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

/*----- Keypad setup -----*/
const byte rows = 4;
const byte columns = 4;

const char Keys[rows][columns] = {
  {'7', '8', '9', '/'},
  {'4', '5', '6', '*'},
  {'1', '2', '3', '-'},
  {'C', '0', '=', '+'}
};

const byte rowsPin[rows] = {9, 8, 7, 6};
const byte columnsPin[columns] = {5, 4, 3, 2};

Keypad myKeypad = Keypad(makeKeymap(Keys), rowsPin, columnsPin, rows, columns);
/*------------------------*/

LiquidCrystal_I2C lcdDisplay(0x27,12,2);//lcd setup

/*========== Creating number ==========*/
long int createNumber(char symbol, long int Num){
  int tempMemory = (int)symbol - 48;//tempMemory = char's value changed to int 

  if(tempMemory == 0) {
    return Num * 10;//return number with 0 in the end example(Num = 123 => return 1230)
  } else {
    return Num * 10 + tempMemory;//return number with tempMemory in the end example(Num = 10, tempMemory = 3 => 103)
  }
}

/*============ Calculating result ========*/
long int Calculations(long int num1, char arithmeticSign, long int num2) {
  
  switch(arithmeticSign) {
    case '/':

      if(num1 == 0 || num2 == 0){//num1 or num2 == 0

        lcdDisplay.clear();
        lcdDisplay.print("ERROR");
        delay(1500);
        lcdDisplay.clear();

        return 0;

      } else {

        num1 = num1 / num2;//calculations

        lcdDisplay.clear();//clear lcd
        lcdDisplay.print(num1);//printing result of the calculations

        delay(500);

        return num1;//return result of the calculations
      }


    case '*':
      num1 = num1 * num2;

      lcdDisplay.clear();
      lcdDisplay.print(num1);

      delay(500);

      return num1;

    case '-':
      num1 = num1 - num2;

      lcdDisplay.clear();
      lcdDisplay.print(num1);

      delay(500);

      return num1;

    case '+':
      num1 = num1 + num2;

      lcdDisplay.clear();
      lcdDisplay.print(num1);

      delay(500);

      return num1;

    case 'C':
      lcdDisplay.clear();

      return 0;
  }

}

void setup() {
  lcdDisplay.init();
  lcdDisplay.backlight();

  Serial.begin(9600);
}

long int num1 = 0;//later it is a result
char arithmeticSign;//*, /, -, +, =, C = clear all
long int num2 = 0;

String whichNum = "num1";//specifies which number user types

void loop() { 
  char pressedKey = myKeypad.getKey();

  switch(pressedKey) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':
      lcdDisplay.print(pressedKey);

      if(whichNum == "num1") {

        num1 = createNumber(pressedKey, num1);
        Serial.print("num1 => ");
        Serial.println(num1);

      } else {

        num2 = createNumber(pressedKey, num2);
        Serial.print("num2 => ");
        Serial.println(num2);

      }
      break;

    case '/':
    case '*':
    case '-':
    case '+':

      if(whichNum == "num1") {
        whichNum = "num2";
      } else if(whichNum == "num2") {
        num1 = Calculations(num1, arithmeticSign, num2);
        num2 = 0;

        whichNum = "num2";
      }

      lcdDisplay.print(pressedKey);
      arithmeticSign = pressedKey;

      break;
    case 'C':
      lcdDisplay.clear();
      whichNum = "num1";
      num1 = 0;
      num2 = 0;

      break;

    case '=':
      if(whichNum == "num1") {
        whichNum = "num2";
      } else if(whichNum == "num2") {
        num1 = Calculations(num1, arithmeticSign, num2);
        num2 = 0;

        whichNum = "num1";
      }
      break;
  }

}