/// <summary>
/// Get the radius of the bbox in hexagons - i.e. the radius of a k-ring centered
/// on the bbox center and covering the entire bbox.
/// </summary>
/// <param name="bbox">Bounding box to measure</param>
/// <param name="res">Resolution of hexagons to use in measurement</param>
/// <returns>Radius in hexagons</returns>
public int bboxHexRadius(BBox bbox, int res)
{
var center = new GeoCoord();
// Determine the center of the bounding box
center = bboxCenter();
// Use a vertex on the side closest to the equator, to ensure the longest
// radius in cases with significant distortion. East/west is arbitrary.
double lat = Math.Abs(bbox.north) > Math.Abs(bbox.south) ? bbox.south : bbox.north;
var vertex = new GeoCoord(lat, bbox.east);
// Determine the length of the bounding box "radius" to then use
// as a circle on the earth that the k-rings must be greater than
double bboxRadiusKm = GeoCoord._geoDistKm(center, vertex);
// Determine the radius of the center hexagon
double centerHexRadiusKm = _hexRadiusKm(center.ToH3Index(res));
// The closest point along a hexagon drawn through the center points
// of a k-ring aggregation is exactly 1.5 radii of the hexagon. For
// any orientation of the GeoJSON encased in a circle defined by the
// bounding box radius and center, it is guaranteed to fit in this k-ring
// Rounded *up* to guarantee containment
return((int)Math.Ceiling(bboxRadiusKm / (1.5 * centerHexRadiusKm)));
}
/// <summary>
/// Returns the radius of a given hexagon in km.
/// </summary>
/// <param name="h3Index">The index of the hexagon</param>
/// <returns>The radius of the hexagon in km</returns>
public static double _hexRadiusKm(H3Index h3Index)
{
// There is probably a cheaper way to determine the radius of a
// hexagon, but this way is conceptually simple
GeoCoord h3Center = h3Index.ToGeoCoord();
GeoBoundary h3Boundary = h3Index.ToGeoBoundary();
// TODO: double check this logic
return(GeoCoord._geoDistKm(h3Center, h3Boundary.verts[0]));
}
