Merge pull request #6451 from xFedeT/fix/hand-sway-yaw-wrap

🤖 Agent: queued: reviewed, union-tested green at this head.

crates/horizon-client/src/first_person.rs

@@ -520,6 +520,17 @@ pub fn equip_displacement(height: f32, swap_animation_scale: f32) -> f32 {
     swap_animation_scale * (1.0 - height)
 }
 
+/// `Mth.wrapDegrees`: an angle difference in -180..=180.
+fn wrap_degrees(d: f32) -> f32 {
+    let d = d % 360.0;
+    if d >= 180.0 {
+        d - 360.0
+    } else if d < -180.0 {
+        d + 360.0
+    } else {
+        d
+    }
+}
 /// `LocalPlayer.xBob` and `yBob`: the hands lag behind the view when it
 /// turns. `LocalPlayer.applyInput` eases them halfway to the rotation every
 /// tick; `FirstPersonHandsAndItemsRenderer` turns the hands by a tenth of
@@ -537,7 +548,9 @@ impl HandSway {
         self.y_bob_o = self.y_bob;
         self.x_bob_o = self.x_bob;
         self.x_bob += (x_rot - self.x_bob) * 0.5;
-        self.y_bob += (y_rot - self.y_bob) * 0.5;
+        // Vanilla's yaw runs on without wrapping, so `yRot - yBob` is the short way round. Ours
+        // is kept in -180..180 (a boat turns its rider through it), so take the short way here.
+        self.y_bob += wrap_degrees(y_rot - self.y_bob) * 0.5;
     }
     /// The degrees the hands turn about X, then Y, before either is drawn:
     /// `(viewXRot - xBob) * 0.1` and `(viewYRot - yBob) * 0.1`, the bobs
@@ -545,7 +558,10 @@ impl HandSway {
     pub fn rotation(&self, partial: f32, view_x_rot: f32, view_y_rot: f32) -> [f32; 2] {
         let x_bob = crate::camera::lerp(partial, self.x_bob_o, self.x_bob);
         let y_bob = crate::camera::lerp(partial, self.y_bob_o, self.y_bob);
-        [(view_x_rot - x_bob) * 0.1, (view_y_rot - y_bob) * 0.1]
+        [
+            (view_x_rot - x_bob) * 0.1,
+            wrap_degrees(view_y_rot - y_bob) * 0.1,
+        ]
     }
     /// The hands' view space to camera-relative world space, turned by the
     /// sway. The hand pass draws with the camera's view, whose rotation a
@@ -1384,4 +1400,22 @@ mod action_result_tests {
             );
         }
     }
+
+    /// The hands' sway turns the short way round when the yaw wraps from 179 to -179 degrees (a
+    /// boat turning its rider), instead of swinging them 36 degrees the long way.
+    #[test]
+    fn hand_sway_takes_the_short_way_over_the_yaw_wrap() {
+        let mut sway = HandSway::default();
+        sway.tick(0.0, 179.0);
+        for _ in 0..20 {
+            sway.tick(0.0, 179.0);
+        }
+        // The view is still at 179; one tick later it reads -179 (2 degrees on).
+        sway.tick(0.0, -179.0);
+        let [_, y] = sway.rotation(1.0, 0.0, -179.0);
+        assert!(
+            y.abs() < 0.5,
+            "a 2 degree turn sways the hands by under half a degree: {y}"
+        );
+    }
 }