Function cluster_name 

pub fn cluster_name(keys: &[String]) -> String

What to call a set of files.

The directory they all sit under, when there is one — that is the name a person would use — followed by the two subdirectories most of them sit in, which is what tells two clusters under the same root apart. Files that share no directory get MIXED in the prefix’s place.

Files sitting directly in the shared directory add nothing to it, so a cluster that is exactly one directory deep is named by that directory alone.