Detecting cannot force mate

In the section Detecting draws by FIDE rules the "Draw by insufficient material rule." is explained and implemented. This rule makes it possible to claim a draw due to having insufficient material to win the game. However, that rule states that a draw can only be claimed if there is no way to achieve checkmate by any order of legal moves.

There are some material combinations such as King+Knight vs. King+Knight where a mate can be achieved by legal moves, even though one of the players has to assist in getting mated. In case of these material combinations a draw cannot be claimed under the FIDE rules. When implementing XBoard, the engine has to know about this.

The function to detect this situation is draw_by_insufficient_material_rule().

When searching however, the situation is a bit different. Even though the game cannot be claimed as draw in the King+Knight vs. King+Knight situation, it is, in practice, still a draw because checkmate cannot be forced. We need a different function to detect such conditions, which is then used during the engine's search routine. It can be used to score a position where mate cannot be forced as a draw. The engine will actively try to achieve such a position when it is losing or try to avoid it when it is winning.

This function is sufficient_material_to_force_checkmate().

Checkmate can still be forced if one of the following conditions is true:

• There is still a queen, rook, or pawn on the board. (The pawn can potentially promote to a rook or a queen in the future, which can then be used to force a checkmate.)
• At least one player has the bishop pair. This means that one player has to have two bishops, and they must be on squares of different color. (See: Detecting the bishop pair).
• At least one player has both a bishop and a knight.
• At least one player has at least three knights, which can be achieved by promoting a pawn. Granted, this is a super-rare situation, but with three knights against a lone king, checkmate can be forced.

The following function checks the above conditions. If any of them applies it returns true to indicate that checkmate can still be forced. It can be found in the file draw.rs:

pub fn sufficient_material_to_force_checkmate(&self) -> bool {
    let w = self.get_bitboards(Sides::WHITE);
    let b = self.get_bitboards(Sides::BLACK);

    w[Pieces::QUEEN] > 0
    || b[Pieces::QUEEN] > 0
    || w[Pieces::ROOK] > 0
    || b[Pieces::ROOK] > 0
    || w[Pieces::PAWN] > 0
    || b[Pieces::PAWN] > 0
    || self.has_bishop_pair(Sides::WHITE)
    || self.has_bishop_pair(Sides::BLACK)
    || (w[Pieces::BISHOP] > 0 && w[Pieces::KNIGHT] > 0)
    || (b[Pieces::BISHOP] > 0 && b[Pieces::KNIGHT] > 0)
    || w[Pieces::KNIGHT].count_ones() >= 3
    || b[Pieces::KNIGHT].count_ones() >= 3
}

During the search the engine will call this function after every move. As soon as it returns false, the engine scores the evaluation of the position as a draw and it will be useless to search any deeper.