# C++ Port — Boundaries Batch Generator

Port the `boundaries.ts` pipeline to native C++ for **10–100× speedup** on geometry union + GHS raster enrichment.

> [!NOTE]
> Assumes existing C++ boilerplate for argument parsing, logging, and JSON serialization is available.

---

## Why C++

| Bottleneck (TS) | Root cause | C++ remedy |
|---|---|---|
| `polygon-clipping.union` | Pure-JS, throws on complex polys (DEU) | GEOS `GEOSUnaryUnion` — battle-tested C engine |
| `enrichFeatureWithGHS` PIP loop | GC pressure, obj alloc per pixel | Stack-allocated ray-cast, zero alloc |
| `readRasters` (GeoTIFF) | JS decoder, no tiling | GDAL `RasterIO` — native COG/tiled reads |
| `proj4` per pixel | JS re-init overhead | PROJ C API, reuse `PJ*` transform handle |

---

## Dependencies

| Library | Role | Notes |
|---|---|---|
| **GDAL/OGR 3.x** | GeoPackage read, GeoTIFF read, coordinate transforms | Single dep for 3 concerns |
| **GEOS 3.12+** | `GEOSUnaryUnion`, simplify, valid geometry repair | Used internally by GDAL |
| **nlohmann/json** | JSON output (cache files) | Header-only |
| **SQLite3** | Already bundled with GDAL; direct access if needed | |

On Windows: `vcpkg install gdal geos nlohmann-json` or use the OSGeo4W SDK.

---

## Architecture

```
boundaries-cli
├── main.cpp            # CLI entry, country loop, cache skip logic
├── gpkg_reader.h/cpp   # GeoPackage → GeoJSON features
├── geo_merge.h/cpp     # Geometry union (GEOS)
├── ghs_enrich.h/cpp    # GeoTIFF raster sampling + PIP
├── pip.h               # Inline ray-casting point-in-polygon
└── types.h             # BoundaryFeature, BoundaryResult structs
```

---

## Module Mapping (TS → C++)

### 1. `gpkg_reader` — GeoPackage Reading

**TS source:** [gpkg-reader.ts](file:///c:/Users/zx/Desktop/polymech/pm-pics/packages/gadm/src/gpkg-reader.ts)

- Replace custom WKB parser + `better-sqlite3` with **OGR `GDALOpenEx`** + `OGR_L_GetNextFeature`
- OGR natively reads GeoPackage and returns `OGRGeometry*` objects — no manual WKB parsing
- Group features by `GID_{level}` column, collect geometries per group

```cpp
GDALDatasetH ds = GDALOpenEx("gadm41.gpkg", GDAL_OF_VECTOR, nullptr, nullptr, nullptr);
OGRLayerH layer = GDALDatasetGetLayer(ds, 0);
OGR_L_SetAttributeFilter(layer, "GID_0 = 'DEU'");
OGRFeatureH feat;
while ((feat = OGR_L_GetNextFeature(layer)) != nullptr) {
    OGRGeometryH geom = OGR_F_GetGeometryRef(feat);
    // group by GID_{level}, collect geom clones
}
```

### 2. `geo_merge` — Geometry Union

**TS source:** `mergeGeometries()` in gpkg-reader.ts

- Replace `polygon-clipping` + pairwise fallback with **`GEOSUnaryUnion`**
- GEOS handles all edge cases (self-intersections, degeneracies) that crash JS libs
- For invalid inputs: `GEOSMakeValid` → then union

```cpp
GEOSGeometry* collection = GEOSGeom_createCollection(
    GEOS_GEOMETRYCOLLECTION, geoms.data(), geoms.size());
GEOSGeometry* merged = GEOSUnaryUnion(collection);
if (!merged) {
    // MakeValid fallback
    for (auto& g : geoms) g = GEOSMakeValid(g);
    merged = GEOSUnaryUnion(collection);
}
```

### 3. `ghs_enrich` — GeoTIFF Raster Sampling

**TS source:** [enrich-ghs.ts](file:///c:/Users/zx/Desktop/polymech/pm-pics/packages/gadm/src/enrich-ghs.ts)

- Replace `geotiff.js` with **GDAL `GDALRasterIO`** — supports tiled/COG reads, much faster
- Replace JS `proj4()` per-pixel with **PROJ `proj_trans`** using a cached `PJ*` handle
- Replace Turf PIP with inline ray-casting (already done in TS, trivial in C++)
- Keep stride-based sampling for large windows

```cpp
// One-time setup
PJ* transform = proj_create_crs_to_crs(ctx, "EPSG:54009", "EPSG:4326", nullptr);

// Read raster window
GDALRasterBandH band = GDALGetRasterBand(ds, 1);
std::vector<float> buf(width * height);
GDALRasterIO(band, GF_Read, minPx, minPy, width, height,
             buf.data(), width, height, GDT_Float32, 0, 0);

// Fast pixel loop — no allocations
for (int y = 0; y < height; y += stride) {
    for (int x = 0; x < width; x += stride) {
        float val = buf[y * width + x];
        if (val <= 0 || val == 65535.f) continue;
        PJ_COORD in = proj_coord(originX + (minPx+x)*resX, originY + (minPy+y)*resY, 0, 0);
        PJ_COORD out = proj_trans(transform, PJ_FWD, in);
        if (point_in_geometry(out.xy.x, out.xy.y, geom)) {
            totalVal += val * strideFactor;
            // ... weighted sums, candidate tracking
        }
    }
}
```

### 4. `main.cpp` — CLI + Cache Logic

**TS source:** [boundaries.ts](file:///c:/Users/zx/Desktop/polymech/pm-pics/packages/gadm/scripts/boundaries.ts)

- Read `gadm_continent.json` with nlohmann/json
- Same skip-if-cached logic (check file existence)
- Output identical JSON format to `cache/gadm/boundary_{code}_{level}.json`
- Optional: OpenMP `#pragma omp parallel for` on the country loop (each country is independent)

---

## Expected Performance

| Operation | TS (current) | C++ (expected) | Speedup |
|---|---|---|---|
| Geometry union (DEU L0, 151 polys) | **Crashes / >10min** | ~1–3s | ∞ / ~100× |
| GHS enrich (NGA L0, ~6M px window) | ~210s | ~5–15s | ~15× |
| Full batch (all countries, all levels) | **Hours, may hang** | ~20–40 min | ~10× |

---

## Build & Integration

### vcpkg Manifest — `vcpkg.json`

```json
{
  "name": "gadm-boundaries",
  "version-string": "1.0.0",
  "dependencies": [
    "gdal",
    "geos",
    "proj",
    "nlohmann-json"
  ]
}
```

> [!TIP]
> `gdal` pulls in `proj` and `geos` transitively on most triplets, but listing them explicitly ensures the CMake targets are always available.

### CMake — `CMakeLists.txt`

```cmake
cmake_minimum_required(VERSION 3.20)
project(gadm-boundaries LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# ── Dependencies via vcpkg find_package ──
find_package(GDAL CONFIG REQUIRED)        # target: GDAL::GDAL
find_package(GEOS CONFIG REQUIRED)        # target: GEOS::geos_c
find_package(PROJ CONFIG REQUIRED)        # target: PROJ::proj
find_package(nlohmann_json CONFIG REQUIRED) # target: nlohmann_json::nlohmann_json

# ── OpenMP (optional, for country-level parallelism) ──
find_package(OpenMP)

add_executable(boundaries
    src/main.cpp
    src/gpkg_reader.cpp
    src/geo_merge.cpp
    src/ghs_enrich.cpp
)

target_include_directories(boundaries PRIVATE src)

target_link_libraries(boundaries PRIVATE
    GDAL::GDAL
    GEOS::geos_c
    PROJ::proj
    nlohmann_json::nlohmann_json
)

if(OpenMP_CXX_FOUND)
    target_link_libraries(boundaries PRIVATE OpenMP::OpenMP_CXX)
    target_compile_definitions(boundaries PRIVATE HAS_OPENMP=1)
endif()

# Copy data files needed at runtime
add_custom_command(TARGET boundaries POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy_if_different
        ${CMAKE_SOURCE_DIR}/../data/gadm_continent.json
        $<TARGET_FILE_DIR:boundaries>/gadm_continent.json
)
```

### CMake Presets — `CMakePresets.json`

```json
{
  "version": 6,
  "configurePresets": [
    {
      "name": "vcpkg",
      "displayName": "vcpkg (default)",
      "generator": "Ninja",
      "binaryDir": "${sourceDir}/build",
      "cacheVariables": {
        "CMAKE_TOOLCHAIN_FILE": "$env{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake",
        "CMAKE_BUILD_TYPE": "Release"
      }
    },
    {
      "name": "vcpkg-win",
      "inherits": "vcpkg",
      "displayName": "vcpkg (Windows MSVC)",
      "generator": "Visual Studio 17 2022",
      "cacheVariables": {
        "VCPKG_TARGET_TRIPLET": "x64-windows"
      }
    }
  ],
  "buildPresets": [
    {
      "name": "release",
      "configurePreset": "vcpkg",
      "configuration": "Release"
    }
  ]
}
```

### Build & Run

```bash
# 1. Install dependencies (one-time, from project root with vcpkg.json)
vcpkg install

# 2. Configure + build
cmake --preset vcpkg          # or vcpkg-win on Windows
cmake --build build --config Release

# 3. Run
./build/boundaries --country=all
./build/boundaries --country=DEU --level=0
```

Output goes to the same `cache/gadm/` directory — the Node.js server reads these cache files at runtime, so the C++ tool is a **drop-in replacement** for batch generation.

---

## Implementation Order

1. **`gpkg_reader`** — OGR-based GeoPackage read + feature grouping
2. **`geo_merge`** — GEOS union with MakeValid fallback
3. **`ghs_enrich`** — GDAL raster read + PROJ transform + PIP loop
4. **`main`** — CLI harness, cache logic, JSON output
5. **Parallelism** — OpenMP on country loop (optional, easy win)

Each module can be tested independently against the existing JSON cache files for correctness.