From 65b130822bef9a7dccd6477643a412f99cfa6aad Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Thu, 2 Apr 2020 02:37:57 -0400
Subject: [PATCH] Optimize Collision Chunk lookup and avoid loading far chunks

Try to use a faster chunk lookup for collision detection, and only
fall back to the original for nearby chunks.

The collision code takes an AABB and generates a cuboid of checks rather
than a cylinder, so at high velocity this can generate a lot of chunk checks.

diff --git a/src/main/java/net/minecraft/server/ICollisionAccess.java b/src/main/java/net/minecraft/server/ICollisionAccess.java
index f851ed11df..d154487294 100644
--- a/src/main/java/net/minecraft/server/ICollisionAccess.java
+++ b/src/main/java/net/minecraft/server/ICollisionAccess.java
@@ -83,15 +83,19 @@ public interface ICollisionAccess extends IBlockAccess {
                 }
 
                 while (cursorposition.a()) {
-                    int k1 = cursorposition.b();
-                    int l1 = cursorposition.c();
-                    int i2 = cursorposition.d();
+                    int k1 = cursorposition.b();int x = k1; // Paper
+                    int l1 = cursorposition.c();int y = l1; // Paper
+                    int i2 = cursorposition.d();int z = i2; // Paper
                     int j2 = cursorposition.e();
 
                     if (j2 != 3) {
                         int k2 = k1 >> 4;
                         int l2 = i2 >> 4;
-                        IBlockAccess iblockaccess = ICollisionAccess.this.c(k2, l2);
+                        // Paper start - ensure we don't load chunks
+                        boolean far = entity != null && MCUtil.distance(entity.locX(), entity.locY(), entity.locZ(), x, y, z) > 32;
+                        IBlockAccess iblockaccess = ICollisionAccess.this instanceof WorldServer ? ((WorldServer) ICollisionAccess.this).getChunkProvider().getChunkAtIfLoadedMainThread(k2, l2) : null;
+                        if (!far && iblockaccess == null) iblockaccess = ICollisionAccess.this.c(k2, l2);
+                        // Paper end
 
                         if (iblockaccess != null) {
                             blockposition_mutableblockposition.d(k1, l1, i2);
-- 
2.25.1