UnifiedEnum -- union construction of enumerates

UnifiedEnum.h

@@ -0,0 +1,129 @@
+static_assert( __cplusplus > 2023'00 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include "Enum.h"
+
+namespace Alepha::Hydrogen  ::detail::  UnifiedEnum_m
+{
+	inline namespace exports {}
+
+	template< typename T >
+	constexpr bool is_alepha_enum_v= false;
+
+	using detail::Enum_m::EnumValueString;
+
+	template< detail::Enum_m::EnumValueString ... values >
+	constexpr bool is_alepha_enum_v< Enum< values... > >{ true };
+
+	template< typename T >
+	concept EnhancedEnumerate= is_alepha_enum_v< T >;
+
+	namespace exports
+	{
+		template< EnhancedEnumerate ... >
+		class UnifiedEnum;
+	}
+
+	template< EnumValueString cs_strs >
+	struct StringWrapper;
+
+	template< EnhancedEnumerate ... enums >
+	struct unify_values;
+
+	template< detail::Enum_m::EnumValueString ... cs_str >
+	struct unify_values< Enum< cs_str... > >
+	{
+		using type= TypeList< StringWrapper< cs_str >... >;
+	};
+
+	template< detail::Enum_m::EnumValueString ... cs_str, EnhancedEnumerate ... next >
+	struct unify_values< Enum< cs_str... >, next... >
+	{
+		using type= list_cat_t< TypeList< StringWrapper< cs_str >... >,
+				typename unify_values< next... >::type >;
+	};
+
+	// Because of the way that this works, I do wind up with a possibility of duplicates
+	// in the enumerate list.  This is (hopefully) benign.  The underlying `Enum`
+	// type doesn't check for duplicates for the same reason we'll not check here.
+	// It's hopefully benign as the duplicated enumerate values simply become an
+	// unused index by the `get_index` and `set_index` functions.  For the most
+	// part users of the class shouldn't be calling those functions.  If they are,
+	// it should merely be for serialization.  As such, skipping a value in the
+	// sequential indexing shouldn't be observable.
+	//
+	// Without good enough support for a compiletime search and dedupe on types,
+	// we have to resort to O( N ** 2 ) or worse algorithms to remove duplicates.
+	// With so much compiletime algorithm pressure already, this would become a
+	// significant pain point for TUs which have a lot of `Enum`s.
+	//
+	// An unfortunate side-effect of this design choice is that when/if this design
+	// is changed to support deduplication of enumerate values (strings), that
+	// will create a massive but subtle ABI break:
+	// `UnifiedEnum< Enum< "a"_value >, Enum< "a"_value > >` would now use a
+	// different index scheme, despite having the same typename.
+	template< TypeListType > struct build_enum;
+
+	template< detail::Enum_m::EnumValueString ... cs_str >
+	struct build_enum< TypeList< StringWrapper< cs_str >... > >
+	{
+		using type= Enum< cs_str... >;
+	};
+
+	template< EnhancedEnumerate ... EnumType >
+	class exports::UnifiedEnum
+		: protected build_enum< typename unify_values< EnumType... >::type >::type
+	{
+		private:
+			using Base= typename build_enum< typename unify_values< EnumType... >::type >::type;
+
+		public:
+			using Base::Base;
+			using Base::name;
+			using Base::accepts;
+			using Base::set_index;
+			using Base::get_index;
+
+			template< typename Enumerate >
+			requires( list_contains_v< TypeList< EnumType... >, Enumerate > )
+			UnifiedEnum( const Enumerate e )
+			{
+				unsigned value= 0;
+				for( const auto &next: this->keys() )
+				{
+					if( next == e.key() )
+					{
+						set_index( value );
+						return;
+					}
+					++value;
+				}
+
+				// It should not be possible to reach here.
+				abort();
+			}
+
+			friend std::ostream &
+			operator << ( std::ostream &os, const UnifiedEnum &rhs )
+			{
+				return os << static_cast< const Base & >( rhs );
+			}
+
+			friend std::istream &
+			operator >> ( std::istream &is, UnifiedEnum &rhs )
+			{
+				return is >> static_cast< Base & >( rhs );
+			}
+
+			friend constexpr bool
+			operator == ( const UnifiedEnum &, const UnifiedEnum & )= default;
+	};
+}
+
+namespace Alepha::Hydrogen::inline exports::inline UnifiedEnum_m
+{
+	using namespace detail::UnifiedEnum_m::exports;
+}