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 488ebd443903af812913437f1ade3002093f2470..a043ac10834562d357ef0b5aded2e916e2a0d056 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 * 4096.0);
+        return Math.round(value * 4096.0); // Paper - Fix MC-4; diff on change
     }
 
     @VisibleForTesting
     static double decode(long value) {
-        return (double)value / 4096.0;
+        return value / 4096.0; // Paper - Fix MC-4; 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 dae7643897b47dba304cbea56112445df2736cff..bed376337035545e7ec677f2f7fe3372a3c9ea25 100644
--- a/src/main/java/net/minecraft/world/entity/Entity.java
+++ b/src/main/java/net/minecraft/world/entity/Entity.java
@@ -4260,6 +4260,16 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
             return;
         }
         // Paper end - Block invalid positions and bounding box
+        // 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 * 4096.0) * (1 / 4096.0);
+                y = Mth.lfloor(y * 4096.0) * (1 / 4096.0);
+                z = Mth.lfloor(z * 4096.0) * (1 / 4096.0);
+            }
+        }
+        // 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);