From 0000000000000000000000000000000000000000 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 to not load 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.

Treat an unloaded chunk as a collision for entities, and also for players if
the "prevent moving into unloaded chunks" setting is enabled.

If that serting is not enabled, collisions will be ignored for players, since
movement will load only the chunk the player enters anyways and avoids loading
massive amounts of surrounding chunks due to large AABB lookups.

diff --git a/src/main/java/net/minecraft/server/players/PlayerList.java b/src/main/java/net/minecraft/server/players/PlayerList.java
index 614ee0c22fcb2f909c53e2deff03f6343b2ad803..3b7db01c51868c8a820b87cd13f69cd2bbce3e85 100644
--- a/src/main/java/net/minecraft/server/players/PlayerList.java
+++ b/src/main/java/net/minecraft/server/players/PlayerList.java
@@ -805,6 +805,7 @@ public abstract class PlayerList {
         entityplayer1.forceSetPositionRotation(location.getX(), location.getY(), location.getZ(), location.getYaw(), location.getPitch());
         // CraftBukkit end
 
+        worldserver1.getChunkSource().addRegionTicket(net.minecraft.server.level.TicketType.POST_TELEPORT, new net.minecraft.world.level.ChunkPos(location.getBlockX() >> 4, location.getBlockZ() >> 4), 1, entityplayer.getId()); // Paper
         while (avoidSuffocation && !worldserver1.noCollision((Entity) entityplayer1) && entityplayer1.getY() < (double) worldserver1.getMaxBuildHeight()) {
             entityplayer1.setPos(entityplayer1.getX(), entityplayer1.getY() + 1.0D, entityplayer1.getZ());
         }
diff --git a/src/main/java/net/minecraft/world/entity/Entity.java b/src/main/java/net/minecraft/world/entity/Entity.java
index 6b19894c4353cf96d853d00128614b6d49c7a3c1..1e4699372b22c3fe3eed6859c3f15f9b70a537c0 100644
--- a/src/main/java/net/minecraft/world/entity/Entity.java
+++ b/src/main/java/net/minecraft/world/entity/Entity.java
@@ -236,6 +236,7 @@ public abstract class Entity implements Nameable, EntityAccess, CommandSource {
     public org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason spawnReason; // Paper
 
     public com.destroystokyo.paper.loottable.PaperLootableInventoryData lootableData; // Paper
+    public boolean collisionLoadChunks = false; // Paper
     private CraftEntity bukkitEntity;
 
     public @org.jetbrains.annotations.Nullable net.minecraft.server.level.ChunkMap.TrackedEntity tracker; // Paper
diff --git a/src/main/java/net/minecraft/world/level/BlockCollisions.java b/src/main/java/net/minecraft/world/level/BlockCollisions.java
index 8390ce194ccc692139c0e870c16a7fb76ac8ba68..be578f14146b0184d5419d5b961c5d681f9ba7a3 100644
--- a/src/main/java/net/minecraft/world/level/BlockCollisions.java
+++ b/src/main/java/net/minecraft/world/level/BlockCollisions.java
@@ -66,22 +66,41 @@ public class BlockCollisions extends AbstractIterator<VoxelShape> {
     protected VoxelShape computeNext() {
         while(true) {
             if (this.cursor.advance()) {
-                int i = this.cursor.nextX();
-                int j = this.cursor.nextY();
-                int k = this.cursor.nextZ();
+                int i = this.cursor.nextX(); final int x = i; // Paper
+                int j = this.cursor.nextY(); final int y = j; // Paper
+                int k = this.cursor.nextZ(); final int z = k; // Paper
                 int l = this.cursor.getNextType();
                 if (l == 3) {
                     continue;
                 }
+                // Paper start - ensure we don't load chunks
+                final @Nullable Entity source = this.context instanceof net.minecraft.world.phys.shapes.EntityCollisionContext entityContext ? entityContext.getEntity() : null;
+                boolean far = source != null && io.papermc.paper.util.MCUtil.distanceSq(source.getX(), y, source.getZ(), x, y, z) > 14;
+                this.pos.set(x, y, z);
 
-                BlockGetter blockGetter = this.getChunk(i, k);
-                if (blockGetter == null) {
+                BlockState blockState;
+                if (this.collisionGetter instanceof net.minecraft.server.level.WorldGenRegion) {
+                    BlockGetter blockGetter = this.getChunk(x, z);
+                    if (blockGetter == null) {
+                       continue;
+                    }
+                    blockState = blockGetter.getBlockState(this.pos);
+                } else if ((!far && source instanceof net.minecraft.server.level.ServerPlayer) || (source != null && source.collisionLoadChunks)) {
+                    blockState = this.collisionGetter.getBlockState(this.pos);
+                } else {
+                    blockState = this.collisionGetter.getBlockStateIfLoaded(this.pos);
+                }
+
+                if (blockState == null) {
+                    if (!(source instanceof net.minecraft.server.level.ServerPlayer) || source.level.paperConfig().chunks.preventMovingIntoUnloadedChunks) {
+                        return Shapes.create(far ? source.getBoundingBox() : new AABB(new BlockPos(x, y, z)));
+                    }
+                    // Paper end
                     continue;
                 }
 
-                this.pos.set(i, j, k);
-                BlockState blockState = blockGetter.getBlockState(this.pos);
-                if (this.onlySuffocatingBlocks && !blockState.isSuffocating(blockGetter, this.pos) || l == 1 && !blockState.hasLargeCollisionShape() || l == 2 && !blockState.is(Blocks.MOVING_PISTON)) {
+                // Paper - moved up
+                if (/*this.onlySuffocatingBlocks && (!blockState.isSuffocating(blockGetter, this.pos)) ||*/ l == 1 && !blockState.hasLargeCollisionShape() || l == 2 && !blockState.is(Blocks.MOVING_PISTON)) { // Paper - onlySuffocatingBlocks is only true on the client, so we don't care about it here
                     continue;
                 }
 
diff --git a/src/main/java/net/minecraft/world/level/CollisionGetter.java b/src/main/java/net/minecraft/world/level/CollisionGetter.java
index 4c373d6c8ddd9f5db88888cd8dbbfc24eb5df793..56d94c94fb0d4dc468bb5d69be655ddd5c6b5360 100644
--- a/src/main/java/net/minecraft/world/level/CollisionGetter.java
+++ b/src/main/java/net/minecraft/world/level/CollisionGetter.java
@@ -44,11 +44,13 @@ public interface CollisionGetter extends BlockGetter {
     }
 
     default boolean noCollision(@Nullable Entity entity, AABB box) {
+        try { if (entity != null) entity.collisionLoadChunks = true; // Paper
         for(VoxelShape voxelShape : this.getBlockCollisions(entity, box)) {
             if (!voxelShape.isEmpty()) {
                 return false;
             }
         }
+        } finally { if (entity != null) entity.collisionLoadChunks = false; } // Paper
 
         if (!this.getEntityCollisions(entity, box).isEmpty()) {
             return false;