const int ledPins[] = {2, 3, 4, 5}; //LED Pins

int delayTime = 500; //Initial delaytime in ms
int previousDelayTime = -1;
const int fasterButton = 6; // faster pin button
const int slowerButton = 7; //slower pin button

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200); 
  Serial.println("Hello Arduino\n");
  for (int  i = 0; i < 4; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
  pinMode(fasterButton, INPUT_PULLUP);
  pinMode(slowerButton, INPUT_PULLUP);

  attachInterrupt(digitalPinToInterrupt(6), fasterInterrupt, FALLING);
  attachInterrupt(digitalPinToInterrupt(7), slowerInterrupt, FALLING);
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i = 0; i < 4; i++) {
    digitalWrite(ledPins[i], HIGH);
  }
  delay(delayTime);
  for (int i = 0; i < 4; i++) {
    digitalWrite(ledPins[i],LOW);
  }
  delay(delayTime);
  //if (delayTime != previousDelayTime) {
    Serial.println(delayTime);
  //  previousDelayTime = delayTime;
  //}
}

void fasterInterrupt() {
  if (delayTime > 55) {
    noInterrupts();
    delayTime -= 50;
    interrupts();
  }
}

void slowerInterrupt() {
  if (delayTime < 10000) {
    noInterrupts();
    delayTime += 50;
    interrupts();
  }
}