Cleanup: Deduplicate OffsetIndices utility for meshes and curves

The "reverse map" of corners to faces and points to curves is the same
for meshes and curves now. Move it to the offset indices header to
reflect this.

This unification can go further in the future, but I'd rather wait
until the design is clearer for now.

Pull Request: blender/blender#106570

source/blender/blenkernel/intern/curves_geometry.cc

@@ -549,13 +549,8 @@ IndexMask CurvesGeometry::indices_for_curve_type(const CurveType type,
 
 Array<int> CurvesGeometry::point_to_curve_map() const
 {
-  const OffsetIndices points_by_curve = this->points_by_curve();
   Array<int> map(this->points_num());
-  threading::parallel_for(this->curves_range(), 1024, [&](const IndexRange range) {
-    for (const int i_curve : range) {
-      map.as_mutable_span().slice(points_by_curve[i_curve]).fill(i_curve);
-    }
-  });
+  offset_indices::build_reverse_map(this->points_by_curve(), map);
   return map;
 }
 

source/blender/blenkernel/intern/mesh_mapping.cc

@@ -526,11 +526,7 @@ namespace blender::bke::mesh_topology {
 Array<int> build_loop_to_poly_map(const OffsetIndices<int> polys)
 {
   Array<int> map(polys.total_size());
-  threading::parallel_for(polys.index_range(), 1024, [&](IndexRange range) {
-    for (const int64_t poly_i : range) {
-      map.as_mutable_span().slice(polys[poly_i]).fill(int(poly_i));
-    }
-  });
+  offset_indices::build_reverse_map(polys, map);
   return map;
 }
 

source/blender/blenlib/BLI_offset_indices.hh

@@ -97,6 +97,12 @@ template<typename T> class OffsetIndices {
  */
 void accumulate_counts_to_offsets(MutableSpan<int> counts_to_offsets, int start_offset = 0);
 
+/**
+ * Create a map from indexed elements to the source indices, in other words from the larger array
+ * to the smaller array.
+ */
+void build_reverse_map(OffsetIndices<int> offsets, MutableSpan<int> r_map);
+
 }  // namespace blender::offset_indices
 
 namespace blender {

source/blender/blenlib/intern/offset_indices.cc

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0-or-later */
 
 #include "BLI_offset_indices.hh"
+#include "BLI_task.hh"
 
 namespace blender::offset_indices {
 
@@ -16,4 +17,13 @@ void accumulate_counts_to_offsets(MutableSpan<int> counts_to_offsets, const int
   counts_to_offsets.last() = offset;
 }
 
+void build_reverse_map(OffsetIndices<int> offsets, MutableSpan<int> r_map)
+{
+  threading::parallel_for(offsets.index_range(), 1024, [&](const IndexRange range) {
+    for (const int64_t i : range) {
+      r_map.slice(offsets[i]).fill(i);
+    }
+  });
+}
+
 }  // namespace blender::offset_indices