[eyelidlessness]

If I encountered this test, here’s what I’d think (or hope) is being evaluated:

- Do you understand the problem, or seek better understanding if not? (Obviously the problem here isn’t extrapolating arithmetic, but identifying the importance of an atomic operation.)

- Do you recognize the value of the change, or question it if it feels non-obvious?

- How do you think about approaching an unfamiliar codebase/code path?

- How do you think about the challenges you encountered while working on the problem?

- What else felt important to you that I’m not looking for?

All of these questions provide a lot more information than “can you write code?”

Especially meaningful for evaluating actual fit is reactions to walking into pre-existing code. In fact I think this has been, at least subconsciously, my best heuristic for assessing colleagues’ skill maturity. “Juniors” will stare at a problem or ask a lot of easily answered questions; “mid level” devs will bang their heads trying to answer without asking; “senior” devs will have a good intuition for which challenges are discoverable and which are best solved by asking a human or a google or whatever, or at least for recognizing the distinction after some time exploring the problem.

[XorNot]

Still winds up just feeling like a trick question: because the immediate response in any real world scenario is "why does the client need this feature?" when a CAS operation is right there.

But probably not what the interviewer wants to hear - maybe.

[teraflop]

Aside from being a lot simpler to use, a dedicated "multiply" command could end up being dramatically more efficient.

If multiple clients are simultaneously trying to update the same value, then locking allows them to take turns with relatively little overhead. With compare-and-set, all but one of the clients would fail at the "compare" step and need to retry, requiring additional network round-trips.

[eyelidlessness]

That’s not or shouldn’t be a trick question! “An affordance for what I’m asked to do already exists” is an excellent answer. Probably the best answer in most scenarios, and the one I’d most hope/expect to encounter from a “senior”.