From fd0bebd7be8d71fca58653b72aae2d3f763ce233 Mon Sep 17 00:00:00 2001
From: _Redstone_c_ <myredstone1024@gmail.com>
Date: Wed, 30 Mar 2022 12:07:48 +0800
Subject: [PATCH] feat(miscellaneous): add TSynthThreeWay and the corresponding
 testing

---
 .../Private/Testing/MiscellaneousTesting.cpp  | 19 ++++++
 .../Source/Public/Miscellaneous/Compare.h     | 60 ++++++++++++++++++-
 2 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/Redcraft.Utility/Source/Private/Testing/MiscellaneousTesting.cpp b/Redcraft.Utility/Source/Private/Testing/MiscellaneousTesting.cpp
index f72faa4..403086e 100644
--- a/Redcraft.Utility/Source/Private/Testing/MiscellaneousTesting.cpp
+++ b/Redcraft.Utility/Source/Private/Testing/MiscellaneousTesting.cpp
@@ -108,6 +108,14 @@ struct FTestStrongOrdering
 	friend strong_ordering operator<=>(FTestStrongOrdering LHS, FTestStrongOrdering RHS) { return LHS.Num <=> RHS.Num; }
 };
 
+struct FTestSynth
+{
+	int32 A;
+	FTestSynth(int32 InA) : A(InA) { }
+	friend bool operator==(FTestSynth LHS, FTestSynth RHS) { return LHS.A == RHS.A; }
+	friend bool operator< (FTestSynth LHS, FTestSynth RHS) { return LHS.A <  RHS.A; }
+};
+
 NAMESPACE_UNNAMED_END
 
 void TestCompare()
@@ -200,6 +208,17 @@ void TestCompare()
 	always_check((TCompareThreeWay<int32>()(0, 0)   == strong_ordering::equal));
 	always_check((TCompareThreeWay<void>() (0, 0.0) == strong_ordering::equal));
 
+	
+	always_check(TSynthThreeWay{}(FTestPartialOrdering(-1), FTestPartialOrdering( 0)) == partial_ordering::less);
+	always_check(TSynthThreeWay{}(FTestPartialOrdering( 0), FTestPartialOrdering( 0)) == partial_ordering::equivalent);
+	always_check(TSynthThreeWay{}(FTestPartialOrdering( 0), FTestPartialOrdering(-1)) == partial_ordering::greater);
+
+	always_check(TSynthThreeWay{}(FTestPartialOrdering( 0, true), FTestPartialOrdering( 0, false)) == partial_ordering::unordered);
+	
+	always_check(TSynthThreeWay{}(FTestSynth(-1), FTestSynth( 0)) == weak_ordering::less);
+	always_check(TSynthThreeWay{}(FTestSynth( 0), FTestSynth( 0)) == weak_ordering::equivalent);
+	always_check(TSynthThreeWay{}(FTestSynth( 0), FTestSynth(-1)) == weak_ordering::greater);
+	
 	always_check((StrongOrder(0, 0)                 == strong_ordering::equal));
 	always_check((WeakOrder(0, 0)                   == strong_ordering::equal));
 	always_check((PartialOrder(0, 0)                == strong_ordering::equal));
diff --git a/Redcraft.Utility/Source/Public/Miscellaneous/Compare.h b/Redcraft.Utility/Source/Public/Miscellaneous/Compare.h
index f969b68..326036f 100644
--- a/Redcraft.Utility/Source/Public/Miscellaneous/Compare.h
+++ b/Redcraft.Utility/Source/Public/Miscellaneous/Compare.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "CoreTypes.h"
+#include "Templates/Utility.h"
 #include "Concepts/Concepts.h"
 #include "TypeTraits/TypeTraits.h"
 
@@ -52,7 +53,7 @@ concept CThreeWayComparableWith = CWeaklyEqualityComparableWith<T, U> && CPartia
 	CThreeWayComparable<T, OrderingType> &&	CThreeWayComparable<U, OrderingType> &&
 	CCommonReferenceWith<const typename TRemoveReference<T>::Type&, const typename TRemoveReference<U>::Type&> &&
 	CThreeWayComparable<typename TCommonReference<const typename TRemoveReference<T>::Type&, const typename TRemoveReference<U>::Type&>::Type, OrderingType> &&
-	requires(const TRemoveReference<T>::Type& A, const TRemoveReference<T>::Type& B)
+	requires(const TRemoveReference<T>::Type& A, const TRemoveReference<U>::Type& B)
 	{
 		{ A <=> B } -> CThreeWayComparesAs<OrderingType>;
 		{ B <=> A } -> CThreeWayComparesAs<OrderingType>;
@@ -67,7 +68,7 @@ struct TCompareThreeWayResult<T, U>
 	using Type = decltype(DeclVal<const typename TRemoveReference<T>::Type&>() <=> DeclVal<const typename TRemoveReference<U>::Type&>());
 };
 
-template <typename T = void> requires CSameAs<T, void> || CThreeWayComparable<T>
+template <typename T = void> requires (CSameAs<T, void> || CThreeWayComparable<T>)
 struct TCompareThreeWay
 {
 	constexpr auto operator()(T&& LHS, T&& RHS) const
@@ -86,6 +87,61 @@ struct TCompareThreeWay<void>
 	}
 };
 
+template <typename T, typename OrderingType = partial_ordering>
+concept CSynthThreeWayComparable = CThreeWayComparable<T> ||
+	requires(const TRemoveReference<T>::Type& A, const TRemoveReference<T>::Type& B)
+	{
+		{ A < B } -> CBooleanTestable;
+		{ B < A } -> CBooleanTestable;
+	};
+
+template <typename T, typename U, typename OrderingType = partial_ordering>
+concept CSynthThreeWayComparableWith = CThreeWayComparableWith<T, U> ||
+	requires(const TRemoveReference<T>::Type& A, const TRemoveReference<U>::Type& B)
+	{
+		{ A < B } -> CBooleanTestable;
+		{ B < A } -> CBooleanTestable;
+	};
+
+template <typename T = void> requires (CSameAs<T, void> || CSynthThreeWayComparable<T>)
+struct TSynthThreeWay
+{
+	constexpr auto operator()(T&& LHS, T&& RHS) const
+	{
+		if constexpr (CThreeWayComparable<T>)
+		{
+			return Forward<T>(LHS) <=> Forward<T>(RHS);
+		}
+		else
+		{
+			return Forward<T>(LHS) < Forward<T>(RHS) ? weak_ordering::less : Forward<T>(RHS) < Forward<T>(LHS) ? weak_ordering::greater : weak_ordering::equivalent;
+		}
+	}
+};
+
+template <>
+struct TSynthThreeWay<void>
+{
+	template <typename T, typename U> requires CSynthThreeWayComparableWith<T, U>
+	constexpr auto operator()(T&& LHS, U&& RHS) const
+	{
+		if constexpr (CThreeWayComparableWith<T, U>)
+		{
+			return Forward<T>(LHS) <=> Forward<U>(RHS);
+		}
+		else
+		{
+			return Forward<T>(LHS) < Forward<U>(RHS) ? weak_ordering::less : Forward<U>(RHS) < Forward<T>(LHS) ? weak_ordering::greater : weak_ordering::equivalent;
+		}
+	}
+};
+
+template <typename T, typename U = T>
+struct TSynthThreeWayResult
+{
+	using Type = decltype(TSynthThreeWay{}(DeclVal<const typename TRemoveReference<T>::Type&>(), DeclVal<const typename TRemoveReference<U>::Type&>()));
+};
+
 NAMESPACE_UNNAMED_BEGIN
 
 inline constexpr decltype(NAMESPACE_STD::strong_order)                   StrongOrder;