pick_var, pick_seq

Summary: Variations on existing pick, using type_pack_element for acceleration and minimizing class template instantiations. In particular, `pick_var` is equivalent to existing `pick`, but for now we avoid modifying the existing implementations.

Reviewed By: Alfus

Differential Revision: D37254884

fbshipit-source-id: 5a94df9dba4605a655becb79fdc5b49b403b345e

fatal/type/impl/slice.h

@@ -54,6 +54,37 @@ using type_pack_element = type_pack_element_fallback<I, Ts...>;
 
 #endif
 
+template <typename List>
+struct pick_var_;
+template <
+  template <typename...> class Variadic,
+  typename... Args
+>
+struct pick_var_<Variadic<Args...>> {
+  template <std::size_t i>
+  using t = type_pack_element<i, Args...>;
+  template <std::size_t... i>
+  using apply = Variadic<t<i>...>;
+};
+template <
+  template <typename V, V...> class Variadic,
+  typename T, T... Args
+>
+struct pick_var_<Variadic<T, Args...>> {
+  template <std::size_t i>
+  using t = type_pack_element<i, std::integral_constant<T, Args>...>;
+  template <std::size_t... i>
+  using apply = Variadic<T, t<i>::value...>;
+};
+
+template <typename Seq>
+struct pick_seq_;
+template <std::size_t... Indexes>
+struct pick_seq_<index_sequence<Indexes...>> {
+  template <typename List>
+  using apply = typename pick_var_<List>::template apply<Indexes...>;
+};
+
 template <std::size_t, typename> struct a;
 
 template <template <typename> class Single, typename T>

fatal/type/slice.h

@@ -21,6 +21,12 @@ namespace fatal {
 
 using i_at::type_pack_element;
 
+template <typename List, std::size_t... Indexes>
+using pick_var = typename i_at::pick_var_<List>::template apply<Indexes...>;
+
+template <typename List, typename Seq>
+using pick_seq = typename i_at::pick_seq_<Seq>::template apply<List>;
+
 template <typename T, std::size_t Index>
 using at = typename i_at::a<Index, T>::type;
 

fatal/type/test/slice_test.cpp

@@ -49,6 +49,46 @@ FATAL_TEST(type_pack_element, list) {
   FATAL_EXPECT_FALSE((detect_v<test::native_ic, zero>));
 }
 
+FATAL_TEST(pick_var, list) {
+  FATAL_EXPECT_SAME<
+    pick_var<
+      list<int, float, char, double, long>,
+      3, 1
+    >,
+    list<double, float>
+  >();
+}
+
+FATAL_TEST(pick_var, seq) {
+  FATAL_EXPECT_SAME<
+    pick_var<
+      char_sequence<'a', 'b', 'c', 'd', 'e'>,
+      3, 1
+    >,
+    char_sequence<'d', 'b'>
+  >();
+}
+
+FATAL_TEST(pick_seq, list) {
+  FATAL_EXPECT_SAME<
+    pick_seq<
+      list<int, float, char, double, long>,
+      index_sequence<3, 1>
+    >,
+    list<double, float>
+  >();
+}
+
+FATAL_TEST(pick_seq, seq) {
+  FATAL_EXPECT_SAME<
+    pick_seq<
+      char_sequence<'a', 'b', 'c', 'd', 'e'>,
+      index_sequence<3, 1>
+    >,
+    char_sequence<'d', 'b'>
+  >();
+}
+
 FATAL_TEST(index_of, list) {
   using ls = list<int, double, bool, void, float, char, unsigned>;
   FATAL_EXPECT_EQ(0, index_of<ls, int>::value);