code: public IntTrie compactToTrieWithRowIndexes() {
          PVecToTrieCompactHandler compactor = new PVecToTrieCompactHandler();
          compact(compactor);
          return compactor.builder.serialize(new DefaultGetFoldedValue(
                  compactor.builder), new DefaultGetFoldingOffset());
      }
docs: /*
  Call compact(), create a IntTrie with indexes into the compacted
  vectors array.