gulme.c

enum custom_keycodes {
  LAUGH = SAFE_RANGE
};

static uint16_t random_value = 157;
const uint16_t random_chars[9] = {
  KC_A, KC_S, KC_D, KC_F, KC_G, KC_H, KC_J, KC_K, KC_L
};

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
  switch(keycode) {
    case LAUGH:
      if (record->event.pressed) {
        uint8_t clockbyte=0;
        clockbyte = TCNT1 % 256;
        uint8_t rval;

        random_value = ((random_value + randadd) * randmul) % randmod;
        uint8_t len = ((random_value ^ clockbyte) % 4) + 4;
      
        for (uint8_t i = 0; i <= len; i++) {
          random_value = ((random_value + randadd) * randmul) % randmod;
          rval = (random_value ^ clockbyte) % 9;
          register_code(random_chars[rval]);
          unregister_code(random_chars[rval]);
        }
      }
      return false;
  }

  return true;
}

#define randadd 53
#define randmul 181
#define randmod 167