int index, last;

char int2char(int n) {
  return (n > 9) ? ' ' : n+'0';
}

int char2int(char c) {
  return (isdigit(c)) ? int(c-'0') : -1;
}


void setup() {
  Serial.begin(9600);
  
  for (byte i=0; i<255; i++)
    Serial.print(i),
    Serial.print(": "),
    Serial.println((char)(i+127));

  last = -1;
  index = 0;
}

void loop(){
  if (Serial.available() > 0) {
    char c = Serial.read();

    if (isprint(c)) {
      int val = char2int(c);
      byte x = (val+48);
      index = (val == last) ? index+1 : 0;

      index = (index == 16) ? 178 : ((index == 182) ? 0 : index);
      x = (index > 0) ? x-(index+val) : x;
    
      Serial.print(val); Serial.print(" -> "); Serial.print(index);
      Serial.print(", "); Serial.print((char)x);
      Serial.print(" "); Serial.println(x);
      last = val;
    }
  }
}