[WS3 | Phase 3] DistanceEngine — Flat relation-keyed matrices (→ WS5)

[oxy86]

Part of: [WS3] Domain Model Split (to be linked)
Roadmap: docs/roadmaps/roadmap_domain_model_split.md · docs/roadmaps/roadmap_matrices_modernization.md
Depends on: #242 (Phase 2)

Goal

Replace GraphVertex::m_distance and GraphVertex::m_shortestPaths (per-vertex QHash stores, keyed by target vertex and relation) with a centralised QHash<int, Matrix> on Graph (keyed by relation), using the existing SocNetV Matrix class.

Why

• Eliminates per-vertex QHash lookup overhead in the back-propagation inner loop (shortestPaths(v1) called per predecessor per vertex per source)
• Makes APSP write-back in Phase 2 a flat array write: distMatrix[rel][si * N + ti] = d
• Removes the per-vertex mutex array introduced in Phase 2 (each source owns its own row — no contention at all)
• Aligns with WS5 goals: cancellable, testable matrix subsystem

Tasks

(To be detailed when Phase 2 is complete and WS5 is scoped)

• Define centralised QHash<int, Matrix> on Graph for APSP results
• Update DistanceEngine write-back to use flat matrix rows
• Remove per-vertex m_distance / m_shortestPaths hashes from GraphVertex
• Remove per-vertex mutex array from Phase 2
• Update all callers of the vertex distance/shortest-paths API
• All regression baselines pass

Note

This phase is delegated to WS5 (Matrices Modernization). It is listed here to show the full arc and to mark the dependency.