XOR interrupt

const byte XORout_pin = 2;     // XOR output pin
const byte chanA_pin = 4;      // XOR channel A pin
const byte chanB_pin = 5;      // XOR channel B pin

int chanA, chanB, XOR;

long pos_counter = 0;          // position counter

short XORstate = 4;
short XORstate_ = XORstate;    // buffer

boolean hasChanged = 0;        // checking for changes in the position counter

void setup() {
  pinMode(XORout_pin, INPUT);
  pinMode(chanA_pin, INPUT);
  pinMode(chanB_pin, INPUT);

  attachInterrupt(digitalPinToInterrupt(XORout_pin), trigger, CHANGE);
  Serial.begin(9600);
}

void trigger() {
  hasChanged = true;
}

void loop() {
  if(hasChanged) {
    hasChanged = false;
    XOR = digitalRead(XORout_pin);
    chanA = digitalRead(chanA_pin);
    chanB = digitalRead(chanB_pin);

    XORstate = (1 << 2) | ((chanA != 0) << 1) | (chanB != 0);

    if (XORstate_ == XORstate) {
      Serial.println("XOR state is identical");
    } else if ((~XORstate_ & 3) == (XORstate & 3)) {
      Serial.println("XOR state is inverted");
    } else if ((XORstate_ & 1) == ((XORstate & 2) >> 1)) {
      pos_counter--;
    } else {
      pos_counter++;
    }

    XORstate_ = XORstate;
    Serial.println(pos_counter);
  }
}