#include <Keypad.h>
#include <LiquidCrystal_I2C.h>

const int ROW_NUM = 4;
const int COLUMN_NUM = 4;

char keys[ROW_NUM][COLUMN_NUM] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};

byte pin_rows[ROW_NUM] = {9, 8, 7, 6};
byte pin_column[COLUMN_NUM] = {5, 4, 3, 2};

Keypad keypad = Keypad(makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM);

LiquidCrystal_I2C lcd(0x27, 16, 2);

String inputNumber = "";

void setup() {
  Serial.begin(9600);
  lcd.begin(16, 2);
  lcd.print("Enter date:");
}

void loop() {
  char key = keypad.getKey();
  
  if (key) {
    if (key == '#') {
      // Reset the process
      inputNumber = "";
      lcd.clear();
      lcd.print("Enter date:");
    } 
    else if (key == '*') {
      if (inputNumber.length() == 8) {
        // Display preloader
        displayPreloader();

        // Convert the input string to integers for day, month, and year
        int day = inputNumber.substring(0, 2).toInt();
        int month = inputNumber.substring(2, 4).toInt();
        int year = inputNumber.substring(4, 8).toInt();

        // Check if the date is valid
        if (isValidDate(day, month, year)) {
          // Calculate the day of the week using Zeller's Congruence
          String result = zellersCongruence(day, month, year);

          // Display the result on the LCD
          lcd.clear();
          lcd.print("Day of week:");
          lcd.setCursor(0, 1);
          lcd.print(result);
        } else {
          // Display "Invalid" on the LCD for invalid dates
          lcd.clear();
          lcd.print("Invalid date");
        }

        // Reset the input for the next round
        inputNumber = "";
      }
    } 
    else if (key == 'A' && inputNumber.length() > 0) {
      // Backspace functionality, remove the last entered digit
      inputNumber.remove(inputNumber.length() - 1);
      
      // Display the entered number on the LCD
      lcd.clear();
      lcd.print("Enter date:");
      lcd.setCursor(0, 1);
      lcd.print(inputNumber);
    }
    else if (inputNumber.length() < 8 && key >= '0' && key <= '9') {
      // Append the pressed key to the input number, only if the length is less than 8 and key is a digit
      inputNumber += key;

      // Display the entered number on the LCD
      lcd.clear();
      lcd.print("Enter date:");
      lcd.setCursor(0, 1);
      lcd.print(inputNumber);
    }
  }
}

void displayPreloader() {
  lcd.clear();
  lcd.print("Processing...");
  delay(2000); // Simulate preloader delay
}

String zellersCongruence(int day, int month, int year) {
  if (month < 3) {
    month += 12;
    year -= 1;
  }

  int K = year % 100;
  int J = year / 100;

  int h = (day + ((13 * (month + 1)) / 5) + K + (K / 4) + (J / 4) - 2 * J) % 7;

  // Map the result to the corresponding day names
  switch (h) {
    case 0: return "Saturday";
    case 1: return "Sunday";
    case 2: return "Monday";
    case 3: return "Tuesday";
    case 4: return "Wednesday";
    case 5: return "Thursday";
    case 6: return "Friday";
    default: return "Invalid";
  }
}

bool isValidDate(int day, int month, int year) {
  // Basic checks for valid day, month, and year ranges
  if (month < 1 || month > 12 || day < 1 || day > 31 || year < 1000 || year > 9999) {
    return false;
  }

  // Additional checks for specific month-day combinations
  if ((month == 4 || month == 6 || month == 9 || month == 11) && day > 30) {
    return false; // April, June, September, November have 30 days
  } else if (month == 2) {
    if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) {
      return day <= 29; // February in a leap year has 29 days
    } else {
      return day <= 28; // February in a non-leap year has 28 days
    }
  }

  return true; // All other months have 31 days
}