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) 2015-2017 Marco Costalba, Joona Kiiski, Gary Linscott, 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
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 };
+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 };
// 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* bb, Square to, Bitboard stmAttackers,
+PieceType min_attacker(const Bitboard* byTypeBB, Square to, Bitboard stmAttackers,
Bitboard& occupied, Bitboard& attackers) {
- Bitboard b = stmAttackers & bb[Pt];
+ Bitboard b = stmAttackers & byTypeBB[Pt];
if (!b)
- return min_attacker<Pt + 1>(bb, to, stmAttackers, occupied, attackers);
+ return min_attacker<Pt + 1>(byTypeBB, to, stmAttackers, occupied, attackers);
- occupied ^= b & ~(b - 1);
+ 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) & (bb[BISHOP] | bb[QUEEN]);
+ attackers |= attacks_bb<BISHOP>(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]);
if (Pt == ROOK || Pt == QUEEN)
- attackers |= attacks_bb<ROOK>(to, occupied) & (bb[ROOK] | bb[QUEEN]);
+ attackers |= attacks_bb<ROOK>(to, occupied) & (byTypeBB[ROOK] | byTypeBB[QUEEN]);
- attackers &= occupied; // After X-ray that may add already processed pieces
+ // 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;
}
while ((ss >> token) && !isspace(token))
{
if (isdigit(token))
- sq += Square(token - '0'); // Advance the given number of files
+ sq += (token - '0') * EAST; // Advance the given number of files
else if (token == '/')
- sq -= Square(16);
+ sq += 2 * SOUTH;
else if ((idx = PieceToChar.find(token)) != string::npos)
{
// 5-6. Halfmove clock and fullmove number
ss >> std::skipws >> st->rule50 >> gamePly;
- // Convert from fullmove starting from 1 to ply starting from 0,
+ // 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);
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]);
+ si->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), si->pinners[BLACK]);
+ si->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), si->pinners[WHITE]);
Square ksq = square<KING>(~sideToMove);
}
-/// Position::game_phase() calculates the game phase interpolating total non-pawn
-/// material between endgame and midgame limits.
-
-Phase Position::game_phase() const {
-
- Value npm = st->nonPawnMaterial[WHITE] + st->nonPawnMaterial[BLACK];
-
- npm = std::max(EndgameLimit, std::min(npm, MidgameLimit));
-
- return Phase(((npm - EndgameLimit) * PHASE_MIDGAME) / (MidgameLimit - EndgameLimit));
-}
-
-
/// 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
Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
- Bitboard result = 0;
+ Bitboard blockers = 0;
pinners = 0;
// Snipers are sliders that attack 's' when a piece is removed
Square sniperSq = pop_lsb(&snipers);
Bitboard b = between_bb(s, sniperSq) & pieces();
- if (!more_than_one(b))
+ if (b && !more_than_one(b))
{
- result |= b;
+ blockers |= b;
if (b & pieces(color_of(piece_on(s))))
pinners |= sniperSq;
}
}
- return result;
+ return blockers;
}
// 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 !(pinned_pieces(us) & from)
+ return !(blockers_for_king(us) & from)
|| aligned(from, to_sq(m), square<KING>(us));
}
return true;
// Is there a discovered check?
- if ( (discovered_check_candidates() & from)
+ if ( (st->blockersForKing[~sideToMove] & from)
&& !aligned(from, to, square<KING>(~sideToMove)))
return true;
if ( (int(to) ^ int(from)) == 16
&& (attacks_from<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
{
- st->epSquare = (from + to) / 2;
+ st->epSquare = to - pawn_push(us);
k ^= Zobrist::enpassant[file_of(st->epSquare)];
}
assert(is_ok(m));
- // Castling moves are implemented as king capturing the rook so cannot be
- // handled correctly. Simply assume the SEE value is VALUE_ZERO that is always
- // correct unless in the rare case the rook ends up under attack.
- if (type_of(m) == CASTLING)
+ // 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 stm = ~color_of(piece_on(from)); // First consider opponent's move
- Value balance; // Values of the pieces taken by us minus opponent's ones
- Bitboard occupied, stmAttackers;
+ 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
- if (type_of(m) == ENPASSANT)
- {
- occupied = SquareBB[to - pawn_push(~stm)]; // Remove the captured pawn
- balance = PieceValue[MG][PAWN];
- }
- else
- {
- balance = PieceValue[MG][piece_on(to)];
- occupied = 0;
- }
+ // 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 < threshold)
+ if (balance < VALUE_ZERO)
return false;
- if (nextVictim == KING)
- return true;
-
+ // Now assume the worst possible result: that the opponent can
+ // capture our piece for free.
balance -= PieceValue[MG][nextVictim];
- if (balance >= threshold)
+ // 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;
- bool relativeStm = true; // True if the opponent is to move
- occupied ^= pieces() ^ from ^ to;
-
- // Find all attackers to the destination square, with the moving piece removed,
- // but possibly an X-ray attacker added behind it.
+ // 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 pieces except the king as long all
- // pinners are on their original square.
- if (!(st->pinnersForKing[stm] & ~occupied))
+ // Don't allow pinned pieces to attack (except the king) as long as
+ // all pinners are on their original square.
+ if (!(st->pinners[~stm] & ~occupied))
stmAttackers &= ~st->blockersForKing[stm];
+ // If stm has no more attackers then give up: stm loses
if (!stmAttackers)
- return relativeStm;
+ break;
- // Locate and remove the next least valuable attacker
+ // 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);
- if (nextVictim == KING)
- return relativeStm == bool(attackers & pieces(~stm));
-
- balance += relativeStm ? PieceValue[MG][nextVictim]
- : -PieceValue[MG][nextVictim];
+ stm = ~stm; // Switch side to move
- relativeStm = !relativeStm;
+ // Negamax the balance with alpha = balance, beta = balance+1 and
+ // add nextVictim's value.
+ //
+ // (balance, balance+1) -> (-balance-1, -balance)
+ //
+ assert(balance < VALUE_ZERO);
- if (relativeStm == (balance >= threshold))
- return relativeStm;
+ balance = -balance - 1 - PieceValue[MG][nextVictim];
- stm = ~stm;
+ // 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
}
{
stp = stp->previous->previous;
- // At root position ply is 1, so return a draw score if a position
- // repeats once earlier but strictly after the root, or repeats twice
- // before or at the root.
+ // 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 - 1 > i) == 2)
+ && ++cnt + (ply > i) == 2)
return true;
}
}
+// Position::has_repeated() 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;
+ while (true)
+ {
+ int i = 4, e = std::min(stc->rule50, stc->pliesFromNull);
+
+ if (e < i)
+ return false;
+
+ StateInfo* stp = st->previous->previous;
+
+ do {
+ stp = stp->previous->previous;
+
+ if (stp->key == stc->key)
+ return true;
+
+ i += 2;
+ } while (i <= e);
+
+ stc = stc->previous;
+ }
+}
+
+
/// Position::flip() flips position with the white and black sides reversed. This
/// is only useful for debugging e.g. for finding evaluation symmetry bugs.
bool Position::pos_is_ok() const {
- const bool Fast = true; // Quick (default) or full check?
+ constexpr bool Fast = true; // Quick (default) or full check?
if ( (sideToMove != WHITE && sideToMove != BLACK)
|| piece_on(square<KING>(WHITE)) != W_KING
projects / stockfish / blobdiff