export type LatLng = { lat: number; lng: number }

/**
 * Storsirkel-avstand mellom to koordinater, i meter. Ren klient-side
 * beregning -- ingen Mapbox-kall (ADR-048 kostnadskontroll-invariant B).
 */
export function haversineMeters(a: LatLng, b: LatLng): number {
  const R = 6371000
  const toRad = (d: number) => (d * Math.PI) / 180
  const dLat = toRad(b.lat - a.lat)
  const dLng = toRad(b.lng - a.lng)
  const h =
    Math.sin(dLat / 2) ** 2 +
    Math.cos(toRad(a.lat)) * Math.cos(toRad(b.lat)) * Math.sin(dLng / 2) ** 2
  return 2 * R * Math.asin(Math.sqrt(h))
}