Implement builtin functions fade-in(), fade-out(), opacify(), and transparentize()

src/builtin/color.rs

@@ -227,4 +227,52 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         Some(Value::Color(color.desaturate(amount)))
     });
+    decl!(f "opacify", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `opacify()`")
+        };
+        let amount = match arg!(args, 1, "amount").eval() {
+            Value::Dimension(n, Unit::None) => n,
+            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
+            _ => todo!("expected either unitless or % number for amount"),
+        };
+        Some(Value::Color(color.fade_in(amount)))
+    });
+    decl!(f "fade-in", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `fade-in()`")
+        };
+        let amount = match arg!(args, 1, "amount").eval() {
+            Value::Dimension(n, Unit::None) => n,
+            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
+            _ => todo!("expected either unitless or % number for amount"),
+        };
+        Some(Value::Color(color.fade_in(amount)))
+    });
+    decl!(f "transparentize", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `transparentize()`")
+        };
+        let amount = match arg!(args, 1, "amount").eval() {
+            Value::Dimension(n, Unit::None) => n,
+            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
+            _ => todo!("expected either unitless or % number for amount"),
+        };
+        Some(Value::Color(color.fade_out(amount)))
+    });
+    decl!(f "fade-out", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `fade-out()`")
+        };
+        let amount = match arg!(args, 1, "amount").eval() {
+            Value::Dimension(n, Unit::None) => n,
+            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
+            _ => todo!("expected either unitless or % number for amount"),
+        };
+        Some(Value::Color(color.fade_out(amount)))
+    });
 }

src/color/mod.rs

@@ -171,6 +171,20 @@ impl Color {
         self.alpha.clone()
     }
 
+    /// Makes a color more opaque.
+    /// Takes a color and a number between 0 and 1,
+    /// and returns a color with the opacity increased by that amount.
+    pub fn fade_in(self, amount: Number) -> Self {
+        Color::from_rgba(self.red, self.green, self.blue, self.alpha + amount)
+    }
+
+    /// Makes a color more transparent.
+    /// Takes a color and a number between 0 and 1,
+    /// and returns a color with the opacity decreased by that amount.
+    pub fn fade_out(self, amount: Number) -> Self {
+        Color::from_rgba(self.red, self.green, self.blue, self.alpha - amount)
+    }
+
     /// Create RGBA representation from HSLA values
     /// Algorithm adapted from <http://www.niwa.nu/2013/05/math-behind-colorspace-conversions-rgb-hsl/>
     pub fn from_hsla(

tests/color.rs

@@ -309,3 +309,23 @@ test!(
     "a {\n  color: desaturate(plum, 14%);\n}\n",
     "a {\n  color: #d4a9d4;\n}\n"
 );
+test!(
+    transparentize,
+    "a {\n  color: transparentize(rgba(0, 0, 0, 0.5), 0.1);\n}\n",
+    "a {\n  color: rgba(0, 0, 0, 0.4);\n}\n"
+);
+test!(
+    fade_out,
+    "a {\n  color: fade-out(rgba(0, 0, 0, 0.8), 0.2);\n}\n",
+    "a {\n  color: rgba(0, 0, 0, 0.6);\n}\n"
+);
+test!(
+    opacify,
+    "a {\n  color: opacify(rgba(0, 0, 0, 0.5), 0.1);\n}\n",
+    "a {\n  color: rgba(0, 0, 0, 0.6);\n}\n"
+);
+test!(
+    fade_in,
+    "a {\n  color: opacify(rgba(0, 0, 17, 0.8), 0.2);\n}\n",
+    "a {\n  color: #000011;\n}\n"
+);