/*
  Forum: https://forum.arduino.cc/t/how-to-read-bytes-from-a-rfid-card-and-convert-them-into-an-integer/1158246
  Wokwi: https://wokwi.com/projects/373072344139685889

*/

byte buf[8] = {1, 0, 127, 0, 255, 0, 0, 1};


void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("Start");
  Serial.println(IntFromBytes(buf, 0));
  Serial.println(IntFromBytes(buf, 2));
  Serial.println(IntFromBytes(buf, 4));
  Serial.println(IntFromBytes(buf, 6));
  writeIntToBytes(20, 0);
  writeIntToBytes(40, 2);
  writeIntToBytes(120, 4);
  writeIntToBytes(512, 6);
  Serial.println(IntFromBytes(buf, 0));
  Serial.println(IntFromBytes(buf, 2));
  Serial.println(IntFromBytes(buf, 4));
  Serial.println(IntFromBytes(buf, 6));
}

void loop() {
  // put your main code here, to run repeatedly:

}

int IntFromBytes(byte *b, int offset) {
  return b[offset] + b[offset + 1] * 256;
}

void writeIntToBytes(int v, int offset) {
  buf[offset]   = v & 0xFF;
  buf[offset + 1] = v >> 8;
}