Raymond Moay Raymond Moay - 1 year ago 71
Swift Question

Does a larger number limit on Arc4random slows down loading speed?

Hi I'm new to programming here and I am curious if a larger limit number generator cause it to be slower when generating a random number? For example:

which one generates a random number faster?




Could it be the same speed?


Answer Source

Assuming your are talking about arc4random_uniform instead of arc4random since the later does not have an upper bound you can specify.

The answer is: it might!

Looking at the source and documentation:

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.

That means that the speed of the random number generation depends on the ratio between the upper limit of arc4random and the afore mentioned modulo remainder.

In your example:

2^32 % 10 = 4
2^32 % 1000000 = 967.296

Creating a random number using arc4random() one time yields for example 768.649 which is smaller than the second value. That means it would have to create a second random number calling arc4random() a second time for the later case - the first case is already done creating random numbers.

But that runtime difference is entirely up to change.

A second try might yield 1.316.166.055 on the first try which causes both calls to take the same amount of time.

Since arc4random is uniformly distributed the probability of the second one taking longer is somewhat close to 967.296 / 2^32 which is 0.00022521615 => in 0.02% of the calls the second one takes longer than the first one.

In this calculation I ignored the fact that theoretically the arc4random can even produce a number smaller than 4 which would trigger even the first call to requery). But it should still give you an understanding of how improbable a difference will be.

If you want to get the probably "slowest" call you would have to perform arc4random_uniform(2^31+1) which has a ~50% chance of failing on the first call to arc4random.

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