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

From: Jos Hickson (jos.hickson_at_[hidden])
Date: 2007-06-12 05:24:34

On 11/06/07, Andy <atompkins_at_[hidden]> wrote:
> "Jos Hickson" <jos.hickson_at_[hidden]> wrote in
> news:fc03d05a0706060823v59628c64xd1bcda61e9fd5f48_at_[hidden]:
>
> > On 06/06/07, Peter Dimov <pdimov_at_[hidden]> wrote:
> >> Jos Hickson wrote:
> >> > On 05/06/07, Peter Dimov <pdimov_at_[hidden]> wrote:
> >> >> [snip] ... and if Engine
> >> >> is a template parameter, we probably need to support engines that
> >> >> only take a 32 bit seed anyway.
> >> >
> >> > Isn't it the case that the maximum seed size is related (equal?) to
> >> > the difference between the max and min values produced by a
> >> > particular engine and also that all the engines in Boost Random
> >> > provide the iterator version of seed?
> >>
> >> The seed for mt19937 is 624 words.
> > Aah, a bit bigger then!
>
> I have been thinking about this for awhile now, and maybe I should use
> Peter's suggestion and only support random number generators that can be
> seeded with a single number.  And xor with rd_.
>

IMHO such a restriction would be to the detriment of the library
whereas the solution below would seem a lot better to me.

Really what we need is a Boost.Seed library to compliment Boost.Random
so that anybody like yourself using the latter doesn't have to spend
all this time wondering how to seed things generically.

> I have been playing with another solution.  I wrapped the seed function
> in a class that models the random number generator concept.  Then used
> Boost.Iterators' function output iterator, and feed it into the seed
> function that takes iterators of the random number generator.
>
> // not that this is not quite complete
> class seed_rng
> {
> public:
>   seed_rng() : rd_index_(5) {}
>
>   unsigned operator()() {
>     if (rd_index_ >= 5) {
>       sha1_random_digest_(); // fill rd_ with new data
>       rd_index_ = 0;
>     }
>     return rd_[rd_index_++];
>   }
> private:
>   sha1_random_digest_();
>
>   unsigned rd_[5];
>   int rd_index_;
> };
>
> template <typename Engine=boost::mt19937>
> class uuid_generator {
> public:
>   uuid_generator() {
>     boost::seed_rgn e;
>     boost::generator_iterator<boost::seed_rng> begin(&e);
>     boost::generator_iterator<boost::seed_rnd> end;
>     en_.seed(begin, end);
>   }
>
>   boost::uuid operator()() {
>     // get random data from en_
>     // return uuid based on this data
>   }
> private:
>   Engine en_;
> };
>
> Now this works fine, the slow seed_rng is used to fully seed the random
> number generator used.  It can be specialized for boost::random_device
> since boost::random_device does not have a seed function.
>
> Andy.
>