From 0ab9c79a28fe9924bb59978e02a28a987e316066 Mon Sep 17 00:00:00 2001
From: themode <themode@outlook.fr>
Date: Fri, 13 Nov 2020 03:26:35 +0100
Subject: [PATCH] Prevent array resizing during server tick

---
 .../java/net/minestom/server/MinecraftServer.java   |  4 ++--
 .../server/thread/PerGroupChunkProvider.java        | 13 ++++++++++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/src/main/java/net/minestom/server/MinecraftServer.java b/src/main/java/net/minestom/server/MinecraftServer.java
index 7c1be1707..c4b33591e 100644
--- a/src/main/java/net/minestom/server/MinecraftServer.java
+++ b/src/main/java/net/minestom/server/MinecraftServer.java
@@ -112,10 +112,10 @@ public final class MinecraftServer {
     @Setter
     private static boolean fixLighting = true;
 
-    // Rate Limiting
+    // Network monitoring
     private static int rateLimit = 300;
     private static int maxPacketSize = 30_000;
-
+    // Network
     private static PacketListenerManager packetListenerManager;
     private static NettyServer nettyServer;
 
diff --git a/src/main/java/net/minestom/server/thread/PerGroupChunkProvider.java b/src/main/java/net/minestom/server/thread/PerGroupChunkProvider.java
index 347fa6cdd..f54c73083 100644
--- a/src/main/java/net/minestom/server/thread/PerGroupChunkProvider.java
+++ b/src/main/java/net/minestom/server/thread/PerGroupChunkProvider.java
@@ -120,7 +120,17 @@ public class PerGroupChunkProvider extends ThreadProvider {
     @NotNull
     @Override
     public List<Future<?>> update(long time) {
-        List<Future<?>> futures = new ArrayList<>();
+
+        List<Future<?>> futures;
+        int potentialSize = 0;
+
+        // Compute the potential array size
+        {
+            for (Map<LongSet, Instance> longSetInstanceMap : instanceInstanceMap.values()) {
+                potentialSize += 1 + longSetInstanceMap.size();
+            }
+            futures = new ArrayList<>(potentialSize);
+        }
 
         instanceInstanceMap.forEach((instance, instanceMap) -> {
 
@@ -147,6 +157,7 @@ public class PerGroupChunkProvider extends ThreadProvider {
                 chunksIndexes.forEach((long chunkIndex) -> processChunkTick(instance, chunkIndex, time));
             })));
         });
+
         return futures;
     }