From eca2b21e6a092380441956a42f85952afa0fcb64 Mon Sep 17 00:00:00 2001
From: Jonathan Johnson <jon@khonsulabs.com>
Date: Sat, 11 Nov 2023 17:57:26 -0800
Subject: [PATCH] contrast_between is now smarter

---
 src/animation.rs |   6 +++
 src/styles.rs    | 106 ++++++++++++++++++++++++++++++++++++++---------
 2 files changed, 93 insertions(+), 19 deletions(-)

diff --git a/src/animation.rs b/src/animation.rs
index 2509b8444..14d680e39 100644
--- a/src/animation.rs
+++ b/src/animation.rs
@@ -800,6 +800,12 @@ impl ZeroToOne {
         Self(value.clamp(0., 1.))
     }
 
+    /// Returns the difference between `self` and `other` as a positive number.
+    #[must_use]
+    pub fn difference_between(self, other: Self) -> Self {
+        Self((self.0 - other.0).abs())
+    }
+
     /// Returns the contained floating point value.
     #[must_use]
     pub fn into_f32(self) -> f32 {
diff --git a/src/styles.rs b/src/styles.rs
index e9476cf35..19a137b46 100644
--- a/src/styles.rs
+++ b/src/styles.rs
@@ -1150,6 +1150,32 @@ impl ColorSource {
             Okhsl::new(self.hue, *self.saturation, *lightness.into_lightness()).into_color();
         Color::new_f32(rgb.red, rgb.blue, rgb.green, 1.0)
     }
+
+    /// Calculates an approximate ratio between 0.0 and 1.0 of how contrasting
+    /// these colors are, with perfect constrast being two clors that are
+    /// opposite of each other on the hue circle and one fully desaturated and
+    /// the other fully saturated.
+    #[must_use]
+    pub fn contrast_between(self, other: Self) -> ZeroToOne {
+        let saturation_delta = self.saturation.difference_between(other.saturation);
+        let self_hue = self.hue.into_positive_degrees();
+        let other_hue = other.hue.into_positive_degrees();
+        // Calculate the shortest distance between the hues, taking into account
+        // that 0 and 359 are one degree apart.
+        let hue_delta = ZeroToOne::new(
+            if self_hue < other_hue {
+                let hue_delta_a = other_hue - self_hue;
+                let hue_delta_b = self_hue + 360. - other_hue;
+                hue_delta_a.min(hue_delta_b)
+            } else {
+                let hue_delta_a = self_hue - other_hue;
+                let hue_delta_b = other_hue + 360. - self_hue;
+                hue_delta_a.min(hue_delta_b)
+            } / 180.,
+        );
+
+        saturation_delta * hue_delta
+    }
 }
 
 /// A value that can represent the lightness of a color.
@@ -1197,29 +1223,31 @@ pub trait ColorExt: Copy {
         self.into_source_and_lightness().1
     }
 
+    /// Returns the contrast between this color and the components provided.
+    ///
+    /// To achieve a contrast of 1.0:
+    ///
+    /// - `self`'s hue and `check_source.hue` must be 180 degrees apart.
+    /// - `self`'s saturation and `check_source.saturation` must be different by
+    ///   1.0.
+    /// - `self`'s lightness and `check_lightness` must be different by 1.0.
+    /// - `self`'s alpha and `check_alpha` must be different by 1.0.
+    ///
+    /// The algorithm currently used is purposely left undocumented as it will
+    /// likely change. It should be a reasonable heuristic until someone smarter
+    /// than @ecton comes along.
+    fn contrast_between(
+        self,
+        check_source: ColorSource,
+        check_lightness: ZeroToOne,
+        check_alpha: ZeroToOne,
+    ) -> ZeroToOne;
+
     /// Returns the color in `others` that contrasts the most from `self`.
     #[must_use]
     fn most_contrasting(self, others: &[Self]) -> Self
     where
-        Self: Copy,
-    {
-        // TODO this currently only checks lightness. We should probably factor
-        // in hue/saturation changes too.
-        let check = self.lightness();
-
-        let mut others = others.iter().copied();
-        let mut most_contrasting = others.next().expect("at least one comparison");
-        let mut most_contrast_amount = (*most_contrasting.lightness() - *check).abs();
-        for other in others {
-            let contrast_amount = (*other.lightness() - *check).abs();
-            if contrast_amount > most_contrast_amount {
-                most_contrasting = other;
-                most_contrast_amount = contrast_amount;
-            }
-        }
-
-        most_contrasting
-    }
+        Self: Copy;
 }
 
 impl ColorExt for Color {
@@ -1234,4 +1262,44 @@ impl ColorExt for Color {
             ZeroToOne::new(hsl.lightness * self.alpha_f32()),
         )
     }
+
+    fn contrast_between(
+        self,
+        check_source: ColorSource,
+        check_lightness: ZeroToOne,
+        check_alpha: ZeroToOne,
+    ) -> ZeroToOne {
+        let (other_source, other_lightness) = self.into_source_and_lightness();
+        let lightness_delta = other_lightness.difference_between(check_lightness);
+
+        let source_change = check_source.contrast_between(other_source);
+
+        let other_alpha = ZeroToOne::new(self.alpha_f32());
+        let alpha_delta = check_alpha.difference_between(other_alpha);
+
+        lightness_delta * source_change * alpha_delta
+    }
+
+    fn most_contrasting(self, others: &[Self]) -> Self
+    where
+        Self: Copy,
+    {
+        let (check_source, check_lightness) = self.into_source_and_lightness();
+        let check_alpha = ZeroToOne::new(self.alpha_f32());
+
+        let mut others = others.iter().copied();
+        let mut most_contrasting = others.next().expect("at least one comparison");
+        let mut most_contrast_amount =
+            most_contrasting.contrast_between(check_source, check_lightness, check_alpha);
+        for other in others {
+            let contrast_amount =
+                other.contrast_between(check_source, check_lightness, check_alpha);
+            if contrast_amount > most_contrast_amount {
+                most_contrasting = other;
+                most_contrast_amount = contrast_amount;
+            }
+        }
+
+        most_contrasting
+    }
 }