# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Project Overview

**SimVulcan** is a minimal **Vulkan 1.3 / C++20** 3D model editor. It renders a
3D editor space — three reference grid planes (XY, XZ, YZ) through the origin plus
coloured X/Y/Z axes — viewed through an orbit camera, and loads/displays `.obj`
models with selectable display modes (solid, wireframe, solid + wireframe).

The C++ side runs no simulation: it is a focused rendering skeleton with an ImGui
interface (a Viewport control panel and a Mesh load panel). The repository *also*
carries an unrelated Python research prototype under `docs/theory/` — see
[Research prototype](#research-prototype-docstheory) below; it is not part of the
CMake build.

## Build

Prerequisites: **Vulkan SDK 1.3.290+** (provides `glslangValidator` for offline
shader compilation), **CMake 3.26+**, **Ninja**, a C++20 compiler (MSVC 19.36+,
gcc 11+, clang 14+). On macOS, Vulkan is via MoltenVK (Apple Silicon only).

```sh
cmake --preset windows-msvc-release
cmake --build --preset windows-msvc-release
```

Presets: `windows-msvc-debug`, `windows-msvc-release`, `linux-gcc-release`,
`linux-clang-release`, `macos-arm64-release` (all Ninja, one dir per preset under
`build/<presetName>/`). The first configure fetches dependencies via FetchContent
(GLFW, GLM, volk, vk-bootstrap, VulkanMemoryAllocator, Dear ImGui, spdlog,
tinyobjloader, Catch2) and needs network access. `VK_NO_PROTOTYPES` is set
project-wide; Vulkan entry points load through **volk**.

Warnings come from `simv_set_warnings` (`/W4 /permissive-`, or
`-Wall -Wextra -Wpedantic -Wshadow -Wold-style-cast …`); they are **not** errors.
`cmake/Sanitizers.cmake` defines `simv_enable_sanitizers` (Debug-only ASan/UBSan)
but no target currently calls it — wire it in manually when chasing memory bugs.

## Running

The executable resolves SPIR-V relative to the working directory (`FindSpvPath`
probes `spirv/<rel>` and `current_path()/spirv/<rel>`). The `SimVulcan` POST_BUILD
step copies the compiled `spirv/` tree and `assets/` next to the executable, so
**run from the executable's own directory** (`build/<preset>/src/app/`). `main.cpp`
also probes several `../` ancestors for `assets/meshes`. Meshes load at runtime
through the ImGui Mesh panel.

Running writes two files into the CWD: `pipeline_cache.bin` (serialised
`VkPipelineCache`, reloaded on the next start) and ImGui's `imgui.ini`. Both are
disposable — delete them if pipeline creation or the panel layout misbehaves.

`ContextOptions::enableValidation` / `enableDebugUtils` default to **true** in
every build config, so the Khronos validation layer is requested even in Release;
messages (error + warning severity) go through spdlog.

`run.bat` at the repo root is **stale**: it launches
`build/vs2022/src/app/Release/SimVulcan.exe`, a path the Ninja presets never
produce. Do not point users at it without fixing the path first.

## Tests

Catch2 unit tests, pure CPU/math (mesh bounds + welding). No GPU required.

```sh
cmake --build --preset windows-msvc-debug --target simv_tests
ctest --preset windows-msvc-debug
```

`windows-msvc-debug` is the only preset with a `testPreset` — for the other
configs, invoke `ctest` in `build/<preset>/` directly.

Single test / subset — either through CTest (each `TEST_CASE` is registered
individually by `catch_discover_tests`):

```sh
ctest --preset windows-msvc-debug -R "WeldVertices" -V
```

or by running the binary with a Catch2 name or tag filter:

```sh
./build/windows-msvc-debug/tests/simv_tests.exe "[decimator]"
./build/windows-msvc-debug/tests/simv_tests.exe --list-tests
```

## Architecture

### Library / target map

```
SimVulcan (exe) → simv_core, simv_vk, simv_mesh, simv_editor
simv_core   → simv_vk            (App owns Window + vk::Renderer; no Vulkan calls)
simv_editor → simv_core, simv_mesh   (Camera, input, ImGui panels; no Vulkan)
simv_mesh   → simv_core              (CPU mesh only; no Vulkan)
simv_vk     → third-party (volk, vk-bootstrap, VMA, GLFW, GLM, spdlog, imgui)
simv_shaders → glslangValidator (GLSL → SPIR-V)
```

Namespaces follow directories: `simv::core`, `simv::vk`, `simv::mesh`,
`simv::editor`. Each library exports `src/` as its include root, so includes are
written module-qualified (`#include "mesh/Mesh.h"`, `#include "vk/Renderer.h"`).

### Frame loop

`main.cpp` creates `core::App`, which owns the `core::Window` and a
`vk::Renderer`, then runs the loop. Each frame `Renderer::DrawFrame` calls the UI
callback (between ImGui NewFrame/Render), then records the scene: grid + mesh into
one dynamic-rendering pass with a depth attachment, followed by ImGui, then
presents (2 frames in flight, sync2 submits).

`main.cpp` wires the editor via the UI callback: it draws the panels, applies
mouse input to the `editor::Camera`, and pushes the resulting state into the
renderer (`SetViewProj`, `SetRenderMode`, `SetGridVisible`). Mesh loads go through
`MeshLoadPanel`'s callback → `Renderer::SetMeshCpu`.

### Mesh load path

`MeshLoadPanel` lists `*.obj` in the mesh directory and kicks off
`mesh::LoadObjAsync` (worker thread, `std::future`). The future is **drained on
the main thread** at the top of `MeshLoadPanel::Draw`, so the `OnLoaded` callback
— and therefore the GPU upload — always runs on the render thread. Loader
exceptions surface as the panel's status string.

`main.cpp`'s `OnLoaded` welds the mesh (`WeldVertices`, tolerance `1e-4`), logs
the counts, uploads via `SetMeshCpu`, and reframes the camera to the bbox radius.
Despite the file name, `mesh/MeshDecimator.h` implements **only** spatial-hash
vertex welding (collapse near-duplicates, drop degenerate triangles, recompute
bounds) — there is no LOD/decimation.

### Non-obvious invariants (read before editing)

- **All Vulkan lives in `src/vk/`.** `core/`, `mesh/`, `editor/` and `app/` make no
  Vulkan API calls. `vk::Renderer`'s public header is deliberately Vulkan-free
  (pImpl + glm/`RenderMode` only) so `App` can own it without pulling in volk. Keep
  it that way — do not leak `Vk*` types into the public interfaces of those modules.
- **Single render pass with depth.** Scene (grid + mesh) and ImGui draw into one
  `vkCmdBeginRendering` pass that has both a colour and a `D32_SFLOAT` depth
  attachment. The ImGui backend is initialised with `depthAttachmentFormat` set so
  its pipeline matches the pass; the depth buffer is recreated with the swapchain.
- **Wireframe needs `fillModeNonSolid`.** `MeshRenderer` builds a fill pipeline and
  a `VK_POLYGON_MODE_LINE` pipeline; the line pipeline uses a small depth bias so
  the overlay sits on top of the fill. The device feature is requested in `Context`.
  Culling is off (`VK_CULL_MODE_NONE`) — loaded models may have mixed winding.
- **Camera is fixed on the origin.** `editor::Camera` orbits (yaw/pitch/distance)
  the world origin; loaded models are recentred there via a translate-only model
  matrix. Projection uses Vulkan clip space (`GLM_FORCE_DEPTH_ZERO_TO_ONE` + Y flip,
  isolated to `Camera.cpp`).
- **Buffers.** `vk::GpuMesh` owns the model's vertex/index buffers (staged upload on
  the transfer queue). `GridRenderer` builds a static host-visible line buffer once.
  Both scene renderers use only a push constant — no descriptor sets.
- **Push-constant layout is a cross-file contract.** `MeshRenderer.cpp`'s anonymous
  `MeshPC { mat4 mvp; vec4 color; }` must stay byte-identical to the
  `push_constant` block in `mesh.vert`/`mesh.frag`; `color.a` is a *flag*, not
  alpha (1 = flat-shaded, 0 = constant colour for wireframe). `GridRenderer` pushes
  a bare `mat4` (vertex stage only). Change either side and you must change both.
- **Swapchain recreation rebuilds sync objects.** `RecreateSwapDependent` recreates
  the per-frame `imageAvailable` semaphores (a failed acquire can leave one
  signalled) *and* the per-swapchain-image `renderFinished` semaphores (the image
  count may change), then re-ensures both pipelines. Keep that ordering if you
  touch resize handling.
- **Mesh upload stalls the device.** `SetMeshCpu`/`ClearMesh` call `WaitIdle`
  before touching `GpuMesh` — acceptable because loads are rare; do not copy that
  pattern into per-frame paths.

### Shaders

GLSL under `shaders/editor/` (`mesh.{vert,frag}`, `grid.{vert,frag}`). `simv_shaders`
compiles each to `build/<preset>/spirv/editor/<name>.spv` targeting `vulkan1.3`,
with `shaders/` as the `-I` root (so `#include "common/foo.glsl"` would resolve).
Adding a shader under that globbed dir is picked up automatically
(`CONFIGURE_DEPENDS`). `mesh.frag` reconstructs a flat normal from screen-space
derivatives, so the vertex stream carries only positions (tight `float3`, one
binding, one attribute).

## Logging

`simv::core::Logger` (spdlog-backed, singleton) with category-based throttling.
Log via `LogFmt(LogCategory, LogLevel, fmt, args...)`. Categories: `Core`,
`Vulkan`, `MeshIO`, `UI`, `Test`. Per-category level, throttle interval and
on/off are settable at runtime (`SetMinLevel` / `SetThrottle` / `SetEnabled`).
Some low-level Vulkan code still calls `spdlog::` directly.

## Research prototype (`docs/theory/`)

Separate from the C++ application and from the CMake build: a Python **Lattice
Boltzmann (D2Q9, KBC-N1)** research codebase — cylinder flow with a ×2 nested AMR
patch and SDF+Bouzidi boundaries. **GPU/CuPy only, no CPU fallback.** Its
documentation and the running experiment log are in Russian.

- `docs/theory/solver_2x_sdf/` — the component-split solver (`run.py`,
  `run_blockage.py`, `run_factors.py`); its `README.md` is the authoritative
  status/experiment log and maps every file to the physics it owns.
- `docs/theory/demos_gpu/` — demo runs that produce the notebook's figures/GIFs;
  they import physics from `solver_2x_sdf`, never duplicate it.
- `docs/theory/kbc_lbm.ipynb` — the write-up; it embeds pre-rendered artefacts and
  does not execute the simulations.
- `docs/origins/` — source PDFs behind the theory.

Do not fold this into the CMake build, and do not treat it as dead code — it is
active research with a documented experiment history.