Add min_by, max_by

[kklingenberg]

This PR is a naive attempt to implement the missing min_by and max_by filters. I'm unsure about the part on run_if_ok and checking whether an error happened after reduction, but in the end I decided to imitate the sort_by and group_by functions on that, since I'm not very sure about how things work in the library, generally.

I was in doubt about sending the PR because this seems like a pair of functions that could have been implemented directly in std.jq using a map-reduce approach, but having tried that and benchmarked against gojq, I saw that it had to be provided from Rust to be comparable. In the end, and since I don't have much of a clue on how to make these things fast, I "ported" my jq implementation to Rust 😅.

Hopefully this is useful.

[01mf02]

Hi @kklingenberg, thanks a lot for your PR!
I discovered a case where your implementation diverges from jq;
[{a: 1, b: 2}, {a: 1, b: 3}] | max_by(.a) | .b yields 3 in jq, but 2 in jaq.
Furthermore, it would be good to add a test where the function passed to max_by yields multiple outputs.
When you address both these issues, I will be happy to merge your PR.

[kklingenberg]

Hello @01mf02 ! I've updated the PR and it should now follow your suggestions. Thank you! And thank you again for building jaq!

[01mf02]

I made some benchmarks myself, and compared your native implementation with:

def min_by(f): reduce .[] as $x (.[0] | [., [f]]; ($x | [f]) as $y | if $y < .[1] then [$x, $y] else . end) | .[0];
def max_by(f): reduce .[] as $x (.[0] | [., [f]]; ($x | [f]) as $y | if $y < .[1] then . else [$x, $y] end) | .[0];

The filter [range(5000000)] | min_by(.) terminated in about 1.2 seconds with your native implementation, whereas the implementation by definition took about 10 seconds. This is clearly in support of having a native implementation, even if it adds quite some code.

[01mf02]

I reworked your implementation by making it abort on the first error, instead of continuing until the end of the array and only then yielding the error.
Furthermore, because the tests are now for a native function, I moved most of them to jaq-core/tests.
Thanks again for your contribution!