reikhelm/.dev/2026-05-28-procgen-map-core/tasks/09-mst-connect.md

Task 9: MstConnect Pass

Depends on: Task 6

Context Files

• .dev/2026-05-28-procgen-map-core/spec/design.md — §4.7 (Connector role), §4.5 (ConnGraph), §5 (data flow).
• .dev/2026-05-28-procgen-map-core/README.md — inter-pass data contract (MstConnect row).
• reikhelm-core/src/pass.rs — Pass, GenContext (Task 6).
• reikhelm-core/src/region.rs — Region, RegionKind::Room, ConnGraph, Edge (Task 5).
• reikhelm-core/src/passes/mod.rs — append the module declaration here.

Files

• Create: reikhelm-core/src/passes/connect.rs — ConnectConfig, MstConnect.
• Modify: reikhelm-core/src/passes/mod.rs — append pub mod connect; and re-export MstConnect, ConnectConfig.

What to Build

A connector (spec §4.7) that decides which rooms connect, without carving anything. It reads the real Room regions — those with kind == Room and non-empty cells (an empty-cells Room is an uncarved leaf; see Task 8) — builds a minimum spanning tree over their centers (edge weight = distance between centers, e.g. Manhattan), then adds a configurable fraction of extra edges between nearby rooms to create loops (so the dungeon isn't a strict tree). Each connection is pushed to ctx.graph as an Edge { a, b, at: None } — the door location is filled later by DoorPlacer. Operates only on ctx.regions/ctx.graph; carves no tiles.

Interface Dependencies

• Consumes: Pass, GenContext, Rng (Task 6); Region, RegionKind, ConnGraph (Task 5).
• Produces:
  • struct ConnectConfig { extra_edge_ratio: f64 } (+ Default; 0.0 = pure MST).
  • struct MstConnect { /* holds ConnectConfig */ } — new(cfg) -> Self; impl Pass with name() == "mst_connect".
  • Writes to context: MST + extra edges in ctx.graph, all with at: None.

Tests

• Given a GenContext with N (≥2) Room regions, after running: the MST portion has exactly N−1 edges and connects all rooms (graph is connected — verify by union-find/BFS over edges).
• extra_edge_ratio: 0.0 produces exactly N−1 edges; a positive ratio adds the expected extra count.
• Every edge references valid RegionIds of Room regions and has at == None.
• A single room (N=1) produces zero edges without panic.
• Determinism: same seed/sub-stream → identical edge set.

Success Criteria

• MstConnect builds a connected MST over room centers plus configurable loop edges, all with at: None.
• Carves no tiles; edge endpoints are valid Room region ids.
• pub mod connect; appended to passes/mod.rs.
• Tests pass: cargo test -p reikhelm-core connect

Commit

"feat(core): MstConnect pass"