const byte SEG_PIN[3] = {15, 2, 16};
byte Data[3] = {0, 0, 0};

const byte dataPin = 21;
const byte latchPin = 22;
const byte clockPin = 23;

const byte LEDs[10] = {0xFC,0x60,0xDA,0xF2,0x66,0xB6,0xBE,0xE0,0xFE,0xF6};

int countdownValue = 0;
bool startCount = false;

unsigned long lastCountTime = 0;     // exact 1 second
unsigned long lastMuxMicros = 0;     // 1 ms multiplex
byte muxDigit = 0;

// --------------------------------------------------------------------
void setup() {
  Serial.begin(115200);

  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);

  for (byte i = 0; i < 3; i++)
    pinMode(SEG_PIN[i], OUTPUT);

  Serial.println("Enter number (0-999) to start countdown:");
}

// --------------------------------------------------------------------
void loop() {

  // ---------------- READ SERIAL INPUT ----------------
  if (Serial.available()) {
    String s = Serial.readStringUntil('\n');
    s.trim();

    int n = s.toInt();
    if (n >= 0 && n <= 999) {
      countdownValue = n;
      startCount = true;
      lastCountTime = millis();      // reset timer baseline
      Serial.print("Countdown: ");
      Serial.println(countdownValue);
    }
  }

  // ---------------- 100% EXACT 1-SECOND COUNTDOWN ----------------
  if (startCount) {
    unsigned long now = millis();
    if (now - lastCountTime >= 1000) {
      lastCountTime += 1000;        // drift-free correction

      if (countdownValue > 0) countdownValue--;
      else {
        startCount = false;
        Serial.println("Finished.");
      }
    }
  }

  // ---------------- ALWAYS REFRESH DISPLAY ----------------
  refreshDisplay(countdownValue);
}

// --------------------------------------------------------------------
void refreshDisplay(int number) {

  Data[0] = number % 10;
  Data[1] = (number / 10) % 10;
  Data[2] = (number / 100) % 10;

  // Switch digit every 1000 microseconds (1 ms)
  if (micros() - lastMuxMicros >= 1000) {
    lastMuxMicros += 1000;

    // turn off all digits
    digitalWrite(SEG_PIN[0], HIGH);
    digitalWrite(SEG_PIN[1], HIGH);
    digitalWrite(SEG_PIN[2], HIGH);

    // output segment data
    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, LSBFIRST, LEDs[Data[muxDigit]]);
    digitalWrite(latchPin, HIGH);

    // enable digit (common anode → LOW)
    digitalWrite(SEG_PIN[muxDigit], LOW);

    // next digit
    muxDigit++;
    if (muxDigit > 2) muxDigit = 0;
  }
}