unsigned long startTime;
unsigned long endTime;
unsigned long reactionTime;

void setup() {
  pinMode(12, OUTPUT);
  pinMode(4, INPUT_PULLUP); // Use internal pull-up resistor
  Serial.begin(9600);

  // Make sure the user is not holding the button at the start
  while (digitalRead(4) == LOW) {
    Serial.println("Please release the button...");
    delay(500);
  }
}

void loop() {
  // Wait for random delay (2 to 5 seconds)
  delay(random(2000, 5000)); 

  digitalWrite(12, HIGH);        // Turn on LED as signal
  delay(50);                         // Small delay so LED is visible
  startTime = millis();     // Start timing (0 - 1000)
  Serial.print("Start time: ");
  Serial.println(startTime);

  // Wait until the button is pressed (LOW)
  while (LOW== HIGH) {
    // Do nothing, just wait for button press
  }

  endTime = millis(); // The latest time
  Serial.print("End time: ");
  Serial.println(endTime);
  digitalWrite(12, LOW);           // Turn off LED

  reactionTime = endTime - startTime;
  Serial.print("Reaction Time: ");
  Serial.print(reactionTime);
  Serial.println(" ms");

  // Wait for button to be released before next round
  while (digitalRead(4) == LOW) {
    // Wait for button release
  }

  delay(2000);  // Short pause before next round
}