i use it regularly. it's an extension of cp/scp that does lots of stuff like scanning the directory tree and reading filesizes and datestamps and - i'm not sure, i don't think it includes generating hashes.

IMO, filesystems have several advancements they need but we are still stuck in late 70s unix filesystem designs. A few technologies work with the base and take it further but sadly not enough. A number of things that people associate with Macintosh relate to this. Apple did a lot of effort with making better filesystems but sadly their user-oriented advances have not been widely made use of. It seems to me like, for example, that all files should have a hash in their record that can be quickly accessed. But no. All such systems only create mirror images of the filesystem, if at best, inside a database, no filesystem exists afaik, that keeps a current hash of the file consist.