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/net/minecraft/network/protocol/game/VecDeltaCodec.java b/src/main/java/net/minecraft/network/protocol/game/VecDeltaCodec.java
index 05ac41e136da43284fb24a6b698ebd36318278fb..33d9131e9c75ef23cd637f5d6c39a2704a0c92a5 100644
--- a/src/main/java/net/minecraft/network/protocol/game/VecDeltaCodec.java
+++ b/src/main/java/net/minecraft/network/protocol/game/VecDeltaCodec.java
@@ -9,12 +9,12 @@ public class VecDeltaCodec {
 
     @VisibleForTesting
     static long encode(double value) {
-        return Math.round(value * 4096.0D);
+        return Math.round(value * 4096.0D); // Paper - diff on change
     }
 
     @VisibleForTesting
     static double decode(long value) {
-        return (double)value / 4096.0D;
+        return (double)value / 4096.0D; // Paper - diff on change
     }
 
     public Vec3 decode(long x, long y, long z) {
diff --git a/src/main/java/net/minecraft/world/entity/Entity.java b/src/main/java/net/minecraft/world/entity/Entity.java
index 90621d3dfe8660f9b389ed292a3582a5dbb726bd..e68874f8d3d7628d11c1164849a4f835399d2452 100644
--- a/src/main/java/net/minecraft/world/entity/Entity.java
+++ b/src/main/java/net/minecraft/world/entity/Entity.java
@@ -4269,6 +4269,16 @@ public abstract class Entity implements Nameable, EntityAccess, CommandSource, S
             return;
         }
         // Paper end - rewrite chunk system
+        // Paper start - fix MC-4
+        if (this instanceof ItemEntity) {
+            if (io.papermc.paper.configuration.GlobalConfiguration.get().misc.fixEntityPositionDesync) {
+                // encode/decode from ClientboundMoveEntityPacket
+                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);