sclass BollingerBands {
  replace Candle with TradingCandle.
  
  new Welford welford;
  settable int length = 20; // Bollinger length (how many candles to look at)
  settable double deviation = 2;
  
  // result
  settable DoubleRange range;
  
  simplyCached CircularFifoBuffer<Candle> candles() {
    ret new CircularFifoBuffer(length);
  }
  
  gettable AverageAndStandardDeviation as;
  
  Candle lastCandle() { ret candles().last(); }
  
  // add completed candles only
  void add(TradingCandle candle) {
    if (lastCandle() != null && candle.startTime <= lastCandle().startTime)
      ret;
    candles().removeToSize(length-1);
    candles().add(candle);
    
    if (l(candles) >= length) {
      var bCandles = takeLast(length, candles());
      var prices = mapToDoubleArray(bCandles, ->.endPrice);
      as = averageAndStandardDeviation(prices);
      range = bollingerRange(as, deviation);
    }
  }
}