Polygon.swift

import MapKit

struct Polygon {
    let coordinates: [CLLocationCoordinate2D]
    let mapPoints: [MKMapPoint]
    
    init(coordinates: [CLLocationCoordinate2D]) {
        self.coordinates = coordinates
        self.mapPoints = coordinates.compactMap(MKMapPointForCoordinate)
    }
    
    // http://www.faqs.org/faqs/graphics/algorithms-faq/ 2.03, How do I find if a point lies within a polygon
    func contains(coordinate: CLLocationCoordinate2D) -> Bool {
        guard CLLocationCoordinate2DIsValid(coordinate) else { return false }
        let p = MKMapPointForCoordinate(coordinate)
        var isInside = false
        var j = mapPoints.count - 1
        for i in 0..<mapPoints.count {
            if ((mapPoints[i].y > p.y) != (mapPoints[j].y > p.y)),
                (p.x < (mapPoints[j].x - mapPoints[i].x) * (p.y - mapPoints[i].y) / (mapPoints[j].y - mapPoints[i].y) + mapPoints[i].x) {
                isInside = !isInside
            }
            j = i
        }
        return isInside
    }
}