Keys 0..2047 that land in segment 0 of 16: how many distinct buckets of that segment's 128-slot HashMap do they use?
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(bucket = (h ^ (h >>> 16)) & 127, the formula java.util.HashMap uses; computed arithmetically, not by reflection)

LOW_BITS  keys in segment 0: 128   buckets used:   8 of 128   longest bucket: 16 keys
MIXED     keys in segment 0: 129   buckets used:  65 of 128   longest bucket:  4 keys

Each of those 8 buckets holds 16 keys: past the treeify threshold of 8 (the table has >= 64 slots), so java.util.HashMap turns them into trees.