Major Description
Int taxonomy facets today always count into a dense int[], which is wasteful in cases where the number of unique facet labels is high and the size of the current result set is small.
I factored the native hash map from LUCENE-7927 and use a simple heuristic (customizable by the user by subclassing) to decide up front whether to count sparse or dense. I also made loading of the large children and siblings int[] lazy, so that they are only instantiated if you really need them.