Add enum fusing function (#127)

README.md

@@ -32,6 +32,7 @@ Header-only C++17 library provides static reflection for enums, work with any en
 * `enum_index` obtains index in enum value sequence from enum value.
 * `enum_contains` checks whether enum contains enumerator with such value.
 * `enum_type_name` returns name of enum type.
+* `enum_fuse` allows multidimensional switch/cases.
 * `is_unscoped_enum` checks whether type is an [Unscoped enumeration](https://en.cppreference.com/w/cpp/language/enum#Unscoped_enumeration).
 * `is_scoped_enum` checks whether type is an [Scoped enumeration](https://en.cppreference.com/w/cpp/language/enum#Scoped_enumerations).
 * `underlying_type` improved UB-free "SFINAE-friendly" [underlying_type](https://en.cppreference.com/w/cpp/types/underlying_type).
@@ -137,6 +138,16 @@ enum class Color { RED = 2, BLUE = 4, GREEN = 8 };
   // color_entries[0].second -> "RED"
   ```
   
+* Enum fusion for multi-level switch/case statements
+
+  ```cpp
+  switch (magic_enum::enum_fuse(color1, color2)) {
+  case magic_enum::enum_fuse(RED, BLUE): // ...
+  case magic_enum::enum_fuse(RED, RED):  // ...
+  // ...
+  }
+  ```
+
 * Ostream operator for enum
 
   ```cpp

doc/reference.md

@@ -12,6 +12,7 @@
 * [`enum_index` obtains index in enum value sequence from enum value.](#enum_index)
 * [`enum_contains` checks whether enum contains enumerator with such value.](#enum_contains)
 * [`enum_type_name` returns type name of enum.](#enum_type_name)
+* [`enum_fuse` returns a bijective mix of enum values.](#enum_fuse)
 * [`is_unscoped_enum` checks whether type is an Unscoped enumeration.](#is_unscoped_enum)
 * [`is_scoped_enum` checks whether type is an Scoped enumeration.](#is_scoped_enum)
 * [`underlying_type` improved UB-free "SFINAE-friendly" underlying_type.](#underlying_type)
@@ -325,6 +326,25 @@ constexpr string_view enum_type_name() noexcept;
   // color_name -> "Color"
   ```
 
+## `enum_fuse`
+
+```cpp
+template<typename ... Es>
+[[nodiscard]] constexpr size_t enum_fuse(Es ... values);
+```
+
+* Returns a bijective mix of several enum values with [Cantor pairing function](https://en.wikipedia.org/wiki/Pairing_function). This can be used to emulate 2D switch/case statements.
+
+* Examples
+
+  ```cpp
+  switch (magic_enum::enum_fuse(color1, color2)) {
+  case magic_enum::enum_fuse(RED, BLUE): // ...
+  case magic_enum::enum_fuse(RED, RED):  // ...
+  // ...
+  }
+  ```
+
 ## `is_unscoped_enum`
 
 ```cpp

include/magic_enum.hpp

@@ -609,6 +609,15 @@ struct underlying_type {};
 template <typename T>
 struct underlying_type<T, true> : std::underlying_type<std::decay_t<T>> {};
 
+constexpr size_t cantor_pair(size_t v1, size_t v2) noexcept {
+  return (((v1 + v2) * (v1 + v2 + 1)) >> 1) + v2;
+}
+
+template<typename ... Ts>
+constexpr size_t cantor_pair(size_t v1, size_t head, Ts ... tail) noexcept {
+  return cantor_pair(cantor_pair(v1, head), tail...);
+}
+
 } // namespace magic_enum::detail
 
 // Checks is magic_enum supported compiler.
@@ -878,6 +887,13 @@ template <typename E>
   return {}; // Invalid value or out of range.
 }
 
+template<typename ... Es>
+[[nodiscard]] constexpr size_t enum_fuse(Es ... enums) {
+  static_assert(sizeof...(Es) >= 2, "magic_enum::enum_fuse requires at least 2 enums");
+  // Add 1 to prevent matching 2D fusions with 3D fusions etc.
+  return detail::cantor_pair((enum_index(enums).value() + 1)...);
+}
+
 // Checks whether enum contains enumerator with such enum value.
 template <typename E>
 [[nodiscard]] constexpr auto enum_contains(E value) noexcept -> detail::enable_if_enum_t<E, bool> {

test/test.cpp

@@ -1014,4 +1014,44 @@ TEST_CASE("constexpr_for") {
   });
 }
 
+static int switch_case_2d(Color color, Directions number)
+{
+  switch (magic_enum::enum_fuse(color, number))
+  {
+  case magic_enum::enum_fuse(Color::RED, Directions::Up):
+    return 1;
+  case magic_enum::enum_fuse(Color::BLUE, Directions::Down):
+    return 2;
+  default:
+    return 0;
+  }
+}
+
+enum class Index { zero = 0, one = 1, two = 2 };
+
+static int switch_case_3d(Color color, Directions number, Index index)
+{
+  switch (magic_enum::enum_fuse(color, number, index))
+  {
+  case magic_enum::enum_fuse(Color::RED, Directions::Up, Index::zero):
+    return 1;
+  // model accidental removal of last index, must not match anything
+  case magic_enum::enum_fuse(Color::BLUE, Directions::Up):
+    return 2;
+  default:
+    return 0;
+  }
+}
+
+TEST_CASE("multdimensional-switch-case") {
+  REQUIRE(switch_case_2d(Color::RED, Directions::Up) == 1);
+  REQUIRE(switch_case_2d(Color::RED, Directions::Down) == 0);
+  REQUIRE(switch_case_2d(Color::BLUE, Directions::Up) == 0);
+  REQUIRE(switch_case_2d(Color::BLUE, Directions::Down) == 2);
+  REQUIRE(switch_case_3d(Color::RED, Directions::Up, Index::zero) == 1);
+  REQUIRE(switch_case_3d(Color::BLUE, Directions::Up, Index::zero) == 0);
+  REQUIRE(switch_case_3d(Color::BLUE, Directions::Up, Index::one) == 0);
+  REQUIRE(switch_case_3d(Color::BLUE, Directions::Up, Index::two) == 0);
+}
+
 #endif