done

sorted by their occurence

should be

sorted by their occurrence

OK, let's do it.

Thanks! I think I fixed all mentioned.

I've added complete test set.

IIHHHH -> HI  # H: 3; I: 2;

I see 4 Hs there. Some other typos:

occurences -> occurrences
more then -> mora than
perforamnce -> performance
unmutable -> immutable

Unless it's a puzzle type kata (which this one isn't), the tests should encompass most of the specification. The point of Codewars isn't to write your own tests.

Also, you should take inspiration from katas created after 2018. Many of the ones created before 2017 would not be good enough in their current state if they were to be approved today.

Please share your opinion how kata should work (and why :-)).
From what I think you mean I tend to disagree.
There seem to be many variants of kata on this site.
The one with only initial tests seem quite common and I liked them.
But I may be biased, I didn't solve that much of them.
I think it involes some good practices if you have to write your own tests (a little like TDD approach).

Reraise the issue below.

Thanks but you still didn't address the first part of the issue.

I've added a note that input is correct in context of the cipher algorithm. Thanks for checking this kata out.

Ah ok, I may have missed it, but if not, then you should probably make that clear in the description.

Such case will never happen. :-)

Writing your own test cases is part of the work on the solution (tests are implementation)

imo, that's not really how katas should work.

For the second comment, I mean if you have the code: BBAA, should the correct answer be BA or AB, or will a case like this never happen?

Increased the tests to 110