#include 
#include 

// LCD settings
LiquidCrystal_I2C lcd(0x27, 16, 2);

// Red backspace button (pin 4 with internal pull-up)
const int BACKSPACE_PIN = 4;

// 4×4 keypad setup
const byte ROWS = 4, COLS = 4;
char keys[ROWS][COLS] = {
  { '1','2','3','A' },
  { '4','5','6','B' },
  { '7','8','9','C' },
  { '*','0','#','D' }
};
byte rowPins[ROWS] = {5, 6, 7, 8};
byte colPins[COLS] = {9, 10, 11, 12};
Adafruit_Keypad keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

// Store the math expression as a string
String expression = "";

void setup() {
  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  keypad.begin();

  pinMode(BACKSPACE_PIN, INPUT_PULLUP);  // use internal pull-up

  lcd.setCursor(0, 0);
  lcd.print("Calculator Ready");
  delay(2000);  // show for 2 seconds
  lcd.clear();
}

void loop() {
  keypad.tick();

  // Check keypad input
  if (keypad.available()) {
    auto e = keypad.read();
    if (e.bit.EVENT == KEY_JUST_PRESSED) {
      char key = e.bit.KEY;

      if (isDigit(key)) {
        // Add digit to expression
        expression += key;
      } else if (key == 'A') {
        expression += '+';
      } else if (key == 'B') {
        expression += '-';
      } else if (key == 'C') {
        expression += '*';
      } else if (key == 'D') {
        expression += '/';
      } else if (key == '*') {
        // Clear everything
        expression = "";
        lcd.clear();
      } else if (key == '#') {
        // Calculate result
        float result = evaluateExpression(expression);
        lcd.setCursor(0, 1);
        lcd.print("= ");
        lcd.print(result, 2);  // show 2 decimal places
        return;  // return to top of loop
      }
      updateLCD();
    }
  }

  // Backspace button: remove last character
  static bool lastButtonState = HIGH;
  bool buttonState = digitalRead(BACKSPACE_PIN);
  if (lastButtonState == HIGH && buttonState == LOW) {
    if (expression.length() > 0) {
      expression.remove(expression.length() - 1);
      updateLCD();
    }
  }
  lastButtonState = buttonState;
}

// Show expression on line 0, clear line 1
void updateLCD() {
  lcd.setCursor(0, 0);
  lcd.print("                ");  // clear line 0
  lcd.setCursor(0, 0);
  lcd.print(expression);
  lcd.setCursor(0, 1);
  lcd.print("                ");  // clear line 1
}

// Support multiple operators: left-to-right evaluation without precedence
float evaluateExpression(String exp) {
  float result = 0;
  float num = 0;
  char op = '+';  // start with addition

  for (int i = 0; i < exp.length(); i++) {
    char c = exp[i];
    bool isLast = (i == exp.length() - 1);

    if (isDigit(c)) {
      // build the current number
      num = num * 10 + (c - '0');
    }

    // if we reach an operator or the end of the string, apply the previous op
    if (!isDigit(c) || isLast) {
      switch (op) {
        case '+': result += num; break;
        case '-': result -= num; break;
        case '*': result *= num; break;
        case '/': result = (num != 0 ? result / num : 0); break;
      }
      num = 0;     // reset number
      op = c;      // set new operator (may be digit on last iteration, but it's not used further)
    }
  }
  return result;
}