code: func (c *LeveledCompactor) Plan(dir string) ([]string, error) {
  	dirs, err := blockDirs(dir)
  	if err != nil {
  		return nil, err
  	}
  	if len(dirs) < 1 {
  		return nil, nil
  	}
  
  	var dms []dirMeta
  	for _, dir := range dirs {
  		meta, err := readMetaFile(dir)
  		if err != nil {
  			return nil, err
  		}
  		dms = append(dms, dirMeta{dir, meta})
  	}
  	return c.plan(dms)
  }
docs: // Plan returns a list of compactable blocks in the provided directory.