const EARTH_RADIUS_KM = 6371;

// Great-circle distance between two WGS84 points (haversine), in km.
export function haversineKm(lat1: number, lon1: number, lat2: number, lon2: number): number {
  const toRad = (d: number) => (d * Math.PI) / 180;
  const dLat = toRad(lat2 - lat1);
  const dLon = toRad(lon2 - lon1);
  const a =
    Math.sin(dLat / 2) ** 2 +
    Math.cos(toRad(lat1)) * Math.cos(toRad(lat2)) * Math.sin(dLon / 2) ** 2;
  return 2 * EARTH_RADIUS_KM * Math.asin(Math.min(1, Math.sqrt(a)));
}