Mostly habit. The safety of not depending on null terminators and instead being explicit.

Any comments about building up a series of katas which get progressivly harder? Like caesars cypher which just takes lower case words, and then one which takes capitals and punctuation, and then one which is rotX instead of rot13?

As a series, they'd be pretty easy steps from one to the next. On one hand that's good for teach people, on the other, it's like farming rep on here.

overflow. That's more than 20 char.

Except brute force DOES work. On my hardware, you have to use parameters in the 100,000 range before it takes a minute. 10,000 is done in 5 seconds. As is, this is a 7 or 8 kyu. If you want it to be a 4 kyu, all that needs to be done is to add test cases where the brute force times out the server and you have to optimize to have a complete solution.

ooooOOOOOOoooh, man I was wondering what the hell they were smoking betwee that first equation and the second. Thanks.