The first stage of the interview is a coding challenge at HackerRank. Once started it must be finished within 2 days to alleviate the time pressure for a candidate. The questions are not that tough even though implementing some of the tasks does require some time - one would likely to spend at least 2 hours. However, I'm clueless about what criteria they apply to the whole experience since regardless of me having solved all their tasks with all the tests passed they replied to me that they won't proceed with the process any further. So, without any feedback - both positive and negative - this entire challenge seems like a complete waste of time.