// https://forum.arduino.cc/t/nano-classic-to-nano-every-sketch-compiling-issues/1458406

const int rel[] = {8, 9, 10, 11}, led[] = {4, 5, 6, 7}, enc[] = {2, 3};
volatile bool ISRflag = 0, datapin; // used inside ISR
int input = 0; // input selector

void setup() {
  for (int i = 4; i < 12; i++) // all relays ans leds...
    pinMode(i, OUTPUT); // set to OUTPUT
  pinMode(enc[0], INPUT); //pin 2 ENC CLK
  pinMode(enc[1], INPUT); //pin 3 ENC DAT

  bool relstate[] = {1, 0, 0, 0}; // set relays
  bool ledstate[] = {1, 0, 0, 0}; // lset leds
  for (int i = 0; i < 4; i++) { // all OFF
    digitalWrite(led[i], relstate[i]);
    digitalWrite(rel[i], ledstate[i]);
  }
  attachInterrupt(digitalPinToInterrupt(enc[1]), ISRfunction, FALLING);
}

void ISRfunction() {
  ISRflag = 1;
  datapin = digitalRead(enc[0]); // read data pin state
}

void loop() {
  if (ISRflag) {
    delay(100); // debounce
    if (datapin == HIGH) {
      input++;
      if (input > 3)
        input = 3;
    }
    if (datapin == LOW) {
      input--;
      if (input < 0)
        input = 0;
    }
    for (int i = 0; i < 4; i++) {
      digitalWrite(led[i], LOW); // leds off
      digitalWrite(rel[i], LOW); // relays off
    }
    digitalWrite(led[input], HIGH); // selected led ON
    digitalWrite(rel[input], HIGH); // selected relay ON
    ISRflag = 0; // reset ISR flag
  }
}