#include <LiquidCrystal.h>

// LCD: RS, E, D4, D5, D6, D7
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

const int greenLEDPin = 8;
const int buttonPin = 7;

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

void setup() {
  lcd.begin(16, 2);
  pinMode(greenLEDPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP); // button goes to GND
  lcd.print("Reaction Timer");
  delay(2000);
  lcd.clear();
}

void loop() {
  lcd.setCursor(0, 0);
  lcd.print("Get Ready...");
  delay(random(2000, 5000)); // random delay between 2-5 seconds

  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("GO!");

  digitalWrite(greenLEDPin, HIGH);
  startTime = millis();
  waitingForReaction = true;

  while (waitingForReaction) {
    if (digitalRead(buttonPin) == LOW) {
      reactionTime = millis() - startTime;
      waitingForReaction = false;
    }
  }

  digitalWrite(greenLEDPin, LOW);
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Your Time:");
  lcd.setCursor(0, 1);
  lcd.print(reactionTime);
  lcd.print(" ms");

  delay(3000); // display result for 3 seconds
  lcd.clear();
}