From b07b08dc56f85bd4f62f5873a1b96d506b321bb9 Mon Sep 17 00:00:00 2001
From: ghostly_zsh <ghostly@e3t.cc>
Date: Sat, 29 Nov 2025 08:48:20 -0600
Subject: [PATCH] fix: trying to fix gravity

---
 crates/unified/src/server/gravity.rs | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/crates/unified/src/server/gravity.rs b/crates/unified/src/server/gravity.rs
index ad3dfdec22749b20e83eb6e95c224061123d7a56..f16722b6698049ba1b4ce8d5aba1a0a566d23a99 100644
--- a/crates/unified/src/server/gravity.rs
+++ b/crates/unified/src/server/gravity.rs
@@ -11,15 +11,16 @@ pub fn newtonian_gravity_plugin(app: &mut App) {
 
 
 fn update_gravity(
-    mut part_query: Query<(&Transform, &Mass, &mut ConstantForce), With<Part>>,
+    mut part_query: Query<(&Transform, &LinearVelocity, &Mass, &mut ConstantForce), With<Part>>,
     planet_query: Query<(&Transform, &Mass), With<Planet>>,
     world_config: Res<WorldConfigResource>,
+    time: Res<Time>,
 ) {
     let Some(world_config) = &world_config.config else {
         return;
     };
 
-    for (part_transform, part_mass, mut forces) in &mut part_query {
+    for (part_transform, part_velocity, part_mass, mut forces) in &mut part_query {
         *forces = ConstantForce::new(0.0, 0.0);
 
         let part_mass = part_mass.0;
@@ -31,8 +32,25 @@ fn update_gravity(
 
             let distance = planet_translation.distance(part_translation);
 
-            let force =
-                world_config.world.gravity * ((part_mass * planet_mass) / distance.squared());
+            const GRAV_ITERS: u32 = 8;
+            let mut x = 0.0;
+            let mut total_f = 0.0;
+            let mut v = part_velocity.0;
+            let dt = time.delta_secs() / GRAV_ITERS as f32;
+            for i in 0..GRAV_ITERS {
+                let f =
+                    world_config.world.gravity * ((part_mass * planet_mass) / distance.squared()-x);
+                let dx = dt*(v.project_onto((planet_translation-part_translation).truncate())).length()
+                    - 1.0/2.0*(f/part_mass)*dt*dt;
+                x += dx;
+                v += f/part_mass*dt;
+                if i == 0 || i == GRAV_ITERS-1 {
+                    total_f += f;
+                } else {
+                    total_f += 2.0*f;
+                }
+            }
+            let force = (dt/2.0*(total_f))/time.delta_secs();
             let direction = (planet_translation - part_translation).normalize() * force;
             forces.x += direction.x;
             forces.y += direction.y;