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 96eea87534b6e28a56c9eea0f30bfb41793440e7..24ff10c4ed69deed2ce9ba25835575419165e2af 100644
--- a/src/main/java/net/minecraft/server/players/PlayerList.java
+++ b/src/main/java/net/minecraft/server/players/PlayerList.java
@@ -868,6 +868,7 @@ public abstract class PlayerList {
         Vec3 vec3d = dimensiontransition.pos();
 
         entityplayer1.forceSetPositionRotation(vec3d.x, vec3d.y, vec3d.z, dimensiontransition.yRot(), dimensiontransition.xRot());
+        worldserver.getChunkSource().addRegionTicket(net.minecraft.server.level.TicketType.POST_TELEPORT, new net.minecraft.world.level.ChunkPos(net.minecraft.util.Mth.floor(vec3d.x()) >> 4, net.minecraft.util.Mth.floor(vec3d.z()) >> 4), 1, entityplayer.getId()); // Paper
         // CraftBukkit end
         if (dimensiontransition.missingRespawnBlock()) {
             entityplayer1.connection.send(new ClientboundGameEventPacket(ClientboundGameEventPacket.NO_RESPAWN_BLOCK_AVAILABLE, 0.0F));
diff --git a/src/main/java/net/minecraft/world/entity/Entity.java b/src/main/java/net/minecraft/world/entity/Entity.java
index 32502664a82dda6220f8d9fa554105740e9bc99a..6b4a7930efabd29f00690a79e8080faaa9767190 100644
--- a/src/main/java/net/minecraft/world/entity/Entity.java
+++ b/src/main/java/net/minecraft/world/entity/Entity.java
@@ -250,6 +250,7 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
     // Paper end - Share random for entities to make them more random
     public org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason spawnReason; // Paper - Entity#getEntitySpawnReason
 
+    public boolean collisionLoadChunks = false; // Paper
     private CraftEntity bukkitEntity;
 
     public CraftEntity getBukkitEntity() {
diff --git a/src/main/java/net/minecraft/world/level/BlockCollisions.java b/src/main/java/net/minecraft/world/level/BlockCollisions.java
index 1c10835b59aaefa3a65ff64f784620bdc54ddcdc..cd89623a44f02d7db77f0d0f87545cf80841f403 100644
--- a/src/main/java/net/minecraft/world/level/BlockCollisions.java
+++ b/src/main/java/net/minecraft/world/level/BlockCollisions.java
@@ -66,18 +66,37 @@ public class BlockCollisions<T> extends AbstractIterator<T> {
     @Override
     protected T computeNext() {
         while (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 - OBFHELPER
+            int j = this.cursor.nextY(); final int y = j; // Paper - OBFHELPER
+            int k = this.cursor.nextZ(); final int z = k; // Paper - OBFHELPER
             int l = this.cursor.getNextType();
             if (l != 3) {
-                BlockGetter blockGetter = this.getChunk(i, k);
-                if (blockGetter != null) {
-                    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 start - ensure we don't load chunks
+                // BlockGetter blockGetter = this.getChunk(i, k);
+                if (true) {
+                    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);
+                    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 this.resultProvider.apply(new BlockPos.MutableBlockPos(x, y, z), Shapes.create(far ? source.getBoundingBox() : new AABB(new BlockPos(x, y, z))));
+                        }
+                        continue;
+                    }
+                    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
+                        // Paper end
                         VoxelShape voxelShape = blockState.getCollisionShape(this.collisionGetter, this.pos, this.context);
                         if (voxelShape == Shapes.block()) {
                             if (this.box.intersects((double)i, (double)j, (double)k, (double)i + 1.0, (double)j + 1.0, (double)k + 1.0)) {
diff --git a/src/main/java/net/minecraft/world/level/CollisionGetter.java b/src/main/java/net/minecraft/world/level/CollisionGetter.java
index e57cb7fe53e915d24246e44c7f49971f5b2ab2cf..1ad0c976c6e2d6d31397dff850a9de7c16d16fba 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;