Powerset count

[Philippe-Cholet]

@phimuemue After counting combinations last week, I thought a simple PR would be nice to start the week.

The first three commits are about Powerset::count while the next four commits are about Powerset::size_hint.

[Philippe-Cholet]

Powerset::count needs n = pool.count() and self.combs.count() which also needs n. Without filling the lazy buffer, we can not compute the real n twice. That's why Combinations::n_and_count exists, to be able to re-use n.

I replaced Powerset::size_hint implementation (and removed the pos field since it is its only purpose) because it removes the fallback (when self.pos overflows, which seems unlikely to me but who knows).
Then I was unsure what to do with size_hint::pow_scalar_base: remove it or allow it to be dead code? I temporarily chose the second.
Feel free to cherry-pick the first three commits if you don't like my size_hint changes.

[phimuemue]

Hi @Philippe-Cholet, thanks for this. I really appreciate the small changes.

I think the computations are correct, although non-tivial. And in some cases I wondered if powerset.count/size_hint could more clearly delegate to combinations.count/size_hint, but so far I cannot point my finger exactly where that could easily be achieved.

However, please add some tests that also test combinations for iterators with unreliable size hints (as it stands, the underlying iterator's in the tests seem to have exact size_hints, so maybe use a filtered one).

[Philippe-Cholet]

Tomorrow, I'll add tests with (0..5).filter(|i| i % 2 == 0).chain(5..10).combinations(k)/powerset() which will have nice size hints. I also thought it would be interesting to test.
Otherwise, I think I answered all points.

[Philippe-Cholet]

I can do a similar test for powerset if you want but I'm gonna wait for your feedback first.

[phimuemue]

Thank you a lot!

bors r+

As for the new tests: I’m not sure if they too closely reimplement the internal logic and assert that the reimplementation matches the original.

I will have a closer look later. I think what I’d test is that the size hints do not become only more exact over time and that they are consistent with count.

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