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/net/minecraft/world/entity/Entity.java b/net/minecraft/world/entity/Entity.java
index a8eaccde3ec9ed912cbc6df0b29e9f8136a46578..0f6ca6ef161ac2934ba761a1eca3215290c7262b 100644
--- a/net/minecraft/world/entity/Entity.java
+++ b/net/minecraft/world/entity/Entity.java
@@ -223,6 +223,7 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
     // Paper end - Share random for entities to make them more random
     public @Nullable org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason spawnReason; // Paper - Entity#getEntitySpawnReason
 
+    public boolean collisionLoadChunks = false; // Paper
     private @Nullable org.bukkit.craftbukkit.entity.CraftEntity bukkitEntity;
 
     public org.bukkit.craftbukkit.entity.CraftEntity getBukkitEntity() {
diff --git a/net/minecraft/world/level/BlockCollisions.java b/net/minecraft/world/level/BlockCollisions.java
index ed6e4f9fd0c7ad1219e66bc1cb4038191dd6edd8..45a20dbb935b12d429153463dba5d6fd3385dd7a 100644
--- a/net/minecraft/world/level/BlockCollisions.java
+++ b/net/minecraft/world/level/BlockCollisions.java
@@ -80,16 +80,37 @@ public class BlockCollisions<T> extends AbstractIterator<T> {
     @Override
     protected T computeNext() {
         while (this.cursor.advance()) {
-            int i = this.cursor.nextX();
-            int i1 = this.cursor.nextY();
-            int i2 = this.cursor.nextZ();
+            int i = this.cursor.nextX(); final int x = i; // Paper - OBFHELPER
+            int i1 = this.cursor.nextY(); final int y = i1; // Paper - OBFHELPER
+            int i2 = this.cursor.nextZ(); final int z = i2; // Paper - OBFHELPER
             int nextType = this.cursor.getNextType();
             if (nextType != 3) {
-                BlockGetter chunk = this.getChunk(i, i2);
-                if (chunk != null) {
-                    this.pos.set(i, i1, i2);
-                    BlockState blockState = chunk.getBlockState(this.pos);
-                    if ((!this.onlySuffocatingBlocks || blockState.isSuffocating(chunk, this.pos))
+                // Paper start - ensure we don't load chunks
+                // BlockGetter blockGetter = this.getChunk(i, k);
+                if (true) {
+                    @Nullable final Entity source = this.context instanceof net.minecraft.world.phys.shapes.EntityCollisionContext entityContext ? entityContext.getEntity() : null;
+                    final 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 (true // onlySuffocatingBlocks is only true on the client, so we don't care about it here
+                    // Paper end - ensure we don't load chunks
                         && (nextType != 1 || blockState.hasLargeCollisionShape())
                         && (nextType != 2 || blockState.is(Blocks.MOVING_PISTON))) {
                         VoxelShape collisionShape = this.context.getCollisionShape(blockState, this.collisionGetter, this.pos);
diff --git a/net/minecraft/world/level/CollisionGetter.java b/net/minecraft/world/level/CollisionGetter.java
index 79af1e4dd1f84580e509ac3e9a77bcd5531c8da6..a031d39854eb049a701f3de9e11c73419883d5ca 100644
--- a/net/minecraft/world/level/CollisionGetter.java
+++ b/net/minecraft/world/level/CollisionGetter.java
@@ -50,11 +50,13 @@ public interface CollisionGetter extends BlockGetter {
     }
 
     default boolean noCollision(@Nullable Entity entity, AABB collisionBox, boolean checkLiquid) {
+        try { if (entity != null) entity.collisionLoadChunks = true; // Paper
         for (VoxelShape voxelShape : checkLiquid ? this.getBlockAndLiquidCollisions(entity, collisionBox) : this.getBlockCollisions(entity, collisionBox)) {
             if (!voxelShape.isEmpty()) {
                 return false;
             }
         }
+        } finally { if (entity != null) entity.collisionLoadChunks = false; } // Paper
 
         if (!this.getEntityCollisions(entity, collisionBox).isEmpty()) {
             return false;