const EARTH_RADIUS_KM = 6371;

// Initial great-circle bearing from point 1 to point 2 (v0.36.0), degrees true [0, 360).
export function initialBearingDeg(lat1: number, lon1: number, lat2: number, lon2: number): number {
  const toRad = (d: number) => (d * Math.PI) / 180;
  const phi1 = toRad(lat1);
  const phi2 = toRad(lat2);
  const dLon = toRad(lon2 - lon1);
  const theta = Math.atan2(
    Math.sin(dLon) * Math.cos(phi2),
    Math.cos(phi1) * Math.sin(phi2) - Math.sin(phi1) * Math.cos(phi2) * Math.cos(dLon),
  );
  return (theta * 180 / Math.PI + 360) % 360;
}

// 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)));
}