Sort by document id in reverse order is not supported currently. Since this option exists in Lucene, it should be supported in bobo as well.
To reproduce the bug, create SortField as follows:
SortField sf = new SortField(null, SortField.DOC, true);
and execute search with this sort field.
I have attached a simple one-liner fix that needs to be added to SortCollector.java
unit test also enhanced with this commit: