Minimal 4 digit counter

// Direct port 4 bit multiplex to test 4 bit BCD to 7 segment decoder(or nixie decoder).
// using port b, Arduino pin 8 as LSB, pin 11 as MSB, pin 12 as Decimal Point
// port d, Arduino pins 2 to 4, as digit commons.
//essence of Blink Without Delay for counter timing.
// a little bit of work one could directly operate the segments (I use 74LS247 decoder)

int counter=0;// main counter
byte digit=0;  // digit counter
byte digit_array[4];

unsigned long previousMillis = 0;
const long interval = 500;


void setup() {
  Serial.begin(9600);
  DDRB=0B00011111;// set lower 5 bits as output for decoder bits
  DDRD=0B00111100;// for anodes/digits
  inttoarray(counter);// pre populate digit array
}

void loop() {
  // actually output to display here, one digit each pass
  PORTB=digit_array[digit]&15;// output to port, with mask of lower 4 bits
  PORTD=4<<digit;// because pin 2 is LSD, start at 4 (0B00000100), move left per digit.

  // digit counter
  digit++;
  digit=digit&3;
  delay (1);


  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;

  //counter stuff. Rolling over at 1023
  counter++;
  counter=counter&1023;
// load counter into display array
  inttoarray(counter);
  }

}// end main loop

void inttoarray(int outnumber){
 for (byte pointer=0;pointer<4;pointer++){
 digit_array[pointer]=(outnumber%10);
 outnumber=outnumber/10;
 }
}