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utils/nearbyPosts.js

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+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,
+}