smooth scroll speed estimation to reduce red-dot spikes

daemon/src/main.rs

@@ -40,11 +40,29 @@ struct Args {
     no_grab: bool,
 }
 
+#[derive(Debug, Default)]
+struct AxisRuntime {
+    last_event_at: Option<Instant>,
+    remainder: f64,
+    smoothed_speed: f64,
+}
+
 #[derive(Debug, Default)]
 struct ScrollState {
-    last_scroll_event_at: Option<Instant>,
-    remainder_v: f64,
-    remainder_h: f64,
+    vertical: AxisRuntime,
+    horizontal: AxisRuntime,
+}
+
+const SPEED_EMA_ALPHA: f64 = 0.25;
+const MIN_DT_SECONDS: f64 = 0.004;
+const SPEED_IDLE_RESET_SECONDS: f64 = 0.2;
+
+fn axis_runtime_mut(state: &mut ScrollState, axis: RelativeAxisCode) -> &mut AxisRuntime {
+    if axis == RelativeAxisCode::REL_HWHEEL || axis == RelativeAxisCode::REL_HWHEEL_HI_RES {
+        &mut state.horizontal
+    } else {
+        &mut state.vertical
+    }
 }
 
 fn main() -> anyhow::Result<()> {
@@ -269,13 +287,14 @@ fn transform_event(
             {
                 return None;
             }
+            let axis_state = axis_runtime_mut(state, axis);
             let now = Instant::now();
-            let dt = state
-                .last_scroll_event_at
+            let dt = axis_state
+                .last_event_at
                 .map(|t| now.saturating_duration_since(t).as_secs_f64())
                 .unwrap_or(0.016)
-                .max(0.001);
-            state.last_scroll_event_at = Some(now);
+                .max(MIN_DT_SECONDS);
+            axis_state.last_event_at = Some(now);
 
             let axis_unit = if axis == RelativeAxisCode::REL_WHEEL_HI_RES
                 || axis == RelativeAxisCode::REL_HWHEEL_HI_RES
@@ -288,7 +307,17 @@ fn transform_event(
             // Normalize hi-res wheel units (120 per detent) to "detent-equivalent"
             // speed so tuning behaves similarly across REL_WHEEL and REL_WHEEL_HI_RES.
             let normalized_value = (value as f64) / axis_unit;
-            let speed = normalized_value.abs() / dt;
+            let raw_speed = normalized_value.abs() / dt;
+
+            // Smooth out bursty evdev timing so the live indicator and accel input
+            // don't jump wildly on single tightly-packed events.
+            let speed = if dt > SPEED_IDLE_RESET_SECONDS {
+                raw_speed
+            } else {
+                SPEED_EMA_ALPHA * raw_speed + (1.0 - SPEED_EMA_ALPHA) * axis_state.smoothed_speed
+            };
+            axis_state.smoothed_speed = speed;
+
             let (gain_x, gain_y) = sensitivity(speed, *mode, params);
             let gain = if axis == RelativeAxisCode::REL_HWHEEL
                 || axis == RelativeAxisCode::REL_HWHEEL_HI_RES
@@ -298,17 +327,9 @@ fn transform_event(
                 gain_y
             };
 
-            let remainder = if axis == RelativeAxisCode::REL_HWHEEL
-                || axis == RelativeAxisCode::REL_HWHEEL_HI_RES
-            {
-                &mut state.remainder_h
-            } else {
-                &mut state.remainder_v
-            };
-
-            let scaled = (value as f64) * gain + *remainder;
+            let scaled = (value as f64) * gain + axis_state.remainder;
             let quantized = scaled.trunc() as i32;
-            *remainder = scaled - (quantized as f64);
+            axis_state.remainder = scaled - (quantized as f64);
 
             let _ = write_speed_stat(speed);