From f9b11fc494edca4b6c42d180fa214cac92fadcdf Mon Sep 17 00:00:00 2001
From: Zach Brown <zach.brown@destroystokyo.com>
Date: Fri, 22 Apr 2016 20:34:21 -0500
Subject: [PATCH] Check entity count additions and mark entities removed


diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index 773b153..4953e97 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -661,12 +661,36 @@ public class Chunk {
             k = this.entitySlices.length - 1;
         }
 
+        // Paper start - Try to catch plugins doing indecent things
+        boolean ignoreAdd = false;
+        if (entity.aa && entity.getChunkX() == this.locX && entity.getChunkY() == k && entity.getChunkZ() == this.locZ) {
+            String chunkName = entity.getWorld().getWorld().getName() + ":" + entity.getChunkX() + "," + entity.getChunkY() + "," + entity.getChunkZ();
+            if (this.entitySlices[k].contains(entity)) {
+                new Throwable("Double Chunk Add to: " + chunkName).printStackTrace();
+                ignoreAdd = true;
+            } else {
+                boolean found = false;
+                for (int i1 = 0; i1 < this.entitySlices.length; i1++) {
+                    if (this.entitySlices[i1].contains(entity)) {
+                        new Throwable("Entity was found in another slice of this chunk, tried: " + k + ", was in: " + chunkName).printStackTrace();
+                        found = true;
+                        ignoreAdd = true;
+                    }
+                }
+                if (!found) {
+                    new Throwable("Improperly detected double chunk add. Was not actually in this chunk.").printStackTrace();
+                }
+            }
+        }
+        // Paper end
+
         entity.aa = true;
         entity.ab = this.locX;
         entity.ac = k;
         entity.ad = this.locZ;
         this.entitySlices[k].add(entity);
         // Paper start - update count
+        if (ignoreAdd) return;
         if (entity instanceof EntityItem) {
             itemCounts[k]++;
         } else if (entity instanceof IInventory) {
@@ -711,6 +735,7 @@ public class Chunk {
         } else if (entity instanceof IInventory) {
             inventoryEntityCounts[i]--;
         }
+        entity.aa = false; // You aren't added to chunk anymore
         // Paper end
         // Spigot start - decrement creature type count
         // Keep this synced up with World.a(Class)
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index 0c20216..2e767e4 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -1749,7 +1749,7 @@ public abstract class World implements IBlockAccess {
             }
 
             int k = MathHelper.floor(entity.locX / 16.0D);
-            int l = MathHelper.floor(entity.locY / 16.0D);
+            int l = Math.min(15, Math.max(0, MathHelper.floor(entity.locY / 16.0D))); // Paper - stay consistent with chunk add/remove behavior
             int i1 = MathHelper.floor(entity.locZ / 16.0D);
 
             if (!entity.aa || entity.ab != k || entity.ac != l || entity.ad != i1) {
-- 
2.8.2