You don't have to use a clean room methodology. Decompiling the code and reimplementing the functionality is a perfectly valid approach in any jurisdiction that permits reverse engineering.
Wait, pardon my ignorance, but isn't a decompiler required for the kind of work they do at react?
Person A's Job:
- Decompile shit.
- Then write down the names of the functions with (1) input, (2) output, (3) a description of what person a think the code is doing (4) any side effect / preconditions / post conditions they can deduce.
Person B's Job:
- Take the spec created by person A and write code.
while(missingFunctionality.hasNext): goto Person A's Job
It is and the split between person A doing the first part and person B doing the second part is important in a "clean room" reimplementation in the US.