/**
 * @param Feature with type `Point`
 * @returns {boolean}
 */
function isPointFeature (feature) {
  return feature.type === 'Feature' && feature.geometry.type === 'Point';
}

/**
 * @param degrees
 * @returns {number}
 */
function toRadians (degrees) {
  return degrees * Math.PI / 180;
}

/**
 * @param {object} Feature
 * @param {object} FeatureCollection
 * @returns {(object|null)} Feature Properties or null if nothing found or malformed reference
 */
function getClosestFeature (referenceFeature, featureCollection) {
  if (!isPointFeature(referenceFeature)) {
    console.error('The reference feature is malformed.');
    return null;
  }

  const {coordinates: [lng1, lat1]} = referenceFeature.geometry;
  const earthRadiusKm = 6371; // Radius of the Earth in kilometers

  let closestDistance = Infinity;
  let closestPlace = null;

  featureCollection.features.forEach((feature) => {
    if (!isPointFeature(feature)) return;

    const {coordinates: [lng2, lat2], properties} = feature.geometry;
    const dLat = toRadians(lat2 - lat1);
    const dLng = toRadians(lng2 - lng1);

    const a = Math.sin(dLat / 2) * Math.sin(dLat / 2) + Math.cos(toRadians(lat1)) * Math.cos(toRadians(lat2)) * Math.sin(dLng / 2) * Math.sin(dLng / 2);

    const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
    const distance = earthRadiusKm * c;

    if (distance < closestDistance) {
      closestDistance = distance;
      closestPlace = properties;
    }
  });

  return closestPlace;
}