First stab at it. Still need to find all the entity position change spots.

src/server/CMakeLists.txt

@@ -8,6 +8,7 @@ set(server_SRCS
 	${CMAKE_CURRENT_SOURCE_DIR}/serveractiveobject.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/serverinventorymgr.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/serverlist.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/spatial_map.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/unit_sao.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/rollback.cpp
 	PARENT_SCOPE)

src/server/activeobjectmgr.cpp

@@ -93,6 +93,7 @@ bool ActiveObjectMgr::registerObject(std::unique_ptr<ServerActiveObject> obj)
 
 	auto obj_id = obj->getId(); 
 	m_active_objects.put(obj_id, std::move(obj));
+	m_spatial_map.insert(obj.get());
 
 	auto new_size = m_active_objects.size();
 	verbosestream << "Server::ActiveObjectMgr::addActiveObjectRaw(): "
@@ -110,6 +111,8 @@ void ActiveObjectMgr::removeObject(u16 id)
 	verbosestream << "Server::ActiveObjectMgr::removeObject(): "
 			<< "id=" << id << std::endl;
 
+	m_spatial_map.remove(m_active_objects.get(id).get());
+	
 	// this will take the object out of the map and then destruct it
 	bool ok = m_active_objects.remove(id);
 	if (!ok) {

src/server/activeobjectmgr.h

@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <vector>
 #include "../activeobjectmgr.h"
 #include "serveractiveobject.h"
+#include "spatial_map.h"
 
 namespace server
 {
@@ -48,5 +49,7 @@ public:
 	void getAddedActiveObjectsAroundPos(v3f player_pos, f32 radius,
 			f32 player_radius, const std::set<u16> &current_objects,
 			std::vector<u16> &added_objects);
+
+	SpatialMap m_spatial_map;
 };
 } // namespace server

src/server/luaentity_sao.cpp

@@ -132,7 +132,7 @@ void LuaEntitySAO::dispatchScriptDeactivate(bool removal)
 		m_env->getScriptIface()->luaentity_Deactivate(m_id, removal);
 }
 
-void LuaEntitySAO::step(float dtime, bool send_recommended)
+void LuaEntitySAO::step(float dtime, bool send_recommended, bool &position_changed)
 {
 	if (!m_properties_sent) {
 		m_properties_sent = true;

src/server/luaentity_sao.h

@@ -40,7 +40,7 @@ public:
 	ActiveObjectType getType() const { return ACTIVEOBJECT_TYPE_LUAENTITY; }
 	ActiveObjectType getSendType() const { return ACTIVEOBJECT_TYPE_GENERIC; }
 	virtual void addedToEnvironment(u32 dtime_s);
-	void step(float dtime, bool send_recommended);
+	void step(float dtime, bool send_recommended, bool &position_changed) override;
 	std::string getClientInitializationData(u16 protocol_version);
 
 	bool isStaticAllowed() const { return m_prop.static_save; }

src/server/player_sao.cpp

@@ -154,7 +154,7 @@ void PlayerSAO::getStaticData(std::string * result) const
 	FATAL_ERROR("This function shall not be called for PlayerSAO");
 }
 
-void PlayerSAO::step(float dtime, bool send_recommended)
+void PlayerSAO::step(float dtime, bool send_recommended, bool &position_changed)
 {
 	if (!isImmortal() && m_drowning_interval.step(dtime, 2.0f)) {
 		// Get nose/mouth position, approximate with eye position
@@ -238,6 +238,7 @@ void PlayerSAO::step(float dtime, bool send_recommended)
 		warningstream << "PlayerSAO::step() id=" << m_id <<
 			" is attached to nonexistent parent. This is a bug." << std::endl;
 		clearParentAttachment();
+		// FIXHERE
 		setBasePosition(m_last_good_position);
 		m_env->getGameDef()->SendMovePlayer(this);
 	}

src/server/player_sao.h

@@ -87,7 +87,7 @@ public:
 	bool shouldUnload() const override { return false; }
 	std::string getClientInitializationData(u16 protocol_version) override;
 	void getStaticData(std::string *result) const override;
-	void step(float dtime, bool send_recommended) override;
+	void step(float dtime, bool send_recommended, bool &position_changed) override;
 	void setBasePosition(v3f position);
 	void setPos(const v3f &pos) override;
 	void addPos(const v3f &added_pos) override;

src/server/serveractiveobject.h

@@ -107,7 +107,7 @@ public:
 			same time so that the data can be combined in a single
 			packet.
 	*/
-	virtual void step(float dtime, bool send_recommended){}
+	virtual void step(float dtime, bool send_recommended, bool &position_changed){}
 
 	/*
 		The return value of this is passed to the client-side object

src/server/spatial_map.cpp

@@ -0,0 +1,120 @@
+/*
+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 "spatial_map.h"
+
+namespace server
+{
+
+// all inserted entires go into the uncached vector
+void SpatialMap::insert(ServerActiveObject* obj)
+{
+	m_uncached.push_back(obj->getId());
+}
+
+// Invalidates upon position update
+void SpatialMap::invalidate(ServerActiveObject* obj)
+{
+	// remove from cache, if present
+	u16 id = obj->getId();
+	bool found = false;
+	auto range = m_cached.equal_range(SpatialKey(obj->getBasePosition()));
+	for (auto it = range.first; it != range.second; ++it) {
+		if (it->second == id) {
+			m_cached.erase(it);
+			found = true;
+			break; // Erase and leave early
+		}
+	}
+
+	if(found) {
+		// place back in uncached
+		insert(obj);
+	}
+}
+
+void SpatialMap::remove(ServerActiveObject* obj)
+{
+	SpatialKey key(obj->getBasePosition());
+	u16 idToRemove = obj->getId();
+	if(m_cached.find(key) != m_cached.end()) {
+		auto range = m_cached.equal_range(key);
+		for (auto it = range.first; it != range.second; ++it) {
+			if (it->second == idToRemove) {
+				m_cached.erase(it);
+				return; // Erase and leave early
+			}
+		}
+	};
+
+	auto it = std::find(m_uncached.begin(), m_uncached.end(), idToRemove);
+	if (it != m_uncached.end()) {
+		m_uncached.erase(it);
+		return;
+	} else {
+		// Error, this shouldn't ever be hit.
+	}
+}
+
+// Only when at least 64 uncached objects or 10% uncached overall
+void SpatialMap::cacheUpdate(ActiveObjectMgr& mgr)
+{
+	bool shouldUpdate = false;
+	if(m_uncached.size() >= 64 || (m_uncached.size() >= 64 && m_uncached.size() * 10 > m_cached.size())) {
+		for(u16& entry : m_uncached) {
+			m_cached.insert(std::pair<SpatialKey,u16>(mgr.getActiveObject(entry)->getBasePosition(), entry));
+		}
+		m_uncached.clear();
+	}
+}
+
+void SpatialMap::getRelevantObjectIds(const aabb3f &box, std::vector<u16> &relevant_objs)
+{
+	if(!m_cached.empty()) {
+		// when searching, we must round to maximum extent of relevant mapblock indexes
+		auto shrinkRnd = [](f32 val) {
+			//return val < 0 ? floor(val) : ceil(val);}
+			s16 rounded = std::lround(val);
+			s16 remainder = (rounded & 0xF) != 0; // same as (val % 16) != 0
+			return (rounded >> 4) + ((rounded < 0) ? -remainder : remainder);
+		};
+
+		v3s16 min(shrinkRnd(box.MinEdge.X), shrinkRnd(box.MinEdge.Y), shrinkRnd(box.MinEdge.Z)),
+			max(shrinkRnd(box.MaxEdge.X), shrinkRnd(box.MaxEdge.Y), shrinkRnd(box.MaxEdge.Z));
+		
+		for (int x = box.MinEdge.X; x < box.MaxEdge.X;x++) {
+			for (int y = box.MinEdge.Y; y < box.MaxEdge.Y;y++) {
+				for (int z = box.MinEdge.Z; z < box.MaxEdge.Z;z++) {
+					SpatialKey key(x,y,z);
+					if (m_cached.find(key) != m_cached.end()) {
+						auto range = m_cached.equal_range(key);
+						relevant_objs.insert(relevant_objs.end(), range.first, range.second);
+					}
+				}
+			}
+		}
+	}
+
+	// add the the rest, uncached objectIDs
+	relevant_objs.insert(relevant_objs.end(), m_uncached.begin(), m_uncached.end());
+}
+
+} // namespace server

src/server/spatial_map.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 <functional>
+#include <vector>
+#include "serveractiveobject.h"
+#include "activeobjectmgr.h"
+
+namespace server
+{
+class SpatialMap
+{
+public:
+	// all inserted entires go into the uncached vector
+	void insert(ServerActiveObject* obj);
+	
+	// Invalidates upon position update or removal
+	void invalidate(ServerActiveObject* obj);
+
+	// On active_object removal, remove.
+	void remove(ServerActiveObject* obj);
+
+	// Only when at least 64 uncached objects or 10% uncached overall
+	void cacheUpdate(ActiveObjectMgr& mgr);
+
+	// Use the same basic algorithm for both area and radius lookups
+	void getRelevantObjectIds(const aabb3f &box, std::vector<u16> &relevant_objs);
+
+protected:
+	typedef struct SpatialKey{
+		u16 padding{0};
+		s16 x;
+		s16 y;
+		s16 z;
+
+		SpatialKey(s16 _x, s16 _y, s16 _z) {
+			x = _x >> 4;
+			y = _y >> 4;
+			z = _z >> 4;
+		}
+		SpatialKey(v3f _pos) : SpatialKey(_pos.X, _pos.Y, _pos.Z){}
+	} SpatialKey;
+
+	std::unordered_multimap<SpatialKey, u16> m_cached;
+	std::vector<u16> m_uncached;
+};
+} // namespace server