Complete Permutations::size_hint
#739
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Known/Overflow variants have the advantage of the readability but it's less handy than options. A doc comment seems enough to me to say that None means it would overflow.
Clearer. And `prefill` extending the buffer seems better performance wise than push items to the buffer one by one with `get_next`.
We are at the very beginning. We prefilled `k` values, but did not generate any item yet. There are `n!/(n-k)!` items to come (safely done in remaining). But `n` might be unknown.
We generated `prev_iteration_count` items (`n` still unknown). So the same as `PermutationState::StartUnknownLen` minus `prev_iteration_count`.
Ideally, the lower bound of the size hint would remain `usize::MAX` but I don't see how we could make this happen. Small bugfix: `n+1` might overflow.
jswrenn
approved these changes
Sep 6, 2023
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@phimuemue
This series of size hint/count improvements ends where it started: with a TODO I wanted to fix.
The end user will mostly enjoy that
(0..n).permutations(k).map(...).collect_vec()will now allocate to the resulting vector in one go (PermutationState::StartUnknownLencase).In the first commit, you probably will want me to unwrap but
panic!(message)is more similar toexpectthanunwrap. Which is why I previously usedexpecttoo.The 2nd commit (about
enough_vals) is off topic but I really don't see why it would not be a (minor) improvement.I have a test
permutations_inexact_size_hintsready if you want (similar tocombinations_inexact_size_hints).