MKPolygon - How to determine if a CLLocationCoordinate2D is in a CLLocationCoordinate2D polygon?

Question

I have the swift code below that draws a polygon and drops an annotation on MKMapView. I am trying to figure out how i could identify if the annotation\'s coordinate is with

Answer 1

- (BOOL)isPoint:(MKMapPoint)point insidePolygon:(MKPolygon *)poly {
  MKMapPoint *a =  poly.points;
  BOOL isInsidePolygon = NO;

  double testx = point.x;
  double testy = point.y;

  NSUInteger i = 0, j = 0, nvert = [poly pointCount];

  for (i = 0,  j = nvert - 1; i < nvert; j = i++) {
    if (((a[i].y >= testy) != (a[j].y >= testy)) &&
        (testx <= (a[j].x - a[i].x) * (testy - a[i].y) / (a[j].y - a[i].y) + a[i].x)) {
          isInsidePolygon = !isInsidePolygon;
        }
  }

  return isInsidePolygon;
}

Because the points x y are in Mercator projection coordinates, this makes sense and you don't need to convert to spherical coordinates for the calculation.

Why this is a correct calculation here.

EDIT: updated so that the calculation also considers a point on the line as inside the poly.