Generic sort functions to be used with set_dupsort() or set_compare().

[mneumann]

Use it like this (for every type that implement FromMdbValue and Ord):

db.set_compare(lmdb::sort::<u64>);
db.set_dupsort(lmdb::sort_reverse::<u64>);

[vhbit]

It would be great to have a test case for it as there is a unsafe conversion from Ordering to u8, even though it's very unlikely will change soon

[mneumann]

Please see my second commit. There, I assert that the ordering conversion is as expected.

[vhbit]

As you use u64 as an example how to use those functions may I also ask you to add a doc comment for them with a note that LMDB has a smart internal integer sorting (it doesn't depends on size of integer), which means if the key has a fixed size it's preferred to set DbIntKey flag instead of setting a custom sort function?

I apologize I haven't though of it earlier.

[mneumann]

I will add a doc comment, but I don't know much about LMDB's internal integer sorting. I only expect it to work for usize and probably u32. Would it work for u16 or u8, too? Where is this documented in LMDB?

Btw, I found out that when you want to sort keys that are > u64 (for example a (u64, u64) tuple), you can do that by encoding it as big-endian and use the string sort of LMDB. This was before you added set_cmp() and set_dupsort() functions, which would be the prefered way.

[vhbit]

I've actually expected it to work only with u32/u64 depending on system but here is a clarification, see MDB_INTEGER section

which would be the preferred way

Tuples are interesting case, I'd try to go with LMDB builtin int comparison