It's been a long time since I spent my time on that, so I do not remember anything specific. But it was difficult figuring out reliably if a post in a given fee

It's been a long time since I spent my time on that, so I do not remember anything specific. But it was difficult figuring out reliably if a post in a given feed matched an already existing (locally cached) post, whether it was an updated version of that post, or whether if it was a completely new, hitherto unseen post, so it had something to do with the post IDs, timestamps and URLs.