You mixed up RSTP with RTSP, I would revisit my Googling skills if I were you.. making a beautiful case though for this kind of interview process.

(before you go on googling again, the algorithm operates over the switch port forwarding fabric, you can't really use a library)

So are you really saying that you could not look at an open source implementation and reimplement?

https://github.com/shemminger/RSTP

Yes, I am saying exactly that. This is a userspace implementation not making use of switch hardware fabric. You can probably hack hardware forwarding into it (haven't looked into this, our implementation predates it by 2 years). Or you can read an RSTP spec and implement it. None of it is secret, like none of CS in general is.

Both approaches however would be beyond the skills of a programmer who can't sort an array. And really it's not a high bar, I don't understand this obsession with anti-intellecualism. Everyone likes plumbing analogies, well I wouldn't trust a plumber who doesn't know that water generally tends to flow downwards - it's this level of CS knowledge we are talking about here.

I’m saying that you don’t need to memorize an algorithm. For one job I had to maintain a proprietary compiler/VM/IDE that never saw the light of day outside of the company. I needed to solve an arbitrarily complex algebraic expression in a string. I had no idea how to do it. I found both algorithms I needed with a quick google search and implemented them in C - one was converting the expression to RPN and the other was implementing the “Shunting Yard Algorithm”. Would I have also been dinged because I didn’t know those two algorithms off the top of my head?

No? I asked to sort a bloody array, that's it.