Calculate the distance of two geo points.

distance_between.js

function distanceBetween(startPoint, endPoint) {
    console.log(startPoint, endPoint)
    let earthRadius = 6371.0

    let startRadians = [
        startPoint.latitude * Math.PI / 180,
        startPoint.longitude * Math.PI / 180
    ]
    let endRadians = [
        endPoint.latitude * Math.PI / 180,
        endPoint.longitude * Math.PI / 180
    ]
    let latitudeDeltas = endRadians[0] - startRadians[0]
    let longitudeDeltas = endRadians[1] - startRadians[1]

    let a = Math.pow(Math.sin(latitudeDeltas / 2), 2) + Math.cos(startRadians[0]) *
        Math.pow(Math.sin(longitudeDeltas / 2), 2) * Math.cos(endRadians[0])
    let c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))

    return c * earthRadius
}

distance_between.swift

// https://github.com/alexreisner/geocoder/blob/master/lib/geocoder/calculations.rb#L84
// https://en.wikipedia.org/wiki/Haversine_formula
private typealias GeoPoint = (latitude: Float, longitude: Float)
private func distanceBetween(_ startPoint: GeoPoint, _ endPoint: GeoPoint) -> Float {
    let earth_radius = 6371.0

    let startRadians = [
        startPoint.latitude * .pi / 180,
        startPoint.longitude * .pi / 180
    ]
    let endRadians = [
        endPoint.latitude * .pi / 180,
        endPoint.longitude * .pi / 180
    ]
    let latitudeDeltas = endRadians[0] - startRadians[0]
    let longitudeDeltas = endRadians[1] - startRadians[1]

    let a = pow(sin(latitudeDeltas / 2), 2) + cos(startRadians[0]) *
        pow(sin(longitudeDeltas / 2), 2) * cos(endRadians[0])
    let c = 2 * atan2(sqrt(a), sqrt(1 - a))
    return c * Float(earth_radius)
}