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

LiquidCrystal_I2C display(0x27, 16, 2);

const byte rows = 4;
const byte columns = 4;

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

byte keyPinRows[rows] = {7,6,5,4};
byte keyPinColumns[columns] = {3,2,1,0};

Keypad thisKeyPad = Keypad(makeKeymap(keys), keyPinRows, keyPinColumns, rows, columns);

char operation = 0;
String first = "";
String second = "";

double math(char operation, double first, double second)
{
  switch(operation){
    case '+':
      return first + second;
      break;
    case '-':
      return first - second;
      break;
    case '*':
      return first * second;
      break;
    case '/':
      return first / second;
      break;
  }
}

void process(char input)
{
  if ('-' == input && second == "") {
    second = "-";
    display.print("-");
    return;
  }

  switch(input){
    case '+':
    case '-':
    case '*':
    case '/':
      if(!operation){
        first = second;
        second = "";
      }
      operation = input;
      display.setCursor(0,1);
      display.print(input);
      display.setCursor(1,1);

      return;
      break;
    case '=':
      float left = first.toFloat();
      float right = second.toFloat();

      first = String(math(operation, left, right));
      second = "";

      display.clear();

      display.setCursor(1,0);
      display.print(first);
      display.setCursor(0,1);
      display.print(operation);
      return;
      break;
    
    
  }
  
  if(input != operation && second == "0"){
    second = String(input);
  } else if(input){
    second += String(input);
  }

  display.print(input);

  if(input == 'C'){
    first="";
    second="";
    operation = 0;
    display.clear();
    display.setCursor(1,0);
  }


}

    


void setup() {
  display.init();
  display.backlight();
  display.setCursor(1,0);

}

void loop() {
  char key = thisKeyPad.getKey();
  if(key != NO_KEY){
    process(key);
  } 
 
}