hash_type calculate_pawn_hash() const {
        hash_type hash_value = 0;
        for (Color c : constants::COLORS) {
            Bitboard bb = piece_type_bb(constants::PAWN, c);
            while (bb) {
                hash_value ^= zobrist::piece_square_key(bb.forward_bitscan(), pt, c);
                bb.forward_popbit();
            }
        }
        auto ep_sq = enpassant_square();
        if (ep_sq) {
            hash_value ^= zobrist::enpassant_key(*ep_sq);
        }
        hash_value ^= zobrist::side_to_move_key(side_to_move());
        return hash_value;
    }