Account for flying in velocity calculation (#1111)

src/main/java/net/minestom/server/entity/Entity.java

@@ -581,8 +581,13 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
         // World border collision
         final Pos finalVelocityPosition = CollisionUtils.applyWorldBorder(instance, position, newPosition);
         final boolean positionChanged = !finalVelocityPosition.samePoint(position);
+        final boolean flying = this instanceof Player player && player.isFlying();
         if (!positionChanged) {
-            if (hasVelocity || newVelocity.isZero()) {
+            if (flying) {
+                this.velocity = Vec.ZERO;
+                sendPacketToViewers(getVelocityPacket());
+                return;
+            } else if (hasVelocity || newVelocity.isZero()) {
                 this.velocity = noGravity ? Vec.ZERO : new Vec(
                         0,
                         -gravityAcceleration * tps * (1 - gravityDragPerTick),
@@ -612,7 +617,7 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
 
         // Update velocity
         if (hasVelocity || !newVelocity.isZero()) {
-            updateVelocity(wasOnGround, positionBeforeMove, newVelocity);
+            updateVelocity(wasOnGround, flying, positionBeforeMove, newVelocity);
         }
         // Verify if velocity packet has to be sent
         if (!(this instanceof Player) && (hasVelocity || gravityTickCount > 0)) {
@@ -620,7 +625,7 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
         }
     }
 
-    protected void updateVelocity(boolean wasOnGround, Pos positionBeforeMove, Vec newVelocity) {
+    protected void updateVelocity(boolean wasOnGround, boolean flying, Pos positionBeforeMove, Vec newVelocity) {
         EntitySpawnType type = entityType.registry().spawnType();
         final double airDrag = type == EntitySpawnType.LIVING || type == EntitySpawnType.PLAYER ? 0.91 : 0.98;
         final double drag;
@@ -631,11 +636,14 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
             }
         } else drag = airDrag;
 
+        double gravity = flying ? 0 : gravityAcceleration;
+        double gravityDrag = flying ? 0.6 : (1 - gravityDragPerTick);
+
         this.velocity = newVelocity
-                // Apply drag
+                // Apply gravity and drag
                 .apply((x, y, z) -> new Vec(
                         x * drag,
-                        !hasNoGravity() ? (y - gravityAcceleration) * (1 - gravityDragPerTick) : y,
+                        !hasNoGravity() ? (y - gravity) * gravityDrag : y,
                         z * drag
                 ))
                 // Convert from block/tick to block/sec

src/test/java/net/minestom/server/entity/EntityVelocityIntegrationTest.java

@@ -8,8 +8,6 @@ import net.minestom.server.instance.Instance;
 import net.minestom.server.utils.chunk.ChunkUtils;
 import org.junit.jupiter.api.Test;
 
-import java.time.Duration;
-
 import static org.junit.jupiter.api.Assertions.*;
 
 @EnvTest
@@ -105,6 +103,31 @@ public class EntityVelocityIntegrationTest {
         });
     }
 
+    @Test
+    public void flyingVelocity(Env env) {
+        var instance = env.createFlatInstance();
+        loadChunks(instance);
+
+        var player = env.createPlayer(instance, new Pos(0, 42, 0));
+        env.tick();
+
+        final double epsilon = 0.0000001;
+
+        assertEquals(player.getVelocity().y(), -1.568, epsilon);
+        double previousVelocity = player.getVelocity().y();
+
+        player.setFlying(true);
+        env.tick();
+
+        // Every tick, the y velocity is multiplied by 0.6, and after 27 ticks it should be 0
+        for (int i = 0; i < 27; i++) {
+            assertEquals(player.getVelocity().y(), previousVelocity * 0.6, epsilon);
+            previousVelocity = player.getVelocity().y();
+            env.tick();
+        }
+        assertEquals(player.getVelocity().y(), 0);
+    }
+
     @Test
     public void testHasVelocity(Env env) {
         var instance = env.createFlatInstance();