Neighbours were selected as the plain closest-M, for both a new node's links
and back-link pruning. On clustered data every link of a node inside a tight
cluster then goes to that same cluster, so clusters become islands a search
entering elsewhere can never reach: recall@10 was 0.87 / 0.67 / 0.31 at
1K / 10K / 100K (384-dim) and flat in ef. Uniform random data — all the
existing tests used — does not show it.
Implement the HNSW paper's Algorithm 4 with keepPrunedConnections: accept a
candidate only if it is closer to the node than to every neighbour already
accepted, then fill spare slots with the closest rejected ones. Recall@10 at
ef=64 is now 1.00 / 1.00 / 0.98 and rises with ef; uniform data improves
slightly. Build is ~3.5x slower at 10K (extra distance evaluations), to be
recovered by the distance-kernel work. The needless rayon fan-out over <=33
distances in prune_connections is gone.
Tests: a clustered-data recall test for bulk build and incremental insert
(scores 0.43 with the old selection), and a unit test of the selection rule.
Harness gains --uniform and --ann-only; before/after in BENCHMARKS.md.
Co-Authored-By: Claude Fable 5.1 <[email protected]>
