Order and sample gradient correctly

Zero-width gradient sections should be ‘ignored’.
The visible endpoints should be precisely the first
and last colors added at that point.

https://html.spec.whatwg.org/multipage/canvas.html#interpolation

content/src/gradient.rs

@@ -15,7 +15,7 @@ use pathfinder_geometry::transform2d::Transform2F;
 use pathfinder_geometry::vector::Vector2F;
 use pathfinder_geometry::util as geometry_util;
 use pathfinder_simd::default::F32x2;
-use std::cmp::{Ordering, PartialOrd};
+use std::cmp::Ordering;
 use std::convert;
 use std::hash::{Hash, Hasher};
 use std::mem;
@@ -110,7 +110,7 @@ impl Gradient {
     #[inline]
     pub fn add(&mut self, stop: ColorStop) {
         let index = self.stops.binary_search_by(|other| {
-            other.offset.partial_cmp(&stop.offset).unwrap()
+            if other.offset <= stop.offset { Ordering::Less } else { Ordering::Greater }
         }).unwrap_or_else(convert::identity);
         self.stops.insert(index, stop);
     }
@@ -138,8 +138,9 @@ impl Gradient {
 
         t = geometry_util::clamp(t, 0.0, 1.0);
         let last_index = self.stops.len() - 1;
+
         let upper_index = self.stops.binary_search_by(|stop| {
-            stop.offset.partial_cmp(&t).unwrap_or(Ordering::Less)
+            if stop.offset < t || stop.offset == 0.0 { Ordering::Less } else { Ordering::Greater }
         }).unwrap_or_else(convert::identity).min(last_index);
         let lower_index = if upper_index > 0 { upper_index - 1 } else { upper_index };
 
@@ -205,3 +206,37 @@ impl RadialGradientLine for Vector2F {
         LineSegment2F::new(self, self)
     }
 }
+
+#[cfg(test)]
+mod test {
+    use crate::gradient::Gradient;
+    use pathfinder_color::ColorU;
+    use pathfinder_geometry::vector::Vector2F;
+
+    #[test]
+    fn stable_order() {
+        let mut grad = Gradient::linear_from_points(Vector2F::default(), Vector2F::default());
+        for i in 0..110 {
+            grad.add_color_stop(ColorU::new(i, 0, 0, 1), (i % 11) as f32 / 10.0);
+        }
+
+        // Check that it sorted stably
+        assert!(grad.stops.windows(2).all(|w| {
+            w[0].offset < w[1].offset || w[0].color.r < w[1].color.r
+        }));
+    }
+
+    #[test]
+    fn never_sample_zero_width() {
+        let mut grad = Gradient::linear_from_points(Vector2F::default(), Vector2F::default());
+        for i in 0..110 {
+            let zero_width = (i == 0) || (11 <= i && i < 99) || (i == 109);
+            grad.add_color_stop(ColorU::new(if zero_width { 255 } else { 0 }, 0, 0, 1), (i % 11) as f32 / 10.0);
+        }
+
+        for i in 0..11 {
+            let sample = grad.sample(i as f32 / 10.0);
+            assert!(sample.r == 0, "{} {}", i, sample.r);
+        }
+    }
+}