const int buttonPin = 2;
const int ledPin = 13;

bool waitingForReaction = false;
unsigned long startTime;
unsigned long reactionTime;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
  randomSeed(analogRead(0)); // for more randomness
}

void loop() {
  if (!waitingForReaction) {
    // Wait for user to start the game
    if (digitalRead(buttonPin) == LOW) {
      delay(500); // debounce
      int delayTime = random(1000, 5000);
      delay(delayTime);
      digitalWrite(ledPin, HIGH);
      startTime = millis();
      waitingForReaction = true;
    }
  } else {
    // Wait for user to react
    if (digitalRead(buttonPin) == LOW) {
      delay(500); // debounce
      reactionTime = millis() - startTime;
      digitalWrite(ledPin, LOW);
      Serial.print("Reaction time: ");
      Serial.print(reactionTime);
      Serial.println(" ms");
      waitingForReaction = false;
    }
  }
}