Implement builtin function mix()

src/builtin/color/opacity.rs

@@ -67,4 +67,4 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         Some(Value::Color(color.fade_out(amount)))
     });
-}
+}

src/builtin/color/rgb.rs

@@ -119,4 +119,22 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             _ => todo!("non-color given to builtin function `blue()`")
         }
     });
-}
+    decl!(f "mix", |args, _| {
+        let color1 = match arg!(args, 0, "color1").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `mix()`")
+        };
+
+        let color2 = match arg!(args, 1, "color2").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `mix()`")
+        };
+
+        let weight = match arg!(args, 2, "weight"=Value::Dimension(Number::ratio(1, 2), Unit::None)) {
+            Value::Dimension(n, Unit::None) => n,
+            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
+            _ => todo!("expected either unitless or % number for $weight")
+        };
+        Some(Value::Color(color1.mix(color2, weight)))
+    });
+}

src/color/mod.rs

@@ -8,6 +8,18 @@ use num_traits::cast::ToPrimitive;
 
 mod name;
 
+macro_rules! clamp {
+    ($c:expr, $min:literal, $max:literal) => {
+        if $c > Number::from($max) {
+            Number::from($max)
+        } else if $c < Number::from($min) {
+            Number::from($min)
+        } else {
+            $c
+        }
+    };
+}
+
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub(crate) struct Color {
     red: Number,
@@ -77,6 +89,32 @@ impl Color {
     pub fn green(&self) -> Number {
         self.green.clone()
     }
+
+    /// Mix two colors together with weight
+    /// Algorithm adapted from 
+    /// <https://github.com/sass/dart-sass/blob/0d0270cb12a9ac5cce73a4d0785fecb00735feee/lib/src/functions/color.dart#L718>
+    pub fn mix(self, other: Color, weight: Number) -> Self {
+        let weight = clamp!(weight, 0, 100);
+        let normalized_weight = weight.clone() * Number::from(2) - Number::from(1);
+        let alpha_distance = self.alpha.clone() - other.alpha.clone();
+
+        let combined_weight1 =
+            if normalized_weight.clone() * alpha_distance.clone() == Number::from(-1) {
+                normalized_weight
+            } else {
+                (normalized_weight.clone() + alpha_distance.clone())
+                    / (Number::from(1) + normalized_weight * alpha_distance)
+            };
+        let weight1 = (combined_weight1 + Number::from(1)) / Number::from(2);
+        let weight2 = Number::from(1) - weight1.clone();
+
+        Color::from_rgba(
+            self.red * weight1.clone() + other.red * weight2.clone(),
+            self.green * weight1.clone() + other.green * weight2.clone(),
+            self.blue * weight1.clone() + other.blue * weight2,
+            self.alpha * weight.clone() + other.alpha * (Number::from(1) - weight),
+        )
+    }
 }
 
 /// HSLA color functions
@@ -207,26 +245,11 @@ impl Color {
     }
 
     /// Create RGBA representation from HSLA values
-    pub fn from_hsla(
-        mut hue: Number,
-        mut saturation: Number,
-        mut luminance: Number,
-        mut alpha: Number,
-    ) -> Self {
-        macro_rules! clamp {
-            ($c:ident, $min:literal, $max:literal) => {
-                if $c > Number::from($max) {
-                    $c = Number::from($max)
-                } else if $c < Number::from($min) {
-                    $c = Number::from($min)
-                }
-            };
-        }
-
-        clamp!(hue, 0, 360);
-        clamp!(saturation, 0, 1);
-        clamp!(luminance, 0, 1);
-        clamp!(alpha, 0, 1);
+    pub fn from_hsla(hue: Number, saturation: Number, luminance: Number, alpha: Number) -> Self {
+        let mut hue = clamp!(hue, 0, 360);
+        let saturation = clamp!(saturation, 0, 1);
+        let luminance = clamp!(luminance, 0, 1);
+        let alpha = clamp!(alpha, 0, 1);
 
         if saturation.clone() == Number::from(0) {
             let luminance = if luminance > Number::from(100) {

tests/color.rs

@@ -356,3 +356,23 @@ test!(
     "a {\n  color: complement(red);\n}\n",
     "a {\n  color: aqua;\n}\n"
 );
+test!(
+    mix_no_weight,
+    "a {\n  color: mix(#f00, #00f);\n}\n",
+    "a {\n  color: purple;\n}\n"
+);
+test!(
+    mix_weight_25,
+    "a {\n  color: mix(#f00, #00f, 25%);\n}\n",
+    "a {\n  color: #4000bf;\n}\n"
+);
+test!(
+    mix_opacity,
+    "a {\n  color: mix(rgba(255, 0, 0, 0.5), #00f);\n}\n",
+    "a {\n  color: rgba(64, 0, 191, 0.75);\n}\n"
+);
+test!(
+    mix_sanity_check,
+    "a {\n  color: mix(black, white);\n}\n",
+    "a {\n  color: gray;\n}\n"
+);