WIP: thread pool class

For now it's missing ability to, you know, actually run tasks.

projectfiles/VC14/wesnoth.vcxproj

@@ -3237,6 +3237,7 @@
       <ObjectFileName Condition="'$(Configuration)|$(Platform)'=='Test_Release|Win32'">$(IntDir)Tests\Utils\</ObjectFileName>
     </ClCompile>
     <ClCompile Include="..\..\src\theme.cpp" />
+    <ClCompile Include="..\..\src\thread_pool.cpp" />
     <ClCompile Include="..\..\src\time_of_day.cpp" />
     <ClCompile Include="..\..\src\tod_manager.cpp" />
     <ClCompile Include="..\..\src\tooltips.cpp" />
@@ -4025,6 +4026,7 @@
       <ExcludedFromBuild Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">true</ExcludedFromBuild>
     </CustomBuild>
     <ClInclude Include="..\..\src\theme.hpp" />
+    <ClInclude Include="..\..\src\thread_pool.hpp" />
     <ClInclude Include="..\..\src\time_of_day.hpp" />
     <ClInclude Include="..\..\src\tod_manager.hpp" />
     <ClInclude Include="..\..\src\serialization\tag.hpp" />

projectfiles/VC14/wesnoth.vcxproj.filters

@@ -1555,6 +1555,7 @@
     <ClCompile Include="..\..\src\ogl\shader.cpp">
       <Filter>OGL</Filter>
     </ClCompile>
+    <ClCompile Include="..\..\src\thread_pool.cpp" />
   </ItemGroup>
   <ItemGroup>
     <ClInclude Include="..\..\src\addon\client.hpp">
@@ -3029,6 +3030,7 @@
     <ClInclude Include="..\..\src\ogl\shader.hpp">
       <Filter>OGL</Filter>
     </ClInclude>
+    <ClInclude Include="..\..\src\thread_pool.hpp" />
   </ItemGroup>
   <ItemGroup>
     <CustomBuild Include="..\..\src\tests\test_sdl_utils.hpp">

source_lists/wesnoth

@@ -347,6 +347,7 @@ syncmp_handler.cpp
 team.cpp
 teambuilder.cpp
 terrain/filter.cpp
+thread_pool.cpp
 tod_manager.cpp
 units/abilities.cpp
 units/animation.cpp

src/thread_pool.cpp

@@ -0,0 +1,90 @@
+/*
+   Copyright (C) 2018 by Jyrki Vesterinen <sandgtx@gmail.com>
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, or
+   (at your option) any later version.
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY.
+
+   See the COPYING file for more details.
+*/
+
+#include "thread_pool.hpp"
+
+#include <algorithm>
+#include <cassert>
+#include <iterator>
+
+thread_pool::thread_pool()
+	: exiting_(false)
+{
+	init();
+
+	std::generate_n(std::back_inserter(threads_), NUM_THREADS,
+		[this]() { return std::thread(&thread_pool::thread_proc, this); });
+}
+
+thread_pool::~thread_pool()
+{
+	{
+		std::unique_lock<std::mutex> lock(mutex_);
+
+		// A thread pool can't exit if work is in progress.
+		assert(!work_);
+
+		exiting_ = true;
+		work_cond_.notify_all();
+	}
+
+	for(std::thread& t : threads_) {
+		t.join();
+	}
+}
+
+void thread_pool::init()
+{
+	num_waiting_threads_ = 0u;
+	num_finished_threads_ = 0u;
+	ready_for_work_ = false;
+	work_ = nullptr;
+	done_ = false;
+}
+
+void thread_pool::thread_proc()
+{
+	std::unique_lock<std::mutex> lock(mutex_);
+
+	++num_waiting_threads_;
+	if(num_waiting_threads_ == NUM_THREADS) {
+		ready_for_work_ = true;
+		ready_for_work_cond_.notify_one();
+	}
+
+	while(!work_ && !exiting_) {
+		work_cond_.wait(lock);
+	}
+
+	num_finished_threads_ = 0u;
+	lock.unlock();
+
+	while(!exiting_) {
+		work_();
+
+		lock.lock();
+		++num_finished_threads_;
+		if(num_finished_threads_ == NUM_THREADS) {
+			done_ = true;
+			done_promise_.set_value();
+			work_ = nullptr;
+		}
+
+		while(!work_ && !exiting_) {
+			work_cond_.wait(lock);
+		}
+
+		num_finished_threads_ = 0u;
+		lock.unlock();
+	}
+}

src/thread_pool.hpp

@@ -0,0 +1,46 @@
+/*
+   Copyright (C) 2018 by Jyrki Vesterinen <sandgtx@gmail.com>
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, or
+   (at your option) any later version.
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY.
+
+   See the COPYING file for more details.
+*/
+
+#pragma once
+
+#include <condition_variable>
+#include <functional>
+#include <future>
+#include <mutex>
+#include <thread>
+#include <vector>
+
+class thread_pool
+{
+	thread_pool();
+	~thread_pool();
+
+private:
+	const unsigned int NUM_THREADS = 16u;
+
+	void thread_proc();
+
+	void init();
+
+	std::vector<std::thread> threads_;
+	std::function<void()> work_;
+	std::mutex mutex_;
+	unsigned int num_waiting_threads_;
+	unsigned int num_finished_threads_;
+	bool ready_for_work_;
+	std::condition_variable ready_for_work_cond_;
+	std::condition_variable work_cond_;
+	bool done_;
+	std::promise<void> done_promise_;
+	bool exiting_;
+};