Add frame transformation overload for Variables

src/DataStructures/Variables/FrameTransform.hpp

@@ -0,0 +1,51 @@
+// Distributed under the MIT License.
+// See LICENSE.txt for details.
+
+#pragma once
+
+#include <cstddef>
+
+#include "DataStructures/Tensor/EagerMath/FrameTransform.hpp"
+#include "DataStructures/Tensor/Metafunctions.hpp"
+#include "DataStructures/Tensor/TypeAliases.hpp"
+#include "DataStructures/Variables.hpp"
+#include "Utilities/Gsl.hpp"
+
+namespace transform {
+
+template <typename... ResultTags, typename... InputTags, size_t Dim,
+          typename SourceFrame, typename TargetFrame>
+void first_index_to_different_frame(
+    const gsl::not_null<Variables<tmpl::list<ResultTags...>>*> result,
+    const Variables<tmpl::list<InputTags...>>& input,
+    const InverseJacobian<DataVector, Dim, SourceFrame, TargetFrame>&
+        inv_jacobian) {
+  EXPAND_PACK_LEFT_TO_RIGHT(
+      first_index_to_different_frame(make_not_null(&get<ResultTags>(*result)),
+                                     get<InputTags>(input), inv_jacobian));
+}
+
+namespace Tags {
+template <typename Tag, typename FirstIndexFrame>
+struct TransformedFirstIndex : db::SimpleTag {
+  using type = TensorMetafunctions::prepend_spatial_index<
+      TensorMetafunctions::remove_first_index<typename Tag::type>,
+      tmpl::front<typename Tag::type::index_list>::dim, UpLo::Up,
+      FirstIndexFrame>;
+};
+}  // namespace Tags
+
+template <typename... InputTags, size_t Dim, typename SourceFrame,
+          typename TargetFrame,
+          typename ResultVars = Variables<tmpl::list<
+              Tags::TransformedFirstIndex<InputTags, SourceFrame>...>>>
+ResultVars first_index_to_different_frame(
+    const Variables<tmpl::list<InputTags...>>& input,
+    const InverseJacobian<DataVector, Dim, SourceFrame, TargetFrame>&
+        inv_jacobian) {
+  ResultVars result{input.number_of_grid_points()};
+  first_index_to_different_frame(make_not_null(&result), input, inv_jacobian);
+  return result;
+}
+
+}  // namespace transform

tests/Unit/DataStructures/Tensor/EagerMath/Test_FrameTransform.cpp

@@ -9,10 +9,20 @@
 #include "DataStructures/Tensor/EagerMath/DeterminantAndInverse.hpp"
 #include "DataStructures/Tensor/EagerMath/FrameTransform.hpp"
 #include "DataStructures/Tensor/Tensor.hpp"
+#include "DataStructures/Variables/FrameTransform.hpp"
 #include "Framework/CheckWithRandomValues.hpp"
 #include "Framework/SetupLocalPythonEnvironment.hpp"
 #include "Helpers/DataStructures/MakeWithRandomValues.hpp"
 
+namespace {
+struct Var1 : db::SimpleTag {
+  using type = tnsr::I<DataVector, 2, Frame::Inertial>;
+};
+struct Var2 : db::SimpleTag {
+  using type = tnsr::Ij<DataVector, 2, Frame::Inertial>;
+};
+}  // namespace
+
 namespace transform {
 
 template <typename SrcTensorType, typename DestTensorType, typename DataType,
@@ -160,6 +170,31 @@ void test_transform_first_index_to_different_frame() {
       CHECK(get<1, 1>(result) == 16.5);
     }
   }
+  {
+    INFO("Variables");
+    const size_t num_points = 3;
+    InverseJacobian<DataVector, 2, Frame::ElementLogical, Frame::Inertial>
+        inv_jacobian{num_points};
+    get<0, 0>(inv_jacobian) = 2.0;
+    get<1, 1>(inv_jacobian) = 3.0;
+    get<0, 1>(inv_jacobian) = 0.5;
+    get<1, 0>(inv_jacobian) = 1.5;
+    Variables<tmpl::list<Var1, Var2>> input{num_points};
+    std::iota(get<Var1>(input).begin(), get<Var1>(input).end(), 1.0);
+    std::iota(get<Var2>(input).begin(), get<Var2>(input).end(), 1.0);
+    CAPTURE(input);
+    const auto result = first_index_to_different_frame(input, inv_jacobian);
+    const auto& var1 =
+        get<Tags::TransformedFirstIndex<Var1, Frame::ElementLogical>>(result);
+    const auto& var2 =
+        get<Tags::TransformedFirstIndex<Var2, Frame::ElementLogical>>(result);
+    CHECK(get<0>(var1) == 3.);
+    CHECK(get<1>(var1) == 7.5);
+    CHECK(get<0, 0>(var2) == 3.);
+    CHECK(get<1, 0>(var2) == 7.5);
+    CHECK(get<0, 1>(var2) == 8.);
+    CHECK(get<1, 1>(var2) == 16.5);
+  }
 }
 
 SPECTRE_TEST_CASE("Unit.PointwiseFunctions.GeneralRelativity.Transform",