On 05/06/07, Peter Dimov <pdimov@mmltd.net> wrote:
Jos Hickson wrote:
On 04/06/07, Andy <atompkins@fastmail.fm> wrote:
I am favoring number 4, unless it will not produce good uuids. Then I favor number 5.
I guess I am looking for direction as to how to proceed.
Andy.
I prefer number 5. All you need do then is provide a get_seed() method that generates a decent seed; with number 4 it might well take a lot more time to implement and test a good source of random numbers and then it might add a extra dimension to maintaining the library. It seems to me that Boost.Random exists then it should be used by default as the source of random number generators unless, of course, what it provides is not good enough but I presume that is not the case.
Number 5 is a good solution for batch UUID generation, assuming that the RNG is seeded well.
That said, I should note that a RNG is not really a source of randomness. Sure, it can be used to produce a sequence that appears sufficiently random for most computing purposes. For UUID generation, however, and specifically in the "one UUID per program" case, a RNG contributes exactly zero random bits. If you have a seed that contains K bits of entropy, and you generate from that seed an UUID by using a RNG, the UUID will contain exactly K bits of entropy. To put it differently, the probability that two seeds collide - 2^-K - is equal to the probability that two UUIDs collide, because equal seeds generate equal UUIDs.
In other words, if you have a "decent" 32 bit seed, you're looking at 2^-32 probability of collisions, no matter how many RNGs you use as an intermediate layer. If you have a decent 128 bit seed, you can use it as an UUID as is, and passing it through a RNG will not change anything. And conversely, if you don't have a good source of randomness to use as a seed, you will not be able to generate a good UUID, so it's not possible to eliminate the "extra dimension".
Understood. However, if the best implementation of uuid::create() depends on which of two usage patterns is assumed ("batch" or "one per program") maybe a better approach is to have both uuid_generator (for batch purposes) and uuid::create() (for the one-per-program usage) with the generator seeded using the randomness that uuid::create() uses. Jos