ConformalLabpp/README.md

# conformallab++

conformallab++ is a modern C++ reimplementation of the [ConformalLab](https://github.com/sechel/conformallab) software by Stefan Sechelmann for experiments in discrete conformal geometry and related mesh transformations.

The long-term goal is a **CGAL package** that brings discrete conformal maps (hyper-ideal, spherical, Euclidean) to the CGAL ecosystem using `CGAL::Surface_mesh` as the underlying half-edge data structure.

> **Status:** Phase 3 complete. Core geometry (Clausen, hyper-ideal, spherical) and the CGAL mesh infrastructure are in place. Solvers and full CLI pipeline are coming next.

---

## Features

| Area | Status |
|------|--------|
| Clausen / Lobachevsky / ImLi₂ functions | ✅ Phase 1 |
| Hyper-ideal geometry (ζ, lᵢⱼ, αᵢⱼ, σᵢ, σᵢⱼ) | ✅ Phase 2 |
| Hyper-ideal functional (energy + gradient, CGAL mesh) | ✅ Phase 3b |
| Spherical functional (energy + gradient, CGAL mesh) | ✅ Phase 3c |
| CGAL `Surface_mesh` infrastructure + mesh builders | ✅ Phase 3a |
| Euclidean functional | 🔜 Phase 4 |
| Solvers (Newton, gradient flow) | 🔜 Phase 4 |
| XML serialisation / mesh I/O | 🔜 Phase 5 |
| Full CLI app | 🔜 Phase 5 |

---

## Build modes

| Mode | CMake flag | What gets built | CI |
|------|-----------|-----------------|-----|
| **Tests only** (default) | *(none)* | `conformallab_tests` · Eigen + GTest | ✅ runs automatically |
| **CGAL tests** | `-DWITH_CGAL=ON` | `conformallab_cgal_tests` · above + CGAL + system Boost | local only |
| **Viewer** | `-DWITH_VIEWER=ON` | `viewer` library · libigl / GLFW / GLAD | local only |
| **Full app** | `-DWITH_CGAL=ON` | `conformallab_core` CLI · all of the above | local only |

External dependencies are bundled as tarballs in `code/deps/tarballs/` and extracted lazily at CMake configure time (GTest is fetched from GitHub via `FetchContent`).

**Note on Boost:** `-DWITH_CGAL=ON` requires a system-installed Boost (header-only use by CGAL). The default `Tests only` mode needs no Boost.

---

## Prerequisites

| Tool | Minimum version |
|------|----------------|
| C++ compiler (GCC or Clang) | C++17 |
| CMake | 3.20 |
| Boost headers | 1.70 *(only with `-DWITH_CGAL=ON`)* |

---

## Getting started

```bash
git clone https://codeberg.org/TMoussa/ConformalLabpp
cd ConformalLabpp
```

### Tests only (CI default — no system deps needed)

```bash
cmake -S code -B build
cmake --build build --target conformallab_tests -j$(nproc)
ctest --test-dir build --output-on-failure
```

### CGAL mesh + functional tests (requires system Boost)

```bash
cmake -S code -B build -DWITH_CGAL=ON
cmake --build build --target conformallab_cgal_tests -j$(nproc)
ctest --test-dir build -R "^cgal\." --output-on-failure
```

Expected output: **30 tests pass, 2 skipped** (the two `@Ignore` Hessian stubs).

### Full CLI app (CGAL + viewer)

```bash
cmake -S code -B build -DWITH_CGAL=ON
cmake --build build -j$(nproc)
./code/bin/conformallab_core --input data/off/example.off --show
```

---

## Public headers (`code/include/`)

| Header | Description |
|--------|-------------|
| `clausen.hpp` | Clausen integral Cl₂, Lobachevsky Л, ImLi₂ |
| `hyper_ideal_geometry.hpp` | Pure-math ζ functions, lᵢⱼ, αᵢⱼ, σᵢ, σᵢⱼ |
| `hyper_ideal_utility.hpp` | Tetrahedron volume (Meyerhoff / Kolpakov–Mednykh) |
| `hyper_ideal_functional.hpp` | Hyper-ideal energy + gradient on `ConformalMesh` |
| `spherical_geometry.hpp` | Spherical arc length, half-angle angle formula |
| `spherical_functional.hpp` | Spherical energy + gradient on `ConformalMesh` |
| `conformal_mesh.hpp` | `ConformalMesh` = `CGAL::Surface_mesh<Point3>`, index types, property-map helpers |
| `mesh_builder.hpp` | Factory meshes: triangle, tetrahedron, quad strip, fan, spherical tetrahedron, octahedron face |
| `discrete_elliptic_utility.hpp` | Discrete elliptic integrals |
| `matrix_utility.hpp` | Small linear-algebra helpers |
| `projective_math.hpp` | Projective geometry utilities |

---

## Project structure

```
code/
├── include/
│   ├── conformal_mesh.hpp          # CGAL mesh type + property-map helpers
│   ├── mesh_builder.hpp            # make_triangle / make_tetrahedron / …
│   ├── hyper_ideal_geometry.hpp    # ζ, lᵢⱼ, αᵢⱼ — pure math
│   ├── hyper_ideal_functional.hpp  # HyperIdealFunctional on ConformalMesh
│   ├── spherical_geometry.hpp      # spherical arc length + angles
│   ├── spherical_functional.hpp    # SphericalFunctional on ConformalMesh
│   ├── clausen.hpp                 # Clausen / Lobachevsky / ImLi₂
│   └── hyper_ideal_utility.hpp     # Tetrahedron volumes
├── src/
│   ├── apps/v0/                    # conformallab_core CLI (requires WITH_CGAL)
│   └── viewer/                     # simple_viewer (requires WITH_VIEWER)
├── tests/
│   ├── CMakeLists.txt
│   ├── *.cpp                       # conformallab_tests (no CGAL)
│   └── cgal/
│       ├── CMakeLists.txt
│       ├── test_conformal_mesh.cpp         # 14 mesh infrastructure tests
│       ├── test_hyper_ideal_functional.cpp # 6 hyper-ideal gradient checks
│       └── test_spherical_functional.cpp   # 8 spherical gradient checks
└── deps/
    ├── tarballs/          # bundled dependency archives
    ├── eigen-3.4.0/       # header-only linear algebra (always extracted)
    ├── CGAL-6.1.1/        # header-only geometry (extracted with WITH_CGAL)
    ├── libigl-2.6.0/      # header-only viewer toolkit (extracted with WITH_VIEWER)
    ├── glfw-3.4/          # windowing (extracted with WITH_VIEWER)
    ├── libigl-glad/       # OpenGL loader (extracted with WITH_VIEWER)
    └── single_includes/   # CLI11, json.hpp
```

---

## Test suites

### `conformallab_tests` (always built, runs in CI)

Pure-math tests requiring only Eigen:

- Clausen function, Lobachevsky function, ImLi₂
- Hyper-ideal geometry (ζ₁₃, ζ₁₄, ζ₁₅, lᵢⱼ, αᵢⱼ)
- Tetrahedron volume formulas (Meyerhoff, Kolpakov–Mednykh)

### `conformallab_cgal_tests` (built with `-DWITH_CGAL=ON`)

CGAL `Surface_mesh` tests (test prefix `cgal.`):

| Suite | Tests | Description |
|-------|-------|-------------|
| `ConformalMeshTopology` | 4 | Euler characteristic, vertex/edge/face counts |
| `ConformalMeshTraversal` | 4 | Halfedge iteration, valence, opposite-halfedge |
| `ConformalMeshProperties` | 5 | Property maps: λ, θ, idx, α, geometry type |
| `ConformalMeshValidity` | 1 | CGAL validity check for all factory meshes |
| `HyperIdealFunctional` | 6 | Finite-difference gradient checks on triangle, tetrahedron, quad strip, fan |
| `SphericalFunctional` | 8 | Angle formula correctness + gradient checks on spherical meshes |

---

## Key design decisions

**CGAL as CoHDS replacement.** `CGAL::Surface_mesh<Point3>` replaces the Java `CoHDS` half-edge data structure. Vertex/edge/face/halfedge descriptors are `Vertex_index`, `Edge_index`, `Face_index`, `Halfedge_index` (typed integers, not raw handles).

**Property maps.** `mesh.add_property_map<Vertex_index, double>("v:lambda", 0.0)` replaces the Java adapter/decorator pattern. Multiple maps can be attached to one mesh without subclassing.

**Energy parameterisation.** Both functionals use a **DOF vector** `x` indexed by `v_idx[v]` / `e_idx[e]` (−1 = pinned). This matches the Java `FunctionalTest` gradient-check convention.

**Spherical edge gradient.** For the spherical parameterisation where `Λᵢⱼ = λ°ᵢⱼ + uᵢ + uⱼ + λₑ` (additive edge DOF), the Schläfli identity gives `∂E/∂λₑ = (2α_opp − S_f)/2` per face (Euclidean limit: `S_f = π`, recovering the familiar `α_opp⁺ + α_opp⁻ − π`).

---

## CI

Tests run automatically on push to `main`, `dev`, and `claude/**` branches via a self-hosted Gitea Actions runner (`eulernest`, ARM64 Raspberry Pi). The pipeline uses a minimal Docker image (`git.eulernest.eu/conformallab/ci-cpp:latest`) with cmake, g++, git, and Node.js 20 pre-installed. **Only `conformallab_tests` runs in CI** (no Boost/CGAL dependency in the CI image).

The Dockerfile for the CI image lives in `.gitea/docker/Dockerfile.ci-cpp`. Build and push it once whenever the image needs updating:

```bash
docker buildx build \
  --platform linux/arm64 \
  -f .gitea/docker/Dockerfile.ci-cpp \
  -t git.eulernest.eu/conformallab/ci-cpp:latest \
  --push \
  .gitea/docker/
```

---

## License

conformallab++ is released under the MIT License (see [LICENSE](LICENSE)).