keymap.c

#include QMK_KEYBOARD_H
enum keycodes{
  KC_CYCLE_LAYERS = QK_USER,
};

#define LAYER_CYCLE_START 0
#define LAYER_CYCLE_END 4

// Add the behaviour of this new keycode
bool process_record_user(uint16_t keycode, keyrecord_t *record) {
  switch (keycode) {
    case KC_CYCLE_LAYERS:
      // Our logic will happen on presses, nothing is done on releases
      if (!record->event.pressed) {
        // We've already handled the keycode (doing nothing), let QMK know so no further code is run unnecessarily
        return false;
      }

      uint8_t current_layer = get_highest_layer(layer_state);

      // Check if we are within the range, if not quit
      if (current_layer > LAYER_CYCLE_END || current_layer < LAYER_CYCLE_START) {
        return false;
      }

      uint8_t next_layer = current_layer - current_layer%2 + 2;
      if (next_layer >= LAYER_CYCLE_END) {
          next_layer = LAYER_CYCLE_START;
      }
      layer_move(next_layer);
      return false;

    // Process other keycodes normally
    default:
      return true;
  }
}

// Place `KC_CYCLE_LAYERS` as a keycode in your keymap

// clang-format off



const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {

    [0] = LAYOUT( /* Base */
        KC_CAPS,   KC_1,     KC_2,     KC_3,   KC_4,   KC_5,   KC_6,    KC_7,   KC_8,   KC_9,     KC_0,     KC_MINS,    KC_EQL,  KC_BSPC,  
        KC_TAB,   KC_Q,     KC_W,     KC_E,   KC_R,   KC_T,   KC_Y,    KC_U,   KC_I,   KC_O,     KC_P,     KC_LBRC,    KC_RBRC, KC_BSLS,
        KC_ESC,  KC_A,     KC_S,     KC_D,   KC_F,   KC_G,   KC_H,    KC_J,   KC_K,   KC_L,     KC_SCLN,  KC_QUOT,    KC_BSLS, KC_ENT ,
        KC_LSFT,  KC_NUBS,  KC_Z,     KC_X,   KC_C,   KC_V,   KC_B,    KC_N,   KC_M,   KC_COMM,  KC_DOT,   KC_SLSH,    KC_INT1, KC_RSFT,
        KC_LCTL,  KC_LGUI,  KC_LALT,                          KC_SPC,                            KC_RALT,  MO(1),      KC_APP,  KC_RCTL
    ),
    [1] = LAYOUT( /* Base */
        KC_CYCLE_LAYERS,   KC_F1,    KC_F2,   KC_F3,   KC_F4,   KC_F5,    KC_F6,   KC_F7,    KC_F8,  KC_F9,   KC_F10,    KC_F11,   KC_F12,   _______,  
        _______,  _______,  KC_UP, _______, _______, _______,  _______, _______, _______, _______,  KC_PSCR,  KC_HOME,  KC_END,   _______,
        KC_GRV,  KC_LEFT,  KC_DOWN, KC_RIGHT, _______, _______,  _______, _______, _______, KC_SCRL,  KC_PGUP,  KC_PGDN, _______,   _______,
        _______,  _______,  _______, _______, _______, _______,  _______, _______, _______, KC_SCRL,  KC_INS,   KC_DEL,  _______,   _______,
        _______,  GU_TOGG,  _______,                             _______,                             _______,  _______,  KC_APP,   _______
    ),
    [2] = LAYOUT( /* Zilted Graphite */
        KC_CAPS,   KC_B,     KC_Q,     KC_L,   KC_D,   KC_4,   KC_5,    KC_6,   KC_O,   KC_U,     KC_SCLN,  KC_J,       KC_7,    KC_BSPC,  
        KC_TAB,   KC_N,     KC_R,     KC_T,   KC_W,   KC_Z,   KC_QUOT, KC_F,   KC_A,   KC_E,     KC_I,     KC_COMM,    KC_EQL,  KC_BSLS,
        KC_ESC,  KC_1,     KC_X,     KC_M,   KC_S,   KC_G,   KC_Y,    KC_H,   KC_DOT, KC_MINS,  KC_SLSH,  KC_8,       KC_BSLS, KC_ENT ,
        KC_LSFT,  KC_NUBS,  KC_2,     KC_3,   KC_C,   KC_V,   KC_0,   KC_K,    KC_P,   KC_LBRC,  KC_RBRC,  KC_9,       KC_INT1, KC_RSFT,
        KC_LCTL,  KC_LGUI,  KC_LALT,                          KC_SPC,                            KC_RALT,  MO(3),      KC_APP,  KC_RCTL
    ),
    [3] = LAYOUT( 
        KC_CYCLE_LAYERS,   KC_F1,    KC_F2,   KC_F3,   KC_F4,   KC_F5,    KC_F6,   KC_F7,    KC_F8,  KC_F9,   KC_F10,    KC_F11,   KC_F12,   _______,  
        _______,  _______,  KC_UP, _______, _______, _______,  _______, _______, _______, _______,  KC_PSCR,  KC_HOME,  KC_END,   _______,
        KC_GRV,  KC_LEFT,  KC_DOWN, KC_RIGHT, _______, _______,  KC_Q, _______, _______, KC_SCRL,  KC_PGUP,  KC_PGDN, _______,   _______,
        _______,  _______,  _______, _______, _______, _______,  _______, _______, _______, KC_SCRL,  KC_INS,   KC_DEL,  _______,   _______,
        _______,  GU_TOGG,  _______,                             _______,                             _______,  _______,  KC_APP,   _______
    ),
};
// clang-format on