Implement builtin function adjust-hue

src/builtin/color.rs

@@ -166,4 +166,17 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             _ => todo!("non-color given to builtin function `alpha()`")
         }
     });
+    decl!(f "adjust-hue", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `adjust-hue()`")
+        };
+        let degrees = match arg!(args, 1, "degrees").eval() {
+            Value::Dimension(n, Unit::None)
+            | Value::Dimension(n, Unit::Percent)
+            | Value::Dimension(n, Unit::Deg) => n,
+            _ => todo!("expected either unitless or % number for alpha"),
+        };
+        Some(Value::Color(color.adjust_hue(degrees)))
+    });
 }

src/color/mod.rs

@@ -102,6 +102,13 @@ impl Color {
         (((min + max) / Number::from(2)) * Number::from(100)).round()
     }
 
+    pub fn adjust_hue(&self, degrees: Number) -> Self {
+        let hue = self.hue();
+        let saturation = Number::ratio(self.saturation(), 100);
+        let luminance = Number::ratio(self.lightness(), 100);
+        Color::from_hsla(hue + degrees, saturation, luminance, self.alpha())
+    }
+
     pub fn alpha(&self) -> Number {
         self.alpha.clone()
     }

src/value/number.rs

@@ -58,6 +58,12 @@ impl From<BigInt> for Number {
     }
 }
 
+impl From<Number> for BigInt {
+    fn from(b: Number) -> Self {
+        b.to_integer()
+    }
+}
+
 macro_rules! from_integer {
     ($ty:ty) => {
         impl From<$ty> for Number {

tests/color.rs

@@ -196,3 +196,24 @@ test!(
     "a {\n  color: invert(white, 20);\n}\n",
     "a {\n  color: #cccccc;\n}\n"
 );
+test!(
+    adjust_hue_positive,
+    "a {\n  color: adjust-hue(hsl(120, 30%, 90%), 60deg);\n}\n",
+    "a {\n  color: #deeded;\n}\n"
+);
+test!(
+    adjust_hue_negative,
+    "a {\n  color: adjust-hue(hsl(120, 30%, 90%), -60deg);\n}\n",
+    "a {\n  color: #ededde;\n}\n"
+);
+test!(
+    adjust_hue_3_hex,
+    "a {\n  color: adjust-hue(#811, 45deg);\n}\n",
+    "a {\n  color: #886a11;\n}\n"
+);
+// blocked on better parsing of call args
+// test!(
+//     adjust_hue_named_args,
+//     "a {\n  color: adjust-hue($color: hsl(120, 30%, 90%), $degrees: 60deg);\n}\n",
+//     "a {\n  color: #deeded;\n}\n"
+// );