solver: order by planes or graph dissection, whichever fills less (#212)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/solver/src/analysis.rs

@@ -15,7 +15,7 @@ use crate::cholesky::{Factor, FactorError, UpperColumns};
 use crate::constraints::{Budget, Reduction, coupling_rows, symmetry_rows};
 use crate::eigen::{Operator, Wanted, lanczos};
 use crate::ldlt::{Kind, Symmetric};
-use crate::ordering::nested_dissection;
+use crate::ordering::fill_reducing_order;
 use crate::sparse::{Csr, NodeGraph, add_block, add_diagonal, add_scalar};
 use crate::system::{
     Mesh, Structural, StructuralKernel, add_thermal_loads, inelastic_warnings, loads, prescribed, structural_body_forces, temperature_loads,
@@ -311,10 +311,10 @@ pub(crate) fn pattern(mesh: &Mesh) -> (NodeGraph, Vec<usize>) {
     // The order covers the mesh nodes; reference point equations follow them.
     let n = mesh.labels.len();
     let order = if graph.adjacency.len() == n {
-        nested_dissection(&mesh.xyz, &graph.adjacency)
+        fill_reducing_order(&mesh.xyz, &graph.adjacency)
     } else {
         let mesh_part: Vec<Vec<usize>> = graph.adjacency.iter().take(n).map(|row| row.iter().copied().filter(|b| *b < n).collect()).collect();
-        nested_dissection(&mesh.xyz, &mesh_part)
+        fill_reducing_order(&mesh.xyz, &mesh_part)
     };
     (graph, order)
 }

crates/solver/src/heat.rs

@@ -28,7 +28,7 @@ use fieldcraft_model::{ElementKind, Load, Model, Procedure, Resolved, SectionKin
 use crate::cholesky::{Factor, FactorError, UpperColumns};
 use crate::constraints::{Budget, Reduction, Row};
 use crate::contact::thickness;
-use crate::ordering::nested_dissection;
+use crate::ordering::fill_reducing_order;
 use crate::sparse::{Csr, NodeGraph, add_block};
 use crate::system::hold;
 use crate::tie::{TieFace, TieNodes, rows_of};
@@ -398,7 +398,7 @@ fn run(model: &Model, resolved: &Resolved, step: &Step, limits: &Limits) -> Resu
     let mut warnings = Vec::new();
     let rows = tie_rows(model, resolved, &mesh, &mut warnings)?;
     let fixed = prescribed(model, resolved, &mesh, step)?;
-    let order = nested_dissection(&mesh.xyz, &graph.adjacency);
+    let order = fill_reducing_order(&mesh.xyz, &graph.adjacency);
     let budget = Budget { max_bytes: limits.max_factor_entries.saturating_mul(8), pattern_entries: graph.entries(&mesh.layout), matrices: 1 };
     let eqs = Reduction::new(&mesh.layout, std::iter::empty(), &order, &rows, |_| Ok(fixed), budget)?;
     if eqs.neq() == 0 {