From 17ef1c2f57d558b46fa8257bf386f4f4faba240e Mon Sep 17 00:00:00 2001
From: Arektor <maxence@symfonye.com>
Date: Fri, 7 Oct 2022 17:19:29 +0200
Subject: [PATCH] Reduce velocity update rate (#1426)

---
 .../net/minestom/server/entity/Entity.java    | 17 +++++++++---
 .../entity/EntityVelocityIntegrationTest.java | 27 +++++++++++++++++++
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/src/main/java/net/minestom/server/entity/Entity.java b/src/main/java/net/minestom/server/entity/Entity.java
index 61959a508..91fab8c45 100644
--- a/src/main/java/net/minestom/server/entity/Entity.java
+++ b/src/main/java/net/minestom/server/entity/Entity.java
@@ -86,6 +86,8 @@ import java.util.function.UnaryOperator;
 public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, EventHandler<EntityEvent>, Taggable,
         PermissionHandler, HoverEventSource<ShowEntity>, Sound.Emitter {
 
+    private static final int VELOCITY_UPDATE_INTERVAL = 1;
+
     private static final Int2ObjectSyncMap<Entity> ENTITY_BY_ID = Int2ObjectSyncMap.hashmap();
     private static final Map<UUID, Entity> ENTITY_BY_UUID = new ConcurrentHashMap<>();
     private static final AtomicInteger LAST_ENTITY_ID = new AtomicInteger();
@@ -106,6 +108,7 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
 
     // Velocity
     protected Vec velocity = Vec.ZERO; // Movement in block per second
+    protected boolean lastVelocityWasZero = true;
     protected boolean hasPhysics = true;
 
     /**
@@ -593,7 +596,12 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
                         -gravityAcceleration * tps * (1 - gravityDragPerTick),
                         0
                 );
-                if (!isPlayer) sendPacketToViewers(getVelocityPacket());
+                if (this.ticks % VELOCITY_UPDATE_INTERVAL == 0) {
+                    if (!isPlayer && !this.lastVelocityWasZero) {
+                        sendPacketToViewers(getVelocityPacket());
+                        this.lastVelocityWasZero = !hasVelocity;
+                    }
+                }
                 return;
             }
         }
@@ -620,8 +628,11 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
             updateVelocity(wasOnGround, flying, positionBeforeMove, newVelocity);
         }
         // Verify if velocity packet has to be sent
-        if (!isPlayer && (hasVelocity || gravityTickCount > 0)) {
-            sendPacketToViewers(getVelocityPacket());
+        if (this.ticks % VELOCITY_UPDATE_INTERVAL == 0) {
+            if (!isPlayer && (hasVelocity || !lastVelocityWasZero)) {
+                sendPacketToViewers(getVelocityPacket());
+                this.lastVelocityWasZero = !hasVelocity;
+            }
         }
     }
 
diff --git a/src/test/java/net/minestom/server/entity/EntityVelocityIntegrationTest.java b/src/test/java/net/minestom/server/entity/EntityVelocityIntegrationTest.java
index b68bf6b29..3d7c2eb77 100644
--- a/src/test/java/net/minestom/server/entity/EntityVelocityIntegrationTest.java
+++ b/src/test/java/net/minestom/server/entity/EntityVelocityIntegrationTest.java
@@ -9,6 +9,9 @@ import net.minestom.server.network.packet.server.play.EntityVelocityPacket;
 import net.minestom.server.utils.chunk.ChunkUtils;
 import org.junit.jupiter.api.Test;
 
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.BooleanSupplier;
+
 import static org.junit.jupiter.api.Assertions.*;
 
 @EnvTest
@@ -160,6 +163,30 @@ public class EntityVelocityIntegrationTest {
         assertFalse(entity.hasVelocity());
     }
 
+    @Test
+    public void countVelocityPackets(Env env) {
+        final int VELOCITY_UPDATE_INTERVAL = 1;
+
+        var instance = env.createFlatInstance();
+        var viewerConnection = env.createConnection();
+        viewerConnection.connect(instance, new Pos(1, 40, 1)).join();
+        var entity = new Entity(EntityType.ZOMBIE);
+        entity.setInstance(instance, new Pos(0,40,0)).join();
+
+        AtomicInteger i = new AtomicInteger();
+        BooleanSupplier tickLoopCondition = () -> i.getAndIncrement() < Math.max(VELOCITY_UPDATE_INTERVAL, 1);
+
+        var tracker = viewerConnection.trackIncoming(EntityVelocityPacket.class);
+        env.tickWhile(tickLoopCondition, null);
+        tracker.assertEmpty(); // Verify no updates are sent while the entity is not moving
+
+        entity.setVelocity(new Vec(0, 5, 0));
+        tracker = viewerConnection.trackIncoming(EntityVelocityPacket.class);
+        i.set(0);
+        env.tickWhile(tickLoopCondition, null);
+        tracker.assertCount(1); // Verify the update is only sent once
+    }
+
     private void testMovement(Env env, Entity entity, Vec... sample) {
         final double epsilon = 0.003;
         for (Vec vec : sample) {