// Segment pins
const int segmentPins[] = {23, 22, 21, 17, 15, 2, 4}; // a, b, c, d, e, f, g
// Digit selection pins
const int digitPins[] = {19, 18, 5, 16}; // d1, d2, d3, d4

// 7-segment patterns for 0-9 (Common Cathode logic)
// If your display is Common Anode, flip 0s and 1s
const byte numbers[] = {
  0b00111111, // 0
  0b00000110, // 1
  0b01011011, // 2
  0b01001111, // 3
  0b01100110, // 4
  0b01101101, // 5
  0b01111101, // 6
  0b00000111, // 7
  0b01111111, // 8
  0b01101111  // 9
};

unsigned long lastUpdate = 0;
int counter = 0;

void setup() {
  for (int i = 0; i < 7; i++) pinMode(segmentPins[i], OUTPUT);
  for (int i = 0; i < 4; i++) pinMode(digitPins[i], OUTPUT);
}

void displayDigit(int digit, int number) {
  // 1. Turn off all digits (prevent ghosting)
  for (int i = 0; i < 4; i++) digitalWrite(digitPins[i], LOW); 

  // 2. Set the segments for the specific number
  for (int i = 0; i < 7; i++) {
    digitalWrite(segmentPins[i], (numbers[number] >> i) & 0x01);
  }

  // 3. Turn on the specific digit
  digitalWrite(digitPins[digit], HIGH); 
  delay(5); // Short delay for persistence of vision
}

void loop() {
  // Increment counter every 1 second
  if (millis() - lastUpdate > 500) {
    counter++;
    if (counter > 9999) counter = 0;
    lastUpdate = millis();
  }

  // Multiplexing the display
  displayDigit(0, (counter / 1000) % 10); // Thousands
  displayDigit(1, (counter / 100) % 10);  // Hundreds
  displayDigit(2, (counter / 10) % 10);   // Tens
  displayDigit(3, counter % 10);          // Units
}