Ranking cannot be changed

This depends strongly on the language. With e.g. rust, the solution is trivial as the language provides a built-in bit counting function.

visible in the description examples

Yes takes a lot more knowledge of other functions and methods not yet explored.

I agree.

a ha

Please emphasize, that one must count exactly %w[like] and more tests. Nice kata

I don't know why this wasn't approved, my solutions:

a.reject{|x|x==0} + a.reject{|x|x!=0}
a.reverse.inject([]){|arr,o| o ==  0 ? arr.push(0) : arr.unshift(o); arr}

Benchmark:
Rehearsal ----------------------------------------------
reject 0.000000 0.000000 0.000000 ( 0.000016)
inject 0.000000 0.000000 0.000000 ( 0.000032)
select 0.000000 0.000000 0.000000 ( 0.000014)
------------------------------------- total: 0.000000sec

I don't think this kata costs 6kyu, 7 maybe?

I added some emphasis there. Thanks for the feedback!

Yes, that is a great idea. I Will change it tomorrow, I am currently travelling. Thanks for the link!