Please support custom allocators and runtime comparators

[jeaye]

Hi! I hope you're doing well. Thanks so much for making BppTree.

As mentioned here, I've integrated BppTree into jank, the native Clojure dialect on LLVM. There are two main blockers for shipping jank with BppTree.

1. BppTree doesn't support custom allocators, while jank is using a GC; this means that any BppTree allocated in a GC allocated object will never be destructed, which will result in leaks; if BppTree were to support custom allocators, the GC would then be able to follow those pointers and clean things up as needed. Note that immer uses policies for this to good effect: https://sinusoid.es/immer/memory.html#example-tracing-garbage-collection.
2. BppTree doesn't support runtime comparators. That is, we may know the type of a comparator at compile-time, but not the value of the function itself. This is necessary to support functionality such as sorted-map-by, in Clojure.

Is there any interest in supporting these?

[jeffplaisance]

I would be interested in having BppTree support these features as long as they come at zero cost if they are not used. Specifically I'd want to be sure that the object sizes aren't increased in order to store the allocator and/or comparator if the allocator and/or comparator are stateless.

Between my kids and job it will most likely be several years before I have time to work on this, so if you want it any time soon you should probably implement it yourself.