[332] - Did you know that in C++ you can generate jump tables at comp…

…ile-time?

README.md

@@ -12,6 +12,7 @@
 
 ### Tips
 
+* [[332]](https://github.com/QuantlabFinancial/cpp_tip_of_the_week/blob/master/tips/332.md) - Did you know that in C++ you can generate jump tables at compile-time?
 * [[331]](https://github.com/QuantlabFinancial/cpp_tip_of_the_week/blob/master/tips/331.md) - Did you about C++17 std::index_sequence, std::make_index_sequence?
 * [[330]](https://github.com/QuantlabFinancial/cpp_tip_of_the_week/blob/master/tips/330.md) - Did you know that C++17 added std::pmr::polymorphic_allocator?
 * [[329]](https://github.com/QuantlabFinancial/cpp_tip_of_the_week/blob/master/tips/329.md) - Did you know about C++ allows to pass Pointer To Member Function via template parameter?

tips/331.md

@@ -2,6 +2,8 @@
 
 * **Did you about C++17 std::index_sequence, std::make_index_sequence?**
 
+  * https://eel.is/c++draft/utility.syn#lib:index_sequence
+
 </p></details><details open><summary>Example</summary><p>
 
 ```cpp
@@ -51,4 +53,70 @@ static_assert(matrix<Foo>[2][2]);
 > https://godbolt.org/z/3YvfqezPa
 
 </p></details><details><summary>Solutions</summary><p>
+
+```cpp
+template <class T, std::size_t N = 3, class = std::make_index_sequence<N>>
+constexpr const std::array<std::array<int, N>, N> matrix{};
+
+template <class T, auto N, auto... Is>
+constexpr const std::array matrix<T, N, std::index_sequence<Is...>>{
+    []<auto I> -> std::array<int, 3> {
+        return {[]<auto J> -> int {
+            return requires(T v, id<I, J> p) { v.bar(p); };
+        }.template operator()<Is>()...};
+    }.template operator()<Is>()...};
+```
+
+> https://godbolt.org/z/Yq9xdYo3a
+
+```cpp
+template <class T, auto... ns>
+constexpr auto has = []() { return requires(T t) { t.bar(id<ns...>{}); }; };
+
+template <class T, auto R, auto N>
+constexpr auto row = []() {
+    constexpr auto make_row =
+        []<auto... I>(std::index_sequence<I...>) -> std::array<bool, N> {
+        return {has<T, R, I>()...};
+    };
+
+    return make_row(std::make_index_sequence<N>{});
+};
+
+template <class T, auto N>
+constexpr auto rows = []() {
+    constexpr auto make_rows = []<auto... R>(std::index_sequence<R...>)
+        -> std::array<std::array<bool, N>, N> { return {row<T, R, N>()...}; };
+
+    return make_rows(std::make_index_sequence<N>{});
+};
+
+template <class T, auto N = 3>
+constexpr const auto matrix = rows<T, N>();
+```
+
+> https://godbolt.org/z/Gcx4sd5E3
+
+```cpp
+template <typename T, auto N, auto X, auto C = X % N, auto R = (X - C) / N>
+concept hasBar = requires(T t) {
+    { t.bar(std::declval<id<R, C>>()) };
+};
+
+template <auto N, typename... Args>
+constexpr std::array<std::array<long, N>, N> make_symmetric_matrix(
+    Args&&... args) {
+    return {std::forward<Args>(args)...};
+}
+
+template <class T, auto N = 3>
+constexpr const auto matrix =
+    []<auto... Indices>(std::index_sequence<Indices...>)
+    -> std::array<std::array<long, N>, N> {
+    return make_symmetric_matrix<N>(hasBar<T, N, static_cast<int>(Indices)>...);
+}(std::make_index_sequence<N * N>{});
+```
+
+> https://godbolt.org/z/bdjEadn5f
+
 </p></details>

tips/332.md

@@ -0,0 +1,43 @@
+<details open><summary>Info</summary><p>
+
+* **Did you know that in C++ you can generate jump tables at compile-time?**
+
+  * https://en.wikipedia.org/wiki/Branch_table
+
+</p></details><details open><summary>Example</summary><p>
+
+```cpp
+template<auto N> constexpr auto foo() { return N; }
+
+constexpr std::array jump_table{
+    foo<0>,
+    foo<1>,
+    foo<2>,
+};
+
+static_assert(0 == jump_table[0]());
+static_assert(1 == jump_table[1]());
+static_assert(2 == jump_table[2]());
+```
+
+> https://godbolt.org/z/x3xa9erGE
+
+</p></details><details open><summary>Puzzle</summary><p>
+
+* **Can you implemnt dispatch fn which generates jump table for given N?**
+
+```cpp
+template<auto N> constexpr auto foo() { return N; }
+
+template<auto N = 42>
+constexpr auto dispatch(auto n); // TODO
+
+static_assert(1 == dispatch(1));
+static_assert(7 == dispatch(7));
+static_assert(23 == dispatch(23));
+```
+
+> https://godbolt.org/z/4M9x1vjcG
+
+</p></details><details><summary>Solutions</summary><p>
+</p></details>