Only predict projectile higher if there is gravity (#1016)

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

@@ -63,8 +63,10 @@ public class EntityProjectile extends Entity {
         double dx = to.x() - from.x();
         double dy = to.y() - from.y();
         double dz = to.z() - from.z();
+        if (!hasNoGravity()) {
             final double xzLength = Math.sqrt(dx * dx + dz * dz);
             dy += xzLength * 0.20000000298023224D;
+        }
 
         final double length = Math.sqrt(dx * dx + dy * dy + dz * dz);
         dx /= length;

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

@@ -0,0 +1,74 @@
+package net.minestom.server.entity;
+
+import net.minestom.server.api.Env;
+import net.minestom.server.api.EnvTest;
+import net.minestom.server.coordinate.Pos;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@EnvTest
+public class EntityProjectileIntegrationTest {
+    @Test
+    public void gravityVelocity(Env env) {
+        var instance = env.createFlatInstance();
+        var shooter = new EntityCreature(EntityType.SKELETON);
+        shooter.setInstance(instance, new Pos(0, 42, 0)).join();
+        var projectile = new EntityProjectile(shooter, EntityType.ARROW);
+        var from = new Pos(0, 42, 0).add(0,
+                shooter.getEyeHeight(), shooter.getPosition().direction().z());
+        var target = from.add(0, 0, 10);
+        projectile.setInstance(instance, from).join();
+        projectile.shoot(target, 1, 0);
+
+        var before = projectile.getPosition(); // at start
+        var after = projectile.getPosition(); // now - 1 tick, closest to target
+        var smallestDistance = 1e6;
+        while (true) {
+            final var distance = projectile.getPosition().distanceSquared(target);
+            if (distance <= smallestDistance) smallestDistance = distance;
+            else break;
+
+            after = projectile.getPosition();
+            env.tick();
+        }
+
+        // Ensure the position is correct.
+        // x doesn't change
+        // Big delta because ticks aren't very accurate
+        assertEquals(before.x(), after.x());
+        assertEquals(target.y(), after.y(), 0.6);
+        assertEquals(target.z(), after.z(), 0.6);
+    }
+
+    @Test
+    public void noGravityVelocity(Env env) {
+        var instance = env.createFlatInstance();
+        var shooter = new EntityCreature(EntityType.SKELETON);
+        shooter.setInstance(instance, new Pos(0, 42, 0)).join();
+        var projectile = new EntityProjectile(shooter, EntityType.ARROW);
+        var from = new Pos(0, 42, 0).add(0,
+                shooter.getEyeHeight(), shooter.getPosition().direction().z());
+        var target = from.add(0, 0, 10);
+        projectile.setNoGravity(true);
+        projectile.setInstance(instance, from).join();
+        projectile.shoot(target, 1, 0);
+
+        var before = projectile.getPosition(); // at start
+        var after = projectile.getPosition(); // now - 1 tick, closest to target
+        var smallestDistance = 1e6;
+        while (true) {
+            final var distance = projectile.getPosition().distanceSquared(target);
+            if (distance <= smallestDistance) smallestDistance = distance;
+            else break;
+
+            after = projectile.getPosition();
+            env.tick();
+        }
+
+        // x and y don't change (no gravity) and z changes by Σz velocity.
+        assertEquals(before.x(), after.x());
+        assertEquals(before.y(), after.y());
+        assertEquals(target.z(), after.z(), 0.05);
+    }
+}