Add for_each_extents example

examples/CMakeLists.txt

@@ -16,3 +16,4 @@ add_subdirectory(dot_product)
 add_subdirectory(tiled_layout)
 add_subdirectory(restrict_accessor)
 add_subdirectory(aligned_accessor)
+add_subdirectory(for_each_extents)

examples/for_each_extents/CMakeLists.txt

@@ -0,0 +1 @@
+mdspan_add_example(for_each_extents)

examples/for_each_extents/for_each_extents.cpp

@@ -0,0 +1,115 @@
+#include <experimental/mdspan.hpp>
+#include <cassert>
+#include <iostream>
+#include <type_traits>
+
+// "gcc trunk" on godbolt.org as of 2023/03/21
+// (> 12.2) does not define __cpp_lib_ranges_iota,
+// yet std::views::iota works just fine.
+#if defined(__cpp_lib_ranges_cartesian_product) // && defined(__cpp_lib_ranges_iota)
+#  define MDSPAN_EXAMPLE_CAN_USE_STD_RANGES 1
+#endif
+
+#if defined(MDSPAN_EXAMPLE_CAN_USE_STD_RANGES)
+
+// GCC >= 13 ("gcc trunk" on godbolt.org as of 2023/03/21)
+// implements std::views::cartesian_product.
+// If you don't have it, you can use range-v3 instead.
+// Note that mixing std::views::iota with
+// ranges::views::cartesian_product doesn't work.
+// The range-v3 work-around looks like this.
+//
+// #include <range/v3/view/cartesian_product.hpp>
+// #include <range/v3/view/indices.hpp>
+// namespace ranges_views = ranges::views;
+
+#include <ranges>
+namespace ranges_views = std::views;
+
+namespace stdex = std::experimental;
+
+auto print_args = [] <class ... Args> (Args&&... args) {
+  ((std::cout << std::forward<Args>(args) << '\n'), ...);
+};
+
+template<size_t ... Lefts, size_t ... Rights>
+auto concat_index_sequence(std::index_sequence<Lefts...>,
+  std::index_sequence<Rights...>)
+{
+  return std::index_sequence<Lefts..., Rights...>{};
+}
+
+auto reverse_index_sequence(std::index_sequence<> x)
+{
+  return x;  
+}
+
+template<size_t First, size_t ... Rest>
+auto reverse_index_sequence(std::index_sequence<First, Rest...>)
+{
+  return concat_index_sequence(
+    reverse_index_sequence(std::index_sequence<Rest...>{}),
+    std::index_sequence<First>{});
+}
+
+template<size_t N>
+auto make_reverse_index_sequence()
+{
+  return reverse_index_sequence(std::make_index_sequence<N>());
+}
+
+template<class Callable, class IndexType,
+  std::size_t ... Extents, std::size_t ... RankIndices>
+void for_each_in_extents_impl(Callable&& f,
+  stdex::extents<IndexType, Extents...> e,
+  std::index_sequence<RankIndices...> rank_sequence)
+{
+  // In the layout_left case, caller passes in N-1, N-2, ..., 1, 0.
+  // This reverses the order of the Cartesian product,
+  // but also reverses the order of indices in each tuple.
+  [&] <std::size_t ... Indices> (std::index_sequence<Indices...>) {
+    auto v = std::views::cartesian_product(
+      std::views::iota(IndexType(0), e.extent(Indices))...);
+    for (const auto& tuple_of_indices : v) {
+      // In the layout_left case, we undo the reversal of each tuple
+      // by getting its elements in reverse order.
+      [&] <std::size_t ... InnerIndices> (std::index_sequence<InnerIndices...>) {
+          std::forward<Callable>(f)(std::get<InnerIndices>(tuple_of_indices)...);
+        } (rank_sequence);
+    }
+  } (rank_sequence);
+}
+
+template<class Callable, class IndexType, std::size_t ... Extents, class Layout>
+void for_each_in_extents(Callable&& f,
+  stdex::extents<IndexType, Extents...> e,
+  Layout)
+{
+  using layout_type = std::remove_cvref_t<Layout>;
+  if constexpr (std::is_same_v<layout_type, stdex::layout_left>) {
+    for_each_in_extents_impl(std::forward<Callable>(f), e,
+      make_reverse_index_sequence<e.rank()>());
+  }
+  else { // layout_right or any other layout
+    for_each_in_extents_impl(std::forward<Callable>(f), e,
+      std::make_index_sequence<e.rank()>());
+  }
+}
+
+#endif // defined(MDSPAN_EXAMPLE_CAN_USE_STD_RANGES)
+
+int main() {
+
+#if defined(MDSPAN_EXAMPLE_CAN_USE_STD_RANGES)
+  stdex::extents<int, 2, 3> e;
+  auto printer = [] (int i, int j) {
+    std::cout << "(" << i << "," << j << ")\n";
+  };
+  std::cout << "layout_right:\n";
+  for_each_in_extents(printer, e, stdex::layout_right{});
+  std::cout << "\nlayout_left:\n";
+  for_each_in_extents(printer, e, stdex::layout_left{});
+#endif // defined(MDSPAN_EXAMPLE_CAN_USE_STD_RANGES)
+
+  return 0;
+}