Performance Improvement: Use a cache which caches result for getFacePositions.

This greatly reduce the number of std::list generated by caching the result, which is always constant for each radius selected.
In the callgrind map, you will see original:
  * 3.3M calls to std::list for 9700 calls to getFacePositions
In the modified version, you will see:
  * 3.3K calls to std::list for 6900 call to getFacePositions
Callgrind map is here: #2321

it's a huge performance improvement to l_find_node_near

4 files changed, 81 insertions, 68 deletions

diff --git a/src/clientiface.cpp b/src/clientiface.cpp
index 9b952e36a..6180cf5da 100644
--- a/src/clientiface.cpp
+++ b/src/clientiface.cpp
@@ -59,7 +59,7 @@ void RemoteClient::ResendBlockIfOnWire(v3s16 p)
 	}
 }
 
-void RemoteClient::GetNextBlocks(
+void RemoteClient::GetNextBlocks (
 		ServerEnvironment *env,
 		EmergeManager * emerge,
 		float dtime,
@@ -182,18 +182,15 @@ void RemoteClient::GetNextBlocks(
 	//bool queue_is_full = false;
 
 	s16 d;
-	for(d = d_start; d <= d_max; d++)
-	{
+	for(d = d_start; d <= d_max; d++) {
 		/*
 			Get the border/face dot coordinates of a "d-radiused"
 			box
 		*/
-		std::list<v3s16> list;
-		getFacePositions(list, d);
+		std::vector<v3s16> list = FacePositionCache::getFacePositions(d);
 
-		std::list<v3s16>::iterator li;
-		for(li=list.begin(); li!=list.end(); ++li)
-		{
+		std::vector<v3s16>::iterator li;
+		for(li = list.begin(); li != list.end(); ++li) {
 			v3s16 p = *li + center;
 
 			/*
diff --git a/src/script/lua_api/l_env.cpp b/src/script/lua_api/l_env.cpp
index cd5d253ac..2445803d7 100644
--- a/src/script/lua_api/l_env.cpp
+++ b/src/script/lua_api/l_env.cpp
@@ -530,9 +530,8 @@ int ModApiEnvMod::l_find_node_near(lua_State *L)
 	}
 
 	for(int d=1; d<=radius; d++){
-		std::list<v3s16> list;
-		getFacePositions(list, d);
-		for(std::list<v3s16>::iterator i = list.begin();
+		std::vector<v3s16> list = FacePositionCache::getFacePositions(d);
+		for(std::vector<v3s16>::iterator i = list.begin();
 				i != list.end(); ++i){
 			v3s16 p = pos + (*i);
 			content_t c = env->getMap().getNodeNoEx(p).getContent();
diff --git a/src/util/numeric.cpp b/src/util/numeric.cpp
index 6173515ee..19f927134 100644
--- a/src/util/numeric.cpp
+++ b/src/util/numeric.cpp
@@ -20,79 +20,84 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "numeric.h"
 #include "mathconstants.h"
 
-#include "../log.h"
+#include "log.h"
 #include "../constants.h" // BS, MAP_BLOCKSIZE
 #include <string.h>
 #include <iostream>
 
+std::map<u16, std::vector<v3s16> > FacePositionCache::m_cache;
 // Calculate the borders of a "d-radius" cube
-void getFacePositions(std::list<v3s16> &list, u16 d)
+std::vector<v3s16> FacePositionCache::getFacePositions(u16 d)
 {
-	if(d == 0)
-	{
-		list.push_back(v3s16(0,0,0));
+	if (m_cache.find(d) != m_cache.end())
+		return m_cache[d];
+
+	generateFacePosition(d);
+	return m_cache[d];
+
+}
+
+void FacePositionCache::generateFacePosition(u16 d)
+{
+	m_cache[d] = std::vector<v3s16>();
+	if(d == 0) {
+		m_cache[d].push_back(v3s16(0,0,0));
 		return;
 	}
-	if(d == 1)
-	{
+	if(d == 1) {
 		/*
 			This is an optimized sequence of coordinates.
 		*/
-		list.push_back(v3s16( 0, 1, 0)); // top
-		list.push_back(v3s16( 0, 0, 1)); // back
-		list.push_back(v3s16(-1, 0, 0)); // left
-		list.push_back(v3s16( 1, 0, 0)); // right
-		list.push_back(v3s16( 0, 0,-1)); // front
-		list.push_back(v3s16( 0,-1, 0)); // bottom
+		m_cache[d].push_back(v3s16( 0, 1, 0)); // top
+		m_cache[d].push_back(v3s16( 0, 0, 1)); // back
+		m_cache[d].push_back(v3s16(-1, 0, 0)); // left
+		m_cache[d].push_back(v3s16( 1, 0, 0)); // right
+		m_cache[d].push_back(v3s16( 0, 0,-1)); // front
+		m_cache[d].push_back(v3s16( 0,-1, 0)); // bottom
 		// 6
-		list.push_back(v3s16(-1, 0, 1)); // back left
-		list.push_back(v3s16( 1, 0, 1)); // back right
-		list.push_back(v3s16(-1, 0,-1)); // front left
-		list.push_back(v3s16( 1, 0,-1)); // front right
-		list.push_back(v3s16(-1,-1, 0)); // bottom left
-		list.push_back(v3s16( 1,-1, 0)); // bottom right
-		list.push_back(v3s16( 0,-1, 1)); // bottom back
-		list.push_back(v3s16( 0,-1,-1)); // bottom front
-		list.push_back(v3s16(-1, 1, 0)); // top left
-		list.push_back(v3s16( 1, 1, 0)); // top right
-		list.push_back(v3s16( 0, 1, 1)); // top back
-		list.push_back(v3s16( 0, 1,-1)); // top front
+		m_cache[d].push_back(v3s16(-1, 0, 1)); // back left
+		m_cache[d].push_back(v3s16( 1, 0, 1)); // back right
+		m_cache[d].push_back(v3s16(-1, 0,-1)); // front left
+		m_cache[d].push_back(v3s16( 1, 0,-1)); // front right
+		m_cache[d].push_back(v3s16(-1,-1, 0)); // bottom left
+		m_cache[d].push_back(v3s16( 1,-1, 0)); // bottom right
+		m_cache[d].push_back(v3s16( 0,-1, 1)); // bottom back
+		m_cache[d].push_back(v3s16( 0,-1,-1)); // bottom front
+		m_cache[d].push_back(v3s16(-1, 1, 0)); // top left
+		m_cache[d].push_back(v3s16( 1, 1, 0)); // top right
+		m_cache[d].push_back(v3s16( 0, 1, 1)); // top back
+		m_cache[d].push_back(v3s16( 0, 1,-1)); // top front
 		// 18
-		list.push_back(v3s16(-1, 1, 1)); // top back-left
-		list.push_back(v3s16( 1, 1, 1)); // top back-right
-		list.push_back(v3s16(-1, 1,-1)); // top front-left
-		list.push_back(v3s16( 1, 1,-1)); // top front-right
-		list.push_back(v3s16(-1,-1, 1)); // bottom back-left
-		list.push_back(v3s16( 1,-1, 1)); // bottom back-right
-		list.push_back(v3s16(-1,-1,-1)); // bottom front-left
-		list.push_back(v3s16( 1,-1,-1)); // bottom front-right
+		m_cache[d].push_back(v3s16(-1, 1, 1)); // top back-left
+		m_cache[d].push_back(v3s16( 1, 1, 1)); // top back-right
+		m_cache[d].push_back(v3s16(-1, 1,-1)); // top front-left
+		m_cache[d].push_back(v3s16( 1, 1,-1)); // top front-right
+		m_cache[d].push_back(v3s16(-1,-1, 1)); // bottom back-left
+		m_cache[d].push_back(v3s16( 1,-1, 1)); // bottom back-right
+		m_cache[d].push_back(v3s16(-1,-1,-1)); // bottom front-left
+		m_cache[d].push_back(v3s16( 1,-1,-1)); // bottom front-right
 		// 26
 		return;
 	}
 
 	// Take blocks in all sides, starting from y=0 and going +-y
-	for(s16 y=0; y<=d-1; y++)
-	{
+	for(s16 y=0; y<=d-1; y++) {
 		// Left and right side, including borders
-		for(s16 z=-d; z<=d; z++)
-		{
-			list.push_back(v3s16(d,y,z));
-			list.push_back(v3s16(-d,y,z));
-			if(y != 0)
-			{
-				list.push_back(v3s16(d,-y,z));
-				list.push_back(v3s16(-d,-y,z));
+		for(s16 z=-d; z<=d; z++) {
+			m_cache[d].push_back(v3s16(d,y,z));
+			m_cache[d].push_back(v3s16(-d,y,z));
+			if(y != 0) {
+				m_cache[d].push_back(v3s16(d,-y,z));
+				m_cache[d].push_back(v3s16(-d,-y,z));
 			}
 		}
 		// Back and front side, excluding borders
-		for(s16 x=-d+1; x<=d-1; x++)
-		{
-			list.push_back(v3s16(x,y,d));
-			list.push_back(v3s16(x,y,-d));
-			if(y != 0)
-			{
-				list.push_back(v3s16(x,-y,d));
-				list.push_back(v3s16(x,-y,-d));
+		for(s16 x=-d+1; x<=d-1; x++) {
+			m_cache[d].push_back(v3s16(x,y,d));
+			m_cache[d].push_back(v3s16(x,y,-d));
+			if(y != 0) {
+				m_cache[d].push_back(v3s16(x,-y,d));
+				m_cache[d].push_back(v3s16(x,-y,-d));
 			}
 		}
 	}
@@ -100,10 +105,9 @@ void getFacePositions(std::list<v3s16> &list, u16 d)
 	// Take the bottom and top face with borders
 	// -d<x<d, y=+-d, -d<z<d
 	for(s16 x=-d; x<=d; x++)
-	for(s16 z=-d; z<=d; z++)
-	{
-		list.push_back(v3s16(x,-d,z));
-		list.push_back(v3s16(x,d,z));
+	for(s16 z=-d; z<=d; z++) {
+		m_cache[d].push_back(v3s16(x,-d,z));
+		m_cache[d].push_back(v3s16(x,d,z));
 	}
 }
 
diff --git a/src/util/numeric.h b/src/util/numeric.h
index db1eb003e..7fc3d2167 100644
--- a/src/util/numeric.h
+++ b/src/util/numeric.h
@@ -25,10 +25,23 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "../irr_v3d.h"
 #include "../irr_aabb3d.h"
 #include <list>
+#include <map>
+#include <vector>
 #include <algorithm>
 
-// Calculate the borders of a "d-radius" cube
-void getFacePositions(std::list<v3s16> &list, u16 d);
+
+/*
+ * This class permits to cache getFacePosition call results
+ * This reduces CPU usage and vector calls
+ */
+class FacePositionCache
+{
+public:
+	static std::vector<v3s16> getFacePositions(u16 d);
+private:
+	static void generateFacePosition(u16 d);
+	static std::map<u16, std::vector<v3s16> > m_cache;
+};
 
 class IndentationRaiser
 {