- // If one side queens at least three plies before the other, that
- // side wins.
- if (movesToGo[WHITE] <= movesToGo[BLACK] - 3)
- ei.egValue += UnstoppablePawnValue - Value(0x40 * (movesToGo[WHITE]/2));
- else if(movesToGo[BLACK] <= movesToGo[WHITE] - 3)
- ei.egValue -= UnstoppablePawnValue - Value(0x40 * (movesToGo[BLACK]/2));
-
- // We could also add some rules about the situation when one side
- // queens exactly one ply before the other: Does the first queen
- // check the opponent's king, or attack the opponent's queening square?
- // This is slightly tricky to get right, because it is possible that
- // the opponent's king has moved somewhere before the first pawn queens.
+ Color winnerSide = movesToGo[WHITE] < movesToGo[BLACK] ? WHITE : BLACK;
+ Color loserSide = opposite_color(winnerSide);
+
+ // If one side queens at least three plies before the other, that side wins
+ if (movesToGo[winnerSide] <= movesToGo[loserSide] - 3)
+ ei.egValue += Sign[winnerSide] * (UnstoppablePawnValue - Value(0x40 * (movesToGo[winnerSide]/2)));
+
+ // If one side queens one ply before the other and checks the king or attacks
+ // the undefended opponent's queening square, that side wins. To avoid cases
+ // where the opponent's king could move somewhere before first pawn queens we
+ // consider only free paths to queen for both pawns.
+ else if ( !(squares_in_front_of(WHITE, pawnToGo[WHITE]) & pos.occupied_squares())
+ && !(squares_in_front_of(BLACK, pawnToGo[BLACK]) & pos.occupied_squares()))
+ {
+ assert(movesToGo[loserSide] - movesToGo[winnerSide] == 1);
+
+ Square winnerQSq = relative_square(winnerSide, make_square(square_file(pawnToGo[winnerSide]), RANK_8));
+ Square loserQSq = relative_square(loserSide, make_square(square_file(pawnToGo[loserSide]), RANK_8));
+
+ Bitboard b = pos.occupied_squares();
+ clear_bit(&b, pawnToGo[winnerSide]);
+ clear_bit(&b, pawnToGo[loserSide]);
+ b = queen_attacks_bb(winnerQSq, b);
+
+ if ( (b & pos.pieces(KING, loserSide))
+ ||(bit_is_set(b, loserQSq) && !bit_is_set(ei.attacked_by(loserSide), loserQSq)))
+ ei.egValue += Sign[winnerSide] * (UnstoppablePawnValue - Value(0x40 * (movesToGo[winnerSide]/2)));
+ }