// refutes() tests whether a 'first' move is able to defend against a 'second'
// opponent's move. In this case will not be pruned. Normally the second move
// is the threat (the best move returned from a null search that fails low).
private static bool refutes(Position pos, int move, int threat)
{
Debug.Assert(Utils.is_ok_M(move));
Debug.Assert(Utils.is_ok_M(threat));
Square mfrom = Utils.from_sq(move);
Square mto = Utils.to_sq(move);
Square tfrom = Utils.from_sq(threat);
Square tto = Utils.to_sq(threat);
// Don't prune moves of the threatened piece
if (mfrom == tto)
{
return true;
}
// If the threatened piece has value less than or equal to the value of the
// threat piece, don't prune moves which defend it.
if (pos.is_capture(threat)
&& (Position.PieceValue[PhaseC.MG][pos.piece_on(tfrom)] >= Position.PieceValue[PhaseC.MG][pos.piece_on(tto)]
|| Utils.type_of(pos.piece_on(tfrom)) == PieceTypeC.KING))
{
// Update occupancy as if the piece and the threat are moving
var occ = Utils.xor_bit(Utils.xor_bit(Utils.xor_bit(pos.occupied_squares, mfrom), mto), tfrom);
Piece piece = pos.piece_on(mfrom);
// The piece moved in 'to' attacks the square 's' ?
if (Utils.bit_is_set(Position.attacks_from(piece, mto, occ), tto) != 0)
{
return true;
}
// Scan for possible X-ray attackers behind the moved piece
var xray = (Utils.rook_attacks_bb(tto, occ)
& pos.pieces(PieceTypeC.ROOK, PieceTypeC.QUEEN, Utils.color_of(piece)))
| (Utils.bishop_attacks_bb(tto, occ)
& pos.pieces(PieceTypeC.BISHOP, PieceTypeC.QUEEN, Utils.color_of(piece)));
// Verify attackers are triggered by our move and not already existing
if ((xray != 0) && ((xray ^ (xray & pos.attacks_from_QUEEN(tto))) != 0))
{
return true;
}
}
// Don't prune safe moves which block the threat path
if ((Utils.bit_is_set(Utils.between_bb(tfrom, tto), mto) != 0) && pos.see(move, true) >= 0)
{
return true;
}
return false;
}
