From 322bf65400aa205d1ce89e0775df8fc2832ba8e4 Mon Sep 17 00:00:00 2001
From: ADAM David Alan Martin <adam@recursive.engineer>
Date: Thu, 8 Aug 2024 17:16:37 -0400
Subject: [PATCH] Invariant gadget.

---
 CMakeLists.txt                |  1 +
 Invariant.h                   | 44 ++++++++++++++++++++++++
 Invariant.test/0.cc           | 64 +++++++++++++++++++++++++++++++++++
 Invariant.test/CMakeLists.txt |  1 +
 4 files changed, 110 insertions(+)
 create mode 100644 Invariant.h
 create mode 100644 Invariant.test/0.cc
 create mode 100644 Invariant.test/CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index cd8d722..97ff2d0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -39,6 +39,7 @@ add_subdirectory( Exception.test )
 add_subdirectory( word_wrap.test )
 add_subdirectory( string_algorithms.test )
 add_subdirectory( template_for.test )
+add_subdirectory( Invariant.test )
 add_subdirectory( tuplize_args.test )
 add_subdirectory( Thread.test )
 add_subdirectory( assertion.test )
diff --git a/Invariant.h b/Invariant.h
new file mode 100644
index 0000000..dade8d3
--- /dev/null
+++ b/Invariant.h
@@ -0,0 +1,44 @@
+static_assert( __cplusplus > 2023'00 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <cassert>
+
+namespace Alepha::Hydrogen  ::detail::  Invariant_m
+{
+	inline namespace exports
+	{
+		template< typename T >
+		class Invariant
+		{
+			private:
+				const T *this_;
+
+				void
+				check() const
+				{
+					assert( this_->invariant() );
+				}
+
+			public:
+				~Invariant()
+				{
+					check();
+				}
+
+				explicit
+				Invariant( const T *const this_ )
+					: this_( this_ )
+				{
+					check();
+				}
+		};
+	}
+}
+
+namespace Alepha::Hydrogen::inline exports::inline Invariant_m
+{
+	using namespace detail::Invariant_m::exports;
+}
diff --git a/Invariant.test/0.cc b/Invariant.test/0.cc
new file mode 100644
index 0000000..187565b
--- /dev/null
+++ b/Invariant.test/0.cc
@@ -0,0 +1,64 @@
+static_assert( __cplusplus > 2023'00 );
+
+#include "../Invariant.h"
+
+#include <Alepha/Testing/TableTest.h>
+#include <Alepha/Testing/test.h>
+
+static auto init= Alepha::Utility::enroll <=[]
+{
+	using namespace Alepha::Testing::exports;
+
+	"Invariant basic test"_test <=[]
+	{
+		struct ItIsFine {};
+
+		struct FakeVector
+		{
+			int size= 0;
+			int capacity= 0;
+			int *data= nullptr;
+
+			bool
+			invariant() const
+			{
+				return true
+					and ( capacity >= size )
+					and ( capacity < 10 )
+					and ( capacity == 0 or data != nullptr )
+				;
+			}
+
+			void
+			push_back( const int x )
+			{
+				Alepha::Invariant inv{ this };
+
+				if( size == capacity )
+				{
+					// Not fully exception safe, but just
+					// ignore that...
+					const int newCapacity= capacity * 2 + 1;
+					int *const data2= new int [ newCapacity ];
+					if( capacity > 8 ) throw ItIsFine{};
+
+					std::copy( data, data + size, data2 );
+					capacity= newCapacity;
+					delete [] data;
+					data= data2;
+				}
+				assert( size < capacity );
+				data[ size++ ]= x;
+			}
+		};
+
+		FakeVector v;
+
+		for( int i= 0; i < 4; ++i ) try
+		{
+			v.push_back( i );
+		}
+		catch( const ItIsFine & ) {} // It's fine!
+	};
+};
+
diff --git a/Invariant.test/CMakeLists.txt b/Invariant.test/CMakeLists.txt
new file mode 100644
index 0000000..b099603
--- /dev/null
+++ b/Invariant.test/CMakeLists.txt
@@ -0,0 +1 @@
+unit_test( 0 )