/*
  Forum: https://forum.arduino.cc/t/i-dont-really-understand-how-the-millis-work/1229674
  Wokwi: https://wokwi.com/projects/391000332028236801
*/

byte leds[] = {2, 3, 4, 5, 6, 7};
const int noOfLeds = sizeof(leds) / sizeof(leds[0]);

const unsigned long ledDelay = 100;
unsigned long lastChange = 0;
int actLed = 0;
int direction = 1;

void setup() {
  for (int i = 0; i < noOfLeds; i++) {
    pinMode(leds[i], OUTPUT);
  }
}

void loop() {
  ledWave();
}

void ledWave() {
  if (millis() - lastChange > ledDelay) {
    lastChange = millis();
    resetLeds();
    digitalWrite(leds[actLed], HIGH);
    actLed += direction;
    if (actLed == noOfLeds - 1 || actLed == 0) {
      direction = -direction;
    }
  }

}


void resetLeds() {
  for (int i = 0; i < noOfLeds; i++) {
    digitalWrite(leds[i], LOW);
  }
}