#include <Keypad.h>
#include <Wire.h> 
#include <LiquidCrystal.h>
LiquidCrystal lcd(13, 12, 11, 10, 9, 8);

long first = 0;
long second = 0;
double total = 0;
char customKey;
const byte ROWS = 4;
const byte COLS = 4;

char keys[ROWS][COLS] = {
  {'1','2','3','+'},
  {'4','5','6','-'},
  {'7','8','9','*'},
  {'C','0','=','/'}
};
byte rowPins[ROWS] = {7,6,5,4}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {3,2,1,0}; //connect to the column pinouts of the keypad

//initialize an instance of class NewKeypad
Keypad customKeypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS); 

void setup()
{
  lcd.begin(16, 2);               
  for(int i=0;i<=3;i++);
  lcd.setCursor(0,0);
    lcd.print("CALCULATOR");
    lcd.setCursor(0,1);
    lcd.print("GREATECH");
  delay(2500);
  lcd.clear();
  lcd.setCursor(0, 0);
}


void loop()
{
  
    customKey = customKeypad.getKey();
    switch(customKey) 
    {
        case '0' ... '9': 
          lcd.setCursor(0,0);
          first = first * 10 + (customKey - '0');
          lcd.print(first);
          break;
      
        case '+':
          first = (total != 0 ? total : first);
          lcd.setCursor(0,1);
          lcd.print("+");
          second = SecondNumber();
          total = first + second;
          lcd.setCursor(0,3);
          lcd.print(total);
          first = 0, second = 0;
          break;
      
        case '-':
          first = (total != 0 ? total : first);
          lcd.setCursor(0,1);
          lcd.print("-");
          second = SecondNumber();
          total = first - second;
          lcd.setCursor(0,3);
          lcd.print(total);
          first = 0, second = 0;
          break;
      
        case '*':
          first = (total != 0 ? total : first);
          lcd.setCursor(0,1);
          lcd.print("*");
          second = SecondNumber();
          total = first * second;
          lcd.setCursor(0,3);
          lcd.print(total);
          first = 0, second = 0;
          break;
      
        case '/':
          first = (total != 0 ? total : first);
          lcd.setCursor(0,1);
          lcd.print("/");
          second = SecondNumber();
          lcd.setCursor(0,3);
      
          second == 0 ? lcd.print("Invalid") : total = (float)first / (float)second;
      
          lcd.print(total);
          first = 0, second = 0;
          break;
      
        case 'C':
          total = 0;
          lcd.clear();
          break;
    }
}

long SecondNumber()
{
    while( 1 )
    {
          customKey = customKeypad.getKey();
          if(customKey >= '0' && customKey <= '9')
          {
                  second = second * 10 + (customKey - '0');
                  lcd.setCursor(0,2);
                  lcd.print(second);
          }
      
          if(customKey == '=') break;
    }
   return second; 
}