Implement rubber banding for the vertical gesture

src/layout/monitor.rs

@@ -15,12 +15,18 @@ use super::workspace::{
 use super::{LayoutElement, Options};
 use crate::animation::Animation;
 use crate::render_helpers::renderer::NiriRenderer;
+use crate::rubber_band::RubberBand;
 use crate::swipe_tracker::SwipeTracker;
 use crate::utils::output_size;
 
 /// Amount of touchpad movement to scroll the height of one workspace.
 const WORKSPACE_GESTURE_MOVEMENT: f64 = 300.;
 
+const WORKSPACE_GESTURE_RUBBER_BAND: RubberBand = RubberBand {
+    stiffness: 0.5,
+    limit: 0.05,
+};
+
 #[derive(Debug)]
 pub struct Monitor<W: LayoutElement> {
     /// Output for this monitor.
@@ -762,7 +768,8 @@ impl<W: LayoutElement> Monitor<W> {
 
         let min = gesture.center_idx.saturating_sub(1) as f64;
         let max = (gesture.center_idx + 1).min(self.workspaces.len() - 1) as f64;
-        let new_idx = (gesture.center_idx as f64 + pos).clamp(min, max);
+        let new_idx = gesture.center_idx as f64 + pos;
+        let new_idx = WORKSPACE_GESTURE_RUBBER_BAND.clamp(min, max, new_idx);
 
         if gesture.current_idx == new_idx {
             return Some(false);
@@ -783,14 +790,23 @@ impl<W: LayoutElement> Monitor<W> {
             return true;
         }
 
-        let velocity = gesture.tracker.velocity() / WORKSPACE_GESTURE_MOVEMENT;
+        let mut velocity = gesture.tracker.velocity() / WORKSPACE_GESTURE_MOVEMENT;
+        let current_pos = gesture.tracker.pos() / WORKSPACE_GESTURE_MOVEMENT;
         let pos = gesture.tracker.projected_end_pos() / WORKSPACE_GESTURE_MOVEMENT;
 
         let min = gesture.center_idx.saturating_sub(1) as f64;
         let max = (gesture.center_idx + 1).min(self.workspaces.len() - 1) as f64;
-        let new_idx = (gesture.center_idx as f64 + pos).clamp(min, max);
+        let new_idx = gesture.center_idx as f64 + pos;
+
+        let new_idx = WORKSPACE_GESTURE_RUBBER_BAND.clamp(min, max, new_idx);
         let new_idx = new_idx.round() as usize;
 
+        velocity *= WORKSPACE_GESTURE_RUBBER_BAND.clamp_derivative(
+            min,
+            max,
+            gesture.center_idx as f64 + current_pos,
+        );
+
         self.active_workspace_idx = new_idx;
         self.workspace_switch = Some(WorkspaceSwitch::Animation(Animation::new(
             gesture.current_idx,

src/lib.rs

@@ -15,6 +15,7 @@ pub mod layout;
 pub mod niri;
 pub mod protocols;
 pub mod render_helpers;
+pub mod rubber_band;
 pub mod swipe_tracker;
 pub mod ui;
 pub mod utils;

src/rubber_band.rs

@@ -0,0 +1,39 @@
+#[derive(Debug, Clone, Copy)]
+pub struct RubberBand {
+    pub stiffness: f64,
+    pub limit: f64,
+}
+
+impl RubberBand {
+    pub fn band(&self, x: f64) -> f64 {
+        let c = self.stiffness;
+        let d = self.limit;
+
+        (1. - (1. / (x * c / d + 1.))) * d
+    }
+
+    pub fn derivative(&self, x: f64) -> f64 {
+        let c = self.stiffness;
+        let d = self.limit;
+
+        c * d * d / (c * x + d).powi(2)
+    }
+
+    pub fn clamp(&self, min: f64, max: f64, x: f64) -> f64 {
+        let clamped = x.clamp(min, max);
+        let sign = if x < clamped { -1. } else { 1. };
+        let diff = (x - clamped).abs();
+
+        clamped + sign * self.band(diff)
+    }
+
+    pub fn clamp_derivative(&self, min: f64, max: f64, x: f64) -> f64 {
+        if min <= x && x <= max {
+            return 1.;
+        }
+
+        let clamped = x.clamp(min, max);
+        let diff = (x - clamped).abs();
+        self.derivative(diff)
+    }
+}