[DB] Implement countMatchingEntries() in FederatedObsStore (#325)

* Implement countMatchingEntries() in FederatedObsStore

* remove atomic long reference

Author: earocorn

sensorhub-core/src/main/java/org/sensorhub/impl/database/registry/FederatedObsStore.java

@@ -240,6 +240,29 @@ public Stream<Entry<BigId, IObsData>> selectEntries(ObsFilter filter, Set<ObsFie
     }
 
 
+    @Override
+    public long countMatchingEntries(ObsFilter filter)
+    {
+        // if any kind of internal IDs are used, we need to dispatch the correct filter
+        // to the corresponding DB so we create this map
+        var filterDispatchMap = getFilterDispatchMap(filter);
+
+        if (filterDispatchMap != null)
+        {
+            return filterDispatchMap.values().stream()
+                    .mapToLong(v -> v.db.getObservationStore().countMatchingEntries((ObsFilter)v.filter))
+                    .reduce(0, Long::sum);
+        }
+
+        // otherwise scan all DBs
+        else
+        {
+            return parentDb.getAllObsDatabases().stream()
+                    .mapToLong(db -> db.getObservationStore().countMatchingEntries(filter))
+                    .reduce(0, Long::sum);
+        }
+    }
+
     @Override
     public Stream<BigId> selectObservedFois(ObsFilter filter)
     {