From 95ec6e4dd373f58eebf3119e470ead4156c724b4 Mon Sep 17 00:00:00 2001
From: Jacques Lucke <jacques@blender.org>
Date: Fri, 24 Sep 2021 16:02:59 +0200
Subject: Geometry Nodes: make index field more reusable

Some inputs will be the index field implicitly, so we want this
class to be available outside of `node_geo_input_index.cc`.
---
 source/blender/functions/FN_field.hh                 |  9 +++++++++
 source/blender/functions/intern/field.cc             | 15 +++++++++++++++
 .../nodes/geometry/nodes/node_geo_input_index.cc     | 20 +-------------------
 3 files changed, 25 insertions(+), 19 deletions(-)

diff --git a/source/blender/functions/FN_field.hh b/source/blender/functions/FN_field.hh
index d4375b625ce..3ce0993da59 100644
--- a/source/blender/functions/FN_field.hh
+++ b/source/blender/functions/FN_field.hh
@@ -484,4 +484,13 @@ template<typename T> Field<T> make_constant_field(T value)
 
 GField make_field_constant_if_possible(GField field);
 
+class IndexFieldInput final : public FieldInput {
+ public:
+  IndexFieldInput();
+
+  const GVArray *get_varray_for_context(const FieldContext &context,
+                                        IndexMask mask,
+                                        ResourceScope &scope) const final;
+};
+
 }  // namespace blender::fn
diff --git a/source/blender/functions/intern/field.cc b/source/blender/functions/intern/field.cc
index 599e4d4595a..fbd35c2c377 100644
--- a/source/blender/functions/intern/field.cc
+++ b/source/blender/functions/intern/field.cc
@@ -526,6 +526,21 @@ const GVArray *FieldContext::get_varray_for_input(const FieldInput &field_input,
   return field_input.get_varray_for_context(*this, mask, scope);
 }
 
+IndexFieldInput::IndexFieldInput() : FieldInput(CPPType::get<int>(), "Index")
+{
+}
+
+const GVArray *IndexFieldInput::get_varray_for_context(const fn::FieldContext &UNUSED(context),
+                                                       IndexMask mask,
+                                                       ResourceScope &scope) const
+{
+  /* TODO: Investigate a similar method to IndexRange::as_span() */
+  auto index_func = [](int i) { return i; };
+  return &scope.construct<
+      fn::GVArray_For_EmbeddedVArray<int, VArray_For_Func<int, decltype(index_func)>>>(
+      mask.min_array_size(), mask.min_array_size(), index_func);
+}
+
 /* --------------------------------------------------------------------
  * FieldOperation.
  */
diff --git a/source/blender/nodes/geometry/nodes/node_geo_input_index.cc b/source/blender/nodes/geometry/nodes/node_geo_input_index.cc
index f18b0ab090a..7fcbaf429dd 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_input_index.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_input_index.cc
@@ -23,27 +23,9 @@ static void geo_node_input_index_declare(NodeDeclarationBuilder &b)
   b.add_output<decl::Int>("Index").field_source();
 }
 
-class IndexFieldInput final : public fn::FieldInput {
- public:
-  IndexFieldInput() : FieldInput(CPPType::get<int>(), "Index")
-  {
-  }
-
-  const GVArray *get_varray_for_context(const fn::FieldContext &UNUSED(context),
-                                        IndexMask mask,
-                                        ResourceScope &scope) const final
-  {
-    /* TODO: Investigate a similar method to IndexRange::as_span() */
-    auto index_func = [](int i) { return i; };
-    return &scope.construct<
-        fn::GVArray_For_EmbeddedVArray<int, VArray_For_Func<int, decltype(index_func)>>>(
-        mask.min_array_size(), mask.min_array_size(), index_func);
-  }
-};
-
 static void geo_node_input_index_exec(GeoNodeExecParams params)
 {
-  Field<int> index_field{std::make_shared<IndexFieldInput>()};
+  Field<int> index_field{std::make_shared<fn::IndexFieldInput>()};
   params.set_output("Index", std::move(index_field));
 }
 
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