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

LiquidCrystal_I2C lcd(0x27, 20, 4);
const byte ROWS = 4;
const byte COLS = 4;

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

byte colPins[COLS] = {5,4,3,2};
byte rowPins[ROWS] = {9,8,7,6};
String operation = "";
int number = 0;

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

int Num1, Num2, Number, vysledok;
char key, action;
boolean result = false;

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.setCursor(3,0);
  lcd.print("Vitajte v");
  lcd.setCursor(3,1);
  lcd.print("kalkulacke");
  delay(2000);
  lcd.clear();
}

void loop() {
  key = keypad.getKey();
  
  if (key != NO_KEY) {
    DetectButtons();
  }
  
  if (result == true) {
    CalculateResult();
  }
  
  DisplayResult();
}

void DetectButtons() {
  if (key == 'C') {
    lcd.clear();
    lcd.print(vysledok);
  }

  if (key == '0') {
    Number = 0;
  }

  if (key == '1') {
    Number = 1;
  }

  if (key == '2') {
    Number = 2;
  }

  if (key == '3') {
    Number = 3;
  }

  if (key == '4') {
    Number = 4;
  }

  if (key == '5') {
    Number = 5;
  }

  if (key == '6') {
    Number = 6;
  }

  if (key == '7') {
    Number = 7;
  }

  if (key == '8') {
    Number = 8;
  } 

  if (key == '9') {
    Number = 9;
  }

  if (key == '=') {
    Num2 = Number;
    Number = 0;
    result = true;
  }

  if (key == '+' || key == '-' || key == '*' || key == '/') {
    Num1 = Number;
    Number = 0;

    if (key == '+') {
      action = '+';
    }

    if (key == '-') {
      action = '-';
    }

    if (key == '*') {
      action = '*';
    }  

    if (key == '/') {
      action = '/';
    }
  }
}

void CalculateResult() {
  if (action == '+') {
    vysledok = Num1 + Num2;
  }

  if (action == '-') {
    vysledok = Num1 - Num2;
  }

  if (action == '*') {
    vysledok = Num1 * Num2;
  }

  if (action == '/') {
    vysledok = Num1 / Num2;
  }
}

void DisplayResult() {
  lcd.setCursor(0,0);

  lcd.print(Num1);
  lcd.print(action);
  lcd.print(Num2);

  if (result == true) {
    lcd.print(" = ");
    lcd.print(vysledok);
  }
}