Add a generalized %unique

[eternaleye]

One thing I've found myself wanting is to be able to conditionally add disambiguators in paths at granularities other than the album or singleton level. I think a generalized %unique path format function would be helpful for this; it could work like so:

Given %unique{<keys>,<fields>,<disambiguators>,<brackets>}, it will resolve to nothing if the values of keys are sufficient to resolve the tuple of fields to a unique value. If not, it will add fields from disambiguators, enclosed in brackets, until that condition holds.

This is strictly more expressive than the existing %aunique and %sunique functions, as (IIUC) they can be implemented atop it:

• %aunique{<keys>,<disambiguators>,<brackets>} => %unique{<keys>,album_id,<disambiguators> album_id,<brackets>}
• %sunique{<keys>,<disambiguators>,<brakcets>} => %unique{<keys> album_id singleton,id,<disambiguators> id,<brackets>}

In my particular case, I want to separate distinct artists with a common name, so I want %unique{albumartist,mb_albumartistid,mb_albumartistid} in my album paths and %unique{artist,mb_artistid,mb_artistid} in my singleton paths.

I also nest releases under folders for the release group, so I further want my album paths to have:

• ${original_fulldate} -- $album %unique{albumartist original_fulldate release_group_title,mb_releasegroupid,mb_releasegroupid}

(Given this, it may also be good for disambiguators to default to fields if not present, simplifying the above to %unique{albumartist,mb_albumartistid} and %unique{artist,mb_artistid})

[michaelkrieger]

@eternaleye : Within 'latest' there is Singleton Disambiguation:

It is also possible to have singleton tracks with the same name and the same artist. Beets provides the %sunique{} template to avoid giving these tracks the same file path. It has the same arguments as the %aunique template, but the default values are different. The default identifiers are artist title and the default disambiguators are year trackdisambig.

I'd love, however for it to be able to use an autoincrement to make unique paths:
Lets say I have two copies of the single song "Artist - Song". They're different versions of the song and frankly just need to be named better (ie: Club Remix or Album Version or Radio Edit) but I don't yet know the detailed track information. These are often historical files and used in playlists. So bringing them all in, I'd want a %unique as follows:
singleton: Singles/$year/$artist - $title%unique(year, artist, title, '()')
My expectation is that I could get:
Singles/2024/Artist - Title and Singles/2024/Artist - Title (2) and Singles/2024/Artist - Title (3) out of three tracks.
At this point I have to set trackdisambig for each track.

[eternaleye]

I was under the impression you could use id, the beets library database numeric primary key, for this? Similar to how albums with %aunique fall back to the album_id number. It's not quite an autoincrement, but the semantics of an autoincrement are... somewhat fraught, because if your path template changes you'd need to assign a canonical ordering to the new set of tracks it conflicts with.