I think OpenBitSet is trying to satisfy too many audiences, and it's
confusing/error-proned as a result. It has int/long variants of many
methods. Some methods require in-bound access, others don't; of those
others, some methods auto-grow the bits, some don't. OpenBitSet
doesn't always know its numBits.
I'd like to factor out a more "focused" bit set impl whose primary
target usage is a cached Lucene Filter, ie a bit set indexed by docID
(int, not long) whose size is known and fixed up front (backed by
final long) and is always accessed in-bounds.