Random sampling in Phobos

[jerro]

> Hmmm... I see now that randomSample() is defined without ref in 
> Phobos as well.  Would you care to check whether my suggestion 
> fixes your problem?  If so, it is a bug in Phobos that should 
> be fixed.
>
> -Lars

It is a bug in Phobos but it isn't related to passing by value
(see the thread in D.learn).

I agree that copying random generators is error prone but
randomSampler taking the generator by ref wouldn't be enough
to fix that problem. The generator gets saved in a member in
a RandomSampler struct, so that member would have to be a
pointer to avoid copying (so RandomSampler would have to be
instantiated with Random* instead of Random). I usually avoid
this kind of problems by using pointers to random generators
instead of random generators everywhere.