(I made rkyv) I took a look at lite3/tron because I was interested in seeing how it approached some of the issues I ran into with rkyv. I ended up with some additional information related to points 1 and 2 I figured I'd write down:

1) You're correct, rkyv uses a B-tree with a default branching factor of 6 [1]. I don't think I've ever implemented a binary tree in any version of rkyv - maybe one of the earliest versions used a sorted vec instead of a B-tree? I hate the B-tree map implementation because it is so complicated, even though it actually used to be even worse in 0.7 when it was implemented as a B+ tree. Sorry for making you read any of it!

2) Arbitrary mutations would definitely be an improvement on rkyv, and I was most interested to see what kind of approach lite3/tron took! Unfortunately growing the buffer is not supported [2] and freed memory isn't reclaimed [3]. These are the two actually difficult problems standing in the way of rkyv getting more comprehensive mutation support, so unfortunately I think there's nothing valuable to take away here.

The issue with growing the buffer is that you invalidate any pointers into the buffer and have to manually fix them up (e.g. by re-crawling the buffer). There's not a decent approach I'm aware of which manages these pointer fixups in a way that limits complexity and avoids really nasty API surfaces.

The issue with memory reclamation is that you effectively have to add an allocator to your buffer, and that allocator has to also be persistable. I actually got this part working in rel while that was still a project worth hacking on [4]. I just ran out of steam and willingness to play the type-tac-toe required to get a safe API out of it. The main problem I ran into there was related to value branding, since you shouldn't be allowed to make references to objects which cross between contiguous buffers.

This project is neat, but it does basically just look like an rkyv-converted `BTreeMap<String, JsonValue>`.

[1]: https://github.com/rkyv/rkyv/blob/main/rkyv/src/collections/... [2]: https://github.com/fastserial/lite3/blob/main/src/lite3.c#L5... [3]: https://lite3.io/design_and_limitations.html#autotoc_md29 (see note at end of section) [4]: https://github.com/rkyv/rel/blob/main/rel_example/src/main.r...