// Include the libraries:
// LiquidCrystal_I2C.h: https://github.com/johnrickman/LiquidCrystal_I2C
#include <Wire.h> // Library for I2C communication
#include <LiquidCrystal_I2C.h> // Library for LCD
#include <Keypad.h> // Library for Keypad

// User Marco
#define MAX 32 // Max size of stack

// ==== Init Part ====
// View Init
LiquidCrystal_I2C lcd = LiquidCrystal_I2C(0x27, 16, 2);

// Control Init
const uint8_t ROWS = 4;
const uint8_t COLS = 4;
char keys[ROWS][COLS] = {
  { '1', '2', '3', '+' },
  { '4', '5', '6', '-' },
  { '7', '8', '9', '*' },
  { '.', '0', '#', '/' }
};

uint8_t colPins[COLS] = { 8, 7, 6, 5 }; // Pins connected to C1, C2, C3, C4
uint8_t rowPins[ROWS] = { 12, 11, 10, 9 }; // Pins connected to R1, R2, R3, R4

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

// Model Init
/*
  `p` points to the edited number;
  `stack` stores the numbers
*/
int stack[MAX];
int *p;

void setup() {
  // Initiate the LCD:
  lcd.init();
  lcd.backlight();

  // Serial setup
  Serial.begin(9600); // 9
  // while (!Serial);
  Serial.println("\nSimple Calculator");

  // stack setup
  p = stack;
}

void loop () {
  // Model

  // View

  // Control
  if (handle_input()) refresh_lcd();
}

// View Functions
void refresh_lcd() {
  int *p0, row;
  p0 = stack;
  row = 0;

  lcd.clear();

  if (p - p0 >= 1) {
    lcd.setCursor(1, row);
    lcd.print(*(p - 1));
    row++;
  }

  lcd.setCursor(0, row);
  lcd.print('>');
  lcd.print(*p);
}

// Control Functions
int handle_input () {
  char c = keypad.getKey();

  // no key press
  if (c == NO_KEY) return false;

  // number input
  if ('0' <= c && c <= '9') *p = (*p) * 10 + (c - '0');

  // add number to stack
  if (c == '#') append_stack();

  // operations
  if (c == '+') add();
  if (c == '-') sub();
  if (c == '*') mul();
  if (c == '/') div();

  return true;
}

void append_stack () {
  // test if appendable
  p++;
  *p = 0;
}

void add () {
  *(p - 1) = *(p - 1) + *p;
  *p = 0;
}

void sub () {
  *(p - 1) = *(p - 1) - *p;
  *p = 0;
}

void mul () {
  *(p - 1) = *(p - 1) * *p;
  *p = 0;
}

void div () { // divide INT; TODO: float
  *(p - 1) = *(p - 1) / *p;
  *p = 0;
}