markers.js

// https://github.com/salsita/geo-tree
// Red-black tree for geo coordinates, scalar indices with z-curve
import GeoTree from 'geo-tree';

const gt = new GeoTree();

// Items sorted by relevance, data holds itemId
const items = [
  {lat: 48.85886, lng:  2.34706, data: 3849712},
  {lat: 52.50754, lng: 13.42614, data: 234910},
  {lat: 50.05967, lng: 14.46562, data: 115502},
  ...
];

// Index items by coordinates
gt.insert(items);

// Items that will be shown on map
const validItems = [];

// Keeps track of seen items
const itemsSeenById = {}

// Minimal distance between items
const minDistance = 10.0;

for (let i = 0, l = items.lenght; i < l; i++) {
  const item = items[i];
  
  // This item was marked as seen, ignore
  if (itemsSeenById[data]) continue;
  
  validItems.push(item);

  // Find items that violate min distance and mark them as seen
  const neighbours = gt.find(item.lat, item.lng, minDistance);
  for (let j = 0, k = neighbours.length; j < k; j++) {
    const neighbour = neighbours[j];
    itemsSeenById[neighbour.data] = true;
  }
}