From e315d63d13a4c5fc8bb7d410b6b89edec975cf68 Mon Sep 17 00:00:00 2001
From: ADAM David Alan Martin <adam@recursive.engineer>
Date: Sun, 7 Sep 2025 17:40:00 -0400
Subject: [PATCH] Add some tuple algorithms.

Mostly car/cdr and reversal.
---
 Utility/CMakeLists.txt                       |  1 +
 Utility/tuple_algorithms.h                   | 76 ++++++++++++++++++++
 Utility/tuple_algorithms.test/0.cc           | 61 ++++++++++++++++
 Utility/tuple_algorithms.test/CMakeLists.txt |  1 +
 4 files changed, 139 insertions(+)
 create mode 100644 Utility/tuple_algorithms.h
 create mode 100644 Utility/tuple_algorithms.test/0.cc
 create mode 100644 Utility/tuple_algorithms.test/CMakeLists.txt

diff --git a/Utility/CMakeLists.txt b/Utility/CMakeLists.txt
index 6410db3..f8e8e4f 100644
--- a/Utility/CMakeLists.txt
+++ b/Utility/CMakeLists.txt
@@ -6,3 +6,4 @@ add_subdirectory( derived_pointer_cast.test )
 add_subdirectory( print_number.test )
 add_subdirectory( NamedOperator.test )
 add_subdirectory( NamedComparator.test )
+add_subdirectory( tuple_algorithms.test )
diff --git a/Utility/tuple_algorithms.h b/Utility/tuple_algorithms.h
new file mode 100644
index 0000000..f86238f
--- /dev/null
+++ b/Utility/tuple_algorithms.h
@@ -0,0 +1,76 @@
+static_assert( __cplusplus >= 2023'02 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <tuple>
+#include <type_traits>
+
+#include <Alepha/Concepts.h>
+
+namespace Alepha::Hydrogen::Utility  ::detail::  tuple_algorithms_m
+{
+	namespace tuple
+	{
+		template< typename First, typename ... Args >
+		auto car( std::tuple< First, Args... > tuple );
+
+		auto cdr( Concepts::Tuple auto tuple );
+
+		template< typename ... Args >
+		auto reverse( std::tuple< Args... > tuple );
+	}
+
+	template< typename First, typename ... Args >
+	auto
+	tuple::car( std::tuple< First, Args... > tuple )
+	{
+		return std::get< 0 >( tuple );
+	}
+
+	template< std::size_t first, std::size_t ... rest >
+	auto
+	cdr_index_sequence( std::index_sequence< first, rest... > )
+	{
+		return std::index_sequence< rest... >{};
+	}
+
+	template< typename ... Args >
+	auto
+	cdr_indices( const std::tuple< Args... > & )
+	{
+		return cdr_index_sequence( std::make_index_sequence< sizeof...( Args ) >{} );
+	}
+
+	template< std::size_t ... ints >
+	auto
+	get_indices( Concepts::Tuple auto tuple, std::index_sequence< ints... > )
+	{
+		return std::tuple{ std::get< ints >( tuple )... };
+	}
+
+	auto
+	tuple::cdr( Concepts::Tuple auto tuple )
+	{
+		return get_indices( tuple, cdr_indices( tuple ) );
+	}
+
+	template< typename ... Args >
+	auto
+	tuple::reverse( std::tuple< Args... > tuple )
+	{
+		if constexpr( sizeof...( Args ) == 0 ) return std::tuple{};
+		else return std::tuple_cat( tuple::reverse( cdr( tuple ) ), std::tuple{ car( tuple ) } );
+	}
+
+	inline namespace exports
+	{
+		namespace tuple= tuple_algorithms_m::tuple;
+	}
+}
+
+namespace Alepha::Hydrogen::Utility::inline exports::inline tuple_algorithms_m
+{
+	using namespace detail::tuple_algorithms_m::exports;
+}
diff --git a/Utility/tuple_algorithms.test/0.cc b/Utility/tuple_algorithms.test/0.cc
new file mode 100644
index 0000000..2daa233
--- /dev/null
+++ b/Utility/tuple_algorithms.test/0.cc
@@ -0,0 +1,61 @@
+static_assert( __cplusplus >= 2023'02 );
+
+#include <Alepha/Utility/tuple_algorithms.h>
+
+#include <Alepha/Testing/test.h>
+#include <Alepha/Testing/TableTest.h>
+
+static auto init= Alepha::Utility::enroll <=[]
+{
+	using namespace Alepha::Testing::exports;
+	using namespace Alepha::Testing::literals;
+
+	"Does an empty tuple reverse?"_test <=TableTest
+	<
+		Alepha::Utility::tuple::reverse<>
+	>
+	::Cases
+	{
+		{ "No arguments", { {} }, {} },
+	};
+
+	"Does an single entry reverse?"_test <=TableTest
+	<
+		Alepha::Utility::tuple::reverse< int >
+	>
+	::Cases
+	{
+		{ "No arguments (42)", { { 42 } }, { 42 } },
+		{ "No arguments (2600)", { { 2600 } }, { 2600 } },
+	};
+
+	"Does a pair of entries reverse?"_test <=TableTest
+	<
+		Alepha::Utility::tuple::reverse< int, std::string >
+	>
+	::Cases
+	{
+		{ "No arguments (42, 'Hello')", { { 42, "Hello" } }, { "Hello", 42 } },
+		{ "No arguments (2600, 'Goodbye')", { { 2600, "Goodbye" } }, { "Goodbye", 2600 } },
+	};
+
+	"Does a trio of entries reverse (first and last same)?"_test <=TableTest
+	<
+		Alepha::Utility::tuple::reverse< int, std::string, int >
+	>
+	::Cases
+	{
+		{ "No arguments (42, 'Hello')", { { 42, "Hello", 127 } }, { 127, "Hello", 42 } },
+		{ "No arguments (2600, 'Goodbye')", { { 2600, "Goodbye", 77 } }, { 77, "Goodbye", 2600 } },
+	};
+
+	"Does a trio of entries reverse (first and last differ)?"_test <=TableTest
+	<
+		Alepha::Utility::tuple::reverse< int, std::string, double >
+	>
+	::Cases
+	{
+		{ "No arguments (42, 'Hello')", { { 42, "Hello", 2.71828 } }, { 2.71828, "Hello", 42 } },
+		{ "No arguments (2600, 'Goodbye')", { { 2600, "Goodbye", 3.14159 } }, { 3.14159, "Goodbye", 2600 } },
+	};
+};
diff --git a/Utility/tuple_algorithms.test/CMakeLists.txt b/Utility/tuple_algorithms.test/CMakeLists.txt
new file mode 100644
index 0000000..b099603
--- /dev/null
+++ b/Utility/tuple_algorithms.test/CMakeLists.txt
@@ -0,0 +1 @@
+unit_test( 0 )