encoder

keymap.c

static uint8_t encoder_state = 0;
static int8_t encoder_value = 0;
static int8_t encoder_LUT[] = { 0, -1, 1, 0, 1, 0, 0, -1, -1, 0, 0, 1, 0, 1, -1, 0 }; 

void matrix_init_user(void) {
    encoder_state = PIND & 0x3;
}

void matrix_scan_user(void) {
    encoder_state <<= 2;
    encoder_state |= (PIND & 0x3);
    encoder_value += encoder_LUT[encoder_state & 0xF];
    if (encoder_value >= 4) {
        register_code(KC_PGUP);
        unregister_code(KC_PGUP);
    }
    if (encoder_value <= -4) {
        register_code(KC_PGDN);
        unregister_code(KC_PGDN);
    }
    encoder_value %= 4;
}