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

LiquidCrystal_I2C lcd(0x27, 16, 2);

const byte ROWS = 4;
const byte COLS = 4;

char keys[ROWS][COLS] = {
  {'1', '2', '3', '+'},
  {'4', '5', '6', '-'},
  {'7', '8', '9', '*'},
  {'*', '0', '#', '/'}
};

byte rowPins[ROWS] = { 13, 12, 11, 10 };
byte colPins[COLS] = { 9, 8, 7, 6 };

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

String num1 = "";
String num2 = "";
char operatorChar = 0;
boolean isSecondNum = false;

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.clear();
}

void loop() {
  char key = keypad.getKey();
  
  if (key) {
    if (key >= '0' && key <= '9') {
      if (!isSecondNum) {
        num1 += key;
        lcd.setCursor(0, 0);
        lcd.print(num1);
      } else {
        num2 += key;
        lcd.setCursor(0, 0);
        lcd.print(num1 + operatorChar + num2);
      }
    } else if (key == '+' || key == '-' || key == '*' || key == '/') {
      if (!isSecondNum && num1.length() > 0) {
        operatorChar = key;
        isSecondNum = true;
        lcd.setCursor(0, 0);
        lcd.print(num1 + operatorChar);
      }
    } else if (key == '#') {
      if (num1.length() > 0 && num2.length() > 0) {
        float number1 = num1.toFloat();
        float number2 = num2.toFloat();
        float result = 0;
        
        switch (operatorChar) {
          case '+': result = number1 + number2; break;
          case '-': result = number1 - number2; break;
          case '*': result = number1 * number2; break;
          case '/': result = number1 / number2; break;
        }
        
        lcd.setCursor(0, 1);
        lcd.print(result);
        
        delay(900);
        resetCalculator();
      }
    } else if (key == '*') {
      resetCalculator();
    }
  }
}

void resetCalculator() {
  num1 = "";
  num2 = "";
  operatorChar = 0;
  isSecondNum = false;
  lcd.clear();
  lcd.setCursor(0, 0);
}