This kata is very poorly defined. A lot of things are left out and, although it is not difficult, it involves a lot of trial and error to figure it out it is either a certain code convention to be followed or implmentations details. This exlplains the low number of solutions.

Hmm, I swear from now on I would let pass at least one hour between unfruitful attempts and posting a question. You are exactly right! My bad and thanks!

I am dying in here! I get 3 tests failing always a diff of 1. I cannot figure it out. For example upsidedown('86199', '32414333') should return 547 according to unit tests. My implementation returns 548 and for the life of me if I see where I am wrong! Consider this:

1. 86199 (len = 5): 23 combinations possible greater or equal the given number (I even created them by hand)
2. XXXXXX (len = 6): 100 combinations
3. XXXXXXX (len = 7): 300 combinations
4. 32414333(len = 8): 125 combinations possible less or equal the given number (it is equivalent with 1XXXXXX1 which is 1 * XXXXXX/4 * 5)

Total 548. How is this 547?