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No statute defines "clean room RE" and there's basically no relevant litigation establishing case law.You seem confused here? "Clean Room RE" is simply a descriptive result of copyright law in the US, and is well established. In the US there is no "sweat of the brow doctrine", pure facts simply cannot be copyrighted at all, nor underlying methods/ideas (that's patents). And as a direct result of the latter, again unlike patents, if two people truly independently write the same thing, both get separate copyright.
"Clean room reverse engineering" is just applying that to make any legal fight much cheaper or discourage it from every being bothered with in the first place. You have an RE "dirty" team that goes through copyrighted software and makes a specification that is purely factual, ie, "function 1 takes (int16)X input and does equation ABC on it, function 2 takes (long)X & (long)Y inputs and does LMN" etc. A factual/math spec like that cannot be copyrighted, so it breaks any derivation chain. They then give that, and only that with no other communications, to the clean team. If someone with no other exposure to the software being RE'd uses that spec to write a new piece of software themselves, then by definition the result cannot have any copyright of the original. It's a brand new work based off a list of uncopyrightable facts & math/ideas (though worth noting it could still get tripped up by abomination that is software patents).
There are other ways to argue against copyright infringement successfully, but they're much more subjective, which legally speaking reads "expensive, uncertain". CRRE simply trades that for two clear binary questions: was the spec copyrightable, and did the people writing a new piece of software from that spec have any previous exposure to the original? If the answers to that are no, then all arguments about function names being similar or whatever are no longer relevant.
On the subject of LLMs, one curious twist is that proprietary software might have less effective protection then open source depending on how it all goes. A coding LLM almost certain trained on all open source software available on the web. So there's at least some argument it's tainted. But proprietary software is proprietary. If the LLM has never had any of the source code in its training input, then isn't it clean in terms of producing anything derivative of that? That'll be kinda interesting, and might leave some companies with a bit of a conundrum (or I guess pushing the patent side even harder, but that has a much more sane 20 year expiration date vs copyright).