const unsigned char combo[] = {0, 1, 2, 3, 4,};

void setup() {
  Serial.begin(115200);   // hello, it's 2022
  Serial.println("Hello Combination World!\n");

  DDRD &= ~0xf8;
  PORTD |= 0xf8;
}

void loop() {
  static unsigned long lastTime;
  static unsigned char lastEntry;
  static unsigned char next;

  unsigned long now = millis();

  if (now - lastTime < 20) return;    // throttled loop. get over it.
  lastTime = now;

  unsigned char entry = (PIND ^ 0xff) >> 3;
  unsigned char delta = entry & ~lastEntry;
  lastEntry = entry;

  if (!delta)
    return;

  if (delta == (1 << combo[next])) next++;
  else {
    Serial.println("Bzzzt!");
    next = 0;
  }

  if (next == 5) {
    Serial.println("OKAY!");
    next = 0;
  }
}