qmk-firmware/keyboards/cannonkeys/ortho75/ortho75.c

#include "ortho75.h"

#define MEDIA_KEY_DELAY 10

uint8_t layer = 0;

uint32_t layer_state_set_kb(uint32_t state) {
  state = layer_state_set_user(state);
  layer = biton32(state);
  return state;
}

bool encoder_update_kb(uint8_t index, bool clockwise) {
    if (!encoder_update_user(index, clockwise)) return false;
  uint16_t mapped_code = 0;
  if (index == 0) {
    if (clockwise) {
        switch(layer){
            case 0:
            default:
                mapped_code = KC_VOLU;
                break;
            case 1:
                mapped_code = KC_MEDIA_NEXT_TRACK;
                break;
            case 2:
                mapped_code = KC_PGDN;
                break;
        }
    } else {
        switch(layer){
            case 0:
            default:
                mapped_code = KC_VOLD;
                break;
            case 1:
                mapped_code = KC_MEDIA_PREV_TRACK;
                break;
            case 2:
                mapped_code = KC_PGUP;
                break;
        }
    }
    uint16_t held_keycode_timer = timer_read();
    register_code(mapped_code);
    while (timer_elapsed(held_keycode_timer) < MEDIA_KEY_DELAY){ /* no-op */ }
    unregister_code(mapped_code);
  }
  return true;
}