Implement builtin functions saturate() and desaturate()

src/builtin/color.rs

@@ -203,4 +203,28 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         Some(Value::Color(color.darken(amount)))
     });
+    decl!(f "saturate", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `saturate()`")
+        };
+        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.saturate(amount)))
+    });
+    decl!(f "desaturate", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `desaturate()`")
+        };
+        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.desaturate(amount)))
+    });
 }

src/color/mod.rs

@@ -157,6 +157,16 @@ impl Color {
         Color::from_hsla(hue, saturation, luminance - amount, alpha)
     }
 
+    pub fn saturate(&self, amount: Number) -> Self {
+        let (hue, saturation, luminance, alpha) = self.as_hsla();
+        Color::from_hsla(hue, saturation + amount, luminance, alpha)
+    }
+
+    pub fn desaturate(&self, amount: Number) -> Self {
+        let (hue, saturation, luminance, alpha) = self.as_hsla();
+        Color::from_hsla(hue, saturation - amount, luminance, alpha)
+    }
+
     pub fn alpha(&self) -> Number {
         self.alpha.clone()
     }

tests/color.rs

@@ -252,3 +252,35 @@ test!(
     // blocked on recognizing when to use 3-hex over 6-hex
     "a {\n  color: #220000;\n}\n"
 );
+// blocked on better parsing of call args
+// test!(
+//     saturate_named_args,
+//     "a {\n  color: saturate($color: hsl(25, 100%, 80%), $amount: 30%);\n}\n",
+//     "a {\n  color: #ff6a00;\n}\n"
+// );
+test!(
+    saturate_basic,
+    "a {\n  color: saturate(hsl(120, 30%, 90%), 20%);\n}\n",
+    "a {\n  color: #d9f2d9;\n}\n"
+);
+test!(
+    saturate_3_hex,
+    "a {\n  color: saturate(#855, 20%);\n}\n",
+    "a {\n  color: #9e3f3f;\n}\n"
+);
+// blocked on better parsing of call args
+// test!(
+//     desaturate_named_args,
+//     "a {\n  color: desaturate($color: hsl(25, 100%, 80%), $amount: 30%);\n}\n",
+//     "a {\n  color: #ff6a00;\n}\n"
+// );
+test!(
+    desaturate_basic,
+    "a {\n  color: desaturate(hsl(120, 30%, 90%), 20%);\n}\n",
+    "a {\n  color: #e3e8e3;\n}\n"
+);
+test!(
+    desaturate_3_hex,
+    "a {\n  color: desaturate(#855, 20%);\n}\n",
+    "a {\n  color: #726b6b;\n}\n"
+);