sclass RollingAverage {
  int windowSize = 50;
  int recalcEvery = 1000; // to counter floating point errors
  
  int nRecalc;
  double sum;
  double[] list;
  int start, len;
  
  *() {}
  *(int *windowSize) {}
  
  synchronized void add(double d) {
    init();
    
    if (++nRecalc >= recalcEvery) {
      nRecalc = 0;
      sum = 0;
      for i to len: sum += _get(i);
    }
    
    if (len == windowSize) {
      sum -= _get(0);
      len--;
      start = (start+1) % windowSize;
    }
      
    list[(start+len) % windowSize] = d;
    ++len;
    sum += d;
  }
  
  double _get(int i) { ret list[(start+i) % windowSize]; }
  
  synchronized double get() {
    init();
    ret doubleRatio(sum, len);
  }
  
  void init {
    if (list == null) list = new double[windowSize];
  }
}