To the StackOverflow topic I previously linked to, someone suggested the function below.

To be honest I don't understand how it works, the code comments seem to be in discordance with the code itself.

It appears to me that the code goes in Undefined Behavior territory.

`/*`

* Calculate a uniformly distributed random number less than upper_bound

* avoiding "modulo bias".

*

* Uniformity is achieved by generating new random numbers until the one

* returned is outside the range [0, 2**32 % upper_bound). This

* guarantees the selected random number will be inside

* [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)

* after reduction modulo upper_bound.

*/

u_int32_t

arc4random_uniform(u_int32_t upper_bound)

{

u_int32_t r, min;

if (upper_bound < 2)

return 0;

/* 2**32 % x == (2**32 - x) % x */

min = -upper_bound % upper_bound;

/*

* This could theoretically loop forever but each retry has

* p > 0.5 (worst case, usually far better) of selecting a

* number inside the range we need, so it should rarely need

* to re-roll.

*/

for (;;) {

r = arc4random();

if (r >= min)

break;

}

return r % upper_bound;

}

Additionally, if the function is compiled on Pelles C, a warning about "unary -" pops up.

I've fixed that by using `0-x` instead of just `-x` but I'm concerned I changed the meaning of the code.

Moderators: is there a way to enable syntax highlighting in code?