D3 Examples - Sample to display GeoJSON as cluster in Openlayers 3. derived from http://openlayers.org/en/v3.6.0/examples/cluster.html?mode=advanced

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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Clustering example</title>
<script src="https://code.jquery.com/jquery-1.11.2.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/ol3/3.6.0/ol.css" type="text/css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/ol3/3.6.0/ol.js"></script>
</head>
<body>
<div class="container-fluid">
​
<div class="row-fluid">
  <div class="span12">
    <div id="map" class="map"></div>
  </div>
</div>
​
</div>
<script>
​
var source = new ol.source.Vector({
    url: 'sample.geojson',
    format: new ol.format.GeoJSON()
});
​
var clusterSource = new ol.source.Cluster({
  distance: 40,
  source: source
});
​
var styleCache = {};
var clusters = new ol.layer.Vector({
  source: clusterSource,
  style: function(feature, resolution) {
    var size = feature.get('features').length;
    var style = styleCache[size];
    if (!style) {
      style = [new ol.style.Style({
        image: new ol.style.Circle({
          radius: 10,
          stroke: new ol.style.Stroke({
            color: '#fff'
          }),
          fill: new ol.style.Fill({
            color: '#3399CC'
          })
        }),
        text: new ol.style.Text({
          text: size.toString(),
          fill: new ol.style.Fill({
            color: '#fff'
          })
        })
      })];
      styleCache[size] = style;
    }
    return style;
  }
});
​
var raster = new ol.layer.Tile({
  source: new ol.source.MapQuest({layer: 'sat'})
});
​
var raw = new ol.layer.Vector({
  source: source
});
​
var map = new ol.Map({
  layers: [raster, clusters],
  renderer: 'canvas',
  target: 'map',
  view: new ol.View({
    center: [0, 0],
    zoom: 2
  })
});
​
</script>
</body>
</html>
    