Implement Map-Reduce (1.4.3)

ass1-doc/src/main/java/dst/ass1/doc/impl/DocumentQuery.java

@@ -56,6 +56,33 @@ public class DocumentQuery implements IDocumentQuery {
 
     @Override
     public List<Document> getDocumentStatistics() {
-        return null;
+        MongoCollection<Document> collection = db.getCollection(COLL_LOCATION_DATA);
+
+        String map =
+                "function() {" +
+                        "if (this.type === 'place') {" +
+                            "emit(this.category, 1);" +
+                        "}" +
+                "}";
+
+        String reduce =
+                "function(key, values) {" +
+                        "sum = 0;" +
+                        "values.forEach(value => {sum += value});" +
+                        "return sum;" +
+                "}";
+
+        List<Document> result = new ArrayList<>();
+        MongoCursor<Document> cursor = collection.mapReduce(map, reduce).iterator();
+
+        try {
+            while (cursor.hasNext()) {
+                result.add(cursor.next());
+            }
+        } finally {
+            cursor.close();
+        }
+
+        return result;
     }
 }