#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int buttonPin1 = 2;
const int buttonPin2 = 3;
const int buttonPin3 = 4;
const int buttonPin4 = 5;

volatile int hours = 0;
volatile int minutes = 0;
volatile int seconds = 0;

volatile unsigned long previousMillis = 0;
volatile unsigned long interval = 1000;

volatile boolean setTimeFlag = false;

LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  lcd.begin(16, 2);
  lcd.setCursor(0, 0);
  lcd.print("Set Time: ");
  
  pinMode(buttonPin1, INPUT_PULLUP);
  pinMode(buttonPin2, INPUT_PULLUP);
  pinMode(buttonPin3, INPUT_PULLUP);
  pinMode(buttonPin4, INPUT_PULLUP);
  
  attachInterrupt(digitalPinToInterrupt(buttonPin1), changeHour, FALLING);
  attachInterrupt(digitalPinToInterrupt(buttonPin2), changeMinute, FALLING);
  attachInterrupt(digitalPinToInterrupt(buttonPin3), changeSecond, FALLING);
  attachInterrupt(digitalPinToInterrupt(buttonPin4), toggleSetTimeFlag, FALLING);
}

void loop() {
  unsigned long currentMillis = millis();
  
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    
    if (!setTimeFlag) {
      seconds++;
      
      if (seconds >= 60) {
        seconds = 0;
        minutes++;
        
        if (minutes >= 60) {
          minutes = 0;
          hours++;
          
          if (hours >= 24) {
            hours = 0;
          }
        }
      }
    }
    
    displayTime();
  }
}

void displayTime() {
  lcd.setCursor(10, 0);
  lcd.print(hours);
  lcd.print(":");
  
  if (minutes < 10) {
    lcd.print("0");
  }
  
  lcd.print(minutes);
  lcd.print(":");
  
  if (seconds < 10) {
    lcd.print("0");
  }
  
  lcd.print(seconds);
}

void changeHour() {
  if (setTimeFlag) {
    hours++;
    
    if (hours >= 24) {
      hours = 0;
    }
    
    displayTime();
  }
}

void changeMinute() {
  if (setTimeFlag) {
    minutes++;
    
    if (minutes >= 60) {
      minutes = 0;
    }
    
    displayTime();
  }
}

void changeSecond() {
  if (setTimeFlag) {
    seconds++;
    
    if (seconds >= 60) {
      seconds = 0;
    }
    
    displayTime();
  }
}

void toggleSetTimeFlag() {
  setTimeFlag = !setTimeFlag;
  
  if (!setTimeFlag) {
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("Set Time: ");
    displayTime();
  }
}