Introduce Valhalla costing options

[Max-Leopold]

I was looking into how to add some custom costing options for Valhalla routing and noticed a TODO comment to add more tunable parameters so I decided to give it a shot. I'm a bit rusty in Rust and normally don't work with Kotlin so please excuse any obvious errors.

[ianthetechie]

Thanks for this PR @Max-Leopold! This is actually something that's been on my list of things to do for a while. I'll give a bit more thought to this tomorrow. The wall that I hit previously (which made me punt on it till later) is that the values can actually be more than strings, and I wasn't sure the best way to surface this in a way that crosses the FFI boundary well (JSON doesn't actually transfer directly).

Here's a stream of consciousness brain dump of the approaches I've considered so far:

• It's actually possible that Valhalla gracefully converts strings to a relevant type... I'll have to check into that; not super hopeful TBH, but if so, then your PR looks like it'd probably do the job!
• Punt on the arbitrary options for now and just add the parameters you need. This is basically what we're doing so far, but it doesn't live up to my aspirations for extensibility.
• Swinging the other direction, we have an OpenAPI spec for costing options in here: https://api.stadiamaps.com/openapi.yaml. This would give a really nice strongly typed interface for all standard parameters.
• Some hybrid builder pattern... Internally we can represent this as a JSON object. But maybe we need to expose a few different methods to set "key paths."
• Allow more dynamic versions like [String: Any] at the platform layer (Swift / Kotlin) and use JSON strings to cross the FFI boundary (need to think about communicating failure in these cases though).

The last one might actually be the last practical in the interest of moving quickly... curious if you have any opinions.

[ianthetechie]

We had a discussion about this on our weekly call with the core devs and we're all leaning toward the last option. I'll give it a pass (probably this weekend) to see if it works out later and, if it works out, will make the modifications on your branch.

[Max-Leopold]

Hey @ianthetechie, sorry for only answering now. Thanks for having a look. I had a similar thought process to you and my hope was that Valhalla would just convert the strings to the appropriate type. Thats why I just implemented it as a String: String map. However, I never actually tested that hypothesis 🤦‍♂️

Allow more dynamic versions like [String: Any] at the platform layer (Swift / Kotlin) and use JSON strings to cross the FFI boundary (need to think about communicating failure in these cases though).

Using a String: Any was what I wanted to do initially but I wasn't sure how to represent the Any type in Rust. Using a JSON string to pass the map from Swift/Kotlin to Rust makes sense. I agree that this sounds like a sensible approach 👍

I won't have much time to look into this further over the next couple of days so please feel free to poke around. Otherwise I might pick this up again end of next week.

[Max-Leopold]

I just had a bit of time in my lunch break so did a first try of using a JSON string to pass FFI instead of the String map. Let me know if this is what you had in mind. As I said, I won't have time the coming days so please feel free to modify the PR.

[ianthetechie]

Wow, thanks @Max-Leopold! Yes, this is what I had in mind! I'll review properly over the weekend to make sure I haven't missed anything, but I'm quite optimistic about getting this merged quickly :) And also quite happy to have the feature for my own app, as I have recently realized that AVSpeechSynthesizer + Valhalla voice instructions as they are today do NOT do well with mixed-language results, so I'd rather just change the language on my requests haha.