text: // Copyright 2021-2022 Google LLC
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     https://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  //
  //
  // For full documentation, see
  // https://getreuer.info/posts/keyboards/custom-shift-keys
  
  #include "custom_shift_keys.h"
  
  bool process_custom_shift_keys(uint16_t keycode, keyrecord_t *record) {
    static uint16_t registered_keycode = KC_NO;
  
    // If a custom shift key is registered, then this event is either releasing
    // it or manipulating another key at the same time. Either way, we release
    // the currently registered key.
    if (registered_keycode != KC_NO) {
      unregister_code16(registered_keycode);
      registered_keycode = KC_NO;
    }
  
    if (record->event.pressed) {  // Press event.
      const uint8_t mods = get_mods();
  #ifndef NO_ACTION_ONESHOT
      if ((mods | get_weak_mods() | get_oneshot_mods()) & MOD_MASK_SHIFT) {
  #else
      if ((mods | get_weak_mods()) & MOD_MASK_SHIFT) {  // Shift is held.
  #endif  // NO_ACTION_ONESHOT
        // Search for a custom key with keycode equal to `keycode`.
        for (int i = 0; i < NUM_CUSTOM_SHIFT_KEYS; ++i) {
          if (keycode == custom_shift_keys[i].keycode) {
            // Continue default handling if this is a tap-hold key being held.
            if (((QK_MOD_TAP <= keycode && keycode <= QK_MOD_TAP_MAX) ||
                 (QK_LAYER_TAP <= keycode && keycode <= QK_LAYER_TAP_MAX)) &&
                record->tap.count == 0) {
              return true;
            }
  #ifndef NO_ACTION_ONESHOT
            del_oneshot_mods(MOD_MASK_SHIFT);
  #endif  // NO_ACTION_ONESHOT
            del_mods(MOD_MASK_SHIFT);
            del_weak_mods(MOD_MASK_SHIFT);
            send_keyboard_report();
            registered_keycode = custom_shift_keys[i].shifted_keycode;
            register_code16(registered_keycode);
            set_mods(mods);  // Restore the mods.
            return false;
          }
        }
      }
    }
  
    return true;  // Continue with default handling.
  }
  
  