BadFish/src/position.cpp

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2024 The Stockfish developers (see AUTHORS file)

  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 "position.h"

#include <algorithm>
#include <atomic>
#include <cassert>
#include <cctype>
#include <cstddef>
#include <cstring>
#include <initializer_list>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <string_view>
#include <utility>

#include "bitboard.h"
#include "misc.h"
#include "movegen.h"
#include "nnue/nnue_common.h"
#include "syzygy/tbprobe.h"
#include "thread.h"
#include "tt.h"
#include "uci.h"

using std::string;

namespace Stockfish {

namespace Zobrist {

Key psq[PIECE_NB][SQUARE_NB];
Key enpassant[FILE_NB];
Key castling[CASTLING_RIGHT_NB];
Key side, noPawns;
}

namespace {

constexpr std::string_view PieceToChar(" PNBRQK  pnbrqk");

constexpr 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};
}  // namespace


// 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 << " | " << (1 + r) << "\n +---+---+---+---+---+---+---+---+\n";
    }

    os << "   a   b   c   d   e   f   g   h\n"
       << "\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;
        ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);

        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;
}


// Implements Marcel van Kervinck's cuckoo algorithm to detect repetition of positions
// for 3-fold repetition draws. The algorithm uses two hash tables with Zobrist hashes
// to allow fast detection of recurring positions. For details see:
// http://web.archive.org/web/20201107002606/https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf

// First and second hash functions for indexing the cuckoo tables
inline int H1(Key h) { return h & 0x1fff; }
inline int H2(Key h) { return (h >> 16) & 0x1fff; }

// Cuckoo tables with Zobrist hashes of valid reversible moves, and the moves themselves
Key  cuckoo[8192];
Move cuckooMove[8192];


// 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] = rng.rand<Key>();

    Zobrist::side    = rng.rand<Key>();
    Zobrist::noPawns = rng.rand<Key>();

    // Prepare the cuckoo tables
    std::memset(cuckoo, 0, sizeof(cuckoo));
    std::memset(cuckooMove, 0, sizeof(cuckooMove));
    [[maybe_unused]] int count = 0;
    for (Piece pc : Pieces)
        for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
            for (Square s2 = Square(s1 + 1); s2 <= SQ_H8; ++s2)
                if ((type_of(pc) != PAWN) && (attacks_bb(type_of(pc), s1, 0) & s2))
                {
                    Move move = Move(s1, s2);
                    Key  key  = Zobrist::psq[pc][s1] ^ Zobrist::psq[pc][s2] ^ Zobrist::side;
                    int  i    = H1(key);
                    while (true)
                    {
                        std::swap(cuckoo[i], key);
                        std::swap(cuckooMove[i], move);
                        if (move == Move::none())  // Arrived at empty slot?
                            break;
                        i = (i == H1(key)) ? H2(key) : H1(key);  // Push victim to alternative slot
                    }
                    count++;
                }
    assert(count == 3668);
}


// 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. Following X-FEN standard, 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));
    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 square is invalid or not on side to move relative rank 6.
    bool enpassant = false;

    if (((ss >> col) && (col >= 'a' && col <= 'h'))
        && ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
    {
        st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));

        // En passant square will be considered only if
        // a) side to move have a pawn threatening epSquare
        // b) there is an enemy pawn in front of epSquare
        // c) there is no piece on epSquare or behind epSquare
        enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
                 && (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
                 && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
    }

    if (!enpassant)
        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();

    assert(pos_is_ok());

    return *this;
}


// 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);
    CastlingRights cr    = c & (kfrom < rfrom ? KING_SIDE : QUEEN_SIDE);

    st->castlingRights |= cr;
    castlingRightsMask[kfrom] |= cr;
    castlingRightsMask[rfrom] |= cr;
    castlingRookSquare[cr] = rfrom;

    Square kto = relative_square(c, cr & KING_SIDE ? SQ_G1 : SQ_C1);
    Square rto = relative_square(c, cr & KING_SIDE ? SQ_F1 : SQ_D1);

    castlingPath[cr] = (between_bb(rfrom, rto) | between_bb(kfrom, kto)) & ~(kfrom | rfrom);
}


// Sets king attacks to detect if a move gives check
void Position::set_check_info() const {

    update_slider_blockers(WHITE);
    update_slider_blockers(BLACK);

    Square ksq = square<KING>(~sideToMove);

    st->checkSquares[PAWN]   = pawn_attacks_bb(~sideToMove, ksq);
    st->checkSquares[KNIGHT] = attacks_bb<KNIGHT>(ksq);
    st->checkSquares[BISHOP] = attacks_bb<BISHOP>(ksq, pieces());
    st->checkSquares[ROOK]   = attacks_bb<ROOK>(ksq, pieces());
    st->checkSquares[QUEEN]  = st->checkSquares[BISHOP] | st->checkSquares[ROOK];
    st->checkSquares[KING]   = 0;
}


// 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
void Position::set_state() const {

    st->key = st->materialKey  = 0;
    st->pawnKey                = Zobrist::noPawns;
    st->nonPawnMaterial[WHITE] = st->nonPawnMaterial[BLACK] = VALUE_ZERO;
    st->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);

    set_check_info();

    for (Bitboard b = pieces(); b;)
    {
        Square s  = pop_lsb(b);
        Piece  pc = piece_on(s);
        st->key ^= Zobrist::psq[pc][s];

        if (type_of(pc) == PAWN)
            st->pawnKey ^= Zobrist::psq[pc][s];

        else if (type_of(pc) != KING)
            st->nonPawnMaterial[color_of(pc)] += PieceValue[pc];
    }

    if (st->epSquare != SQ_NONE)
        st->key ^= Zobrist::enpassant[file_of(st->epSquare)];

    if (sideToMove == BLACK)
        st->key ^= Zobrist::side;

    st->key ^= Zobrist::castling[st->castlingRights];

    for (Piece pc : Pieces)
        for (int cnt = 0; cnt < pieceCount[pc]; ++cnt)
            st->materialKey ^= Zobrist::psq[pc][cnt];
}


// Overload to initialize the position object with the given endgame code string
// like "KBPKN". It's 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[0] == 'K');

    string sides[] = {code.substr(code.find('K', 1)),                                // Weak
                      code.substr(0, std::min(code.find('v'), code.find('K', 1)))};  // Strong

    assert(sides[0].length() > 0 && sides[0].length() < 8);
    assert(sides[1].length() > 0 && sides[1].length() < 8);

    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);
}


// Returns a FEN representation of the position. In case of
// Chess960 the Shredder-FEN notation is used. This is mainly a debugging function.
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_OO))) : 'K');

    if (can_castle(WHITE_OOO))
        ss << (chess960 ? char('A' + file_of(castling_rook_square(WHITE_OOO))) : 'Q');

    if (can_castle(BLACK_OO))
        ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OO))) : 'k');

    if (can_castle(BLACK_OOO))
        ss << (chess960 ? char('a' + file_of(castling_rook_square(BLACK_OOO))) : 'q');

    if (!can_castle(ANY_CASTLING))
        ss << '-';

    ss << (ep_square() == SQ_NONE ? " - " : " " + UCI::square(ep_square()) + " ") << st->rule50
       << " " << 1 + (gamePly - (sideToMove == BLACK)) / 2;

    return ss.str();
}

// Calculates st->blockersForKing[c] and st->pinners[~c],
// which store respectively the pieces preventing king of color c from being in check
// and the slider pieces of color ~c pinning pieces of color c to the king.
void Position::update_slider_blockers(Color c) const {

    Square ksq = square<KING>(c);

    st->blockersForKing[c] = 0;
    st->pinners[~c]        = 0;

    // Snipers are sliders that attack 's' when a piece and other snipers are removed
    Bitboard snipers = ((attacks_bb<ROOK>(ksq) & pieces(QUEEN, ROOK))
                        | (attacks_bb<BISHOP>(ksq) & pieces(QUEEN, BISHOP)))
                     & pieces(~c);
    Bitboard occupancy = pieces() ^ snipers;

    while (snipers)
    {
        Square   sniperSq = pop_lsb(snipers);
        Bitboard b        = between_bb(ksq, sniperSq) & occupancy;

        if (b && !more_than_one(b))
        {
            st->blockersForKing[c] |= b;
            if (b & pieces(c))
                st->pinners[~c] |= sniperSq;
        }
    }
}


// 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 (pawn_attacks_bb(BLACK, s) & pieces(WHITE, PAWN))
         | (pawn_attacks_bb(WHITE, s) & pieces(BLACK, PAWN))
         | (attacks_bb<KNIGHT>(s) & pieces(KNIGHT))
         | (attacks_bb<ROOK>(s, occupied) & pieces(ROOK, QUEEN))
         | (attacks_bb<BISHOP>(s, occupied) & pieces(BISHOP, QUEEN))
         | (attacks_bb<KING>(s) & pieces(KING));
}


// Tests whether a pseudo-legal move is legal
bool Position::legal(Move m) const {

    assert(m.is_ok());

    Color  us   = sideToMove;
    Square from = m.from_sq();
    Square to   = m.to_sq();

    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 (m.type_of() == EN_PASSANT)
    {
        Square   ksq      = square<KING>(us);
        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));
    }

    // Castling moves generation does not check if the castling path is clear of
    // enemy attacks, it is delayed at a later time: now!
    if (m.type_of() == CASTLING)
    {
        // After castling, the rook and king final positions are the same in
        // Chess960 as they would be in standard chess.
        to             = relative_square(us, to > from ? SQ_G1 : SQ_C1);
        Direction step = to > from ? WEST : EAST;

        for (Square s = to; s != from; s += step)
            if (attackers_to(s) & pieces(~us))
                return false;

        // In case of Chess960, verify if the Rook blocks some checks.
        // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
        return !chess960 || !(blockers_for_king(us) & m.to_sq());
    }

    // If the moving piece is a king, check whether the destination square is
    // attacked by the opponent.
    if (type_of(piece_on(from)) == KING)
        return !(attackers_to(to, pieces() ^ from) & 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, square<KING>(us));
}


// 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 = m.from_sq();
    Square to   = m.to_sq();
    Piece  pc   = moved_piece(m);

    // Use a slower but simpler function for uncommon cases
    // yet we skip the legality check of MoveList<LEGAL>().
    if (m.type_of() != NORMAL)
        return checkers() ? MoveList<EVASIONS>(*this).contains(m)
                          : MoveList<NON_EVASIONS>(*this).contains(m);

    // Is not a promotion, so the promotion piece must be empty
    assert(m.promotion_type() - KNIGHT == NO_PIECE_TYPE);

    // 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 ((Rank8BB | Rank1BB) & to)
            return false;

        if (!(pawn_attacks_bb(us, from) & 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
                 && (relative_rank(us, from) == RANK_2) && empty(to) && empty(to - pawn_push(us))))
            return false;
    }
    else if (!(attacks_bb(type_of(pc), from, pieces()) & 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 interposition or a capture of the checking piece
            if (!(between_bb(square<KING>(us), lsb(checkers())) & to))
                return false;
        }
        // In case of king moves under check we have to remove the 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;
}


// Tests whether a pseudo-legal move gives a check
bool Position::gives_check(Move m) const {

    assert(m.is_ok());
    assert(color_of(moved_piece(m)) == sideToMove);

    Square from = m.from_sq();
    Square to   = m.to_sq();

    // Is there a direct check?
    if (check_squares(type_of(piece_on(from))) & to)
        return true;

    // Is there a discovered check?
    if (blockers_for_king(~sideToMove) & from)
        return !aligned(from, to, square<KING>(~sideToMove)) || m.type_of() == CASTLING;

    switch (m.type_of())
    {
    case NORMAL :
        return false;

    case PROMOTION :
        return attacks_bb(m.promotion_type(), 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 EN_PASSANT : {
        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));
    }
    default :  //CASTLING
    {
        // Castling is encoded as 'king captures the rook'
        Square rto = relative_square(sideToMove, to > from ? SQ_F1 : SQ_D1);

        return check_squares(ROOK) & rto;
    }
    }
}


// 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(m.is_ok());
    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;

    // Used by NNUE
    st->accumulatorBig.computed[WHITE]     = st->accumulatorBig.computed[BLACK] =
      st->accumulatorSmall.computed[WHITE] = st->accumulatorSmall.computed[BLACK] = false;
    auto& dp                                                                      = st->dirtyPiece;
    dp.dirty_num                                                                  = 1;

    Color  us       = sideToMove;
    Color  them     = ~us;
    Square from     = m.from_sq();
    Square to       = m.to_sq();
    Piece  pc       = piece_on(from);
    Piece  captured = m.type_of() == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);

    assert(color_of(pc) == us);
    assert(captured == NO_PIECE || color_of(captured) == (m.type_of() != CASTLING ? them : us));
    assert(type_of(captured) != KING);

    if (m.type_of() == CASTLING)
    {
        assert(pc == make_piece(us, KING));
        assert(captured == make_piece(us, ROOK));

        Square rfrom, rto;
        do_castling<true>(us, from, to, rfrom, rto);

        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 (m.type_of() == EN_PASSANT)
            {
                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));
            }

            st->pawnKey ^= Zobrist::psq[captured][capsq];
        }
        else
            st->nonPawnMaterial[them] -= PieceValue[captured];

        dp.dirty_num = 2;  // 1 piece moved, 1 piece captured
        dp.piece[1]  = captured;
        dp.from[1]   = capsq;
        dp.to[1]     = SQ_NONE;

        // Update board and piece lists
        remove_piece(capsq);

        // Update material hash key and prefetch access to materialTable
        k ^= Zobrist::psq[captured][capsq];
        st->materialKey ^= Zobrist::psq[captured][pieceCount[captured]];

        // 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]))
    {
        k ^= Zobrist::castling[st->castlingRights];
        st->castlingRights &= ~(castlingRightsMask[from] | castlingRightsMask[to]);
        k ^= Zobrist::castling[st->castlingRights];
    }

    // Move the piece. The tricky Chess960 castling is handled earlier
    if (m.type_of() != CASTLING)
    {
        dp.piece[0] = pc;
        dp.from[0]  = from;
        dp.to[0]    = to;

        move_piece(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
            && (pawn_attacks_bb(us, to - pawn_push(us)) & pieces(them, PAWN)))
        {
            st->epSquare = to - pawn_push(us);
            k ^= Zobrist::enpassant[file_of(st->epSquare)];
        }

        else if (m.type_of() == PROMOTION)
        {
            Piece promotion = make_piece(us, m.promotion_type());

            assert(relative_rank(us, to) == RANK_8);
            assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);

            remove_piece(to);
            put_piece(promotion, to);

            // Promoting pawn to SQ_NONE, promoted piece from SQ_NONE
            dp.to[0]               = SQ_NONE;
            dp.piece[dp.dirty_num] = promotion;
            dp.from[dp.dirty_num]  = SQ_NONE;
            dp.to[dp.dirty_num]    = to;
            dp.dirty_num++;

            // 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 material
            st->nonPawnMaterial[us] += PieceValue[promotion];
        }

        // Update pawn hash key
        st->pawnKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];

        // Reset rule 50 draw counter
        st->rule50 = 0;
    }

    // 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();

    // Calculate the repetition info. It is the ply distance from the previous
    // occurrence of the same position, negative in the 3-fold case, or zero
    // if the position was not repeated.
    st->repetition = 0;
    int end        = std::min(st->rule50, st->pliesFromNull);
    if (end >= 4)
    {
        StateInfo* stp = st->previous->previous;
        for (int i = 4; i <= end; i += 2)
        {
            stp = stp->previous->previous;
            if (stp->key == st->key)
            {
                st->repetition = stp->repetition ? -i : i;
                break;
            }
        }
    }

    assert(pos_is_ok());
}


// 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(m.is_ok());

    sideToMove = ~sideToMove;

    Color  us   = sideToMove;
    Square from = m.from_sq();
    Square to   = m.to_sq();
    Piece  pc   = piece_on(to);

    assert(empty(from) || m.type_of() == CASTLING);
    assert(type_of(st->capturedPiece) != KING);

    if (m.type_of() == PROMOTION)
    {
        assert(relative_rank(us, to) == RANK_8);
        assert(type_of(pc) == m.promotion_type());
        assert(type_of(pc) >= KNIGHT && type_of(pc) <= QUEEN);

        remove_piece(to);
        pc = make_piece(us, PAWN);
        put_piece(pc, to);
    }

    if (m.type_of() == CASTLING)
    {
        Square rfrom, rto;
        do_castling<false>(us, from, to, rfrom, rto);
    }
    else
    {
        move_piece(to, from);  // Put the piece back at the source square

        if (st->capturedPiece)
        {
            Square capsq = to;

            if (m.type_of() == EN_PASSANT)
            {
                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());
}


// 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);

    if (Do)
    {
        auto& dp     = st->dirtyPiece;
        dp.piece[0]  = make_piece(us, KING);
        dp.from[0]   = from;
        dp.to[0]     = to;
        dp.piece[1]  = make_piece(us, ROOK);
        dp.from[1]   = rfrom;
        dp.to[1]     = rto;
        dp.dirty_num = 2;
    }

    // Remove both pieces first since squares could overlap in Chess960
    remove_piece(Do ? from : to);
    remove_piece(Do ? rfrom : rto);
    board[Do ? from : to] = board[Do ? rfrom : rto] =
      NO_PIECE;  // remove_piece does not do this for us
    put_piece(make_piece(us, KING), Do ? to : from);
    put_piece(make_piece(us, ROOK), Do ? rto : rfrom);
}


// Used to do 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, offsetof(StateInfo, accumulatorBig));

    newSt.previous = st;
    st             = &newSt;

    st->dirtyPiece.dirty_num               = 0;
    st->dirtyPiece.piece[0]                = NO_PIECE;  // Avoid checks in UpdateAccumulator()
    st->accumulatorBig.computed[WHITE]     = st->accumulatorBig.computed[BLACK] =
      st->accumulatorSmall.computed[WHITE] = st->accumulatorSmall.computed[BLACK] = false;

    if (st->epSquare != SQ_NONE)
    {
        st->key ^= Zobrist::enpassant[file_of(st->epSquare)];
        st->epSquare = SQ_NONE;
    }

    st->key ^= Zobrist::side;
    ++st->rule50;
    prefetch(TT.first_entry(key()));

    st->pliesFromNull = 0;

    sideToMove = ~sideToMove;

    set_check_info();

    st->repetition = 0;

    assert(pos_is_ok());
}


// Must be used to undo a "null move"
void Position::undo_null_move() {

    assert(!checkers());

    st         = st->previous;
    sideToMove = ~sideToMove;
}


// 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     = m.from_sq();
    Square to       = m.to_sq();
    Piece  pc       = piece_on(from);
    Piece  captured = piece_on(to);
    Key    k        = st->key ^ Zobrist::side;

    if (captured)
        k ^= Zobrist::psq[captured][to];

    k ^= Zobrist::psq[pc][to] ^ Zobrist::psq[pc][from];

    return (captured || type_of(pc) == PAWN) ? k : adjust_key50<true>(k);
}


// Tests if the SEE (Static Exchange Evaluation)
// 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, int threshold) const {

    assert(m.is_ok());

    // Only deal with normal moves, assume others pass a simple SEE
    if (m.type_of() != NORMAL)
        return VALUE_ZERO >= threshold;

    Square from = m.from_sq(), to = m.to_sq();

    int swap = PieceValue[piece_on(to)] - threshold;
    if (swap < 0)
        return false;

    swap = PieceValue[piece_on(from)] - swap;
    if (swap <= 0)
        return true;

    assert(color_of(piece_on(from)) == sideToMove);
    Bitboard occupied  = pieces() ^ from ^ to;  // xoring to is important for pinned piece logic
    Color    stm       = sideToMove;
    Bitboard attackers = attackers_to(to, occupied);
    Bitboard stmAttackers, bb;
    int      res = 1;

    while (true)
    {
        stm = ~stm;
        attackers &= occupied;

        // If stm has no more attackers then give up: stm loses
        if (!(stmAttackers = attackers & pieces(stm)))
            break;

        // Don't allow pinned pieces to attack as long as there are
        // pinners on their original square.
        if (pinners(~stm) & occupied)
        {
            stmAttackers &= ~blockers_for_king(stm);

            if (!stmAttackers)
                break;
        }

        res ^= 1;

        // Locate and remove the next least valuable attacker, and add to
        // the bitboard 'attackers' any X-ray attackers behind it.
        if ((bb = stmAttackers & pieces(PAWN)))
        {
            if ((swap = PawnValue - swap) < res)
                break;
            occupied ^= least_significant_square_bb(bb);

            attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
        }

        else if ((bb = stmAttackers & pieces(KNIGHT)))
        {
            if ((swap = KnightValue - swap) < res)
                break;
            occupied ^= least_significant_square_bb(bb);
        }

        else if ((bb = stmAttackers & pieces(BISHOP)))
        {
            if ((swap = BishopValue - swap) < res)
                break;
            occupied ^= least_significant_square_bb(bb);

            attackers |= attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN);
        }

        else if ((bb = stmAttackers & pieces(ROOK)))
        {
            if ((swap = RookValue - swap) < res)
                break;
            occupied ^= least_significant_square_bb(bb);

            attackers |= attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN);
        }

        else if ((bb = stmAttackers & pieces(QUEEN)))
        {
            if ((swap = QueenValue - swap) < res)
                break;
            occupied ^= least_significant_square_bb(bb);

            attackers |= (attacks_bb<BISHOP>(to, occupied) & pieces(BISHOP, QUEEN))
                       | (attacks_bb<ROOK>(to, occupied) & pieces(ROOK, QUEEN));
        }

        else  // KING
              // If we "capture" with the king but the opponent still has attackers,
              // reverse the result.
            return (attackers & ~pieces(stm)) ? res ^ 1 : res;
    }

    return bool(res);
}

// 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;

    // Return a draw score if a position repeats once earlier but strictly
    // after the root, or repeats twice before or at the root.
    return st->repetition && st->repetition < ply;
}


// Tests whether there has been at least one repetition
// of positions since the last capture or pawn move.
bool Position::has_repeated() const {

    StateInfo* stc = st;
    int        end = std::min(st->rule50, st->pliesFromNull);
    while (end-- >= 4)
    {
        if (stc->repetition)
            return true;

        stc = stc->previous;
    }
    return false;
}


// Tests if the position has a move which draws by repetition,
// or an earlier position has a move that directly reaches the current position.
bool Position::has_game_cycle(int ply) const {

    int j;

    int end = std::min(st->rule50, st->pliesFromNull);

    if (end < 3)
        return false;

    Key        originalKey = st->key;
    StateInfo* stp         = st->previous;

    for (int i = 3; i <= end; i += 2)
    {
        stp = stp->previous->previous;

        Key moveKey = originalKey ^ stp->key;
        if ((j = H1(moveKey), cuckoo[j] == moveKey) || (j = H2(moveKey), cuckoo[j] == moveKey))
        {
            Move   move = cuckooMove[j];
            Square s1   = move.from_sq();
            Square s2   = move.to_sq();

            if (!((between_bb(s1, s2) ^ s2) & pieces()))
            {
                if (ply > i)
                    return true;

                // For nodes before or at the root, check that the move is a
                // repetition rather than a move to the current position.
                // In the cuckoo table, both moves Rc1c5 and Rc5c1 are stored in
                // the same location, so we have to select which square to check.
                if (color_of(piece_on(empty(s1) ? s2 : s1)) != side_to_move())
                    continue;

                // For repetitions before or at the root, require one more
                if (stp->repetition)
                    return true;
            }
        }
    }
    return false;
}


// 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());
}


// Performs some consistency checks for the position object
// and raise an assert if something wrong is detected.
// This is meant to be helpful when debugging.
bool Position::pos_is_ok() const {

    constexpr 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");


    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 (Color c : {WHITE, BLACK})
        for (CastlingRights cr : {c & KING_SIDE, c & QUEEN_SIDE})
        {
            if (!can_castle(cr))
                continue;

            if (piece_on(castlingRookSquare[cr]) != make_piece(c, ROOK)
                || castlingRightsMask[castlingRookSquare[cr]] != cr
                || (castlingRightsMask[square<KING>(c)] & cr) != cr)
                assert(0 && "pos_is_ok: Castling");
        }

    return true;
}

}  // namespace Stockfish