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))) {
            result ~= elem;
        }
        counts[result]++;
    }
    foreach(k, v; counts) {
        writeln(k, "\t", v);
    }
}

Output:

2 3 1   0
3 2 1   0
1 3 2   0
3 1 2   0
1 2 3   4929
2 1 3   5071

Note that when I use randomShuffle instead of randomCover, I get a fairly uniform distribution on the permutation space.

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

== Quote from Andrei Alexandrescu (SeeWebsiteForEmail@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?