function haversineMeters(lat1, lon1, lat2, lon2) {
  const toRad = (deg) => (deg * 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 6371000 * 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))
}

function filterPostsByDistance(posts, viewerLat, viewerLng, radiusMeters) {
  const parsedLat = parseFloat(viewerLat)
  const parsedLng = parseFloat(viewerLng)
  if (!Number.isFinite(parsedLat) || !Number.isFinite(parsedLng)) return []

  return posts.filter((post) => {
    const postLat = parseFloat(post.lat)
    const postLng = parseFloat(post.lng)
    if (!Number.isFinite(postLat) || !Number.isFinite(postLng)) return false
    return haversineMeters(parsedLat, parsedLng, postLat, postLng) <= radiusMeters
  })
}

module.exports = {
  haversineMeters,
  filterPostsByDistance,
}