I worked for months on this one and even so there was never a shortage of ideas or new approaches to try. For me it needed constant rework and/or reinivention to progress, right up to passing the very last tests. I think it is an excellent problem encompassing a wide range of programming and mathematical concepts along the way - it is also entirely fair and achievable although I doubted that many times. Big respect to the setter for pushing the problem so far from the original.