feat: add dup_sort_comparator

[oXtxNt9U]

Pull Request

What does this PR do?

Expose http://www.lmdb.tech/doc/group__internal.html#gacef4ec3dab0bbd9bc978b73c19c879ae via new dup_sort_comparator function to set a custom Comparator.

It basically works like key_comparator, except that for calling mdb_set_compare it calls mdb_set_dupsort on the database.

It can be used like this:

enum DescendingIntCmp {}

impl Comparator for DescendingIntCmp {
    fn compare(a: &[u8], b: &[u8]) -> Ordering {
        b.cmp(&a)
    }
}

//

let db = env
    .database_options()
    .types::<Str, U128<BigEndian>>()
    .flags(DatabaseFlags::DUP_SORT)
    .dup_sort_comparator::<DescendingIntCmp>()
    .create(&mut wtxn)?;

Also added an example and updated the cookbook (resolves #284).

[Kerollmops]

Hey @oXtxNt9U 👋

They're already is a way to use custom comparison functions in a safe way in heed. You can see that in the database open options, you just have to use the Compare trait.

However, it was hard, even for me, to find that in the documentation. Would you mind adding a small guide (in the top three) to the cookbook to show people that we support it and how to use it? There already are some examples in this PR (whoops, nope).

Have a nice day and please close this PR if I was right 😊

[oXtxNt9U]

@Kerollmops Thank you for pointing out the Compare trait. Upon closer inspection, it only calls mdb_set_compare but what I need is mdb_set_dupsort because only then it passes the values to the compare function instead of the keys.

Basically, what I need is something like this inside raw_open_dbi:

unsafe {
    mdb_result(ffi::mdb_dbi_open(raw_txn, name_ptr, flags, &mut dbi))?;
    if TypeId::of::<C>() != TypeId::of::<DefaultComparator>() {
        if AllDatabaseFlags::from_bits(flags)
            .is_some_and(|f| f.contains(AllDatabaseFlags::DUP_SORT))
        {
            mdb_result(ffi::mdb_set_dupsort(
                raw_txn,
                dbi,
                Some(custom_key_cmp_wrapper::<C>),
            ))?;
        } else {
            mdb_result(ffi::mdb_set_compare(
                raw_txn,
                dbi,
                Some(custom_key_cmp_wrapper::<C>),
            ))?;
        }
    }
};

However this makes it impossible to use a different compare for both keys and values since it reuses C. So I could update my PR to introduce a new type parameter CDUP which if present would hook up a custom mdb_set_dupsort.

let db = env
    .database_options()
    .types::<StorageKey, StorageValue>()
    .dup_sort_comparator::<CustomComparator>() // sets CDUP
    .flags(DatabaseFlags::DUP_SORT)
    .create(&mut wtxn)
    .expect("db");

What do you think?

[Kerollmops]

So I could update my PR to introduce a new type parameter CDUP which if present would hook up a custom mdb_set_dupsort [..] What do you think?

Yeah! That could be much better. Also, not breaking APIs could be great too but may be impossible.
I also created an issue about documenting the custom comparison functions more, if you want to take a look.

[oXtxNt9U]

@Kerollmops Hey, I updated the PR and included examples for the cookbook too. Please let me know if anything is still missing.

[oXtxNt9U]

@Kerollmops Friendly ping! 😄