The author have a list of secret good questions, and that is not helping anyone. He expects the candidate to think loud and be explicit, but are himself very implicit in his requirements. What I consider production code is without a doubt different from what he things is production code.
> Your code should be commented.
Then put this requirement, because judging from the example, the authors expectations, is what I fail students and junior devs for. Comments are complementary and explain things like "the why" ("pad to multiple of 256, otherwise libX will hang"). Don't expect people to automatically know your implicit requirements, it's a recipie for disaster.
> You should have error handling or at least logging.
Again, put this in the requirements. Some environments need no error handling, because the error handling is external. And don't get me started on logging. Yes it's nice in a debug situation, but in many high volume production systems you have engineers with the sole responsibility of creating a logging system that impact performance as little as possible.
> You should have a test harness.
If you say so, otherwise I'm not going to make one. Be explicit.
> Runs fast.
> Doesn’t take up more memory than it needs to.
> Is stable and easy to maintain.
1. and 2. are secondary. 3. is king, unless you have empirical evidence that warrant 1. and 2. This is something many developers struggle with, including the auther by the sound of it.
> RegEx
No, just no - the imfamous quote is "I have a problem, let's use a RegEx. Now you have two problems" and it's true 98% of the time, including testing if a string is a palindrome. Besides, regular expressions cannot identify palindromes - this is first year CS stuff. I'm well aware that some implementations allow back references, thus they are not really regular expressions but leaning on CFGs. For this particular problem, reversing the string and then compare is by far the most readable and understandable solution.
> For senior devs, I want an optimal solution, clean, maintainable code, error handling, comments, a full suite of tests. And for bonus points I want you to flag any of your assumptions in the comments.
What is optimal? With regard to execution time? Memory usage? Maintainability? Program size? What kind of error handling?