OneCAD-Simulation/docs/architecture.md

# Architecture

ModelCraft turns geometry into finite element models, writes them for the solvers engineers
already use, and solves linear problems itself.

```
SolidCraft B-rep ──tessellate──▶ boundary surface ──▶ volume mesh ──▶ FE model ──▶ Abaqus .inp
 (solidcraft-kernel)           (welded, watertight,   tet (Delaunay)   materials,     Nastran .bdf
                                face-tagged)          hex (core goal)  sections,      LS-DYNA .k
                                                                       BCs, loads,  ──▶ own linear
                                                                       steps            solver
```

## Crates and layers

`cargo xtask layers` enforces the table in `xtask/src/layers.rs`: a crate may depend only on
crates in strictly lower layers.

| Layer | Crate | Role | Status |
|---|---|---|---|
| L0 | `modelcraft-model` | Solver-neutral FE model: nodes and elements by label, their geometry tags, sets, materials, sections, couplings, BCs, loads and steps, the model check and the resolved view the writers read. Knows no file format and no geometry type. | started |
| L1 | `modelcraft-geometry` | The only door to SolidCraft: opens `.solidcraft` designs (rebuilt from their timeline, bodies with persistent face names), B-rep bodies, measures, tessellation into a face-tagged `SurfaceMesh`; face-tagged 3MF files read into the same type without the kernel (docs/formats/tagged-mesh.md). No SolidCraft type in its API. | started |
| L1 | `modelcraft-mesh` | Surface preparation (weld, watertightness, feature edges), robust predicates, Delaunay tetrahedralization and refinement, Voronoi-based optimization, hex meshing, quality metrics. | started |
| L1 | `modelcraft-fem` | Element kernels shared by every structural solver: the element table (tet4, tet10, wedge6, hex8 full and incompatible modes, hex20 full and reduced), shape functions, quadrature, element stiffness, consistent and lumped mass, body and pressure loads, stresses and their extrapolation, isotropic and orthotropic elasticity, and the uniform-strain (one-point) hex8 and tet4 kinematics, hourglass shape vectors and element frequency bounds of the explicit solver. Pure functions, no solver loop. | started |
| L1 | `modelcraft-decks` | Abaqus input, Nastran bulk data and LS-DYNA keyword readers and writers, to and from `model`. Text out, no file system; `deck.write` saves it. | started |
| L1 | `modelcraft-render` | Display data without a GPU dependency: the visible faces, element edges and nodes of a model, the orbit camera, a CPU rasterizer for headless snapshots; contour maps later. | started |
| L2 | `modelcraft-solver` | Implicit solver: assembly from the resolved model, boundary conditions, sparse direct solution, linear static and modal analysis, stresses and reactions. The lattice Boltzmann solver (`modelcraft-lbm`) joins this layer. | started |
| L2 | `modelcraft-explicit` | Explicit dynamics: central-difference time integration with lumped mass, one-point hex8 with hourglass control, nodal-pressure tet4, bulk viscosity, rigid walls, energy account, histories and field frames. | started |
| L3 | `modelcraft-fsi` | Fluid–structure coupling between the L2 solvers (later). | planned |
| L4 | `modelcraft-doc` | The document: geometry, mesh, model, results, history. | planned |
| L5 | `modelcraft-io` | `.modelcraft` files (docs/formats/modelcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery. Bytes in, bytes out; the file system only in `fs` and `recovery`. Result files later. | started |
| L6 | `modelcraft-engine` | Commands (`CommandSpec`), `Session::execute` with rollback and the panic guard, scripts. | started |
| L7 | `modelcraft-ui-egui`, `modelcraft-mcp` | Desktop UI (egui + wgpu viewport, model tree, commands, status bar) and control channel requests (docs/control-protocol.md); MCP server. | ui started |
| testkit | `modelcraft-testkit` | Test tooling, a dev-dependency only: the meshing test corpus (`tests/corpus/*/recipe.json`, analytic shapes) and its runner (`cargo xtask corpus`). | started |
| apps | `modelcraft`, `modelcraft-cli`, `modelcraft-web` | Desktop, command line, browser. | desktop, cli started |

Rules beyond the layer order:
- Nothing below L7 uses egui, eframe, winit, wgpu or rfd.
- Only `modelcraft-geometry` uses `solidcraft-*`, pinned by rev in the workspace `Cargo.toml`.
  SolidCraft's `[patch.crates-io]` (its fixed truck crates) does not reach dependents, so the
  workspace repeats it from the same rev; bump both together. The same goes for SolidCraft's
  patched `egui-wgpu` (MSAA clamped to what the device supports, 1x on OpenGL adapters, so the
  desktop app's 4x never fails pipeline validation at start).
- Everything below the apps builds for `wasm32-unknown-unknown`.
- `modelcraft-geometry`'s non-default `scripts` feature builds designs from SolidCraft command
  scripts (`script_design`, through `solidcraft-engine`) for the test corpus; the library opens
  designs without it.

## Meshing pipeline

1. **Boundary surface.** SolidCraft tessellates each body; every triangle carries its B-rep face,
   so feature edges and face-based loads and BCs survive meshing. Faces of an opened design also
   carry SolidCraft's persistent names (`F2:side:l4`, from the feature and sketch curve that made
   them), so a load on a named face follows it when the design is edited and opened again.
   Bodies of components are placed in the world through their occurrences (one body per
   placement), as SolidCraft displays and exports them.
2. **Surface preparation** (`modelcraft-mesh::surface`, `mesh.surface.check`). Weld the tessellation's per-face
   vertex copies (at twice SolidCraft's linear tolerance), check the surface is closed, manifold, consistently and
   outwardly oriented (voids inwards) and free of self-intersections (exact triangle–triangle tests), and find
   feature edges (face boundaries, sharp dihedral angles). Defects come back located by triangle, vertex and
   position.
3. **Tetrahedral meshing.** Bowyer–Watson Delaunay on Shewchuk's adaptive predicates (symbolic
   perturbation for degeneracies), boundary recovery by the robust constrained Delaunay method of
   Diazzi et al. 2023 (implicit Steiner points on indirect predicates, gift-wrapping facet
   recovery; `modelcraft-mesh::cdt`, command `mesh.tet.generate`), so every boundary triangle and
   its B-rep face tag survives. Then constrained Delaunay refinement to a
   size field, ODT smoothing interleaved as in Tournois et al. 2009, and the Klingner–Shewchuk 2007
   improvement schedule against slivers.
4. **Hexahedral meshing (the core).** Octree/grid dual all-hex first, the one family that meshes
   any input validly (Livesu et al. 2021 dual schemes, Pitzalis et al. 2021 refinement, Gao et al.
   2019 feature projection and padding, Garanzha et al. 2021 untangling). Then hex-dominant by tet
   recombination (Pellerin et al. 2018), then polycube (DualCube labelling) for blocky parts;
   frame-field/integer-grid-map methods stay research. Validity uses a certified trilinear Jacobian
   check (Marschner et al. 2020), not corner Jacobians alone.
5. **Structured hex and face meshing.** Faces get all-quad meshes on shared, prescribed edge
   nodes (`modelcraft-mesh::quad`): transfinite interpolation for four-sided faces (Gordon and
   Hall 1973), and for free faces a half-resolution constrained Delaunay triangulation,
   recombination (Lee and Lo 1994) and one uniform split that lands on the boundary nodes, then
   smoothing and untangling; every quad is checked with exact predicates. One `SizeField` trait
   (`modelcraft-mesh::size`) sets interior sizes and edge divisions, with even counts per loop.
   Mapped and swept hex build on these face meshes.
6. **Quality.** Scaled Jacobian, aspect ratio, dihedral angles; every generated mesh is checked
   for positive Jacobians and conserved volume before it reaches the model.

## Model definition

`modelcraft-model` holds the analysis model in no solver's vocabulary (the writers map it to
element names, cards and keywords):

- **Geometry tags.** Geometry is referred to by name only: a body and, inside it, the persistent
  name of a face, edge or vertex (`{"body": "Body1", "face": "F2:top"}`). A mesher tags what it
  makes: each element with the body (solids), face (shells) or edge (beams) it fills, each
  boundary element face with its geometry face, element edges with geometry edges, nodes with
  vertices (`Model::geometry_tag`, `tag_element`, `tag_element_face`, …; `mesh.tag` by hand).
  Replacing or removing an element drops its tags; a remesh clears them and writes new ones.
- **Sets** are defined by geometry names, explicit labels or both, and resolve against the
  current mesh into sorted nodes, elements and element faces. A face name also selects the pieces
  a kernel splits it into (`name#0`, `name#1`). Because the definition names geometry, a remesh,
  or an edit of the design followed by a remesh, keeps every set, section and load in place.
- **Element faces** are numbered per kind (`ElementKind::faces`): solids' faces are oriented with
  the right-hand normal pointing out; shells have a positive and a negative side.
- **Definitions** (materials, sections with their element formulation, reference points,
  couplings, boundary conditions, loads, steps) refer to each other by name. A reference must
  exist when it is made, and a definition still referred to (a load listed by a step included)
  cannot be deleted; what depends on
  the mesh is checked when the model is resolved.
- **Steps** list the boundary conditions and loads active in them. Nothing propagates from one
  step to the next.
- **Units** are a label (`mm-t-s-N-MPa`, …); numbers are never converted, and the check warns
  when a modulus, density or gravity is implausible for the chosen system.
- **Check and resolve.** `Model::check` lists errors and warnings, each naming its item (a set
  member not on the mesh, an element with no section or in two, a moment on solid nodes, a
  missing density for a frequency step, …). `Model::resolve` returns what writers and the solver
  read besides the definitions: every set resolved (each element then lies in exactly one
  section's set), the node label of every reference point, and every force, moment and traction
  as nodal values. It fails when the check finds an error.
- **Loads that keep their meaning on any mesh.** A force or moment on a set is a total by
  default: split by the consistent nodal weights `∫ N_i dA` of the set's faces (Gauss quadrature
  on each face's own linear or quadratic geometry), or equally over a set of nodes only. Applying
  it at every node is an explicit, mesh-dependent opt-in (`per_node`). A traction is force per
  area on faces. The expansion lives in `resolve`, so the deck writers and the solver share it.
- **Reference point labels** are stored with the point: chosen from the top of the label range
  when it is created, so remeshing or adding nodes and points never renumbers them.
- **Files.** A model deserializes by replaying its contents through the same checked methods the
  commands use, so a file cannot hold what commands would refuse.

## Implicit solver

`modelcraft-solver` solves a model's linear static and frequency steps (`solve.static`,
`solve.frequency`; `solve.probe` reads the stored results). Methods and papers are cited in the
module docs; ADR 0001 records why the linear algebra is our own.

1. **System.** `Model::resolve()`; each solid section's elements take their material's
   elasticity matrix and the section's formulation (`modelcraft_fem::element::ELEMENT_TYPES`, one
   row per element kind: tet4, tet10, wedge6, hex8, hex20). Every reference point a coupling
   ties to the mesh gets six degrees of freedom (translations, rotations) after the mesh's. Shells
   and beams return "not supported yet". The step's boundary conditions prescribe translations
   (rotations do not exist on solid nodes) and a coupled point's six directions; its loads become
   a load vector: forces and tractions from `Resolved::nodal_loads`, forces and moments on coupled
   reference points, pressure as consistent face loads, gravity and body forces through the
   element shape functions.
2. **Assembly** on the node graph (3 × 3 blocks, elements in label order, so sums are
   reproducible bit for bit).
3. **Ordering and elimination** (`constraints`). Nested dissection of the nodes by coordinate
   bisection. Couplings become linear constraint rows: kinematic = rigid link of each coupled
   translation to the point's translation and rotation; distributing = the point follows the
   least-squares rigid fit of the set's translations (equal weights, the deck writers' choice).
   Prescribed values and rows are eliminated master–slave in a fixed order, one dependent degree
   of freedom per row (the coupled node or the distributing point while it is free, else the
   largest coefficient), giving u = T_f q + T_p u_p; the reduced matrix T_fᵀ K T_f stays
   symmetric positive definite (no penalty, no multipliers). Free mesh equations follow the
   node order, reference point equations come last. Without couplings T is a selection and the
   numbers are those of plain elimination, bit for bit.
4. **Static solution.** Up-looking sparse Cholesky (the factor size is computed and capped before
   any value), reactions T_pᵀ (K u − f) (at nodes and at prescribed reference points, forces and
   moments), integration-point stresses extrapolated to the nodes and averaged, von Mises. A zero
   pivot names the node or reference point and direction that can move freely.
5. **Frequency.** Shift-invert block Lanczos (block 6, full reorthogonalization) on
   K − σM, σ = 0 for a supported structure and slightly negative for a free one, so rigid-body
   modes come out at zero; mass-normalized shapes, participation factors and effective masses.
6. **Results** stay in the session per step, stamped with the session revision they were solved
   at; `Session::result` hands out only those of the current revision, so an edit makes them
   stale and undoing back to the solved state makes them current again.

Verification (`crates/solver/tests/verification.rs`, analytic only): displacement and traction
patch tests for every element and formulation (exact to round-off), cantilever tip deflection
against Timoshenko with convergence rates of the linear elements, Lamé's cylinder, Kirsch's
plate with a hole (Kt → 3), cantilever and free–free beam frequencies with six rigid-body modes.
`crates/solver/tests/couplings.rs`: a block given a rigid motion through a coupled point
(kinematic and distributing), clamping through a coupled point equals clamping (static and
modes), tension through a distributing coupling against the equivalent pressure away from the
end, pure bending by a moment through a distributing coupling, torsion of a tube by a moment on
a kinematically coupled point (twist and shear stress).

## Explicit solver

`modelcraft-explicit` integrates a model's `explicit_dynamic` steps in time (`solve.explicit`;
`solve.history` and `solve.probe` read the stored histories and frames). Methods and papers are
cited in the module docs and in docs/solvers/literature.md (Part H).

1. **System.** `Model::resolve()`; hex8 sections with reduced integration become one-point
   uniform-strain hexahedra with Flanagan–Belytschko hourglass control, tet4 sections Bonet–Burton
   average nodal pressure tetrahedra (`modelcraft_fem::uniform` holds their kinematics). Other
   element kinds and formulations, couplings, reference points and moments return "not supported
   yet". Masses are row-sum lumped. A zero-valued boundary condition holds a degree of freedom; a
   displacement value is reached at constant velocity over the step. Loads act in full from the
   start. The step lists initial velocities and fixed frictionless rigid planes by set.
2. **Time step.** Each element's highest frequency is bounded in closed form (exact for a cube);
   the increment is the step's scale (0.9 by default) times the smallest element increment with
   Wilkins' bulk-viscosity correction, recomputed every increment. The ten controlling elements
   are reported; a step that would need more increments than its cap is refused before it runs.
3. **Integration.** Central differences in the half-step form, elements in label order, no
   parallel reductions (bit-for-bit repeatable). Rigid walls and prescribed degrees of freedom are
   kinematic velocity constraints whose impulses are the reactions.
4. **Energy account.** Kinetic, internal (stress work), hourglass, bulk viscosity, external and
   constraint work, and their balance; a run warns when the balance is off by more than 1% of the
   largest energy or the hourglass energy exceeds 10% of the internal energy.
5. **Results.** Histories (energies, momentum, constraint force, chosen nodes) at the history
   interval and field frames (displacement, velocity, element stress and strain) at the output
   interval, capped in size; kept in the session like the implicit results.

The first version is geometrically linear with linear elastic materials. Verification
(`crates/explicit/tests/verification.rs`, analytic only): stress waves in hex8 and tet4 bars
(front at c·t, σ = ρ c v, work input), a bar striking a rigid wall (contact time 2L/c, rebound
speed, plateau force ρ c v₀ A), the energy balance, bit-for-bit repeatability; in the engine
(`solve.rs`), a cantilever's free-vibration period against `solve.frequency`.

## Commands

Every user-visible action is a `CommandSpec` in `crates/engine/src/cmd/<area>.rs` with a JSON
form. `Session::execute` clones the model, runs the command under `catch_unwind`, and restores
the model on an error or a panic; commands that changed the model are journalled, so the
journal replays as a script. Each journalled command is also an undo step (the state before it):
`edit.undo` restores that state and moves the command off the journal onto the redo stack,
`edit.redo` puts it back, so the journal always rebuilds the current model. History is bounded by
step count and by an estimate of the memory the kept states hold (oldest steps go first).
Commands of kind `Session` (`file.*`, `edit.*`) are never journalled. `file.save` and `file.open`
write and read `.modelcraft` files; the desktop app autosaves unsaved work for `file.recover`.

## Test corpus

Meshers are measured on our own parts (`docs/adr/0001-libraries.md`, test geometry). Each part is
a SolidCraft command script in `tests/corpus/<name>/recipe.json` (the file `solidcraft-cli run`
takes, plus a `corpus` object with its categories and, when it has one, its closed-form volume)
with a one-line `README.md`; analytic shapes (sphere, torus, cylinder, thick-walled pipe, plate
with a hole) are generated in code. `modelcraft-testkit` runs a recipe in SolidCraft's engine,
saves the design and opens it through `open_design`, the path a person's design takes, so no
geometry is stored. `cargo xtask corpus` (a release build of the testkit's `corpus` binary)
builds every part, tessellates it and runs each stage of `modelcraft_testkit::runner`, writing
docs/meshing/corpus.md. A mesher joins by implementing `runner::Stage` (its columns, and a run
over the part's tessellated surfaces) and adding itself to `runner::stages()`.