Add degraded search

[nnethercott]

Fixes #105

Separates layer traversal logic from Reader to new struct Visitor. Cancellation is performed by checking an impl Fn()->bool in a hot loop inside Visitor::visit. We also add 2 new methods to the Reader api for degraded search by vector and by item.

[Kerollmops]

I am wondering if returning a tuple is the best option or if we plan to return other things and therefore returning a struct would be more future-proof? 🤔

[nnethercott]

returning a struct would be more future-proof

Doing something like below is certainly "nicer", but would require also updating the api's for QueryBuilder.by_vector and QueryBuider.by_item and not just their cancellable variants.

I kinda like how simple the methods are already, but I'm not sure if you guys have a preference at meili.

@dureuill thoughts ?

#[non_exhaustive]
pub struct HannoyResult {
  pub neighbours: Vec<(u32, f32)>,
  pub did_cancel: bool 
}

pub fn by_vector_with_cancellation(
    &self,
    rtxn: &RoTxn,
    vector: &'a [f32],
    cancel_fn: impl Fn() -> bool,
) -> Result<HannoyResult> { todo!() }

[Kerollmops]

Ho! Crap! I thought you were changing the output of the original search methods. Indeed doing that is great as it doesn't impact other users.

However, I was more into proposing something similar to the indexing part where we set the cancellation function with the builder and don't introduce any other API. The search methods are not returning the boolean you introduced. We can directly check if the search took too long at Meilisearch 👀

[dureuill]

Hello @nnethercott , thank you for taking this one ☺️

I much prefer the struct version, it is both more readable and more future-proof. I'm fine with the added methods, I don't think we'll need many much more in the future (cancellation is kind of the only support function I can think of)

I'm not exactly sure if I like the non-exhaustive: while I understand it from a semver perspective, I think I prefer the Meilisearch code to break at call sites when a new field is added, which will not be possible with the non-exhaustive AFAIU.

Thanks again, looking forward to this ❤️

[dureuill]

FYI @nnethercott we took the code of this PR and added it to the branch in #103 which we use in Meilisearch.