Implement compute_if_present

[voidc]

This pull request contains the compute_if_present method, which is the simplest one among the reservation-based methods mentioned in #12 since it does not require ReservationNodes.

I took most of the code from put and remove.
For now, I only added basic tests but I also want to port some of the Java tests.

[jonhoo]

This looks quite good! I left a couple of inline notes.

It makes me want to try to figure out how we might be able to unify the code for put, compute_if_present, remove, and maybe even some others. They all have roughly the same "outer" code.. But that's for another time.

[jonhoo]

@voidc Hmm, why do you want the semantics to be that it returns the new value rather than the old? Or, I guess, it could return both?

[voidc]

Hi! Thanks for the comments!
I changed the function to return the computed, new value so it matches the Java version.
But your are right, it could return both.

[jonhoo]

Ohh, I see, I missed that! Yes, I think returning references to both old and new would be good.

[domenicquirl]

@voidc Hi, I just submitted #35 to deal with #9 and noticed you are also matching against BinEntry::Moved and have copied over the corresponding safety comment. Would you mind to change the comment to reference the new safety statement (it's now on the definition of Moved) so we don't spread the FIXME further? 😊

[jonhoo]

@voidc Unless there's more you want to do with this, I think I'm happy to merge once you bring it up to date with master!

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 1.55%.
The diff coverage is 96.29%.

Impacted Files	Coverage Δ
src/iter/iter.rs	92.5% <ø> (ø)	⬆️
src/node.rs	45.45% <100%> (ø)	⬆️
src/map.rs	88.76% <96.15%> (+2.14%)	⬆️

[voidc]

Thanks for the approval! Feel free to merge.
Points of (future) consideration:

• Also return the old value (although this would mean being inconsistent with the other compute_* methods as they don't necessarily have an old value)
• Possibly unify code with replace_node and put.