qmk_firmware/keyboards/thevankeyboards/roadkit/keymaps/default/keymap.c

#include QMK_KEYBOARD_H

extern keymap_config_t keymap_config;

// Each layer gets a name for readability, which is then used in the keymap matrix below.
// The underscores don't mean anything - you can have a layer called STUFF or any other name.
// Layer names don't all need to be of the same length, obviously, and you can also skip them
// entirely and just use numbers.

#define _NP 0

enum custom_keycodes {
  NUMPAD = SAFE_RANGE
};

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
  [_NP] = LAYOUT_numpad_4x4( /* Numpad */
    KC_P7, KC_P8,   KC_P9,   KC_PPLS, \
    KC_P4, KC_P5,   KC_P6,            \
    KC_P1, KC_P2,   KC_P3,   KC_PENT, \
    KC_P0,          KC_PDOT           \
  ),
};

void persistent_default_layer_set(uint16_t default_layer) {
  eeconfig_update_default_layer(default_layer);
  default_layer_set(default_layer);
}

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
      switch(keycode) {
        case NUMPAD:
          if (record->event.pressed) {
            persistent_default_layer_set(1UL<<_NP);
          }
          return false;
        default:
          return true;
      }
    return true;
};