FunC

() recv_internal(int msg_value, cell in_msg, slice in_msg_body) impure {
    var cs = in_msg.begin_parse();
    var flags = cs~load_uint(4);
    slice sender_address = cs~load_msg_addr();

    int op = in_msg_body~load_uint(32);

    slice ds = get_data().begin_parse();
    int counter = ds~load_uint(64);

    if (op == 1) {
        counter += 1;
    } elseif (op == 2) {
        counter -= 1;
    }

    set_data(begin_cell()
        .store_uint(counter, 64)
        .end_cell());
}

int get_counter() method_id {
    slice ds = get_data().begin_parse();
    return ds~load_uint(64);
}