SubdivCCG: Add to_index helper method

This commit adds a method to compute the position of a given
SubdivCCGCoord in an appropriately sized array.

Pull Request: https://projects.blender.org/blender/blender/pulls/125462

source/blender/blenkernel/BKE_subdiv_ccg.hh

@@ -68,6 +68,14 @@ struct SubdivCCGCoord {
 
   /* Coordinate within the grid. */
   short x, y;
+
+  /* Returns the index for the coordinate in an array sized to contain all grid vertices (including
+   * duplicates) */
+  int to_index(const CCGKey &key) const
+  {
+    return key.grid_area * this->grid_index +
+           CCG_grid_xy_to_index(key.grid_size, this->x, this->y);
+  }
 };
 
 /* Definition of an edge which is adjacent to at least one of the faces. */

source/blender/blenkernel/CMakeLists.txt

@@ -849,6 +849,7 @@ if(WITH_GTESTS)
     intern/lib_remap_test.cc
     intern/main_test.cc
     intern/nla_test.cc
+    intern/subdiv_ccg_test.cc
     intern/tracking_test.cc
     intern/volume_test.cc
   )

source/blender/blenkernel/intern/subdiv_ccg_test.cc

@@ -0,0 +1,36 @@
+/* SPDX-FileCopyrightText: 2024 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include "testing/testing.h"
+
+#include "BKE_subdiv_ccg.hh"
+#include "BKE_subsurf.hh"
+
+namespace blender::bke::tests {
+TEST(subdiv_ccg_coord, to_index)
+{
+  CCGKey key;
+  key.level = 2;
+  key.elem_size = sizeof(float);
+
+  key.has_normals = false;
+  key.has_mask = false;
+
+  key.normal_offset = -1;
+  key.mask_offset = -1;
+
+  key.grid_size = BKE_ccg_gridsize(key.level);   /* 3 */
+  key.grid_area = key.grid_size * key.grid_size; /* 9 */
+  key.grid_bytes = key.grid_area * key.elem_size;
+
+  SubdivCCGCoord coord;
+  coord.grid_index = 2;
+  coord.x = 1;
+  coord.y = 1;
+
+  /* (grid_index * grid_area) + y * grid_size + x */
+  /* (2 * 9) + (1 * 3) + 1 */
+  EXPECT_EQ(coord.to_index(key), 22);
+}
+}  // namespace blender::bke::tests