Alternative of let's split keymap, for foobar keyboard. Use with QMK. MIT License.

keymap.c

// Copyright Megumi Tomita <tomykaira@gmail.com>.
// MIT License.

#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 _QWERTY 0

#define _CFG 1
#define _SYMBOL 2
#define _NUM 3
#define _MEDIA 4

enum custom_keycodes {
  CFG,
  ST_SYM,
  ST_NUM,
  ST_MDIA,
  ST_CTRL,
  ST_SHFT,
  ST_META,
  ST_ALT,
  KC_INEN,
  KC_INJA,
};

// Fillers to make layering more clear
#define _______ KC_TRNS
#define XXXXXXX KC_NO

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
[_QWERTY] = LAYOUT_ortho_4x12(
   KC_Q,    KC_W,    KC_E,    KC_R,    KC_T,    _______, _______, KC_Y,    KC_U,    KC_I,    KC_O,    KC_P,   \
   KC_A,    KC_S,    KC_D,    KC_F,    KC_G,    _______, _______, KC_H,    KC_J,    KC_K,    KC_L,    CFG,    \
   KC_Z,    KC_X,    KC_C,    KC_V,    KC_B,    _______, _______, KC_N,    KC_M,    KC_COLN, KC_DOT,  KC_SPC, \
   _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______ \
),

};

const uint16_t PROGMEM virtual_keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
 [_CFG] = LAYOUT_ortho_4x12( \
  XXXXXXX, XXXXXXX, ST_SYM,  ST_NUM,  ST_MDIA, XXXXXXX, XXXXXXX, KC_ESC,  KC_SPC,  KC_TAB,  XXXXXXX, XXXXXXX, \
  XXXXXXX,  ST_ALT, ST_CTRL, ST_SHFT, ST_META, XXXXXXX, XXXXXXX, KC_ENT,  KC_BSPC, XXXXXXX, XXXXXXX, CFG,     \
  XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, KC_INEN, KC_INJA, KC_SCLN, XXXXXXX, XXXXXXX, \
  XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX, XXXXXXX \
),

 [_SYMBOL] = LAYOUT_ortho_4x12( \
  KC_EXCLAIM, KC_DQUO, KC_HASH, KC_DLR,  KC_PERC, _______, _______, KC_AMPR, KC_QUOTE,KC_LPRN, KC_RPRN, KC_MINS, \
  KC_TAB,     KC_ASTR, KC_GRV,  KC_PIPE, KC_LGUI, _______, _______, KC_LCBR, KC_RCBR, KC_LBRC, KC_RBRC, CFG,     \
  KC_PLUS,    KC_TILD, KC_AT,   KC_CIRC, KC_LSFT, _______, _______, KC_UNDS, KC_EQL,  KC_SCLN, KC_BSLS, KC_SLSH, \
  _______,    _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______ \
),

[_NUM] = LAYOUT_ortho_4x12( \
  KC_F1,      KC_F2,   KC_F3,   KC_F4,   KC_F5,   _______, _______, KC_F6,   KC_F7,   KC_F8,   KC_F9,   KC_F10,   \
  KC_1,       KC_2,    KC_3,    KC_4,    KC_5,    _______, _______, KC_6,    KC_7,    KC_8,    KC_9,    CFG,      \
  KC_F11,     KC_F12,  _______, KC_COMMA,KC_DOT,  _______, _______, KC_0,    KC_PLUS, KC_MINUS, KC_ASTR, KC_SLASH, \
  _______,    _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______ \
),

[_MEDIA] = LAYOUT_ortho_4x12( \
  KC_F1,      KC_F2,   KC_F3,   KC_F4,   KC_F5,   _______, _______, KC_F6,   KC_F7,   KC_F8,   KC_F9,   KC_F10,   \
  KC_1,       KC_2,    KC_3,    KC_4,    KC_5,    _______, _______, KC_6,    KC_7,    KC_8,    KC_9,    CFG,      \
  KC_F11,     KC_F12,  _______, KC_COMMA,KC_DOT,  _______, _______, _______, KC_PLUS, KC_MINUS, KC_ASTR, KC_SLASH, \
  _______,    _______, _______, _______, _______, _______, _______, _______, _______, _______, _______, _______ \
),
};

static uint8_t on_cfg = false;
static uint8_t cur_map = _QWERTY;
static uint16_t cur_modifiers = 0;

uint16_t keymap_key_to_keycode(uint8_t layer, keypos_t key)
{
  // Read entire word (16bits)
  if (cur_map) {
    return pgm_read_word(&virtual_keymaps[(cur_map)][(key.row)][(key.col)]) | cur_modifiers;
  } else {
    return pgm_read_word(&keymaps[(layer)][(key.row)][(key.col)]) | cur_modifiers;
  }
}

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
  if ((keycode ^ cur_modifiers) == CFG) {
    if (record->event.pressed && cur_map == _QWERTY && cur_modifiers == 0) {
      on_cfg = true;
    } else if (!record->event.pressed) {
      cur_map = _QWERTY;
      cur_modifiers = 0;
      on_cfg = false;
    }
    return false;
  }
  if (on_cfg) {
    uint16_t vk = pgm_read_word(&virtual_keymaps[(_CFG)][(record->event.key.row)][(record->event.key.col)]);
    switch (vk) {
      case ST_SYM:
        if (record->event.pressed) {
          cur_map = _SYMBOL;
        } else {
          on_cfg = false;
        }
        return false;
      case ST_NUM:
        if (record->event.pressed) {
          cur_map = _NUM;
        } else {
          on_cfg = false;
        }
        return false;
      case ST_MDIA:
        if (record->event.pressed) {
          cur_map = _MEDIA;
        } else {
          on_cfg = false;
        }
        return false;
      case ST_CTRL:
        if (record->event.pressed) {
          cur_modifiers |= QK_LCTL;
        } else {
          on_cfg = false;
        }
        return false;
      case ST_SHFT:
        if (record->event.pressed) {
          cur_modifiers |= QK_LSFT;
        } else {
          on_cfg = false;
        }
        return false;
      case ST_META:
        if (record->event.pressed) {
          cur_modifiers |= QK_LGUI;
        } else {
          on_cfg = false;
        }
        return false;
      case ST_ALT:
        if (record->event.pressed) {
          cur_modifiers |= QK_LALT;
        } else {
          on_cfg = false;
        }
        return false;
      case KC_INEN:
        if (record->event.pressed) {
          register_code16(KC_SCLN | QK_LCTL | QK_LSFT);
          unregister_code16(KC_SCLN | QK_LCTL | QK_LSFT);
        }
        return false;
      case KC_INJA:
        if (record->event.pressed) {
          register_code16(KC_J| QK_LCTL | QK_LSFT);
          unregister_code16(KC_J| QK_LCTL | QK_LSFT);
        }
        return false;
      default:
        if (record->event.pressed) {
          register_code16(vk | cur_modifiers);
        } else {
          unregister_code16(vk | cur_modifiers);
        }
        return false;
    }
  }
  return true;
}