Add an activeobject manager to hold active objects (#7939)

* Add an activeobject manager to hold active objects
* Add unittests

src/activeobjectmgr.h

@@ -0,0 +1,65 @@
+/*
+Minetest
+Copyright (C) 2010-2018 nerzhul, Loic BLOT <loic.blot@unix-experience.fr>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 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; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+
+#include <unordered_map>
+#include "irrlichttypes.h"
+
+class TestClientActiveObjectMgr;
+class TestServerActiveObjectMgr;
+
+template <typename T> class ActiveObjectMgr
+{
+	friend class ::TestClientActiveObjectMgr;
+	friend class ::TestServerActiveObjectMgr;
+
+public:
+	virtual void step(float dtime, const std::function<void(T *)> &f) = 0;
+	virtual bool registerObject(T *obj) = 0;
+	virtual void removeObject(u16 id) = 0;
+
+	T *getActiveObject(u16 id)
+	{
+		typename std::unordered_map<u16, T *>::const_iterator n =
+				m_active_objects.find(id);
+		return (n != m_active_objects.end() ? n->second : nullptr);
+	}
+
+protected:
+	u16 getFreeId() const
+	{
+		// try to reuse id's as late as possible
+		static thread_local u16 last_used_id = 0;
+		u16 startid = last_used_id;
+		while (!isFreeId(++last_used_id)) {
+			if (last_used_id == startid)
+				return 0;
+		}
+
+		return last_used_id;
+	}
+
+	bool isFreeId(u16 id) const
+	{
+		return id != 0 && m_active_objects.find(id) == m_active_objects.end();
+	}
+
+	std::unordered_map<u16, T *> m_active_objects;
+};