Re: [boost] [uuid] uuid::create() function

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-06-05 16:57:10

Andy wrote:

...

> template <typename Engine>

We can supply a default here:

template<class Engine = mt19937>

It's still slightly less convenient to use

    uuid_generator<> g;

and I'm not quite sure why one would want to replace the default engine, but 
it's good enough, I guess.

> class uuid_generator
> {
>    unsigned rd_[5];
>    Engine en_;
>
>    uuid_generator()
>    {
>        init rd_;
>        seed en_ with rd_[4];
>    }
>
>    uuid operator()
>    {
>        generate a 128-bit number using en_;
>        xor it with rd_[0..3];
>        return a uuid based on it;
>    }
> };
>
> But, how does the xor part help?  It is just for the case where we are
> only producing one uuid?

The xor part just varies the sequence produced by the RNG by rd_[0..3]. The 
RNG seeded by the 32-bit value rd_[4] can produce 2^32 distinct UUID 
sequences. With the xor part - and assuming that the contents of rd_[] are 
random for simplicity - it will produce 2^32 * 2^128 distinct UUID 
sequences. The statistical properties of the UUID sequence will still 
correspond to the original 2^32 source, but this doesn't matter for our 
purposes since UUID's or their bits are not cross-checked for correlations 
with one another, only collisions matter.

In short, the above is a relatively easy way to approximate a RNG seeded 
with the entire contents of rd_ instead of throwing 4/5 of it away. You said 
earlier that you had a problem with the iterator range seed, and if Engine 
is a template parameter, we probably need to support engines that only take 
a 32 bit seed anyway. It also has the advantage of being "obviously" at 
least as random as just using the Engine, so this should help alleviate 
people's concerns. :-)