Fix floating when hitting the ceiling and slope slowdown

rust/src/player/mod.rs

@@ -1,6 +1,10 @@
 use std::{collections::HashSet, f32::consts::PI, ops::Neg};
 
-use godot::{classes::ICharacterBody3D, obj::WithBaseField, prelude::*};
+use godot::{
+    classes::{ICharacterBody3D, KinematicCollision3D},
+    obj::WithBaseField,
+    prelude::*,
+};
 use serde::{Deserialize, Serialize};
 
 use crate::{
@@ -70,25 +74,56 @@ pub trait PlayerCommon {
 
 impl<T: IPlayer + ICharacterBody3D + WithBaseField> PlayerCommon for T {
     fn player_update(&mut self, delta: f64) {
+        // Gravity
         self.set_y_speed(self.get_y_speed() - 9.8 * delta as f32);
         if self.base().is_on_floor() && self.get_y_speed() < 0. {
             self.set_y_speed(0.);
         }
+        let vertical_velocity = Vector3::UP * self.get_y_speed();
 
-        let speed = Vector3::UP * self.get_y_speed()
-            + self
-                .get_movement_dir()
-                .try_normalized()
-                .unwrap_or_default()
-                .rotated(Vector3::UP, self.base().get_rotation().y)
-                * self.get_move_speed();
+        // Figure out the ground plane normal (to move on the right plane)
+        let mut up_vector = Vector3::UP;
+        let mut most_upward_coll: Option<Gd<KinematicCollision3D>> = None;
+        for coll in (0..self.base().get_slide_collision_count())
+            .flat_map(|i| self.base().get_slide_collision(i))
+        {
+            let coll_norm_y = coll.get_normal().y;
+            if let Some(prev_most_upward) = &most_upward_coll {
+                if coll_norm_y > prev_most_upward.get_normal().y {
+                    most_upward_coll = Some(coll);
+                }
+            } else if coll_norm_y > 0.5 {
+                // sin(30 deg) == 0.5, so this only accepts 30 deg or shallower slopes
+                most_upward_coll = Some(coll);
+            }
+        }
+        if let Some(coll) = most_upward_coll {
+            up_vector = coll.get_normal();
+        }
 
+        // Ground movement (along the plane the player is currently on)
+        let input_move_on_xz = self.get_movement_dir().try_normalized().unwrap_or_default();
+        let ground_plane_basis = Basis::from_axis_angle(up_vector, self.base().get_rotation().y);
+        let input_move_on_ground_plane = ground_plane_basis * input_move_on_xz;
+
+        let move_speed = self.get_move_speed();
+        let ground_plane_velocity = input_move_on_ground_plane * move_speed;
+
+        // Commit
         if self.is_dead() {
             self.base_mut().set_velocity(Vector3::ZERO);
         } else {
-            self.base_mut().set_velocity(speed);
+            self.base_mut()
+                .set_velocity(vertical_velocity + ground_plane_velocity);
             self.base_mut().move_and_slide();
         }
+
+        // Check if hitting a ceiling; if so, stop upwards velocity (done here instead of
+        // at the start of the next frame to get the right-after-move_and_slide state
+        // rather than the simulated one).
+        if self.base().get_velocity().y < 0.001 && self.get_y_speed() > 0.001 {
+            self.set_y_speed(0.0);
+        }
     }
 
     fn try_jump(&mut self) -> bool {