Add a new mailbox alternative

Atomic/DeadDrop.h

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+static_assert( __cplusplus > 2020'99 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <cassert>
+
+#include <queue>
+
+#include <Alepha/Thread.h>
+
+namespace Alepha::Hydrogen::Atomic  ::detail::  DeadDrop_m
+{
+	inline namespace exports
+	{
+		template< typename Item >
+		class DeadDrop;
+	}
+
+	template< typename Item >
+	class exports::DeadDrop
+	{
+		private:
+			Alepha::mutex access;
+
+			struct Box
+			{
+				private:
+					Alepha::mutex *access= nullptr;
+					Alepha::condition_variable ready;
+
+					Item original;
+
+					Item *active= &original;
+					std::queue< Item * > next;
+
+					Box *companion= nullptr;
+
+					friend DeadDrop;
+
+					void
+					setup( Alepha::mutex *const access, Box *const companion )
+					{
+						this->access= access;
+						this->companion= companion;
+					}
+
+				public:
+					Item &
+					acquireBox()
+					{
+						if( not active )
+						{
+							[[unlikely]];
+							std::unique_lock lock{ *access };
+							ready.wait( lock, [&]{ return not next.empty(); } );
+							active= next.front();
+							next.pop();
+						}
+						assert( active );
+						return *active;
+					}
+
+					void
+					releaseBox()
+					{
+						std::unique_lock lock{ *access };
+						companion->next.push( active );
+						active= nullptr;
+						companion->ready.notify_one();
+					}
+			};
+
+		public:
+			Box producer;
+			Box consumer;
+
+			explicit
+			DeadDrop()
+			{
+				producer.setup( &this->access, &consumer );
+				consumer.setup( &this->access, &producer );
+			}
+	};
+}
+
+namespace Alepha::Hydrogen::Atomic::inline exports::inline DeadDrop_m
+{
+	using namespace detail::DeadDrop_m::exports;
+}

Atomic/Dropbox.h

@@ -0,0 +1,167 @@
+static_assert( __cplusplus > 2020'99 );
+
+/*! 
+ * @file Dropbox.h
+ * @brief Class which abstracts a "dropbox" metaphor, for threaded programming
+ *
+ * Copyright (C) 2024 Alepha Library. All rights reserved. <BR>
+ * @author ADAM David Alan Martin
+ */
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <vector>
+#include <exception>
+
+#include <Alepha/Thread.h>
+#include <Alepha/Exception.h>
+#include <Alepha/assertion.h>
+#include <Alepha/error.h>
+
+#include "DeadDrop.h"
+
+namespace Alepha::Hydrogen::Atomic  ::detail::  Dropbox_m
+{
+	namespace C
+	{
+		const bool debug= false;
+		const bool debugInterlock= false or C::debug;
+		const bool debugPop= false or C::debug;
+	}
+
+	inline namespace exports
+	{
+		template< typename > class Dropbox;
+
+		using DropboxFinishedException= synthetic_exception< struct finished_exception, FinishedException >;
+		using AnyTaggedDropboxFinishedException= AnyTagged< DropboxFinishedException >;
+		template< typename tag > using TaggedDropboxFinishedException= Tagged< DropboxFinishedException, tag >;
+
+		using DropboxFinishedCondition= synthetic_exception< struct finished_exception, FinishedCondition, DropboxFinishedException >;
+		using AnyTaggedDropboxFinishedCondition= AnyTagged< DropboxFinishedCondition >;
+		template< typename tag > using TaggedDropboxFinishedCondition= Tagged< DropboxFinishedCondition, tag >;
+	}
+
+	template< typename T >
+	std::size_t
+	computeWeight( const T & )
+	{
+		return sizeof( T );
+	}
+
+	/*!
+	 * @brief         The Dropbox class implements a Dropbox metaphor.  Just like anonymous dead-drop boxes,
+	 *                the Dropbox class lets users add items to a box from a single producer thread
+	 *                and in a single consumer thread consume the contents of the mailbox.
+	 *
+	 * @tparam Item   The Item type which is used in the dropbox containers.
+	 *
+	 * @note          Dropboxes only support one consumer and one producer.
+	 *
+	 * @note          Dropboxes only block on attempting to acquire the next `DeadDrop` to use after exhaustion.
+	 *
+	 * @invariant     Dropboxes give out their contents in the order placed in.
+	 *
+	 *                The Dropbox primitive is implemented as DeadDrop object which is rotated periodically.
+	 *                When a producer or consumer exhausts the current DeadDrop port, it `release`es it.  When
+	 *                it needs a new DeadDrop port to actually use, it will `acquire` it.  Only `acquire` blocks.
+	 */
+	template< typename Item >
+	class exports::Dropbox
+	{
+		public:
+			using FinishedException= synthetic_exception< struct finished_exception, DropboxFinishedException >;
+			//using AnyTaggedFinishedException= AnyTagged< FinishedException >;
+			//template< typename tag > using TaggedFinishedException= Tagged< FinishedException, tag >;
+
+			using FinishedCondition= synthetic_exception< struct finished_exception, DropboxFinishedCondition, FinishedException >;
+			//using AnyTaggedFinishedCondition= AnyTagged< FinishedCondition >;
+			//template< typename tag > using TaggedFinishedCondition= Tagged< FinishedCondition, tag >;
+
+		private:
+			const std::size_t boxLimit;
+			std::size_t weight= 0;
+
+			struct Box
+			{
+				std::vector< Item > items;
+				std::vector< Item >::iterator pos= begin( items );
+				std::exception_ptr notification;
+			};
+			DeadDrop< Box > drops;
+
+			void
+			sendoff()
+			{
+				auto &producer= drops.producer.acquireBox();
+				producer.pos= begin( producer.items );
+				weight= 0;
+
+				drops.producer.releaseBox();
+			}
+
+		public:
+			explicit
+			Dropbox( const size_t lim )
+				: boxLimit( lim )
+			{
+				drops.consumer.releaseBox();
+			}
+
+			Item
+			pop()
+			{
+				auto &filled= drops.consumer.acquireBox();
+				if( filled.pos == end( filled.items ) )
+				{
+					[[unlikely]];
+					auto notification= std::move( filled.notification );
+					filled.notification= nullptr;
+					drops.consumer.releaseBox();
+					std::rethrow_exception( std::move( notification ) );
+				}
+				auto &rv= *filled.pos++;
+				if( filled.pos == end( filled.items ) and filled.notification == nullptr )
+				{
+					drops.consumer.releaseBox();
+				}
+				return std::move( rv );
+			}
+
+			bool wouldBlock() { return weight > boxLimit; }
+				
+
+			void
+			push( Item item )
+			{
+				if( wouldBlock() ) [[unlikely]] sendoff();
+				assertion( weight <= boxLimit );
+
+				auto &preparing= drops.producer.acquireBox();
+
+				weight+= computeWeight( item );
+				preparing.items.push_back( std::move( item ) );
+			}
+
+			void
+			notify( auto newNotification )
+			{
+				auto &preparing= drops.producer.acquireBox();
+				preparing.notification= std::make_exception_ptr( std::move( newNotification ) );
+				sendoff();
+			}
+
+			void
+			finish()
+			{
+				notify( build_exception< FinishedCondition >( "" ) );
+			}
+	};
+}
+
+namespace Alepha::Hydrogen::Atomic::inline exports::inline Dropbox_m
+{
+	using namespace detail::Dropbox_m::exports;
+}