srecord noeq SimpleMovingAverage(int length) {
  selfType length(int length) { this.length = length; this; }
  
  simplyCached SimpleCircularBuffer<Double> values() {
    ret new SimpleCircularBuffer(length);
  }
  
  new Average avg;
  
  double get() { ret avg!; }
  
  double add(double x) {
    while (values().size() > max(length-1, 0))
      avg.remove(values().remove());
    values().add(x);
    avg.add(x);
    ret get();
  }
  
  void reset {
    resetFields(this, "avg");
  }
  
  bool complete() {
    ret avg.n() >= length;
  }
}

Began life as a copy of #1036444