Add expression re-ordering to allow linear assignment

Author: spectre-ns

include/xtensor/xview.hpp

@@ -32,6 +32,7 @@
 #include "xtensor.hpp"
 #include "xtensor_config.hpp"
 #include "xtensor_forward.hpp"
+#include "xutils.hpp"
 #include "xview_utils.hpp"
 
 namespace xt
@@ -1821,6 +1822,38 @@ namespace xt
         }
     }
 
+    namespace detail
+    {
+        template <class E1, class E2>
+        struct equal_rank
+        {
+            static constexpr bool value = get_rank<E1>::value == get_rank<E2>::value;
+        };
+
+        template <class E1, class... E>
+        struct all_equal_rank
+        {
+            static constexpr bool value = xtl::conjunction<equal_rank<E1, E>...>::value
+                                          && (get_rank<E1>::value != SIZE_MAX);
+        };
+
+        template <class op, class... E, class... S, size_t... I>
+        inline auto impl_reorder_function(xfunction<op, E...> e, std::index_sequence<I...>, S&&... slices)
+        {
+            return detail::make_xfunction<op>(view(std::get<I>(e.arguments()), std::forward<S>(slices)...)...);
+        }
+    }
+
+    template <class op, class... E, class... S, class = std::enable_if_t<detail::all_equal_rank<std::decay_t<E>...>::value>>
+    inline auto view(xfunction<op, E...> e, S&&... slices)
+    {
+        return detail::impl_reorder_function(
+            e,
+            std::make_index_sequence<sizeof...(E)>(),
+            std::forward<S>(slices)...
+        );
+    }
+
     /**
      * Constructs and returns a view on the specified xexpression. Users
      * should not directly construct the slices but call helper functions

test/CMakeLists.txt

@@ -102,7 +102,7 @@ if (CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU")
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=native")
     endif()
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_cxx_std_flag} -Wunused-parameter -Wextra -Wreorder -Wconversion -Wsign-conversion")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wold-style-cast -Wunused-variable")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wold-style-cast -Wunused-variable -ftemplate-backtrace-limit=0")
     if (XTENSOR_DISABLE_EXCEPTIONS)
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-exceptions")
     endif()

test/test_xview.cpp

@@ -22,6 +22,7 @@
 #endif
 
 #include "xtensor/xarray.hpp"
+#include "xtensor/xassign.hpp"
 #include "xtensor/xbuilder.hpp"
 #include "xtensor/xfixed.hpp"
 #include "xtensor/xmanipulation.hpp"
@@ -30,7 +31,6 @@
 #include "xtensor/xstrided_view.hpp"
 #include "xtensor/xtensor.hpp"
 #include "xtensor/xview.hpp"
-#include "xtensor/xassign.hpp"
 
 namespace xt
 {
@@ -1695,12 +1695,13 @@ namespace xt
         XT_ASSERT_THROW(const auto col = xt::col(arr, 0), std::invalid_argument);
     }
 
-    TEST(xview, linearly_assign)
+    TEST(xview, reorder_function)
     {
-        xtensor<float, 1> a = {1,2,3,4};
-        xtensor<float, 1> b = {5,6,7,8};
+        xtensor<float, 1> a = {1, 2, 3, 4};
+        xtensor<float, 1> b = {5, 6, 7, 8};
         xtensor<float, 1> c = xtensor<float, 1>::from_shape(a);
-        auto v = xt::view(a + b, xt::all());
+        auto f = a + b;
+        auto v = xt::view(f, xt::all());
         auto is_linear_assign = xt::xassign_traits<decltype(c), decltype(v)>::linear_assign(c, v, true);
         EXPECT_TRUE(is_linear_assign);
     }