From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: BillyGalbreath <blake.galbreath@gmail.com>
Date: Tue, 8 Dec 2020 20:24:52 -0600
Subject: [PATCH] MC-4: Fix item position desync

This fixes item position desync (MC-4) by running the item coordinates
through the encode/decode methods of the packet that causes the precision
loss, which forces the server to lose the same precision as the client
keeping them in sync.

diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
index ccc60a5465efa2b71fc2593d75af62c0b64fe66c..1adbceadd5df96e17796561a40eb7b760493440e 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -101,6 +101,11 @@ public class PaperConfig {
         trackPluginScoreboards = getBoolean("settings.track-plugin-scoreboards", false);
     }
 
+    public static boolean fixEntityPositionDesync = true;
+    private static void fixEntityPositionDesync() {
+        fixEntityPositionDesync = getBoolean("settings.fix-entity-position-desync", fixEntityPositionDesync);
+    }
+
     public static void registerCommands() {
         for (Map.Entry<String, Command> entry : commands.entrySet()) {
             MinecraftServer.getServer().server.getCommandMap().register(entry.getKey(), "Paper", entry.getValue());
diff --git a/src/main/java/net/minecraft/network/protocol/game/ClientboundMoveEntityPacket.java b/src/main/java/net/minecraft/network/protocol/game/ClientboundMoveEntityPacket.java
index b30c08bfb8c55161543a4ef09f2e462e0a1fe4ae..ec93f5300cc7d423ec0d292f0f8443f900d72dab 100644
--- a/src/main/java/net/minecraft/network/protocol/game/ClientboundMoveEntityPacket.java
+++ b/src/main/java/net/minecraft/network/protocol/game/ClientboundMoveEntityPacket.java
@@ -21,11 +21,11 @@ public abstract class ClientboundMoveEntityPacket implements Packet<ClientGamePa
     protected final boolean hasPos;
 
     public static long entityToPacket(double coord) {
-        return Mth.lfloor(coord * 4096.0D);
+        return Mth.lfloor(coord * 4096.0D); // Paper - check ItemEntity#setPosRaw on update
     }
 
     public static double packetToEntity(long coord) {
-        return (double)coord / 4096.0D;
+        return (double)coord / 4096.0D; // Paper - check ItemEntity#setPosRaw on update
     }
 
     public Vec3 updateEntityPosition(Vec3 orig) {
diff --git a/src/main/java/net/minecraft/world/entity/Entity.java b/src/main/java/net/minecraft/world/entity/Entity.java
index 78dbc89bb247b2765bd2d7d81013d18dfe35539f..e53d8fe713a4c9920f40bf91d6453d36520b45f7 100644
--- a/src/main/java/net/minecraft/world/entity/Entity.java
+++ b/src/main/java/net/minecraft/world/entity/Entity.java
@@ -3802,6 +3802,16 @@ public abstract class Entity implements Nameable, EntityAccess, CommandSource {
     }
     public final void setPosRaw(double x, double y, double z, boolean forceBoundingBoxUpdate) {
         // Paper end
+        // Paper start - fix MC-4
+        if (this instanceof ItemEntity) {
+            if (com.destroystokyo.paper.PaperConfig.fixEntityPositionDesync) {
+                // encode/decode from PacketPlayOutEntity
+                x = Mth.lfloor(x * 4096.0D) * (1 / 4096.0D);
+                y = Mth.lfloor(y * 4096.0D) * (1 / 4096.0D);
+                z = Mth.lfloor(z * 4096.0D) * (1 / 4096.0D);
+            }
+        }
+        // Paper end - fix MC-4
         if (this.position.x != x || this.position.y != y || this.position.z != z) {
             this.position = new Vec3(x, y, z);
             int i = Mth.floor(x);