It's heavily based on Lolfixer and borrows some things from BG2Fixpack's Super Happy Fun Lucky Modder Pack. Plus, it corrects/notifies many additional errors reported by DLTCEP (although not all - yet).
- How is it different from Lolfixer?
Lolfixer simply tries to find and correct whatever it can.
(Apart being more extensive) Happy IE also categorizes issues by severity: "error", "warning", or "info". That allows you to prioritize.
- How is it different from BG2Fixpack?
Fixpack focuses on one single game: classic BG2. And it's very strict in what's considered "a bug". (As expected from a fixpack, naturally).
Contrary to that, Happy IE follows Lolfixer philosophy: finding and fixing bugs at scale. And not only bugs, but also possible issues, inconsistencies, common gotchas and more stuff like that. (For example, usually areas have a script with matching name assigned to them. But some don't. So Happy creates empty scripts and assigns them to the problematic areas.)
(I can hear you chuckling.)
Both of these are impossible to reach, of course. There's just too many things that can go wrong, not to mention that the whole scope is limited to structure integrity, and bugs happen in many other places.
However, it is possible to get closer, and Happy is a tool that aims to allow to do that.
Real power of Happy lies in running it in override-only mode, so that only files changed by your mod get checked and issues reported. That provides a much more manageable list of issues to look it.
You can run Happy yourself, manually. Alternatively, BGforge Golem can do that automatically for you, and report the results.
To be clear, the suite is far for complete. I think I went over all itm/cre issues reported by DLTCEP at some point, but I could be mistaken. There are many more checks for other file types, and there's also NearInfinity with its own set of checks (some will likely overlap, but probably not all.)
Unfortunately, right now I'm busy with other projects and won't be able to dedicate much time to Happy. So it will depend on community to evolve. Anyone proficient in WeiDU is able and welcome to submit pull requests.