#include <Wire.h>
#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','*'},
  {'.','0','=','/'}
};
byte rowPins[rows] = {0, 1, 2, 3};
byte colPins[cols] = {4, 5, 6, 7};
Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, rows, cols );

String last3 = "";
String last2 = "";
String last1 = "";
String current = "";
char operation = 0;

void updateCursor() {
  if (millis() / 500 % 2 == 0 ) {
    lcd.cursor();
  } else {
    lcd.noCursor();
  }
}

double calc(char op, double in1, double in2) {
  switch(op) {
    case '+': return in1 + in2;
    case '-': return in1 - in2;
    case '*': return in1 * in2;
    case '/': return in1 / in2;
  }
}

void readInput(char key) {
  if(!key) {
    return;
  }

  if(key == '-' && current == "") {
    current = '-';
    lcd.print('-');
    return;
  }

  switch(key) {
    case '+':
    case '-':
    case '*':
    case '/':
      if(!operation) {
        last3 = last2;
        last2 = last1;
        last1 = current;
        current = "";
      }
      operation = key;
      lcd.setCursor(0, 3);
      lcd.print(key);
      lcd.setCursor(current.length() + 1, 3);
      return;

    case '=':
      last3 = last2;
      last2 = last1;
      float num1 = last1.toDouble();
      float num2 = current.toDouble();
      last1 = String(calc(operation, num1, num2));
      current = "";
      lcd.clear();
      lcd.setCursor(1, 0);
      lcd.print(last3);
      lcd.setCursor(1, 1);
      lcd.print(last2);
      lcd.setCursor(1, 2);
      lcd.print(last1);
      lcd.setCursor(1, 3);
      operation = 0;
      return;
  }

  if(key == '.' && current.indexOf('.') >= 0) {
    return;
  }

  if(key == '.' && current == "0") {
    current = String(key);
  } else if (key) {
    current += String(key);
  }

  lcd.print(key);

}

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.cursor_on();
  lcd.setCursor(1,2);
}

String input = "";
String input2 = "";

void loop() {
  updateCursor();
  readInput(keypad.getKey());
}