randomCover not so random?

[dsimcha]

== Quote from Andrei Alexandrescu (SeeWebsiteForEmail at erdani.org)'s article
> dsimcha wrote:
> > Maybe I'm doing something subtly wrong here, but I've checked all the obvious
> > stuff.
> >
> > import std.stdio, std.random, std.array;
> >
> > import dstats.base : Perm;
> >
> > void main() {
> >     uint[int[]] counts;
> >     immutable int[] foo = [1,2,3];
> >
> >     // Make sure every permutation is represented in counts.
> >     foreach(perm; Perm!int(foo.dup)) {
> >         counts[perm.dup] = 0;
> >     }
> >
> >     foreach(i; 0..10_000) {
> >         int[] result;
> >         foreach(elem; randomCover(foo, Random(unpredictableSeed))) {
> Here you reseed the random number generator every pass through the outer
> loop. Although unpredictableSeed is designed to be unpredictable, it is
> _not_ random.
> You'd need to have each cover use the same random generator. Here's
> where a shortcoming in the design of randomCover becomes evident:
> randomCover stores a copy of its generator, which makes it difficult to
> support what you need. I will look into a fix.
> Andrei

Actually, I meant to mention in the original post that I thought of this already
and it's not the problem.  When I re-seed the generator every time but use
randomShuffle, I get something pretty uniform looking.

Furthermore, even after I allow a few more clock ticks to go by and re-run the
program, I still get the same thing when using randomCover.  I also tried things
like keeping a Mersenne Twister outside the loop and using successive iterations
of that to seed the generator that I pass to randomCover.

I actually did find the real problem, though.  It's really simple and I've filed
it in Bugzilla.  See http://d.puremagic.com/issues/show_bug.cgi?id=2865 .

Also, since I remember the discussion a while back on how to make randomCover as
efficient as possible, why are we using bool[] instead of BitArray for it?