This seems too small and specific to matter.
Sure, it is important to manage what is going on inside a loop iterating a million times. But without a good profiling system to know where to work, this could be wasted effort. And good design of a large system will matter more than any single function or loop.
That said, you could do a high level system design and get down into a few details.
On a similar note, in my experience (and this is mainly in C++), reading someone else's code means dealing with hundreds of files and tens of thousands of lines. Getting the system in your head is important, but this is difficult to test in an interview -- because it might take days, not minutes.
My favorite interview question is asking about projects done in spare time. This yields a great deal of information about how inherently interested the person is in hacking.
That said, you could do a high level system design and get down into a few details.