qmk-firmware/keyboards/handwired/6key/6key.c

 /* Copyright 2020 Bratzworth
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation, either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
#include "quantum.h"

bool dip_switch_update_kb(uint8_t index, bool active) {
    if (!dip_switch_update_user(index, active)) { return false; }
    if (index == 0) {
        default_layer_set(1UL << (active ? 1 : 0));
    }
    return true;
}