code: func NewBufferIterator(it SeriesIterator, delta int64) *BufferedSeriesIterator {
  	bit := &BufferedSeriesIterator{
  		buf:   newSampleRing(delta, 16),
  		delta: delta,
  	}
  	bit.Reset(it)
  
  	return bit
  }
docs: // NewBufferIterator returns a new iterator that buffers the values within the
  // time range of the current element and the duration of delta before.