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

const byte row = 4;
const byte column = 4;
byte rowPins[row] = {9, 8, 7, 6};
byte columnPins[column] = {5, 4, 3, 2};
char keys[row][column] = {
  {'1', '2', '3', '+'},
  {'4', '5', '6', '-'},
  {'7', '8', '9', '*'},
  {'.', '0', '=', '/'}};

Keypad keypad = Keypad(makeKeymap(keys), rowPins, columnPins, row, column);
byte value = 0;

LiquidCrystal_I2C lcd(0x27, 16, 2);

String inputString = "";


void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  lcd.begin(16,2);
  lcd.print(" ");
}

char operation = 0;
String memory = "";
String current = "";
uint64_t currentDecimal;
bool decimalPoint = false;

double calculate(char operation, double left, double right) {
  switch (operation) {
    case '+': return left + right;
    case '-': return left - right;
    case '*': return left * right;
    case '/': return left / right;
  }
}

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

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

    case '=':
      float leftNum = memory.toDouble();
      float rightNum = current.toDouble();
      memory = String(calculate(operation, leftNum, rightNum));
      current = "";
      lcd.clear();
      lcd.setCursor(1, 0);
      lcd.print(memory);
      lcd.setCursor(0, 1);
      lcd.print(operation);
      return;

  }

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

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

  lcd.print(key);
}

void loop() {
  // put your main code here, to run repeatedly:
  char key = keypad.getKey();
  if (key) {
    processInput(key);
  }
}