/**
 * Haversine great-circle distance between two WGS-84 points.
 *
 * @param {number} lat1 - Latitude of point A (degrees).
 * @param {number} lon1 - Longitude of point A (degrees).
 * @param {number} lat2 - Latitude of point B (degrees).
 * @param {number} lon2 - Longitude of point B (degrees).
 * @returns {number} Distance in kilometers.
 */
export function haversineKm(lat1, lon1, lat2, lon2) {
  const toRad = (value) => (value * 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 6371 * 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
}