Add comments to pos.see_ge()
In terms of technical changes this patch eliminates the return statements from the main loop of pos.see_ge() and replaces two conditional computations with a single bitwise negation. No functional changepull/1296/head^2
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87452f3a8c
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3f44f5303b
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@ -1003,17 +1003,21 @@ bool Position::see_ge(Move m, Value threshold) const {
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Value balance; // Values of the pieces taken by us minus opponent's ones
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Bitboard occupied, stmAttackers;
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balance = PieceValue[MG][piece_on(to)];
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// The opponent may be able to recapture so this is the best result
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// we can hope for.
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balance = PieceValue[MG][piece_on(to)] - threshold;
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if (balance < threshold)
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if (balance < VALUE_ZERO)
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return false;
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// Now assume the worst possible result: that the opponent can
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// capture our piece for free.
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balance -= PieceValue[MG][nextVictim];
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if (balance >= threshold) // Always true if nextVictim == KING
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if (balance >= VALUE_ZERO) // Always true if nextVictim == KING
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return true;
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bool relativeStm = true; // True if the opponent is to move
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bool opponentToMove = true;
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occupied = pieces() ^ from ^ to;
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// Find all attackers to the destination square, with the moving piece removed,
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@ -1022,6 +1026,12 @@ bool Position::see_ge(Move m, Value threshold) const {
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while (true)
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{
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// The balance is negative only because we assumed we could win
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// the last piece for free. We are truly winning only if we can
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// win the last piece _cheaply enough_. Test if we can actually
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// do this otherwise "give up".
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assert(balance < VALUE_ZERO);
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stmAttackers = attackers & pieces(stm);
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// Don't allow pinned pieces to attack pieces except the king as long all
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@ -1029,25 +1039,40 @@ bool Position::see_ge(Move m, Value threshold) const {
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if (!(st->pinnersForKing[stm] & ~occupied))
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stmAttackers &= ~st->blockersForKing[stm];
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// If we have no more attackers we must give up
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if (!stmAttackers)
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return relativeStm;
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break;
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// Locate and remove the next least valuable attacker
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nextVictim = min_attacker<PAWN>(byTypeBB, to, stmAttackers, occupied, attackers);
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if (nextVictim == KING)
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return relativeStm == bool(attackers & pieces(~stm));
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{
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// Our only attacker is the king. If the opponent still has
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// attackers we must give up. Otherwise we make the move and
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// (having no more attackers) the opponent must give up.
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if (!(attackers & pieces(~stm)))
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opponentToMove = !opponentToMove;
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break;
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}
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balance += relativeStm ? PieceValue[MG][nextVictim]
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: -PieceValue[MG][nextVictim];
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// Assume the opponent can win the next piece for free and switch sides
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balance += PieceValue[MG][nextVictim];
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opponentToMove = !opponentToMove;
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relativeStm = !relativeStm;
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if (relativeStm == (balance >= threshold))
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return relativeStm;
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// If balance is negative after receiving a free piece then give up
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if (balance < VALUE_ZERO)
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break;
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// Complete the process of switching sides. The first line swaps
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// all negative numbers with non-negative numbers. The compiler
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// probably knows that it is just the bitwise negation ~balance.
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balance = -balance-1;
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stm = ~stm;
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}
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// If the opponent gave up we win, otherwise we lose.
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return opponentToMove;
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}
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