int rowPins[] = { 13, 12, 9, 8 };
int columnPins[] = { 7, 6, 5, 4 };
bool prevStates[16];
unsigned long pressTimes[16];
unsigned int delayTime = 20;
bool buttonsFiltered[16];

void setup() {
  for (int i = 0; i < 4; i++) {
    pinMode(rowPins[i], OUTPUT);
    pinMode(columnPins[i], INPUT_PULLUP);
  }
  Serial.begin(9600);
}

void loop() {
  for (int r = 0; r < 4; r++) {
    digitalWrite(rowPins[r], LOW);
    for (int c = 0; c < 4; c++) {
      bool buttonReading = !digitalRead(columnPins[c]);
      int buttonIndex = r * 4 + c;
      if (buttonReading != prevStates[buttonIndex]) {
    pressTimes[buttonIndex] = millis();
  }
  if (millis() - pressTimes[buttonIndex] >= delayTime) {
    if (buttonsFiltered[buttonIndex] != buttonReading) {
      buttonsFiltered[buttonIndex] = buttonReading;
      if (buttonsFiltered[buttonIndex]) {
        Serial.print("Button ");
        Serial.print(buttonIndex);
        Serial.println(" has been pressed (filtered)");
      } else {
        Serial.print("Button ");
        Serial.print(buttonIndex);
        Serial.println(" has been depressed (filtered)");
      }
    }
  }
  prevStates[buttonIndex] = buttonReading;
    }
    digitalWrite(rowPins[r], HIGH);
  }
}