rndGen.position<label>(0, XXX) is not evenly distributed

When using the

rndGen.position<label>(0, nDomains - 1);

to determine random decomposition it seems the entries 0 and nDomains-1 have only half the number of entries.

Replicate: run randomDecomp on a scaled up cavity (e.g. 1000x1000).

Is because it rounds. Anywhere 0.5 to 1.5 becomes 1 (so interval of size 1) except for first and last interval that are only size 0.5.

So extend the range to include some of the -0.5 and (N-1) + 0.5 range as well?

template<>
Foam::label Foam::Random::position(const label& start, const label& end)
{
    return start + round(scalar01()*(0.98 + end - start) - 0.49);
}

mentioned in commit 659e56ea8a329180dde995f772e1083539a75958

closed

Maybe floor instead:

floor(end-start+1)

We don't guarantee that scalar01* is always <, it could be = after floating point operations (?) so we'd need an additional check on that.

reopened

mentioned in commit edd410a744e16eee3aff69fcc91ba43917157da3

closed via commit 8e11cf58b5970a70f13cbb5c02b6e3d931f019c3

@kvm - coincidentally Mattijs stumbled across the same. I've patched in master and pre-release, using the logic in 8e11cf58b5970a70f13cbb5c02b6e3d931f019c3

mentioned in commit 8e11cf58b5970a70f13cbb5c02b6e3d931f019c3

mentioned in commit f0806d5d