Implement builtin function darken()

src/builtin/color.rs

@@ -191,4 +191,16 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         Some(Value::Color(color.lighten(amount)))
     });
+    decl!(f "darken", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `darken()`")
+        };
+        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.darken(amount)))
+    });
 }

src/color/mod.rs

@@ -152,6 +152,11 @@ impl Color {
         Color::from_hsla(hue, saturation, luminance + amount, alpha)
     }
 
+    pub fn darken(&self, amount: Number) -> Self {
+        let (hue, saturation, luminance, alpha) = self.as_hsla();
+        Color::from_hsla(hue, saturation, luminance - amount, alpha)
+    }
+
     pub fn alpha(&self) -> Number {
         self.alpha.clone()
     }

tests/color.rs

@@ -234,3 +234,21 @@ test!(
     // blocked on recognizing when to use 3-hex over 6-hex
     "a {\n  color: #ee0000;\n}\n"
 );
+// blocked on better parsing of call args
+// test!(
+//     darken_named_args,
+//     "a {\n  color: darken($color: hsl(25, 100%, 80%), $amount: 30%);\n}\n",
+//     "a {\n  color: #ff6a00;\n}\n"
+// );
+test!(
+    darken_basic,
+    "a {\n  color: darken(hsl(25, 100%, 80%), 30%);\n}\n",
+    "a {\n  color: #ff6a00;\n}\n"
+);
+test!(
+    darken_3_hex,
+    "a {\n  color: darken(#800, 20%);\n}\n",
+    // eventually, this should become `#200`
+    // blocked on recognizing when to use 3-hex over 6-hex
+    "a {\n  color: #220000;\n}\n"
+);