implement latlng-to-guid cache

code/map_data_render.js

@@ -258,6 +258,8 @@ window.Render.prototype.createPortalEntity = function(ent) {
     data: ent[2]
   };
 
+  window.pushPortalGuidPositionCache(ent[0], ent[2].latE6, ent[2].lngE6);
+
   var marker = createMarker(latlng, dataOptions);
 
   marker.on('click', function() { window.renderPortalDetails(ent[0]); });

code/portal_data.js

@@ -88,7 +88,32 @@ window.findPortalLatLng = function(guid) {
 
   // no luck finding portal lat/lng
   return undefined;
+};
+
+
+(function() {
+  var cache = {};
+  var GC_LIMIT = 5000; // run garbage collector when cache has more that 5000 items
+  var GC_KEEP  = 4000; // keep the 4000 most recent items
+
+  window.findPortalGuidByPositionE6 = function(latE6, lngE6) {
+    var item = cache[latE6+","+lngE6];
+    if(!item) return null;
+    return item[0];
+  };
+
+  window.pushPortalGuidPositionCache = function(guid, latE6, lngE6) {
+    cache[latE6+","+lngE6] = [guid, Date.now()];
+
+    if(Object.keys(cache).length > GC_LIMIT) {
+      Object.keys(cache) // get all latlngs
+        .map(function(latlng) { return [latlng, cache[latlng][1]]; })  // map them to [latlng, timestamp]
+        .sort(function(a,b) { return b[1] - a[1]; }) // sort them
+        .slice(GC_KEEP) // drop the MRU
+        .forEach(function(item) { delete cache[item[0]] }); // delete the rest
     }
+  }
+})();
 
 
 // get the AP gains from a portal, based only on the brief summary data from portals, links and fields