feat(clustering): Leiden community detection (well-connected communities) - #8
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…unities) Builds on Louvain (E2): - E2.0: extract package-private CommunityAggregation (compact / numberOfCommunities / degree-conserving aggregate that compacts labels internally) and refactor LouvainClustering to use it. Behaviour-preserving: Louvain's 17 tests stay green. The internal compaction removes the implicit 'caller must pass compact labels' coupling flagged in review, so Leiden can aggregate its non-compact refined partitions safely. - E2.1: LeidenClustering implementing ClusteringAlgorithm<V>. Multilevel local-moving + a refinement phase whose sub-communities grow only along edges (so each is connected), aggregation over the refined partition, and a final connected-components split that guarantees every reported community is connected -- the property Louvain lacks. Non-negative/zero-weight guards mirror Louvain. - E2.2: 16 tests incl. the headline everyCommunityIsConnected oracle (ConnectivityInspector per induced subgraph, structured + 30 random graphs), clique/ring/complete/weighted parity, determinism, edge cases, and a modularity-vs-Louvain comparison (competitive within 0.02; Leiden trades a sliver of modularity for guaranteed connectivity). 33/33 clustering tests pass; checkstyle + javadoc clean. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Leiden vs Louvain vs LabelPropagation vs GreedyModularity on planted partitions. Leiden costs ~1.4-1.5x Louvain (refinement + connectivity-split passes) while staying 7-9x faster than GreedyModularity; scaling parallels Louvain. Cost (ms/op): 125-node Louvain 0.28 / Leiden 0.42; 500-node Louvain 3.05 / Leiden 4.17. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Superseded by #9 — Louvain + Leiden combined into a single clustering PR. |
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Self-review PR (fork only — not upstream; stacked on the Louvain branch, holds for
jgrapht-dev).What (Workstream E2, stacked on #7)
CommunityAggregation(compact / numberOfCommunities /degree-conserving
aggregatethat compacts labels internally) and refactorLouvainClusteringonto it. Behaviour-preserving: Louvain's 17 tests stay green. Internal compaction removes the
implicit "caller passes compact labels" coupling flagged in the Louvain review, so Leiden can
aggregate its non-compact refined partitions safely.
LeidenClustering implements ClusteringAlgorithm<V>. Multilevel local-moving +refinement (sub-communities grow only along edges → each connected) + aggregation over the
refined partition + a final connected-components split that guarantees every reported community
is connected — the property Louvain lacks (Traag-Waltman-van Eck 2019).
everyCommunityIsConnectedoracle (ConnectivityInspectorper induced subgraph, structured + 30 random graphs).
Verification
ConnectivityInspector.Honest scope notes
guaranteed connectivity (worst observed gap on random graphs: 4×10⁻⁴; test allows 0.02).
singletons each level rather than seeding from the previous partition; the refinement gate is
connectivity + positive gain rather than the resolution-parametrised well-connected test. The
connectivity guarantee is enforced unconditionally by the final connected-components split.
resolutionconstructor parameter deferred.Review focus
CommunityAggregation.aggregateinternal compaction + degree conservation🤖 Generated with Claude Code