X-Git-Url: https://git.sesse.net/?p=stockfish;a=blobdiff_plain;f=src%2Fposition.cpp;h=fdae0ae0bf211999f9c2ee69472c4f151af0f8a0;hp=42c1adff71591a270148adb43b37d2588cd2fe29;hb=2e21aba8d937bdf5eb0eedeac99ac2c2553e9d0d;hpb=ba4e215493de31263b9bd352af79d00193e545bf diff --git a/src/position.cpp b/src/position.cpp index 42c1adff..fdae0ae0 100644 --- a/src/position.cpp +++ b/src/position.cpp @@ -2,7 +2,7 @@ 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 @@ -60,22 +60,27 @@ const Piece Pieces[] = { W_PAWN, W_KNIGHT, W_BISHOP, W_ROOK, W_QUEEN, W_KING, // from the bitboards and scan for new X-ray attacks behind it. template -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(bb, to, stmAttackers, occupied, attackers); + return min_attacker(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(to, occupied) & (bb[BISHOP] | bb[QUEEN]); + attackers |= attacks_bb(to, occupied) & (byTypeBB[BISHOP] | byTypeBB[QUEEN]); if (Pt == ROOK || Pt == QUEEN) - attackers |= attacks_bb(to, occupied) & (bb[ROOK] | bb[QUEEN]); + attackers |= attacks_bb(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; } @@ -211,10 +216,10 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th 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) { @@ -787,7 +792,7 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) { if ( (int(to) ^ int(from)) == 16 && (attacks_from(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)]; } @@ -997,57 +1002,75 @@ bool Position::see_ge(Move m, Value threshold) const { 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 - balance = PieceValue[MG][piece_on(to)]; + // 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; + // Now assume the worst possible result: that the opponent can + // capture our piece for free. balance -= PieceValue[MG][nextVictim]; - if (balance >= threshold) // Always true if nextVictim == KING + // 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. + // 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) - 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(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 }