code: public double d(double x){
  		int intervalNumber =getIntervalNumber(x);
  		if (intervalNumber==0 || intervalNumber==points.length) {
  			return x;
  		}
  		return getIntervalReferencePoint(intervalNumber-1);
  	}
docs: If a given x is into an interval of the partition, this method returns the reference point of the corresponding interval.
  If the given x is not contained in any interval of the partition, this method returns x.
  
  @param x The point of interest.
  @return The discretized value.