Function louvain 

pub fn louvain(graph: &Subgraph) -> BTreeMap<String, usize>

Louvain community detection, local-moving phase (§5.4).

Returns node id -> community index. Communities are renumbered densely from zero in order of first appearance, so the result is stable and comparable.

This is phase one of the two-phase Louvain method: nodes are moved greedily to whichever neighbouring community most increases modularity, repeatedly, until no move helps. It does not then aggregate each community into a single node and recurse, which is what the full method does to find coarser structure.

Why the aggregation phase is absent, and it is not the reason given before

Through 0.7.0 this note said the aggregation phase “would matter on graphs far larger than the byte budget admits”. D-115 raised what the budget admits by 5.8×–6.8×, so the claim was re-measured against the new ceiling — and it is false. examples/louvain_aggregation_probe.rs finds two-phase returning a different partition from 6,144 nodes upward, well inside the budget, with the gap widening as the graph grows.

What the difference is settles it. On clustered — cliques joined by one bridge each, where the right answer is known — phase-one recovers the ground truth exactly at every size up to the ceiling, and two-phase scores a higher Q by merging whole cliques: two per community at 512 cliques, four at 4,096, never splitting one. Its Q also exceeds the ground truth’s. That is the modularity resolution limit (Fortunato & Barthélemy): on a large graph the objective prefers a partition coarser than the true one, so optimising it harder moves away from the answer rather than towards it.

So the aggregation phase is declined because at the sizes this crate serves it changes a correct answer into a merged one — not because it would make no difference. modularity_prefers_a_merged_partition_over_the_true_one_at_scale pins the fact underneath that without needing a two-phase implementation here: the merged partition outscores the truth, so a Q comparison cannot be the criterion.