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sclass SubtractedAudio implements IAudioSample {
  IAudioSample source;
  int channels;
  
  // for now same value in all channels. value is actually positive
  //   in terms of what is added
  new TreeMap<Double> subtractions;
  
  *() {}
  *(IAudioSample *source) {}
  
  void setSource(IAudioSample source) {
    this.source = source;
    channels = source.channels();
  }
  
  public double sampleRate() { ret source.sampleRate(); }
  public int channels() { ret channels; }
  public simplyCached DoubleRange bounds() { ret source.bounds(); }
  
  void subtractPlateau(double start, double end, double intensity) {
    if (start >= end) ret;
    
    _changeSumAt(start, -intensity);
    _changeSumAt(end, intensity);
  }

  // We will probably add plateaus rather randomly throughout an area,
  // so could optimize this doing something akin to LogNArray.
  void _changeSumAt(double time, double intensity) {    
    double value = unnull(floorValue(subtractions, time)) + intensity;
    
    // create, overwrite or delete
    mapPutOrRemove(subtractions, time, nullIfZero(value));
    
    Double next = subtractions.higherKey(time);
    while (next != null) {
      subtractions.put(next, subtractions.get(next) + intensity);
      next = subtractions.higherKey(next);
    }
  }
  
  public double readSumTable(int channel, double t) {
    double diff = unnull(floorValue(subtractions, t));
    ret diff + source.readSumTable(channel, t);
  }
}