code: public void addDocument(String... fields){
          for (int i = 0; i < fields.length; i++) {
              final DataType type = schema.getType(i);
              String field = fields[i];
              final Map<String, Set<Integer>> stringSetMap = this.fields.get(i);
              if(type == DataType.DateYear){ // reformat field value for date year
                   field = field.split("-")[0];
              }
              Set<Integer> integers = stringSetMap.get(field);
              if(integers == null){
                  integers = new TreeSet<>();
              }
              integers.add(docPosition);
              stringSetMap.put(field, integers);
          }
          docPosition++;
      }
docs: add fields on index