Please, stop mark this code as a "Best Practice". It hasn't nothing in common with adequate coding practices. :)

Actually, the absolute majority of programming topics are more or least hard before you spent some time and mental efforts to dive into them. For this particular subject I recommend you "Mastering Regular Expressions" by Jeffrey E. F. Friedl, as a good fundamentals. 👍

But only for 32-bit signed numbers (from -2^31 to 2^31 - 1). Out of this range the result will be different from expected.

"i" -- point-free? :)

💥👍

You are right, in general. But theoretically, if we are in environment which is restricted by RAM, additional memory allocations sometimes are not desired. Secondly, perhaps authors supposed that caller code is responsible for immutability, by cloning passed arrays, for example. Lastly, cleverness also counts, and sometimes we're sacrificing best practicing for higher cleverness (less code in current case). 😉

I think that one of the characteristic of well done kata is when it can be solved in different clever ways. Thanks for great kata! 👍