// for https://forum.arduino.cc/t/how-can-i-run-two-loops-at-the-same-time/1151388/12?u=davex
// https://wokwi.com/projects/371242906116762625

void setup() {
  Serial.begin(115200);
}

void loop(void) {
  // choose one scheme:
  switch (3) {
    case 1: loopTimeLineDelays();  break;// use delays() to run a timeline
    case 2: loopTimeLine(); break; // do stuff during the delays in the timeline
    case 3: loopNonBlocking(); break; // do a non-blocking timeline with a state machine
  }
}

void loopTimeLineDelays() {
  // Serial code
  Serial.print("\nA");
  delay(1000);
  Serial.print('B');
  delay(1500);
  Serial.print('C');
  delay(500);
  Serial.print('D');
  delay(1000);
  // try the other actions after the serial timeline runs:
  otherThing2();
  otherThing3();
  otherThing4();
}

void loopTimeLine() {
  // Serial code with delay()s replaced by special cooperative delay function:
  Serial.print("\nA");
  doOtherThingsWhileDelaying(1000);
  Serial.print('B');
  doOtherThingsWhileDelaying(1500);
  Serial.print('C');
  doOtherThingsWhileDelaying(500);
  Serial.print('D');
  doOtherThingsWhileDelaying(1000);
}

void loopNonBlocking() {
  nonBlockingThing1();
  otherThing2();
  otherThing3();
  otherThing4();
}

void doOtherThingsWhileDelaying(unsigned long msDelay) {
  unsigned long startTime = millis();
  while (millis() - startTime < msDelay) {
    otherThing2();
    otherThing3();
    otherThing4();
    //...
  }
}

bool otherThing2(void) {
  const unsigned long interval = 333;
  static unsigned long last = millis();
  if (millis() - last < interval) return false;
  last = millis();
  Serial.print('2');
  return true;
}

bool otherThing3(void) {
  const unsigned long interval = 777;
  static unsigned long last = millis();
  if (millis() - last < interval) return false;
  last = millis();
  Serial.print('3');
  return true;
}

bool otherThing4(void) {
  if (Serial.available()) {
    char ch = Serial.read();
    Serial.print(ch);
    return true;
  }
  return false;
}

void  nonBlockingThing1(void) {
  static int state = 1;
  static unsigned long interval = 0;
  static unsigned long last = 0;
  if (millis() - last >= interval) {
    last = millis();
    switch (state) {
      case 1:
        Serial.print("\nA");
        interval = 1000;
        state = 2;
        break;
      case 2:
        Serial.print('B');
        interval = 1500;
        state = 3;
        break;
      case 3:
        Serial.print('C');
        interval = 500;
        state = 4;
        break;
      case 4:
        Serial.print('D');
        state = 1;
        interval = 1000;
        break;
    }
  }
}