Sorting methods for Slice variant about soa-derive

[Luthaf]

I agree that sorting is a worth adding to the generated code!

However I would prefer doing something which sorts based on all fields by default, so that sorting XXXStructVec produces the same output as sorting a Vec<XXXStruct>. Then sorting based on a single field can be implemented through sort_by and sort_by_key methods.

We might be able to use the permutation crate here (although adding external dependencies in a proc-macro is tricky. The crate should be added as a dependency of soa-derive, and soa-derive-internal should generate calls to soa_derive::permutations).

I think we should be able generate code that looks like this, making sure StructRef implements PartialOrd

fn sort(&mut self) {
    use soa_derive::Permutation;

    let mut permutation: Vec<usize> = (0..self.len()).collect();
    permutation.sort_by_key(|i| self.index(i));
    
    let permutation = Permutation::oneline(permutation);
    permutation.apply_slice_in_place(&mut self.bar);
    permutation.apply_slice_in_place(&mut self.baz);
}

[Christopher-S-25]

After some experimentation, I found that HRTB works perfectly for this (code includes minor corrections):

impl<'a> FooSliceMut<'a>
where
    for<'b> FooRef<'b>: Ord,
{
    fn sort(&mut self) {
        use soa_derive::Permutation;
    
        let mut permutation: Vec<usize> = (0..self.len()).collect();

        // the trait `SoAIndex<FooSlice<'_>>` is not implemented for `&usize`,
        // but it is implemented for `usize`, so we dereference `i`
        permutation.sort_by_key(|i| self.index(*i));
        
        // We need to invert the permutation in order to sort the slices correctly
        let mut permutation = Permutation::oneline(permutation).inverse();
        permutation.apply_slice_in_place(&mut self.bar);
        permutation.apply_slice_in_place(&mut self.baz);
    }

    // ... Other sorting methods ...
}

Since sort_by_key() requires FooRef to implement Ord, it should be easy enough to obtain the sorting methods by using this attribute: #[soa_attr(Ref, derive(Ord, PartialOrd, Eq, PartialEq))], or by manually implementing (some of) the traits for more complex cases.

I will try to convert it to macro code, and create a PR.