Implement builtin function complement()

src/builtin/color_hsl.rs

@@ -135,6 +135,13 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         Some(Value::Color(color.desaturate(Number::from(1))))
     });
+    decl!(f "complement", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `complement()`")
+        };
+        Some(Value::Color(color.complement()))
+    });
     decl!(f "invert", |args, _| {
         let weight = match arg!(args, 1, "weight"=Value::Dimension(Number::from(100), Unit::Percent)) {
             Value::Dimension(n, Unit::None)

src/color/mod.rs

@@ -340,6 +340,16 @@ impl Color {
             repr,
         }
     }
+
+    pub fn complement(&self) -> Self {
+        let (hue, saturation, luminance, alpha) = self.as_hsla();
+        let hue = if hue > Number::from(180) {
+            Number::from(360) - hue
+        } else {
+            hue + Number::from(180)
+        };
+        Color::from_hsla(hue, saturation, luminance, alpha)
+    }
 }
 
 /// Get the proper representation from RGBA values

tests/color.rs

@@ -351,3 +351,8 @@ test!(
     "a {\n  color: grayscale(red);\n}\n",
     "a {\n  color: gray;\n}\n"
 );
+test!(
+    complement,
+    "a {\n  color: complement(red);\n}\n",
+    "a {\n  color: aqua;\n}\n"
+);