Add MeshAccumulator building interleaved vertex arrays

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/gl/MeshAccumulator.java

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import de.hft.stuttgart.citydoctor2.math.Triangle3d;
+import de.hft.stuttgart.citydoctor2.math.Vector3d;
+
+/**
+ * Builds interleaved-by-attribute vertex arrays from triangles. Vertices are stored per triangle corner
+ * (3 per triangle) so a triangle's vertices occupy a contiguous, predictable range — which lets the
+ * draw index set be built by triangle without dedup bookkeeping.
+ */
+public class MeshAccumulator {
+
+    private final Vector3d center;
+    private final FloatList positions = new FloatList();
+    private final FloatList colors = new FloatList();
+    private final IntList ids = new IntList();
+    private int vertexCount;
+
+    public MeshAccumulator(Vector3d center) {
+        this.center = center;
+    }
+
+    public void addTriangle(Triangle3d t, int id, float r, float g, float b) {
+        addVertex(t.getP1(), id, r, g, b);
+        addVertex(t.getP2(), id, r, g, b);
+        addVertex(t.getP3(), id, r, g, b);
+    }
+
+    private void addVertex(Vector3d p, int id, float r, float g, float b) {
+        positions.add((float) (p.getX() - center.getX()),
+                      (float) (p.getY() - center.getY()),
+                      (float) (p.getZ() - center.getZ()));
+        colors.add(r, g, b);
+        ids.add(id);
+        vertexCount++;
+    }
+
+    public int vertexCount() { return vertexCount; }
+
+    public float[] positions() { return positions.toArray(); }
+
+    public float[] colors() { return colors.toArray(); }
+
+    public int[] ids() { return ids.toArray(); }
+}

CityDoctorParent/Extensions/CityDoctorGUI/src/test/java/de/hft/stuttgart/citydoctor2/gui/gl/MeshAccumulatorTest.java

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import de.hft.stuttgart.citydoctor2.math.Triangle3d;
+import de.hft.stuttgart.citydoctor2.math.Vector3d;
+import org.junit.Test;
+
+public class MeshAccumulatorTest {
+
+    @Test
+    public void emitsNineFloatsAndThreeIdsPerTriangleRecentered() {
+        MeshAccumulator acc = new MeshAccumulator(new Vector3d(1, 0, 0));
+        Triangle3d t = new Triangle3d(
+                new Vector3d(1, 0, 0), new Vector3d(2, 0, 0), new Vector3d(1, 1, 0));
+        acc.addTriangle(t, 7, 0.5f, 0.25f, 0.125f);
+
+        assertEquals(3, acc.vertexCount());
+        assertArrayEquals(new float[]{0,0,0, 1,0,0, 0,1,0}, acc.positions(), 1e-6f);
+        assertArrayEquals(new float[]{0.5f,0.25f,0.125f, 0.5f,0.25f,0.125f, 0.5f,0.25f,0.125f},
+                acc.colors(), 1e-6f);
+        assertArrayEquals(new int[]{7, 7, 7}, acc.ids());
+    }
+
+    @Test
+    public void accumulatesMultipleTriangles() {
+        MeshAccumulator acc = new MeshAccumulator(new Vector3d(0, 0, 0));
+        Triangle3d t = new Triangle3d(
+                new Vector3d(0, 0, 0), new Vector3d(1, 0, 0), new Vector3d(0, 1, 0));
+        acc.addTriangle(t, 1, 1, 1, 1);
+        acc.addTriangle(t, 2, 0, 0, 0);
+        assertEquals(6, acc.vertexCount());
+        assertEquals(18, acc.positions().length);
+        assertArrayEquals(new int[]{1, 1, 1, 2, 2, 2}, acc.ids());
+    }
+}