieces taken by us minus opponent's ones
  
    // The opponent may be able to recapture so this is the best result
    // we can hope for.
    balance = PieceValue[MG][piece_on(to)] - threshold;
  
    if (balance < VALUE_ZERO)
        return false;
  
    // Now assume the worst possible result: that the opponent can
    // capture our piece for free.
    balance -= PieceValue[MG][nextVictim];
  
    // If it is enough (like in PxQ) then return immediately. Note that
    // in case nextVictim == KING we always return here, this is ok
    // if the given move is legal.
    if (balance >= VALUE_ZERO)
        return true;
  
    // Find all attackers to the destination square, with the moving piece
    // removed, but possibly an X-ray attacker added behind it.
    Bitboard occupied = pieces() ^ from ^ to;
    Bitboard attackers = attackers_to(to, occupied) & occupied;
  
    while (true)
    {
        stmAttackers = attackers & pieces(stm);
  
        // Don't allow pinned pieces to attack (except the king) as long as
        // all pinners are on their original square.
        if (!(st->pinnersForKing[stm] & ~occupied))
            stmAttackers &= ~st->blockersForKing[stm];
  
        // If stm has no more attackers then give up: stm loses
        if (!stmAttackers)
            break;
  
        // Locate and remove the next least valuable attacker, and add to
        // the bitboard 'attackers' the possibly X-ray attackers behind it.
        nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
  
        stm = ~stm; // Switch side to move
  
        // Negamax the balance with alpha = balance, beta = balance+1 and
        // add nextVictim's value.
        //
        //      (balance, balance+1) -> (-balance-1, -balance)
        //
        assert(balance < VALUE_ZERO);
  
        balance = -balance - 1 - PieceValue[MG][nextVictim];
  
        // If balance is still non-negative after giving away nextVictim then we
        // win. The only thing to be careful about it is that we should revert
        // stm if we captured with the king when the opponent still has attackers.
        if (balance >= VALUE_ZERO)
        {
            if (nextVictim == KING && (attackers & pieces(stm)))
                stm = ~stm;
            break;
        }
        assert(nextVictim != KING);
    }
    return us != stm; // We break the above loop when stm loses
  }
  
  
  /// Position::is_draw() tests whether the position is drawn by 50-move rule
  /// or by repetition. It does not detect stalemates.
  
  bool Position::is_draw(int ply) const {
  
    if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
        return true;
  
    int end = std::min(st->rule50, st->pliesFromNull);
  
    if (end < 4)
      return false;
  
    StateInfo* stp = st->previous->previous;
    int cnt = 0;
  
    for (int i = 4; i <= end; i += 2)
    {
        stp = stp->previous->previous;
  
        // Return a draw score if a position repeats once earlier but strictly
        // after the root, or repeats twice before or at the root.
        if (   stp->key == st->key
            && ++cnt + (ply > i) == 2)
            return true;
    }
  
    return false;
  }
  
  
  /// Position::flip() flips position with the white and black sides reversed. This
  /// is only useful for debugging e.g. for finding evaluation symmetry bugs.
  
  void Position::flip() {
  
    string f, token;
    std::stringstream ss(fen());
  
    for (Rank r = RANK_8; r >= RANK_1; --r) // Piece placement
    {
        std::getline(ss, token, r > RANK_1 ? '/' : ' ');
        f.insert(0, token + (f.empty() ? " " : "/"));
    }
  
    ss >> token; // Active color
    f += (token == "w" ? "B " : "W "); // Will be lowercased later
  
    ss >> token; // Castling availability
    f += token + " ";
  
    std::transform(f.begin(), f.end(), f.begin(),
                   [](char c) { return char(islower(c) ? toupper(c) : tolower(c)); });
  
    ss >> token; // En passant square
    f += (token == "-" ? token : token.replace(1, 1, token[1] == '3' ? "6" : "3"));
  
    std::getline(ss, token); // Half and full moves
    f += token;
  
    set(f, is_chess960(), st, this_thread());
  
    assert(pos_is_ok());
  }
  
  
  /// Position::pos_is_ok() performs some consistency checks for the
  /// position object and raises an asserts if something wrong is detected.
  /// This is meant to be helpful when debugging.
  
  bool Position::pos_is_ok() const {
  
    const bool Fast = true; // Quick (default) or full check?
  
    if (   (sideToMove != WHITE && sideToMove != BLACK)
        || piece_on(square<KING>(WHITE)) != W_KING
        || piece_on(square<KING>(BLACK)) != B_KING
        || (   ep_square() != SQ_NONE
            && relative_rank(sideToMove, ep_square()) != RANK_6))
        assert(0 && "pos_is_ok: Default");
  
    if (Fast)
        return true;
  
    if (   pieceCount[W_KING] != 1
        || pieceCount[B_KING] != 1
        || attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove))
        assert(0 && "pos_is_ok: Kings");
  
    if (   (pieces(PAWN) & (Rank1BB | Rank8BB))
        || pieceCount[W_PAWN] > 8
        || pieceCount[B_PAWN] > 8)
        assert(0 && "pos_is_ok: Pawns");
  
    if (   (pieces(WHITE) & pieces(BLACK))
        || (pieces(WHITE) | pieces(BLACK)) != pieces()
        || popcount(pieces(WHITE)) > 16
        || popcount(pieces(BLACK)) > 16)
        assert(0 && "pos_is_ok: Bitboards");
  
    for (PieceType p1 = PAWN; p1 <= KING; ++p1)
        for (PieceType p2 = PAWN; p2 <= KING; ++p2)
            if (p1 != p2 && (pieces(p1) & pieces(p2)))
                assert(0 && "pos_is_ok: Bitboards");
  
    StateInfo si = *st;
    set_state(&si);
    if (std::memcmp(&si, st, sizeof(StateInfo)))
        assert(0 && "pos_is_ok: State");
  
    for (Piece pc : Pieces)
    {
        if (   pieceCount[pc] != popcount(pieces(color_of(pc), type_of(pc)))
            || pieceCount[pc] != std::count(board, board + SQUARE_NB, pc))
            assert(0 && "pos_is_ok: Pieces");
  
        for (int i = 0; i < pieceCount[pc]; ++i)
            if (board[pieceList[pc][i]] != pc || index[pieceList[pc][i]] != i)
                assert(0 && "pos_is_ok: Index");
    }
  
    for (Color c = WHITE; c <= BLACK; ++c)
        for (CastlingSide s = KING_SIDE; s <= QUEEN_SIDE; s = CastlingSide(s + 1))
        {
            if (!can_castle(c | s))
                continue;
  
            if (   piece_on(castlingRookSquare[c | s]) != make_piece(c, ROOK)
                || castlingRightsMask[castlingRookSquare[c | s]] != (c | s)
                || (castlingRightsMask[square<KING>(c)] & (c | s)) != (c | s))
                assert(0 && "pos_is_ok: Castling");
        }
  
    return true;
  }
  