#include <Wire.h>
#include <LiquidCrystal_I2C.h>

#define ROWS 4
#define COLS 4

char keys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

byte rowPins[ROWS] = { 2, 3, 4, 5 };
byte colPins[COLS] = { 6, 7, 8, 9 };

LiquidCrystal_I2C lcd(0x27, 16, 2); // I2C address of the LCD

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

void loop() {
  char buffer[11];
  int count = 0;
  
  // Read the keypad
  char key = getKey();
  if (key != 0) {
    if (key == '-') {
      // Clear the buffer
      for (int i = 0; i < 11; i++) {
        buffer[i] = '\0';
      }
      count = 0;
      lcd.clear();
    } else if (key == '\n') {
      // Display the buffer on the LCD
      lcd.clear();
      lcd.setCursor(0, 0);
      lcd.print("Artikelnummer:");
      lcd.setCursor(0, 1);
      lcd.print(buffer);
    } else {
      // Append the key to the buffer
      if (count < 10) {
        buffer[count++] = key;
        lcd.print(key);
      }
    }
  }
}

char getKey() {
  // Scan the keypad
  for (byte col = 0; col < COLS; col++) {
    pinMode(colPins[col], OUTPUT);
    digitalWrite(colPins[col], LOW);

    for (byte row = 0; row < ROWS; row++) {
      pinMode(rowPins[row], INPUT_PULLUP);
      if (digitalRead(rowPins[row]) == LOW) {
        delay(10); // debounce
        while (digitalRead(rowPins[row]) == LOW); // wait for key release
        return keys[row][col];
      }
    }

    pinMode(colPins[col], INPUT);
    digitalWrite(colPins[col], HIGH);
  }

  return 0;
}