docs: @Nullable
    Object convertRootField(OrcStruct struct, String fieldName)
    {
      // this cache is only valid for the root level, to skip the indexOf on fieldNames to get the fieldIndex.
      TypeDescription schema = struct.getSchema();
      final List<String> fields = schema.getFieldNames();
      if (fieldIndexCache == null) {
        fieldIndexCache = new Object2IntOpenHashMap<>(fields.size());
        for (int i = 0; i < fields.size(); i++) {
          fieldIndexCache.put(fields.get(i), i);
        }
      }
      WritableComparable wc = struct.getFieldValue(fieldName);
  
      int fieldIndex = fieldIndexCache.getOrDefault(fieldName, -1);
  
      return convertField(struct, fieldIndex);
    }