geo

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The following code is blocked on https://github.com/rust-lang/rust/issues/51344. Relevant discussion happened in https://github.com/georust/rust-geo/issues/47.

pub struct Line<T> {
    start: (T, T),
    end: (T, T),
}

////////////

pub trait LengthAlgo<T> {
    fn length(line: Line<T>) -> T;
}

////////////

pub enum Haversine {}

impl<T> LengthAlgo<T> for Haversine {
    fn length(line: Line<T>) -> T {
        unimplemented!()
    }
}

pub enum Vincenty {}

impl<T> LengthAlgo<T> for Vincenty {
    fn length(line: Line<T>) -> T {
        unimplemented!()
    }
}

////////////

pub trait Length<T, Algo: LengthAlgo<T> = Haversine> {
    fn length(self) -> T;
}

impl<T, Algo: LengthAlgo<T>> Length<T, Algo> for Line<T> {
    fn length(self) -> T {
        Algo::length(self)
    }
}

////////////

fn main() {
    let line: Line<i32> = Line {
        start: (4, 6),
        end: (2, 23),
    };
    
    // This doesn't compile – error message says `Algo` type needs to be specified
    // let haversine_length = line.length();
    
    // This compiles, despite us not specifying the type for `Algo`
    let haversine_length = Length::<i32>::length(line);
}

There are two topics that need to be discussed here:

• What should the trait look like?
• What should the default algorithm be?

I agree about euclidean distance by default.

The trait here takes ownership of a Line. Would it be better to take a reference to 2 points? To prevent creating and discarding a Line struct each time? (or am I misunderstanding something about rust? 🙂)

On 4 June 2018 14:38:44 CEST, "Stephan Hügel" [email protected] wrote:

I would say the default should be Euclidean, since Geo deals primarily with planar geometry.

-- Sent from my Android device with K-9 Mail. Please excuse my brevity.

I have opinions about euclidean as a default, but short on time right now to expand.

Regarding taking ownership, you're right, it should take &self. Unfortunately can't make much progress here until my Rust issue is addressed :-/

PostGIS has a geography type (as opposed to it's older geometry) which stores everything in lat/lons and distance/area functions use a 'spherical/surface distance' and return values in metres. That's very useful in many cases.

Maybe a feature like that would be very useful for rust-geo, and avoid the "which should it be?" discussion?

As long as the underlying Rust issue remains open, I don't think we can make much progress on this, so I'm going to close.