Add OBJ export functionality to SFCGAL

This commit introduces a new feature to export SFCGAL geometries to OBJ format.
The implementation includes:

- New namespace SFCGAL::io::OBJ with export functions
- Support for all SFCGAL geometry types
- Multiple export methods: to stream, file, string, and C-style buffer
- Comprehensive unit tests

Key features:
1. Exports points, lines, polygons, and complex 3D geometries
2. Handles both 2D (z=0.0) and 3D geometries
3. Provides a C-compatible API for integration with other software
4. Includes error handling for invalid geometries and file operations

The OBJ export functionality allows SFCGAL users to easily convert geometries
to a widely supported 3D format, facilitating interoperability with various
3D modeling and visualization software.

Files added:
- include/SFCGAL/io/OBJ.h
- src/io/OBJ.cpp
- test/unit/SFCGAL/io/OBJTest.cpp
- test/data/objfiles directory

src/io/OBJ.cpp

@@ -0,0 +1,167 @@
+#include "SFCGAL/io/OBJ.h"
+#include "SFCGAL/GeometryCollection.h"
+#include "SFCGAL/LineString.h"
+#include "SFCGAL/MultiLineString.h"
+#include "SFCGAL/MultiPoint.h"
+#include "SFCGAL/MultiPolygon.h"
+#include "SFCGAL/MultiSolid.h"
+#include "SFCGAL/Point.h"
+#include "SFCGAL/Polygon.h"
+#include "SFCGAL/PolyhedralSurface.h"
+#include "SFCGAL/Solid.h"
+#include "SFCGAL/Triangle.h"
+#include "SFCGAL/TriangulatedSurface.h"
+#include <fstream>
+#include <functional>
+#include <sstream>
+#include <vector>
+
+namespace SFCGAL {
+namespace io {
+namespace OBJ {
+
+void
+save(const Geometry &geom, std::ostream &out)
+{
+  std::vector<Point>               all_points;
+  std::vector<std::vector<size_t>> all_faces;
+  std::vector<std::vector<size_t>> all_lines;
+  std::vector<size_t>              all_points_indices;
+
+  std::function<void(const Geometry &)> process_geometry =
+      [&](const Geometry &g) {
+        switch (g.geometryTypeId()) {
+        case TYPE_POINT: {
+          const Point &p = g.as<Point>();
+          all_points.push_back(p);
+          all_points_indices.push_back(all_points.size());
+          break;
+        }
+        case TYPE_LINESTRING: {
+          const LineString   &ls = g.as<LineString>();
+          std::vector<size_t> line;
+          for (size_t i = 0; i < ls.numPoints(); ++i) {
+            all_points.push_back(ls.pointN(i));
+            line.push_back(all_points.size());
+          }
+          all_lines.push_back(line);
+          break;
+        }
+        case TYPE_TRIANGLE: {
+          const Triangle     &tri = g.as<Triangle>();
+          std::vector<size_t> face;
+          for (int i = 0; i < 3; ++i) {
+            all_points.push_back(tri.vertex(i));
+            face.push_back(all_points.size());
+          }
+          all_faces.push_back(face);
+          break;
+        }
+        case TYPE_POLYGON: {
+          const Polygon      &poly = g.as<Polygon>();
+          std::vector<size_t> face;
+          for (size_t i = 0; i < poly.exteriorRing().numPoints() - 1; ++i) {
+            all_points.push_back(poly.exteriorRing().pointN(i));
+            face.push_back(all_points.size());
+          }
+          all_faces.push_back(face);
+          break;
+        }
+        case TYPE_TRIANGULATEDSURFACE: {
+          const TriangulatedSurface &ts = g.as<TriangulatedSurface>();
+          for (size_t i = 0; i < ts.numTriangles(); ++i) {
+            process_geometry(ts.triangleN(i));
+          }
+          break;
+        }
+        case TYPE_POLYHEDRALSURFACE: {
+          const PolyhedralSurface &ps = g.as<PolyhedralSurface>();
+          for (size_t i = 0; i < ps.numPolygons(); ++i) {
+            process_geometry(ps.polygonN(i));
+          }
+          break;
+        }
+        case TYPE_SOLID: {
+          const Solid &solid = g.as<Solid>();
+          process_geometry(solid.exteriorShell());
+          break;
+        }
+        case TYPE_MULTIPOINT:
+        case TYPE_MULTILINESTRING:
+        case TYPE_MULTIPOLYGON:
+        case TYPE_MULTISOLID:
+        case TYPE_GEOMETRYCOLLECTION: {
+          const GeometryCollection &gc = g.as<GeometryCollection>();
+          for (size_t i = 0; i < gc.numGeometries(); ++i) {
+            process_geometry(gc.geometryN(i));
+          }
+          break;
+        }
+        default:
+          throw std::runtime_error("Unsupported geometry type: " +
+                                   g.geometryType());
+        }
+      };
+
+  process_geometry(geom);
+
+  for (const auto &p : all_points) {
+    out << "v " << p.x() << " " << p.y() << " " << (p.is3D() ? p.z() : 0.0)
+        << "\n";
+  }
+
+  for (size_t idx : all_points_indices) {
+    out << "p " << idx << "\n";
+  }
+
+  for (const auto &line : all_lines) {
+    out << "l";
+    for (size_t idx : line) {
+      out << " " << idx;
+    }
+    out << "\n";
+  }
+
+  for (const auto &face : all_faces) {
+    out << "f";
+    for (size_t idx : face) {
+      out << " " << idx;
+    }
+    out << "\n";
+  }
+}
+
+void
+save(const Geometry &geom, const std::string &filename)
+{
+  std::ofstream out(filename);
+  if (!out) {
+    throw std::runtime_error("Unable to open file " + filename +
+                             " for writing.");
+  }
+  save(geom, out);
+}
+
+std::string
+saveToString(const Geometry &geom)
+{
+  std::ostringstream oss;
+  save(geom, oss);
+  return oss.str();
+}
+
+void
+saveToBuffer(const Geometry &geom, char *buffer, size_t *size)
+{
+  std::string result = saveToString(geom);
+  if (buffer && *size >= result.size()) {
+    std::copy(result.begin(), result.end(), buffer);
+    *size = result.size();
+  } else {
+    *size = result.size();
+  }
+}
+
+} // namespace OBJ
+} // namespace io
+} // namespace SFCGAL

src/io/OBJ.h

@@ -0,0 +1,66 @@
+// Copyright (c) 2024-2024, Oslandia.
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#ifndef _SFCGAL_IO_OBJ_H_
+#define _SFCGAL_IO_OBJ_H_
+
+#include <SFCGAL/Geometry.h>
+#include <ostream>
+#include <string>
+
+namespace SFCGAL {
+namespace io {
+namespace OBJ {
+
+/**
+ * @brief Saves a geometry to an OBJ format stream.
+ * @ingroup io
+ *
+ * @param[in] geom The geometry to save
+ * @param[out] out The output stream
+ * @throws std::runtime_error If the geometry is invalid or unsupported
+ */
+void
+save(const Geometry &geom, std::ostream &out);
+
+/**
+ * @brief Saves a geometry to an OBJ file.
+ * @ingroup io
+ *
+ * @param[in] geom The geometry to save
+ * @param[in] filename The name of the file to save to
+ * @throws std::runtime_error If the file cannot be opened or the geometry is
+ * invalid
+ */
+void
+save(const Geometry &geom, const std::string &filename);
+
+/**
+ * @brief Saves a geometry to an OBJ format string.
+ * @ingroup io
+ *
+ * @param[in] geom The geometry to save
+ * @return The OBJ format string
+ * @throws std::runtime_error If the geometry is invalid or unsupported
+ */
+std::string
+saveToString(const Geometry &geom);
+
+/**
+ * @brief Saves a geometry to an OBJ format buffer (C API).
+ * @ingroup io
+ *
+ * @param[in] geom The geometry to save
+ * @param[out] buffer The buffer to write to
+ * @param[in,out] size On input, the size of the buffer. On output, the number
+ * of bytes written (or required if buffer is null)
+ * @throws std::runtime_error If the geometry is invalid or unsupported
+ */
+void
+saveToBuffer(const Geometry &geom, char *buffer, size_t *size);
+
+} // namespace OBJ
+} // namespace io
+} // namespace SFCGAL
+
+#endif

test/data/objfiles/geometrycollection.obj

@@ -0,0 +1,6 @@
+v 2 3 0
+v 0 0 0
+v 1 0 0
+v 1 1 0
+p 1
+f 2 3 4

test/data/objfiles/geometrycollectionz.obj

@@ -0,0 +1,6 @@
+v 2 3 5
+v 0 0 6
+v 1 0 6
+v 1 1 6
+p 1
+f 2 3 4

test/data/objfiles/linestring.obj

@@ -0,0 +1,4 @@
+v 0 0 0
+v 1 1 0
+v 2 2 0
+l 1 2 3

test/data/objfiles/linestringz.obj

@@ -0,0 +1,4 @@
+v 0 0 0
+v 1 1 1
+v 2 2 2
+l 1 2 3

test/data/objfiles/multilinestring.obj

@@ -0,0 +1,6 @@
+v 0 0 0
+v 1 1 0
+v 2 2 0
+v 3 3 0
+l 1 2
+l 3 4

test/data/objfiles/multilinestringz.obj

@@ -0,0 +1,6 @@
+v 0 0 0
+v 1 1 1
+v 2 2 2
+v 3 3 3
+l 1 2
+l 3 4

test/data/objfiles/multipoint.obj

@@ -0,0 +1,4 @@
+v 1 1 0
+v 2 2 0
+p 1
+p 2

test/data/objfiles/multipointz.obj

@@ -0,0 +1,4 @@
+v 1 1 1
+v 2 2 2
+p 1
+p 2

test/data/objfiles/multipolygon.obj

@@ -0,0 +1,10 @@
+v 0 0 0
+v 1 0 0
+v 1 1 0
+v 0 1 0
+v 2 2 0
+v 3 2 0
+v 3 3 0
+v 2 3 0
+f 1 2 3 4
+f 5 6 7 8

test/data/objfiles/multipolygonz.obj

@@ -0,0 +1,10 @@
+v 0 0 0
+v 1 0 0
+v 1 1 0
+v 0 1 0
+v 2 2 2
+v 3 2 2
+v 3 3 2
+v 2 3 2
+f 1 2 3 4
+f 5 6 7 8

test/data/objfiles/multisolid.obj

@@ -0,0 +1,60 @@
+v 0 0 0
+v 0 1 0
+v 1 1 0
+v 1 0 0
+v 0 0 0
+v 1 0 0
+v 1 0 0
+v 0 0 0
+v 0 0 0
+v 1 0 0
+v 1 0 0
+v 0 0 0
+v 1 1 0
+v 0 1 0
+v 0 1 0
+v 1 1 0
+v 1 0 0
+v 1 1 0
+v 1 1 0
+v 1 0 0
+v 0 0 0
+v 0 0 0
+v 0 1 0
+v 0 1 0
+v 2 4 0
+v 2 5 0
+v 3 5 0
+v 3 4 0
+v 2 4 0
+v 3 4 0
+v 3 5 0
+v 2 5 0
+v 2 4 0
+v 3 4 0
+v 3 4 0
+v 2 4 0
+v 3 5 0
+v 2 5 0
+v 2 5 0
+v 3 5 0
+v 3 4 0
+v 3 5 0
+v 3 5 0
+v 3 4 0
+v 2 4 0
+v 2 4 0
+v 2 5 0
+v 2 5 0
+f 1 2 3 4
+f 5 6 7 8
+f 9 10 11 12
+f 13 14 15 16
+f 17 18 19 20
+f 21 22 23 24
+f 25 26 27 28
+f 29 30 31 32
+f 33 34 35 36
+f 37 38 39 40
+f 41 42 43 44
+f 45 46 47 48