Objective C: Modulo bias

Question

Using

   value = arc4random() % x

How can I avoid or eliminate modulo bias?

Thanks all.

BTW, at least according to Wikipedia, modulo bias is an issue when programming games of chance.

Answer 1

arc4random returns a 32-bit unsigned integer (0 to 2³²-1).

There will probably be no noticable modulo bias for small enough x. However, if you want to be really sure, do this:

y = 2^p where 2^p-1 < x ≤ 2^p

val = arc4random() % y;
while(val >= x)
    val = arc4random() % y;

Answer 2

If the maximum value of arc4random mod x is greater than x, ignore any values larger than the largest arc4random-max mod x, calling arc4random again instead.

Answer 3

u_int32_t maxValue = ~((u_int32_t) 0);      // equal to 0xffff...
maxValue -= maxValue % x;                   // make maxValue a multiple of x
while((value = arc4random()) >= maxValue) { // loop until we get 0 ≤ value < maxValue
}
value %= x;

although unless you are using any x under a million (or more) I wouldn't worry about it