ErgoDash keymap implementing “correct” dual role RAISE/LOWER keys without `LT` advanced keycode

keymap.c

#include QMK_KEYBOARD_H

extern keymap_config_t keymap_config;

#define _QWERTY 0
#define _LOWER 1
#define _RAISE 2
#define _ADJUST 16

enum custom_keycodes {
  QWERTY = SAFE_RANGE,
  LOWER,
  RAISE,
  ADJUST,
};

// Fillers to make layering more clear
#define _______ KC_TRNS
#define XXXXXXX KC_NO
#define EISU LALT(KC_GRV)

#define ALT_APP ALT_T(KC_APP)

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

  /* Qwerty
   * ,----------------------------------------------------------------------------------------------------------------------.
   * | Tab  |   Q  |   W  |   E  |   R  |   T  |   -  |                    |   =  |   Y  |   U  |   I  |   O  |   P  | Bksp |
   * |------+------+------+------+------+------+------+--------------------+------+------+------+------+------+------+------|
   * | Ctrl |   A  |   S  |   D  |   F  |   G  |   [  |                    |   ]  |   H  |   J  |   K  |   L  |  ;+  |  :*  |
   * |------+------+------+------+------+------+------+--------------------+------+------+------+------+------+------+------|
   * | Shift|   Z  |   X  |   C  |   V  |   B  | MHNK |                    | HENK |   N  |   M  |  ,<  |  .>  |  /?  |  \_  |
   * |------+------+------+------+------+------+------+--------------------+------+------+------+------+------+------+------|
   * | Esc  | Ctrl | GUI  |||||||| Alt  | Lower| Space|      ||||||||      | Enter|Raise | Left |||||||| Down |  Up  |Right |
   * `----------------------------------------------------------------------------------------------------------------------
   */
  [_QWERTY] = LAYOUT( \
    KC_TAB,  KC_Q,    KC_W,    KC_E,    KC_R,    KC_T,    KC_MINS,                        KC_EQL , KC_Y,    KC_U,    KC_I,    KC_O,    KC_P,    KC_BSPC, \
    KC_LCTL, KC_A,    KC_S,    KC_D,    KC_F,    KC_G,    KC_LBRC,                        KC_RBRC, KC_H,    KC_J,    KC_K,    KC_L,    KC_SCLN, KC_QUOT, \
    KC_LSFT, KC_Z,    KC_X,    KC_C,    KC_V,    KC_B,    KC_MHEN,                        KC_HENK, KC_N,    KC_M,    KC_COMM, KC_DOT,  KC_SLSH, KC_RO  , \
    KC_ESC,  KC_LCTL, KC_LGUI,          ALT_APP, LOWER,   KC_SPC,  KC_SPC ,      KC_ENT , KC_ENT , RAISE,   KC_LEFT,          KC_DOWN, KC_UP,   KC_RGHT  \
  ),

  /* Lower
   * ,----------------------------------------------------------------------------------------------------------------------.
   * | Esc  |   !  |   "  |   #  |   $  |   %  |   _  |                    |   +  |   &  |   '  |   (  |   )  |   ~  | Bksp |
   * |------+------+------+------+------+------+------+---------------------------+------+------+------+------+------+------|
   * | Del  |  F1  |  F2  |  F3  |  F4  |  F5  |   {  |                    |   }  |  F6  |   =  |JIS \ |   {  |   }  |      |
   * |------+------+------+------+------+------+------+--------------------+------+------+------+------+------+------+------|
   * | Shift|  F7  |  F8  |  F9  |  F10 |  F11 | Space|                    | Enter|  F12 |   `  |   ~  |      |      |  \_  |
   * |------+------+------+------+------+------+------+------+------+------+------+------+------+------+------+------+------|
   * | Ctrl |      |      |      |      |      |      |      ||||||||      |      |      | Home |||||||| PgDn | PgUp | End  |
   * `----------------------------------------------------------------------------------------------------------------------'
   */
  [_LOWER] = LAYOUT( \
    KC_ESC,  KC_EXLM, KC_AT,   KC_HASH, KC_DLR,  KC_PERC, KC_UNDS,                        KC_PLUS, KC_CIRC, KC_AMPR, KC_ASTR, KC_LPRN, KC_RPRN, KC_BSPC, \
    KC_DEL,  KC_F1,   KC_F2,   KC_F3,   KC_F4,   KC_F5,   KC_LCBR,                        KC_RCBR, KC_F6,   KC_UNDS, KC_JYEN, KC_RCBR, KC_PIPE, _______, \
    KC_LSFT, KC_F7,   KC_F8,   KC_F9,   KC_F10,  KC_F11,  KC_SPC ,                        KC_ENT , KC_F12,  KC_LCBR, KC_PLUS, _______, _______, _______, \
    KC_LCTL, _______, _______,          _______, _______, _______,_______,        _______,_______, _______, KC_HOME,          KC_PGDN, KC_PGUP, KC_END   \
  ),

  /* Raise
   * ,----------------------------------------------------------------------------------------------------------------------.
   * | Esc  |   1  |   2  |   3  |   4  |   5  |   _  |                    |   +  |   6  |   7  |   8  |   9  |   0  | Bksp |
   * |------+------+------+------+------+------+------+--------------------+------+------+------+------+------+------+------|
   * | Del  |  F1  |  F2  |  F3  |  F4  |  F5  |   {  |                    |   }  |  F6  |   -  |JIS | |   [  |   ]  |      |
   * |------+------+------+------+------+------|------+--------------------+------+------+------+------+------+------+------|
   * | Shift|  F7  |  F8  |  F9  |  F10 |  F11 | Space|                    | Enter|  F12 |   @  |   ^  |      |      |  \_  |
   * |------+------+------+------+------+------+------+------+------+------+------+------+------+------+------+------+------|
   * | Ctrl |      |      |      |      |      |      |      ||||||||      |      |      | Home |||||||| PgDn | PgUp | End  |
   * `----------------------------------------------------------------------------------------------------------------------'
   */
  [_RAISE] = LAYOUT( \
    KC_ESC,  KC_1,    KC_2,    KC_3,    KC_4,    KC_5,    KC_UNDS,                        KC_PLUS, KC_6,    KC_7,    KC_8,     KC_9,    KC_0,    KC_BSPC, \
    KC_DEL,  KC_F1,   KC_F2,   KC_F3,   KC_F4,   KC_F5,   KC_LCBR,                        KC_RCBR, KC_F6,   KC_MINS,S(KC_JYEN),KC_RBRC, KC_BSLS, _______, \
    KC_LSFT, KC_F7,   KC_F8,   KC_F9,   KC_F10,  KC_F11,  KC_SPC ,                        KC_ENT , KC_F12,  KC_LBRC, KC_EQL,   _______, _______, _______, \
    KC_LCTL, _______, _______,          _______, _______, _______,_______,        _______,_______,_______,  KC_HOME,           KC_PGDN, KC_PGUP, KC_END   \
  ),

  /* Adjust
   * ,----------------------------------------------------------------------------------------------------------------------.
   * |      | Reset|RGB ON|  MODE|  HUE-|  HUE+|      |                    |      |  SAT-|  SAT+|  VAL-|  VAL+|      |      |
   * |------+------+------+------+------+------+---------------------------+------+------+------+------+------+------+------|
   * |      |      |      |      |      |      |      |                    |      |      |      |      |      |      |      |
   * |------+------+------+------+------+------+---------------------------+------+------+------+------+------+------+------|
   * |      |      |      |      |      |      |      |                    |      |      |      |      |      |      |      |
   * |-------------+------+------+------+------+------+------+------+------+------+------+------+------+------+-------------|
   * |      |      |      |      ||||||||      |      |      ||||||||      |      |      ||||||||      |      |      |      |
   * ,----------------------------------------------------------------------------------------------------------------------.
   */
  [_ADJUST] = LAYOUT(
    _______, RESET  , RGB_TOG, RGB_MOD, RGB_HUD, RGB_HUI,_______,                       _______, RGB_SAD, RGB_SAI, RGB_VAD, RGB_VAI, _______, _______, \
    _______, _______, BL_TOGG, BL_BRTG, BL_INC , BL_DEC ,_______,                       _______, _______, _______, _______, _______, _______, _______, \
    _______, _______, _______, _______, _______, _______,_______,                       _______, _______, _______, _______, _______, _______, _______, \
    _______, _______, _______, _______,          _______,_______,_______,       _______,_______, _______,          _______, _______, _______, _______  \
  )
};

static bool lower_pressed = false;
static uint16_t lower_pressed_time = 0;
static bool raise_pressed = false;
static uint16_t raise_pressed_time = 0;

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
  switch (keycode) {
    case LOWER:
      if (record->event.pressed) {
        lower_pressed = true;
        lower_pressed_time = record->event.time;

        layer_on(_LOWER);
        update_tri_layer(_LOWER, _RAISE, _ADJUST);
      } else {
        layer_off(_LOWER);
        update_tri_layer(_LOWER, _RAISE, _ADJUST);

        if (lower_pressed && (TIMER_DIFF_16(record->event.time, lower_pressed_time) < TAPPING_TERM)) {
          register_code(KC_LANG2); // for macOS
          register_code(KC_MHEN);
          unregister_code(KC_MHEN);
          unregister_code(KC_LANG2);
        }
        lower_pressed = false;
      }
      return false;
      break;
    case RAISE:
      if (record->event.pressed) {
        raise_pressed = true;
        raise_pressed_time = record->event.time;

        layer_on(_RAISE);
        update_tri_layer(_LOWER, _RAISE, _ADJUST);
      } else {
        layer_off(_RAISE);
        update_tri_layer(_LOWER, _RAISE, _ADJUST);

        if (raise_pressed && (TIMER_DIFF_16(record->event.time, raise_pressed_time) < TAPPING_TERM)) {
          register_code(KC_LANG1); // for macOS
          register_code(KC_HENK);
          unregister_code(KC_HENK);
          unregister_code(KC_LANG1);
        }
        raise_pressed = false;
      }
      return false;
      break;
    case ADJUST:
      if (record->event.pressed) {
        layer_on(_ADJUST);
      } else {
        layer_off(_ADJUST);
      }
      return false;
      break;
    default:
      if (record->event.pressed) {
        // reset the flags
        lower_pressed = false;
        raise_pressed = false;
      }
      break;
  }
  return true;
}