perf(color): sort gradient stops once per diamond texture (#1894)

Gradient::sample cloned, sanitised and sorted every stop on each call,
and the diamond texture sampled it for each of its 65 536 pixels, so a
file's gradient with many stops cost that sort 65 536 times.

sample_sorted samples stops already sorted, finding the segment by binary
search; sample delegates to it and the diamond texture sorts once.

Co-authored-by: Brandon Thomas <echelon@gmail.com>

crates/color/src/lib.rs

@@ -376,26 +376,32 @@ impl Gradient {
         s
     }
 
-    /// The colour at `t` (0..1) along the gradient.
+    /// The colour at `t` (0..1) along the gradient. Sorts the stops on every call: to sample one
+    /// gradient many times, sort once with [`Self::sorted_stops`] and use [`sample_sorted`].
     pub fn sample(&self, t: f32) -> Color {
-        let stops = self.sorted_stops();
-        let t = clamp01(t);
-        let mut prev = match stops.first() {
-            Some(s) => *s,
-            None => return Color::BLACK,
-        };
-        if t <= prev.position {
-            return prev.color;
+        sample_sorted(&self.sorted_stops(), t)
+    }
+}
+
+/// The colour at `t` (0..1) along stops already sorted by [`Gradient::sorted_stops`]: found by a
+/// binary search, so a renderer sampling a file's gradient per pixel doesn't scan or sort its
+/// stops each time.
+pub fn sample_sorted(stops: &[ColorStop], t: f32) -> Color {
+    let t = clamp01(t);
+    let Some(first) = stops.first() else { return Color::BLACK };
+    if t <= first.position {
+        return first.color;
+    }
+    // The first stop after the first one at or past `t`, and the stop before it.
+    let rest = stops.get(1..).unwrap_or_default();
+    let i = rest.partition_point(|s| s.position < t);
+    match (rest.get(i), stops.get(i)) {
+        (Some(s), Some(prev)) => {
+            let span = s.position - prev.position;
+            let f = if span > 1e-6 { (t - prev.position) / span } else { 1.0 };
+            prev.color.lerp(s.color, f)
         }
-        for s in stops.iter().skip(1) {
-            if t <= s.position {
-                let span = s.position - prev.position;
-                let f = if span > 1e-6 { (t - prev.position) / span } else { 1.0 };
-                return prev.color.lerp(s.color, f);
-            }
-            prev = *s;
-        }
-        prev.color
+        _ => stops.last().map_or(Color::BLACK, |s| s.color),
     }
 }
 

crates/color/src/tests.rs

@@ -59,6 +59,44 @@ fn gradient_sampling() {
     assert_eq!(g.sample(0.5).to_hex(), "808080");
 }
 
+#[test]
+fn sampling_sorted_stops_matches_a_scan() {
+    // The scan `sample` used to make: the first stop at or past `t`, mixed with the one before.
+    fn scan(stops: &[ColorStop], t: f32) -> Color {
+        let t = if t.is_nan() { 0.0 } else { t.clamp(0.0, 1.0) };
+        let mut prev = stops[0];
+        if t <= prev.position {
+            return prev.color;
+        }
+        for s in &stops[1..] {
+            if t <= s.position {
+                let span = s.position - prev.position;
+                let f = if span > 1e-6 { (t - prev.position) / span } else { 1.0 };
+                return prev.color.lerp(s.color, f);
+            }
+            prev = *s;
+        }
+        prev.color
+    }
+    let grey = |v: f32| Color { r: v, g: v, b: v, a: 1.0 };
+    let positions = [0.3, 0.0, 0.3, 1.0, 0.7, f32::NAN, 0.5, 0.5, 0.9];
+    let g = Gradient {
+        kind: GradientKind::Diamond,
+        stops: positions.iter().enumerate().map(|(i, p)| ColorStop { position: *p, color: grey(i as f32 / 10.0) }).collect(),
+        from: (0.5, 0.5),
+        to: (1.0, 0.5),
+        width: (0.5, 1.0),
+    };
+    let sorted = g.sorted_stops();
+    for i in -10..=110 {
+        let t = i as f32 / 100.0;
+        assert_eq!(sample_sorted(&sorted, t), scan(&sorted, t), "t = {t}");
+        assert_eq!(g.sample(t), scan(&sorted, t), "t = {t}");
+    }
+    assert_eq!(sample_sorted(&sorted, f32::NAN), scan(&sorted, f32::NAN));
+    assert_eq!(sample_sorted(&[], 0.5), Color::BLACK);
+}
+
 #[test]
 fn paint_serde_shape() {
     let p = Paint::solid(Color::from_hex("FF0000").unwrap());

crates/render/src/paint.rs

@@ -79,11 +79,13 @@ fn diamond_pixmap(g: &Gradient, opacity: f32) -> Option<Arc<Pixmap>> {
     const N: u32 = 256;
     let mut p = Pixmap::new(N, N)?;
     let half = f64::from(N) / 2.0;
+    // Sorted once for the 65 536 samples below, not once per sample.
+    let stops = g.sorted_stops();
     for (i, px) in p.data_mut().as_chunks_mut::<4>().0.iter_mut().enumerate() {
         let x = ((i as u32 % N) as f64 + 0.5 - half) / half;
         let y = ((i as u32 / N) as f64 + 0.5 - half) / half;
         let t = (x.abs() + y.abs()) as f32;
-        let c = g.sample(t);
+        let c = figcraft_color::sample_sorted(&stops, t);
         let a = c.a * opacity;
         let q = |v: f32| (v * a * 255.0).round().clamp(0.0, 255.0) as u8;
         px.copy_from_slice(&[q(c.r), q(c.g), q(c.b), (a * 255.0).round().clamp(0.0, 255.0) as u8]);