That sounds interesting. I took a brief look to find out what information is available to the custom algorithm but didn't find too many details, so I will just ask the question here:

If I want to write an algorithm to sort articles in HackerNews by upvotes and time, is the upvotes information available to Gnus? Assume the RSS feed has a customized field called <RSSBrain:upvotes> for each of the article, does the algorithm have access to that data?

And does Gnus need to get all the articles in a folder/feed and run the sorting algorithm on the fly? If so it seems to be a performance concern. Would love to know how it works in real life.