From 1f0d042377ed506ad460067495e5911b2fc2aacc Mon Sep 17 00:00:00 2001
From: lhofhansl <larsh@apache.org>
Date: Sun, 5 Mar 2023 21:33:41 -0800
Subject: [PATCH] Reduce server CPU consumed by occlusion culling. (#13260)

Cache blocks already occluded at a specific distance. The RemoteClient typically visits the same distance multiple time - especially at larger distances, so this saves significant CPU from recalculating the occlusion state of blocks.
---
 src/clientiface.cpp | 12 +++++++++++-
 src/clientiface.h   | 15 ++++++++++++---
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/src/clientiface.cpp b/src/clientiface.cpp
index bb25a4265..49b668cf0 100644
--- a/src/clientiface.cpp
+++ b/src/clientiface.cpp
@@ -328,8 +328,15 @@ void RemoteClient::GetNextBlocks (
 						continue;
 				}
 
+				/*
+					Check occlusion cache first.
+				 */
+				if (m_blocks_occ.find(p) != m_blocks_occ.end())
+					continue;
+
 				if (m_occ_cull && !block_not_found &&
 						env->getMap().isBlockOccluded(block, cam_pos_nodes)) {
+					m_blocks_occ.insert(p);
 					continue;
 				}
 			}
@@ -395,8 +402,11 @@ queue_full_break:
 		}
 	}
 
-	if (new_nearest_unsent_d != -1)
+	if (new_nearest_unsent_d != -1 && m_nearest_unsent_d != new_nearest_unsent_d) {
 		m_nearest_unsent_d = new_nearest_unsent_d;
+		// if the distance has changed, clear the occlusion cache
+		m_blocks_occ.clear();
+	}
 }
 
 void RemoteClient::GotBlock(v3s16 p)
diff --git a/src/clientiface.h b/src/clientiface.h
index cda01650b..261c09fce 100644
--- a/src/clientiface.h
+++ b/src/clientiface.h
@@ -33,6 +33,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <list>
 #include <vector>
 #include <set>
+#include <unordered_set>
 #include <memory>
 #include <mutex>
 
@@ -379,7 +380,15 @@ private:
 		List of block positions.
 		No MapBlock* is stored here because the blocks can get deleted.
 	*/
-	std::set<v3s16> m_blocks_sent;
+	std::unordered_set<v3s16> m_blocks_sent;
+
+	/*
+		Cache of blocks that have been occlusion culled at the current distance.
+		As GetNextBlocks traverses the same distance multiple times, this saves
+		significant CPU time.
+	 */
+	std::unordered_set<v3s16> m_blocks_occ;
+
 	s16 m_nearest_unsent_d = 0;
 	v3s16 m_last_center;
 	v3f m_last_camera_dir;
@@ -399,7 +408,7 @@ private:
 		Block is removed when GOTBLOCKS is received.
 		Value is time from sending. (not used at the moment)
 	*/
-	std::map<v3s16, float> m_blocks_sending;
+	std::unordered_map<v3s16, float> m_blocks_sending;
 
 	/*
 		Blocks that have been modified since blocks were
@@ -409,7 +418,7 @@ private:
 
 		List of block positions.
 	*/
-	std::set<v3s16> m_blocks_modified;
+	std::unordered_set<v3s16> m_blocks_modified;
 
 	/*
 		Count of excess GotBlocks().