From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Sat, 19 Jun 2021 22:47:17 -0700
Subject: [PATCH] Allow removal/addition of entities to entity ticklist during
 tick

It really doesn't make any sense that we would iterate over removed
entities during tick. Sure - tick entity checks removed, but
does it check if the entity is in an entity ticking chunk?
No it doesn't. So, allowing removal while iteration
ENSURES only entities MARKED TO TICK are ticked.

diff --git a/src/main/java/net/minecraft/world/level/entity/EntityTickList.java b/src/main/java/net/minecraft/world/level/entity/EntityTickList.java
index b27c8db914cca3ff0ea8a24acddb9cb9870ce21d..4814e719e0b898464692075170889fdb2729a26a 100644
--- a/src/main/java/net/minecraft/world/level/entity/EntityTickList.java
+++ b/src/main/java/net/minecraft/world/level/entity/EntityTickList.java
@@ -9,57 +9,42 @@ import javax.annotation.Nullable;
 import net.minecraft.world.entity.Entity;
 
 public class EntityTickList {
-    private Int2ObjectMap<Entity> active = new Int2ObjectLinkedOpenHashMap<>();
-    private Int2ObjectMap<Entity> passive = new Int2ObjectLinkedOpenHashMap<>();
-    @Nullable
-    private Int2ObjectMap<Entity> iterated;
+    private final io.papermc.paper.util.maplist.IteratorSafeOrderedReferenceSet<Entity> entities = new io.papermc.paper.util.maplist.IteratorSafeOrderedReferenceSet<>(true); // Paper - rewrite this, always keep this updated - why would we EVER tick an entity that's not ticking?
 
     private void ensureActiveIsNotIterated() {
-        if (this.iterated == this.active) {
-            this.passive.clear();
-
-            for(Entry<Entity> entry : Int2ObjectMaps.fastIterable(this.active)) {
-                this.passive.put(entry.getIntKey(), entry.getValue());
-            }
-
-            Int2ObjectMap<Entity> int2ObjectMap = this.active;
-            this.active = this.passive;
-            this.passive = int2ObjectMap;
-        }
+        // Paper - replace with better logic, do not delay removals
 
     }
 
     public void add(Entity entity) {
         io.papermc.paper.util.TickThread.ensureTickThread("Asynchronous entity ticklist addition"); // Paper
         this.ensureActiveIsNotIterated();
-        this.active.put(entity.getId(), entity);
+        this.entities.add(entity); // Paper - replace with better logic, do not delay removals/additions
     }
 
     public void remove(Entity entity) {
         io.papermc.paper.util.TickThread.ensureTickThread("Asynchronous entity ticklist removal"); // Paper
         this.ensureActiveIsNotIterated();
-        this.active.remove(entity.getId());
+        this.entities.remove(entity); // Paper - replace with better logic, do not delay removals/additions
     }
 
     public boolean contains(Entity entity) {
-        return this.active.containsKey(entity.getId());
+        return this.entities.contains(entity); // Paper - replace with better logic, do not delay removals/additions
     }
 
     public void forEach(Consumer<Entity> action) {
         io.papermc.paper.util.TickThread.ensureTickThread("Asynchronous entity ticklist iteration"); // Paper
-        if (this.iterated != null) {
-            throw new UnsupportedOperationException("Only one concurrent iteration supported");
-        } else {
-            this.iterated = this.active;
-
-            try {
-                for(Entity entity : this.active.values()) {
-                    action.accept(entity);
-                }
-            } finally {
-                this.iterated = null;
+        // Paper start - replace with better logic, do not delay removals/additions
+        // To ensure nothing weird happens with dimension travelling, do not iterate over new entries...
+        // (by dfl iterator() is configured to not iterate over new entries)
+        io.papermc.paper.util.maplist.IteratorSafeOrderedReferenceSet.Iterator<Entity> iterator = this.entities.iterator();
+        try {
+            while (iterator.hasNext()) {
+                action.accept(iterator.next());
             }
-
+        } finally {
+            iterator.finishedIterating();
         }
+        // Paper end - replace with better logic, do not delay removals/additions
     }
 }