Auto merge of #20189 - servo:squared-distance-for-real, r=emilio

Never store a squared root in SquaredDistance

<!-- Reviewable:start -->
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20189)
<!-- Reviewable:end -->

components/style/properties/helpers/animated_properties.mako.rs

@@ -811,7 +811,7 @@ impl Animate for Visibility {
 impl ComputeSquaredDistance for Visibility {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        Ok(SquaredDistance::Value(if *self == *other { 0. } else { 1. }))
+        Ok(SquaredDistance::from_sqrt(if *self == *other { 0. } else { 1. }))
     }
 }
 
@@ -1921,7 +1921,7 @@ impl ComputeSquaredDistance for Quaternion {
         // so we can get their angle difference by:
         // cos(theta/2) = (q1 dot q2) / (|q1| * |q2|) = q1 dot q2.
         let distance = self.dot(other).max(-1.0).min(1.0).acos() * 2.0;
-        Ok(SquaredDistance::Value(distance * distance))
+        Ok(SquaredDistance::from_sqrt(distance))
     }
 }
 

components/style/values/animated/color.rs

@@ -166,19 +166,19 @@ impl ComputeSquaredDistance for Color {
         // All comments from the Animate impl also applies here.
         if self.foreground_ratio == other.foreground_ratio {
             if self.is_currentcolor() {
-                Ok(SquaredDistance::Value(0.))
+                Ok(SquaredDistance::from_sqrt(0.))
             } else {
                 self.color.compute_squared_distance(&other.color)
             }
         } else if self.is_currentcolor() && other.is_numeric() {
             Ok(
                 RGBA::transparent().compute_squared_distance(&other.color)? +
-                SquaredDistance::Value(1.),
+                SquaredDistance::from_sqrt(1.),
             )
         } else if self.is_numeric() && other.is_currentcolor() {
             Ok(
                 self.color.compute_squared_distance(&RGBA::transparent())? +
-                SquaredDistance::Value(1.),
+                SquaredDistance::from_sqrt(1.),
             )
         } else {
             let self_color = self.effective_intermediate_rgba();

components/style/values/distance.rs

@@ -28,38 +28,43 @@ pub trait ComputeSquaredDistance {
 
 /// A distance between two animatable values.
 #[derive(Clone, Copy, Debug)]
-pub enum SquaredDistance {
+pub struct SquaredDistance {
-    /// Represented as the square root of the squared distance.
+    value: f64,
-    Sqrt(f64),
+}
-    /// Represented as the squared distance itself.
+
-    Value(f64),
+impl SquaredDistance {
+    /// Returns a squared distance from its square root.
+    #[inline]
+    pub fn from_sqrt(sqrt: f64) -> Self {
+        Self { value: sqrt * sqrt }
+    }
 }
 
 impl ComputeSquaredDistance for u16 {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        Ok(SquaredDistance::Sqrt(((*self as f64) - (*other as f64)).abs()))
+        Ok(SquaredDistance::from_sqrt(((*self as f64) - (*other as f64)).abs()))
     }
 }
 
 impl ComputeSquaredDistance for i32 {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        Ok(SquaredDistance::Sqrt((*self - *other).abs() as f64))
+        Ok(SquaredDistance::from_sqrt((*self - *other).abs() as f64))
     }
 }
 
 impl ComputeSquaredDistance for f32 {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        Ok(SquaredDistance::Sqrt((*self - *other).abs() as f64))
+        Ok(SquaredDistance::from_sqrt((*self - *other).abs() as f64))
     }
 }
 
 impl ComputeSquaredDistance for f64 {
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        Ok(SquaredDistance::Sqrt((*self - *other).abs()))
+        Ok(SquaredDistance::from_sqrt((*self - *other).abs()))
     }
 }
 
@@ -77,7 +82,7 @@ impl<T> ComputeSquaredDistance for Option<T>
     fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
         match (self.as_ref(), other.as_ref()) {
             (Some(this), Some(other)) => this.compute_squared_distance(other),
-            (None, None) => Ok(SquaredDistance::Value(0.)),
+            (None, None) => Ok(SquaredDistance::from_sqrt(0.)),
             _ => Err(()),
         }
     }
@@ -94,21 +99,16 @@ impl<T> ComputeSquaredDistance for Size2D<T>
 
 impl SquaredDistance {
     /// Returns the square root of this squared distance.
+    #[inline]
     pub fn sqrt(self) -> f64 {
-        match self {
+        self.value.sqrt()
-            SquaredDistance::Sqrt(this) => this,
-            SquaredDistance::Value(this) => this.sqrt(),
-        }
     }
 }
 
 impl From<SquaredDistance> for f64 {
     #[inline]
     fn from(distance: SquaredDistance) -> Self {
-        match distance {
+        distance.value
-            SquaredDistance::Sqrt(this) => this * this,
-            SquaredDistance::Value(this) => this,
-        }
     }
 }
 
@@ -117,19 +117,15 @@ impl Add for SquaredDistance {
 
     #[inline]
     fn add(self, rhs: Self) -> Self {
-        SquaredDistance::Value(f64::from(self) + f64::from(rhs))
+        SquaredDistance { value: self.value + rhs.value }
     }
 }
 
 impl Sum for SquaredDistance {
-    fn sum<I>(mut iter: I) -> Self
+    fn sum<I>(iter: I) -> Self
     where
         I: Iterator<Item = Self>,
     {
-        let first = match iter.next() {
+        iter.fold(SquaredDistance::from_sqrt(0.), Add::add)
-            Some(first) => first,
-            None => return SquaredDistance::Value(0.),
-        };
-        iter.fold(first, Add::add)
     }
 }

components/style_derive/compute_squared_distance.rs

@@ -28,7 +28,7 @@ pub fn derive(input: DeriveInput) -> Tokens {
         let (this_pattern, this_info) = cg::ref_pattern(&variant, "this");
         let (other_pattern, other_info) = cg::ref_pattern(&variant, "other");
         let sum = if this_info.is_empty() {
-            quote! { ::values::distance::SquaredDistance::Value(0.) }
+            quote! { ::values::distance::SquaredDistance::from_sqrt(0.) }
         } else {
             let mut sum = quote!();
             sum.append_separated(this_info.iter().zip(&other_info).map(|(this, other)| {