code: func (h *Head) compactable() bool {
  	return h.MaxTime()-h.MinTime() > h.chunkRange/2*3
  }
docs: // compactable returns whether the head has a compactable range.
  // The head has a compactable range when the head time range is 1.5 times the chunk range.
  // The 0.5 acts as a buffer of the appendable window.