The spec was clear, but the fact that the test dataset used for the test was the very one given as an example wasn't. plus, it was a bit frustrating not being able to fiddle with the test dataset. I would have done it locally with my compiler, but the necessary boilerplate would have lost me time. So, I didn't test enough, and made a silly error which went undetected because of the trailing zero. I still got 75%, but that looks quite miraculous.

EDIT: It appears I can customize my test dataset, and I didn't see it. Silly me.

Yes, it is even possible to add your own data sets, Codility will tell you what you code returns for them. I guess we should make it more apparent in the interface.