Major Description
When I was iterating on BKD tree originally I remember trying to use this builder (which makes a sparse bit set at first and then upgrades to dense if enough bits get set) and being disappointed with its performance.
I wound up just making a FixedBitSet every time, but this is obviously wasteful for small queries.
It could be the perf was poor because I was always .or'ing in DISIs that had 512 - 1024 hits each time (the size of each leaf cell in the BKD tree)? I also had to make my own DISI wrapper around each leaf cell... maybe that was the source of the slowness, not sure.
I also sort of wondered whether the SmallDocSet in spatial module (backed by a SentinelIntSet) might be faster ... though it'd need to be sorted in the and after building before returning to Lucene.