Here are some unscientific timings of generating 1 million UUIDs:
libcrypto: ~2.3 seconds
libbsd (arc4random): 0.7 seconds
random: 0.6 seconds
create and release 1 million NSObject instances: 0.3 seconds
So, the time taken by random() or arc4random() is negligible; comparable to
instantiating an object. libcrypto is slightly slower but probably not enough
to be a problem.
code:
int i;
NSLog(@"Start");
for (i=0; i<1000000; i++)
{
id a = [[ETUUID alloc] init];
[a release];
}
NSLog(@"Done");
On 2011-11-25, at 2:22 PM, Eric Wasylishen wrote:
> Hey, I updated the random number code in ETUUID.
>
> Now it will use arc4random if available - this function is declared in
> stdlib.h on BSD's, and it looks like an ideal function for us. It seeds
> itself using /dev/urandom, has a large state (I think it's 128 bytes or
> more), and is supposed to be fast. I believe it works something along the
> lines of: encrypt the internal state with the RC4 stream cipher, return the
> RC4 output as random bytes, and mix the output back in to the internal state,
> and repeat. If we wanted to we could use this function on linux as well by
> requiring the libbsd library, but I'm not sure if it's worth it.
>
> Platforms without arc4random will use libcrypto's RNG. This is also supposed
> to seed itself on platforms with /dev/urandom, and on Windows using the
> windows crypto API. My only concern is, if it isn't on one of those
> platforms, it returns an error instead of seeding itself with gettimeofday.
> I'm also not sure how fast/slow libcrypto's RNG is.
>
> The old code used srandom(int x) on non-BSD platforms. The big problem with
> using this to generate UUID's is, assuming sizeof(int)==4, you will only get
> 2^32 possible UUID sequences.In other words, the first UUID generated by
> ETUUID will be one out of a list of ~4 billion which you can easily
> precompute… this is clearly a bad property for a UUID generator to have.
>
> -Eric
>
>
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