// https://forum.arduino.cc/t/resetting-millis/1213723/19
// https://wokwi.com/projects/387534707798036481

const byte led1Pin = 12;
const byte led2Pin =  2;

unsigned long my4DayTimer;
unsigned long myPrintInfoTimer;
//unsigned long myShortTimer;
const unsigned long myInterval = 4000;

boolean first4Days;
boolean second4Days;

void setup() {
  Serial.begin(115200);
  Serial.println("Setup-Start");
  pinMode(led1Pin, OUTPUT);
  pinMode(led2Pin, OUTPUT);

  my4DayTimer = millis();
  myPrintInfoTimer = millis();
  first4Days = true;
  second4Days = false;
}

void loop() {

  // check if 4 days of time have passed by
  if ( TimePeriodIsOver(my4DayTimer, myInterval) == true ) {
    // if REALLY 4 days of time have passed by
    flip_between_First_and_Second();
  }

  // check if boolean variable with name "first4Days"  is true
  if (first4Days == true) {
    printFirst_4_days_message(); // does what the function name says
  }

  // check if boolean variable with name "second4Days"  is true
  if (second4Days == true) {
    printSecond_4_days_message(); // does what the function name says
  }

  printDaysPassedBy(); // does what the functions name says

}



// easy to use helper-function for non-blocking timing
boolean TimePeriodIsOver (unsigned long & startOfPeriod, unsigned long TimePeriod) {
  unsigned long currentMillis  = millis();
  if ( currentMillis - startOfPeriod >= TimePeriod ) {
    // more time than TimePeriod has elapsed since last time if-condition was true
    startOfPeriod = currentMillis; // a new period starts right here so set new starttime
    return true;
  }
  else return false;            // actual TimePeriod is NOT yet over
}


void printDaysPassedBy() {

  static unsigned long myPrintInfoTimer;

  if ( TimePeriodIsOver(myPrintInfoTimer, 1000) ) {
    Serial.print("-day");
    Serial.print( ( millis() - my4DayTimer) / 1000);
  }
}


void printFirst_4_days_message() {
  static unsigned long myShortTimer;

  if ( TimePeriodIsOver(myShortTimer, 500) == true) {
    Serial.print("  1st");
    digitalWrite(led1Pin, !digitalRead(led1Pin)); // invert LOW/HIGH by using the not-operator
  }
}


void printSecond_4_days_message() {
  static unsigned long myShortTimer;

  if ( TimePeriodIsOver(myShortTimer, 500) == true) {
    Serial.print("  2nd");
    digitalWrite(led2Pin, !digitalRead(led2Pin)); // invert LOW/HIGH by using the not-operator
  }
}


void flip_between_First_and_Second() {
  if (first4Days == true) {
    first4Days  = false;
    second4Days = true;
    //Serial.println("change to SECOND 4 days code");
    Serial.println();
    digitalWrite(led1Pin, LOW);
  }
  else {
    first4Days  = true;
    second4Days = false;
    //Serial.println("change to first 4 days code");
    Serial.println();
    digitalWrite(led2Pin, LOW);
  }
}