Is Codility test hard?

No, Good depth knowledge in basic concepts, which can make test easy.

yes ,for giving the test you should have in depth knowledge about the subject

Its just need some basic knowledge in coding language and have to find the logic to solve the question. you can get the logic by doing different coding practice. there is a lot of pages to practice.

I have a PhD in Computer Science, and am a strong "puzzle" solver. The very hardest codility questions are often a two coffee question, I need to think in some unexpected way to get the insight needed for the optimal solution. What is great about some of these questions is that they admit slower, much easier answers, so they provide testing value at different skill levels. I can learn if a candidate just as no clue, or they can get the straightforward solution. but it leave room for the 1-in-20 candidate who can see the deeper insight too. All in one single test question. In designing a good test, it is good to mix these hardest questions with easier ones, to get maximal information from your test.