Add structural graph analysis and fold groups

README.md

@@ -15,11 +15,15 @@ The result is a spectrum:
 
 ## Current Pipeline
 
-1. Assign ranks from directed edges using a longest-path DAG pass.
+1. Analyze graph anatomy: weak components, SCCs, bridges, articulation points,
-2. Preserve partial hierarchy for cyclic leftovers with local edge pressure.
+   edge roles, degree metrics, and cyclic nodes.
-3. Order nodes inside each rank with repeated barycenter sweeps.
+2. Assign ranks from directed edges using a longest-path DAG pass.
-4. Generate rank, order, and edge-direction constraints.
+3. Preserve partial hierarchy for cyclic leftovers with local edge pressure.
-5. Relax the graph with deterministic force iterations.
+4. Order nodes inside each rank with repeated barycenter sweeps.
+5. Generate rank, order, and edge-direction constraints.
+6. Relax the graph with deterministic force iterations.
+7. Emit fold groups for renderer-side collapse, expansion, highlighting, or
+   protein-ish folding.
 
 ## Example
 
@@ -68,6 +72,46 @@ For Bevy, map `NodeLayout3d::as_xyz()` directly into `Vec3::new(x, y, z)`.
 The crate does not depend on Bevy just to borrow its vector type; the layout
 core stays small and renderer-agnostic.
 
+## Graph Analysis And Folding
+
+The crate exposes `analyze(&graph)` for layout-independent structure reads:
+
+- weak components
+- strongly connected components
+- articulation points
+- bridge edges
+- per-node degree metrics
+- edge roles
+- cyclic node flags
+
+`layout_3d` uses that same analysis to add structural pressure:
+
+- SCC cohesion folds cycles and tight feedback regions into local bodies.
+- weak component cohesion keeps broad regions visually legible.
+- bridge hinges keep connector edges readable between bodies.
+- centrality anchoring keeps important nodes from drifting into decorative exile.
+- cycle folding uses depth so feedback does not have to lose every argument
+  against hierarchy.
+
+The result includes `fold_groups`, which are renderer-facing bodies:
+
+```rust
+for group in result.fold_groups {
+    println!(
+        "{:?}: center={:?} radius={} nodes={}",
+        group.kind,
+        group.center,
+        group.radius,
+        group.nodes.len()
+    );
+}
+```
+
+Fold groups currently cover weak components, strongly connected components, and
+cycle bodies. They are meant to be useful handles for Bevy interactions:
+collapse a component, pulse a cycle, dim a subtree, or unfold a dense region
+without asking the raw node soup for permission.
+
 ## Notes
 
 The first release favors a small deterministic core over a sprawling layout

src/lib.rs

@@ -130,6 +130,16 @@ pub struct Layout3dConfig {
     pub horizontal_repulsion: f32,
     /// Extra multiplier for vertical repulsion.
     pub vertical_repulsion: f32,
+    /// Force pulling strongly connected components into foldable bodies.
+    pub component_cohesion_strength: f32,
+    /// Force pulling weakly connected components into broad local regions.
+    pub weak_component_cohesion_strength: f32,
+    /// Force letting bridge edges behave like hinges between folded bodies.
+    pub bridge_hinge_strength: f32,
+    /// Force making cyclic nodes fold into depth instead of fighting hierarchy.
+    pub cycle_fold_strength: f32,
+    /// Force anchoring high-centrality nodes closer to their component center.
+    pub centrality_anchor_strength: f32,
 }
 
 impl Default for Layout3dConfig {
@@ -141,6 +151,11 @@ impl Default for Layout3dConfig {
             depth_strength: 0.025,
             horizontal_repulsion: 1.0,
             vertical_repulsion: 0.35,
+            component_cohesion_strength: 0.045,
+            weak_component_cohesion_strength: 0.012,
+            bridge_hinge_strength: 0.025,
+            cycle_fold_strength: 0.035,
+            centrality_anchor_strength: 0.02,
         }
     }
 }
@@ -186,6 +201,65 @@ impl NodeLayout3d {
 pub struct Layout3d {
     pub nodes: Vec<NodeLayout3d>,
     pub constraints: ConstraintSet,
+    pub analysis: GraphAnalysis,
+    pub fold_groups: Vec<FoldGroup>,
+}
+
+/// Structural analysis used to turn graph anatomy into layout pressure.
+#[derive(Clone, Debug, Default, PartialEq)]
+pub struct GraphAnalysis {
+    pub weak_components: Vec<Component>,
+    pub strongly_connected_components: Vec<Component>,
+    pub articulation_points: Vec<NodeId>,
+    pub bridges: Vec<EdgeId>,
+    pub node_metrics: Vec<NodeMetrics>,
+    pub edge_roles: Vec<EdgeRole>,
+    pub node_to_weak_component: Vec<usize>,
+    pub node_to_strong_component: Vec<usize>,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct Component {
+    pub id: usize,
+    pub nodes: Vec<NodeId>,
+}
+
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
+pub struct NodeMetrics {
+    pub in_degree: usize,
+    pub out_degree: usize,
+    pub undirected_degree: usize,
+    pub degree_centrality: f32,
+    pub is_articulation: bool,
+    pub is_cyclic: bool,
+}
+
+#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
+pub enum EdgeRole {
+    Tree,
+    Back,
+    #[default]
+    Cross,
+    Bridge,
+    IntraComponent,
+}
+
+/// A renderer-facing body that can be folded, collapsed, or highlighted.
+#[derive(Clone, Debug, PartialEq)]
+pub struct FoldGroup {
+    pub id: usize,
+    pub kind: FoldGroupKind,
+    pub nodes: Vec<NodeId>,
+    pub center: [f32; 3],
+    pub radius: f32,
+    pub parent: Option<usize>,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum FoldGroupKind {
+    WeakComponent,
+    StrongComponent,
+    Cycle,
 }
 
 /// Constraints generated by the Sugiyama pass.
@@ -259,15 +333,18 @@ pub fn layout_3d(graph: &Graph, config: &Layout3dConfig) -> Layout3d {
         return Layout3d {
             nodes: Vec::new(),
             constraints: ConstraintSet::default(),
+            analysis: GraphAnalysis::default(),
+            fold_groups: Vec::new(),
         };
     }
 
+    let analysis = analyze(graph);
     let ranks = assign_ranks(graph);
     let orders = order_ranks(graph, &ranks);
     let constraints = build_constraints(graph, &ranks, &orders, &config.base);
     let mut positions = initial_positions_3d(&ranks, &orders, config);
 
-    relax_3d(graph, &constraints, config, &mut positions);
+    relax_3d(graph, &constraints, &analysis, config, &mut positions);
 
     let mut nodes = (0..n)
         .map(|idx| NodeLayout3d {
@@ -281,7 +358,53 @@ pub fn layout_3d(graph: &Graph, config: &Layout3dConfig) -> Layout3d {
         .collect::<Vec<_>>();
     nodes.sort_by_key(|node| node.id.0);
 
-    Layout3d { nodes, constraints }
+    let fold_groups = build_fold_groups(&analysis, &positions);
+
+    Layout3d {
+        nodes,
+        constraints,
+        analysis,
+        fold_groups,
+    }
+}
+
+/// Analyze graph structure without running layout.
+pub fn analyze(graph: &Graph) -> GraphAnalysis {
+    let n = graph.node_count();
+    if n == 0 {
+        return GraphAnalysis::default();
+    }
+
+    let directed = directed_adjacency(graph);
+    let undirected = undirected_adjacency(graph);
+    let weak_components = weak_components(&undirected);
+    let (sccs, node_to_scc) = strongly_connected_components(&directed);
+    let (articulation_points, bridges) = articulation_points_and_bridges(&undirected);
+    let edge_roles = classify_edges(graph, &node_to_scc, &bridges);
+    let node_metrics = node_metrics(
+        graph,
+        &undirected,
+        &articulation_points,
+        &sccs,
+        &node_to_scc,
+    );
+    let mut node_to_weak_component = vec![0usize; n];
+    for component in &weak_components {
+        for node in &component.nodes {
+            node_to_weak_component[node.0] = component.id;
+        }
+    }
+
+    GraphAnalysis {
+        weak_components,
+        strongly_connected_components: sccs,
+        articulation_points,
+        bridges,
+        node_metrics,
+        edge_roles,
+        node_to_weak_component,
+        node_to_strong_component: node_to_scc,
+    }
 }
 
 fn assign_ranks(graph: &Graph) -> Vec<usize> {
@@ -339,6 +462,265 @@ fn assign_ranks(graph: &Graph) -> Vec<usize> {
     ranks
 }
 
+fn directed_adjacency(graph: &Graph) -> Vec<Vec<usize>> {
+    let mut adjacency = vec![Vec::new(); graph.node_count()];
+    for &(source, target) in graph.edges() {
+        adjacency[source.0].push(target.0);
+    }
+    adjacency
+}
+
+fn undirected_adjacency(graph: &Graph) -> Vec<Vec<(usize, EdgeId)>> {
+    let mut adjacency = vec![Vec::new(); graph.node_count()];
+    for (idx, &(source, target)) in graph.edges().iter().enumerate() {
+        let edge = EdgeId(idx);
+        adjacency[source.0].push((target.0, edge));
+        adjacency[target.0].push((source.0, edge));
+    }
+    adjacency
+}
+
+fn weak_components(adjacency: &[Vec<(usize, EdgeId)>]) -> Vec<Component> {
+    let mut components = Vec::new();
+    let mut seen = vec![false; adjacency.len()];
+
+    for start in 0..adjacency.len() {
+        if seen[start] {
+            continue;
+        }
+
+        let mut queue = VecDeque::from([start]);
+        let mut nodes = Vec::new();
+        seen[start] = true;
+
+        while let Some(node) = queue.pop_front() {
+            nodes.push(NodeId(node));
+            for &(next, _) in &adjacency[node] {
+                if !seen[next] {
+                    seen[next] = true;
+                    queue.push_back(next);
+                }
+            }
+        }
+
+        components.push(Component {
+            id: components.len(),
+            nodes,
+        });
+    }
+
+    components
+}
+
+fn strongly_connected_components(adjacency: &[Vec<usize>]) -> (Vec<Component>, Vec<usize>) {
+    struct Tarjan<'a> {
+        adjacency: &'a [Vec<usize>],
+        index: usize,
+        stack: Vec<usize>,
+        on_stack: Vec<bool>,
+        indices: Vec<Option<usize>>,
+        lowlink: Vec<usize>,
+        components: Vec<Component>,
+        node_to_component: Vec<usize>,
+    }
+
+    impl Tarjan<'_> {
+        fn visit(&mut self, node: usize) {
+            self.indices[node] = Some(self.index);
+            self.lowlink[node] = self.index;
+            self.index += 1;
+            self.stack.push(node);
+            self.on_stack[node] = true;
+
+            for &next in &self.adjacency[node] {
+                if self.indices[next].is_none() {
+                    self.visit(next);
+                    self.lowlink[node] = self.lowlink[node].min(self.lowlink[next]);
+                } else if self.on_stack[next] {
+                    self.lowlink[node] = self.lowlink[node].min(self.indices[next].unwrap_or(0));
+                }
+            }
+
+            if self.lowlink[node] == self.indices[node].unwrap_or(0) {
+                let id = self.components.len();
+                let mut nodes = Vec::new();
+                while let Some(member) = self.stack.pop() {
+                    self.on_stack[member] = false;
+                    self.node_to_component[member] = id;
+                    nodes.push(NodeId(member));
+                    if member == node {
+                        break;
+                    }
+                }
+                nodes.sort_by_key(|node| node.0);
+                self.components.push(Component { id, nodes });
+            }
+        }
+    }
+
+    let n = adjacency.len();
+    let mut tarjan = Tarjan {
+        adjacency,
+        index: 0,
+        stack: Vec::new(),
+        on_stack: vec![false; n],
+        indices: vec![None; n],
+        lowlink: vec![0; n],
+        components: Vec::new(),
+        node_to_component: vec![0; n],
+    };
+
+    for node in 0..n {
+        if tarjan.indices[node].is_none() {
+            tarjan.visit(node);
+        }
+    }
+
+    (tarjan.components, tarjan.node_to_component)
+}
+
+fn articulation_points_and_bridges(
+    adjacency: &[Vec<(usize, EdgeId)>],
+) -> (Vec<NodeId>, Vec<EdgeId>) {
+    struct Dfs<'a> {
+        adjacency: &'a [Vec<(usize, EdgeId)>],
+        time: usize,
+        visited: Vec<bool>,
+        discovery: Vec<usize>,
+        low: Vec<usize>,
+        articulation: Vec<bool>,
+        bridges: Vec<EdgeId>,
+    }
+
+    impl Dfs<'_> {
+        fn visit(&mut self, node: usize, parent_edge: Option<EdgeId>) {
+            self.visited[node] = true;
+            self.discovery[node] = self.time;
+            self.low[node] = self.time;
+            self.time += 1;
+
+            let mut child_count = 0usize;
+            for &(next, edge) in &self.adjacency[node] {
+                if Some(edge) == parent_edge {
+                    continue;
+                }
+                if self.visited[next] {
+                    self.low[node] = self.low[node].min(self.discovery[next]);
+                    continue;
+                }
+
+                child_count += 1;
+                self.visit(next, Some(edge));
+                self.low[node] = self.low[node].min(self.low[next]);
+
+                if parent_edge.is_some() && self.low[next] >= self.discovery[node] {
+                    self.articulation[node] = true;
+                }
+                if self.low[next] > self.discovery[node] {
+                    self.bridges.push(edge);
+                }
+            }
+
+            if parent_edge.is_none() && child_count > 1 {
+                self.articulation[node] = true;
+            }
+        }
+    }
+
+    let n = adjacency.len();
+    let mut dfs = Dfs {
+        adjacency,
+        time: 0,
+        visited: vec![false; n],
+        discovery: vec![0; n],
+        low: vec![0; n],
+        articulation: vec![false; n],
+        bridges: Vec::new(),
+    };
+
+    for node in 0..n {
+        if !dfs.visited[node] {
+            dfs.visit(node, None);
+        }
+    }
+
+    let articulation_points = dfs
+        .articulation
+        .iter()
+        .enumerate()
+        .filter_map(|(idx, &is_articulation)| is_articulation.then_some(NodeId(idx)))
+        .collect();
+    dfs.bridges.sort_by_key(|edge| edge.0);
+    (articulation_points, dfs.bridges)
+}
+
+fn classify_edges(graph: &Graph, node_to_scc: &[usize], bridges: &[EdgeId]) -> Vec<EdgeRole> {
+    let mut roles = vec![EdgeRole::Cross; graph.edge_count()];
+    let mut bridge_marks = vec![false; graph.edge_count()];
+    for edge in bridges {
+        bridge_marks[edge.0] = true;
+    }
+
+    for (idx, &(source, target)) in graph.edges().iter().enumerate() {
+        roles[idx] = if bridge_marks[idx] {
+            EdgeRole::Bridge
+        } else if node_to_scc[source.0] == node_to_scc[target.0] {
+            EdgeRole::IntraComponent
+        } else if source.0 < target.0 {
+            EdgeRole::Tree
+        } else {
+            EdgeRole::Back
+        };
+    }
+
+    roles
+}
+
+fn node_metrics(
+    graph: &Graph,
+    undirected: &[Vec<(usize, EdgeId)>],
+    articulation_points: &[NodeId],
+    sccs: &[Component],
+    node_to_scc: &[usize],
+) -> Vec<NodeMetrics> {
+    let n = graph.node_count();
+    let mut in_degree = vec![0usize; n];
+    let mut out_degree = vec![0usize; n];
+    for &(source, target) in graph.edges() {
+        out_degree[source.0] += 1;
+        in_degree[target.0] += 1;
+    }
+
+    let mut articulation = vec![false; n];
+    for node in articulation_points {
+        articulation[node.0] = true;
+    }
+
+    let mut cyclic_scc = vec![false; sccs.len()];
+    for component in sccs {
+        if component.nodes.len() > 1 {
+            cyclic_scc[component.id] = true;
+        }
+    }
+    for &(source, target) in graph.edges() {
+        if source == target {
+            cyclic_scc[node_to_scc[source.0]] = true;
+        }
+    }
+
+    let centrality_denominator = n.saturating_sub(1).max(1) as f32;
+    (0..n)
+        .map(|idx| NodeMetrics {
+            in_degree: in_degree[idx],
+            out_degree: out_degree[idx],
+            undirected_degree: undirected[idx].len(),
+            degree_centrality: undirected[idx].len() as f32 / centrality_denominator,
+            is_articulation: articulation[idx],
+            is_cyclic: cyclic_scc[node_to_scc[idx]],
+        })
+        .collect()
+}
+
 fn normalize_ranks(ranks: &mut [usize]) {
     let mut used = ranks.to_vec();
     used.sort_unstable();
@@ -515,6 +897,7 @@ fn relax(
 fn relax_3d(
     graph: &Graph,
     constraints: &ConstraintSet,
+    analysis: &GraphAnalysis,
     config: &Layout3dConfig,
     positions: &mut [(f32, f32, f32)],
 ) {
@@ -527,6 +910,7 @@ fn relax_3d(
         apply_repulsion_3d(positions, &mut forces, config);
         apply_springs_3d(graph, positions, &mut forces, config);
         apply_constraints_3d(constraints, positions, &mut forces, config);
+        apply_structural_forces_3d(graph, analysis, positions, &mut forces, config);
         apply_grounding_3d(positions, &mut forces, config);
 
         let cooling = 1.0 - (iter as f32 / config.base.iterations.max(1) as f32);
@@ -539,6 +923,242 @@ fn relax_3d(
     }
 }
 
+fn apply_structural_forces_3d(
+    graph: &Graph,
+    analysis: &GraphAnalysis,
+    positions: &[(f32, f32, f32)],
+    forces: &mut [(f32, f32, f32)],
+    config: &Layout3dConfig,
+) {
+    apply_component_cohesion(
+        &analysis.strongly_connected_components,
+        positions,
+        forces,
+        config.component_cohesion_strength,
+    );
+    apply_component_cohesion(
+        &analysis.weak_components,
+        positions,
+        forces,
+        config.weak_component_cohesion_strength,
+    );
+    apply_centrality_anchors(analysis, positions, forces, config);
+    apply_cycle_folds(analysis, positions, forces, config);
+    apply_bridge_hinges(graph, analysis, positions, forces, config);
+}
+
+fn apply_component_cohesion(
+    components: &[Component],
+    positions: &[(f32, f32, f32)],
+    forces: &mut [(f32, f32, f32)],
+    strength: f32,
+) {
+    if strength <= 0.0 {
+        return;
+    }
+
+    for component in components {
+        if component.nodes.len() < 2 {
+            continue;
+        }
+
+        let center = component_center(&component.nodes, positions);
+        for node in &component.nodes {
+            let idx = node.0;
+            forces[idx].0 += (center[0] - positions[idx].0) * strength;
+            forces[idx].1 += (center[1] - positions[idx].1) * strength * 0.25;
+            forces[idx].2 += (center[2] - positions[idx].2) * strength;
+        }
+    }
+}
+
+fn apply_centrality_anchors(
+    analysis: &GraphAnalysis,
+    positions: &[(f32, f32, f32)],
+    forces: &mut [(f32, f32, f32)],
+    config: &Layout3dConfig,
+) {
+    if config.centrality_anchor_strength <= 0.0 {
+        return;
+    }
+
+    for component in &analysis.weak_components {
+        if component.nodes.len() < 3 {
+            continue;
+        }
+
+        let center = component_center(&component.nodes, positions);
+        for node in &component.nodes {
+            let idx = node.0;
+            let metric = analysis.node_metrics[idx];
+            let strength = config.centrality_anchor_strength * metric.degree_centrality;
+            forces[idx].0 += (center[0] - positions[idx].0) * strength;
+            forces[idx].2 += (center[2] - positions[idx].2) * strength;
+        }
+    }
+}
+
+fn apply_cycle_folds(
+    analysis: &GraphAnalysis,
+    positions: &[(f32, f32, f32)],
+    forces: &mut [(f32, f32, f32)],
+    config: &Layout3dConfig,
+) {
+    if config.cycle_fold_strength <= 0.0 {
+        return;
+    }
+
+    for component in &analysis.strongly_connected_components {
+        if component.nodes.len() < 2 {
+            continue;
+        }
+
+        let center = component_center(&component.nodes, positions);
+        for (offset, node) in component.nodes.iter().enumerate() {
+            let idx = node.0;
+            let side = if offset % 2 == 0 { 1.0 } else { -1.0 };
+            let target_z =
+                center[2] + side * config.depth_gap * component.nodes.len() as f32 * 0.35;
+            forces[idx].2 += (target_z - positions[idx].2) * config.cycle_fold_strength;
+            forces[idx].1 -= positions[idx].1 * config.cycle_fold_strength * 0.08;
+        }
+    }
+}
+
+fn apply_bridge_hinges(
+    graph: &Graph,
+    analysis: &GraphAnalysis,
+    positions: &[(f32, f32, f32)],
+    forces: &mut [(f32, f32, f32)],
+    config: &Layout3dConfig,
+) {
+    if config.bridge_hinge_strength <= 0.0 {
+        return;
+    }
+
+    for bridge in &analysis.bridges {
+        let (source, target) = graph.edges()[bridge.0];
+        let source_component = analysis.node_to_weak_component[source.0];
+        let target_component = analysis.node_to_weak_component[target.0];
+        if source_component == target_component {
+            let midpoint = midpoint_3d(positions[source.0], positions[target.0]);
+            for node in [source, target] {
+                let idx = node.0;
+                let multiplier = if analysis.node_metrics[idx].is_articulation {
+                    1.5
+                } else {
+                    1.0
+                };
+                forces[idx].0 +=
+                    (midpoint.0 - positions[idx].0) * config.bridge_hinge_strength * multiplier;
+                forces[idx].2 +=
+                    (midpoint.2 - positions[idx].2) * config.bridge_hinge_strength * multiplier;
+            }
+        }
+    }
+}
+
+fn component_center(nodes: &[NodeId], positions: &[(f32, f32, f32)]) -> [f32; 3] {
+    let mut center = [0.0f32; 3];
+    for node in nodes {
+        center[0] += positions[node.0].0;
+        center[1] += positions[node.0].1;
+        center[2] += positions[node.0].2;
+    }
+    let len = nodes.len().max(1) as f32;
+    center[0] /= len;
+    center[1] /= len;
+    center[2] /= len;
+    center
+}
+
+fn midpoint_3d(a: (f32, f32, f32), b: (f32, f32, f32)) -> (f32, f32, f32) {
+    ((a.0 + b.0) * 0.5, (a.1 + b.1) * 0.5, (a.2 + b.2) * 0.5)
+}
+
+fn build_fold_groups(analysis: &GraphAnalysis, positions: &[(f32, f32, f32)]) -> Vec<FoldGroup> {
+    let mut groups = Vec::new();
+
+    for component in &analysis.weak_components {
+        if component.nodes.is_empty() {
+            continue;
+        }
+        groups.push(fold_group(
+            groups.len(),
+            FoldGroupKind::WeakComponent,
+            component.nodes.clone(),
+            positions,
+            None,
+        ));
+    }
+
+    for component in &analysis.strongly_connected_components {
+        if component.nodes.len() < 2 {
+            continue;
+        }
+        let parent = component
+            .nodes
+            .first()
+            .map(|node| analysis.node_to_weak_component[node.0]);
+        groups.push(fold_group(
+            groups.len(),
+            FoldGroupKind::StrongComponent,
+            component.nodes.clone(),
+            positions,
+            parent,
+        ));
+    }
+
+    for component in &analysis.strongly_connected_components {
+        let is_cycle = component
+            .nodes
+            .iter()
+            .any(|node| analysis.node_metrics[node.0].is_cyclic);
+        if !is_cycle || component.nodes.len() < 2 {
+            continue;
+        }
+        let parent = component
+            .nodes
+            .first()
+            .map(|node| analysis.node_to_weak_component[node.0]);
+        groups.push(fold_group(
+            groups.len(),
+            FoldGroupKind::Cycle,
+            component.nodes.clone(),
+            positions,
+            parent,
+        ));
+    }
+
+    groups
+}
+
+fn fold_group(
+    id: usize,
+    kind: FoldGroupKind,
+    nodes: Vec<NodeId>,
+    positions: &[(f32, f32, f32)],
+    parent: Option<usize>,
+) -> FoldGroup {
+    let center = component_center(&nodes, positions);
+    let mut radius = 0.0f32;
+    for node in &nodes {
+        let dx = positions[node.0].0 - center[0];
+        let dy = positions[node.0].1 - center[1];
+        let dz = positions[node.0].2 - center[2];
+        radius = radius.max((dx * dx + dy * dy + dz * dz).sqrt());
+    }
+
+    FoldGroup {
+        id,
+        kind,
+        nodes,
+        center,
+        radius,
+        parent,
+    }
+}
+
 fn apply_repulsion(positions: &[(f32, f32)], forces: &mut [(f32, f32)], strength: f32) {
     for a in 0..positions.len() {
         for b in (a + 1)..positions.len() {
@@ -827,4 +1447,61 @@ mod tests {
         assert!(result.nodes.is_empty());
         assert!(result.constraints.ranks.is_empty());
     }
+
+    #[test]
+    fn analysis_finds_cycles_bridges_and_articulation_points() {
+        let mut graph = Graph::new();
+        let a = graph.add_node(1.0);
+        let b = graph.add_node(1.0);
+        let c = graph.add_node(1.0);
+        let d = graph.add_node(1.0);
+        graph.add_edge(a, b);
+        graph.add_edge(b, c);
+        graph.add_edge(c, a);
+        let bridge = graph.add_edge(c, d);
+
+        let analysis = analyze(&graph);
+
+        assert_eq!(analysis.weak_components.len(), 1);
+        assert_eq!(analysis.strongly_connected_components.len(), 2);
+        assert!(analysis.node_metrics[a.0].is_cyclic);
+        assert!(analysis.node_metrics[b.0].is_cyclic);
+        assert!(analysis.node_metrics[c.0].is_cyclic);
+        assert!(!analysis.node_metrics[d.0].is_cyclic);
+        assert!(analysis.articulation_points.contains(&c));
+        assert!(analysis.bridges.contains(&bridge));
+        assert_eq!(analysis.edge_roles[bridge.0], EdgeRole::Bridge);
+    }
+
+    #[test]
+    fn layout_3d_exposes_fold_groups() {
+        let mut graph = Graph::new();
+        let a = graph.add_node(1.0);
+        let b = graph.add_node(1.0);
+        let c = graph.add_node(1.0);
+        graph.add_edge(a, b);
+        graph.add_edge(b, c);
+        graph.add_edge(c, a);
+
+        let result = layout_3d(&graph, &Layout3dConfig::default());
+
+        assert!(
+            result
+                .fold_groups
+                .iter()
+                .any(|group| group.kind == FoldGroupKind::WeakComponent)
+        );
+        assert!(
+            result
+                .fold_groups
+                .iter()
+                .any(|group| group.kind == FoldGroupKind::StrongComponent)
+        );
+        assert!(
+            result
+                .fold_groups
+                .iter()
+                .any(|group| group.kind == FoldGroupKind::Cycle)
+        );
+    }
 }