IIFE & NFE & recursion

According to the description "The key has to be shortest of all possible keys that can be used to code the message: i.e. when the possible keys are 12 , 1212, 121212, your solution should return 12.". 19391 and 1939 may be used as a key. 1939 is shorter. I hope it helps.

That kata is a third one in a series. Start from the first one and it will be clear.

I see you meant in the tags, then yes.

This is O(n) for the initial count (first line map/reduce) and then O(n) again for the summarisation, for a total complexity of O(n) where n is the total length of the input.

It's not really any faster than most of the solutions to this problem, however (including yours). At this level it's far less about the Big O complexity of the algorithm and far more about the constant factors and language/algo-specific performance characteristics.

Your solution has a bug and passes unexpected values into some of predicates. It's not a kata issue.