leiden/partition.ts: Partition.deltaCPM/deltaModularityDirected/deltaModularityUndirected/getCandidateCommunityCount are unreachable dead code codegraph's dead-detector misses

[carlos-alm]

Context

Found during Titan-run grind phase 22 (/titan-grind --phase 22) while running the mandatory Step 2c codebase-wide duplicate scan on the newly-extracted computeCpmEdgeWeights[Directed/Undirected] helpers in src/graph/algorithms/leiden/partition.ts (commit 21db9a9). The scan surfaced that computeCpmEdgeWeights* duplicates the exact same directed/undirected branch-splitting formula as the pre-existing diffCPM in src/graph/algorithms/leiden/cpm.ts — which led to tracing who actually calls each path.

Finding

src/graph/algorithms/leiden/optimiser.ts:computeQualityGain (the only caller of CPM/modularity delta computation in the Leiden hot loop) calls the standalone module-level functions directly:

if (quality === 'cpm') {
  return diffCPM(partition, partition.graph, v, c, gamma);
}
return diffModularity(partition, partition.graph, v, c, gamma);

It never calls partition.deltaCPM(...), partition.deltaModularityDirected(...), or partition.deltaModularityUndirected(...) — the Partition interface methods backed by computeDeltaCPM, computeDeltaModularityDirected, computeDeltaModularityUndirected in partition.ts.

Verified via repo-wide grep (not just codegraph) that these three interface methods, plus getCandidateCommunityCount, have zero callers anywhere in src/, tests/, or scripts:

$ grep -rn "\.deltaCPM(\|\.deltaModularityDirected(\|\.deltaModularityUndirected(\|\.getCandidateCommunityCount(" src/ tests/ --include="*.ts" | grep -v "partition.ts:"
(no output)

Every other Partition interface method (moveNodeToCommunity, compactCommunityIds, getCommunityMembers, getCommunityTotalSize, getCommunityNodeCount, getCandidateCommunityAt, getNeighborEdgeWeightToCommunity, getOutEdgeWeightToCommunity, getInEdgeWeightFromCommunity, accumulateNeighborCommunityEdgeWeights, resizeCommunities, initializeAggregates) has 1+ external callers — only these four are orphaned.

makePartition/Partition are not exported from the package's public API (src/index.ts), so this isn't unused-by-design public surface — it's purely internal dead code.

Why codegraph's own dead-code detector misses this

codegraph roles --role dead -T --file src/graph/algorithms/leiden/partition.ts --json does not flag computeDeltaCPM, computeDeltaModularityDirected, computeDeltaModularityUndirected, or the backing arrow functions as dead — because each is referenced as a value inside the object literal returned by the exported makePartition() factory (e.g. deltaCPM: (v, newC, gamma) => computeDeltaCPM(s, v, newC, gamma)). The detector treats "referenced as a property value in a returned object" as sufficient evidence of liveness, without tracing whether the resulting object's property is ever actually invoked by a consumer (partition.deltaCPM(...)). This is a distinct blind spot from #1723 (interface/type declarations) and #1769 (missed call-edge attribution to real callers) — here the bug runs the other direction: a false negative, undercounting dead code through object-literal-property indirection.

Impact

~80 lines of module code (computeDeltaCPM, computeDeltaModularityDirected, computeDeltaModularityUndirected, plus the phase-22-extracted computeCpmEdgeWeights/computeCpmEdgeWeightsDirected/computeCpmEdgeWeightsUndirected sub-helpers) exist purely to back four Partition interface methods that no code path invokes. computeDeltaCPM's logic is also a near-duplicate of cpm.ts's actively-used diffCPM (same directed/undirected formula, different data-access layer: raw PartitionState typed arrays vs. PartitionView getter interface).

Not fixed here because

• Predates phase 22 — computeDeltaCPM/computeDeltaModularityDirected/computeDeltaModularityUndirected existed before this phase; phase 22 only decomposed their internals (behavior-preserving), it didn't introduce the unreachability.
• Two possible fixes with different risk profiles need dedicated investigation, not a drive-by:
  1. Remove the four dead Partition interface methods + their backing implementations (verify nothing external/future depends on the interface shape).
  2. Wire computeQualityGain to call partition.deltaCPM/partition.deltaModularityDirected/partition.deltaModularityUndirected instead of the standalone diffCPM/diffModularity functions, deduplicating the two parallel implementations — but this changes which code path executes in the Leiden optimization hot loop and needs careful behavioral-equivalence verification first (out of scope for a pure-adoption grind pass, which must not change control flow).
• Filed per project scope-discipline rule rather than silently left undocumented.