Implement builtin function change-color()

src/builtin/color/other.rs

@@ -1,7 +1,77 @@
 use std::collections::BTreeMap;
 
 use super::Builtin;
+use crate::color::Color;
+use crate::units::Unit;
+use crate::value::{Number, Value};
 
-pub(crate) fn register(_f: &mut BTreeMap<String, Builtin>) {
+pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
+    decl!(f "change-color", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `change-color()`")
+        };
 
+        let alpha = match arg!(args, -1, "alpha"=Value::Null).eval() {
+            Value::Dimension(n, Unit::None) => Some(n),
+            Value::Dimension(n, Unit::Percent) => Some(n / Number::from(100)),
+            Value::Null => None,
+            _ => todo!("expected either unitless or % number for $alpha")
+        };
+
+        let red = match arg!(args, -1, "red"=Value::Null).eval() {
+            Value::Dimension(n, Unit::None) => Some(n),
+            Value::Dimension(n, Unit::Percent) => Some(n / Number::from(100)),
+            Value::Null => None,
+            _ => todo!("expected either unitless or % number for $red")
+        };
+        let green = match arg!(args, -1, "green"=Value::Null).eval() {
+            Value::Dimension(n, Unit::None) => Some(n),
+            Value::Dimension(n, Unit::Percent) => Some(n / Number::from(100)),
+            Value::Null => None,
+            _ => todo!("expected either unitless or % number for $green")
+        };
+        let blue = match arg!(args, -1, "blue"=Value::Null).eval() {
+            Value::Dimension(n, Unit::None) => Some(n),
+            Value::Dimension(n, Unit::Percent) => Some(n / Number::from(100)),
+            Value::Null => None,
+            _ => todo!("expected either unitless or % number for $blue")
+        };
+
+        if !red.is_none() || !green.is_none() || !blue.is_none() {
+            return Some(Value::Color(Color::from_rgba(red.unwrap_or(color.red()), green.unwrap_or(color.green()), blue.unwrap_or(color.blue()), alpha.unwrap_or(color.alpha()))))
+        }
+
+        let hue = match arg!(args, -1, "hue"=Value::Null).eval() {
+            Value::Dimension(n, Unit::None)
+            | Value::Dimension(n, Unit::Percent)
+            | Value::Dimension(n, Unit::Deg) => Some(n),
+            Value::Null => None,
+            _ => todo!("expected either unitless or % number for hue"),
+        };
+        let saturation = match arg!(args, -1, "saturation"=Value::Null).eval() {
+            Value::Dimension(n, Unit::None)
+            | Value::Dimension(n, Unit::Percent) => Some(n / Number::from(100)),
+            Value::Null => None,
+            _ => todo!("expected either unitless or % number for saturation"),
+        };
+        let luminance = match arg!(args, -1, "lightness"=Value::Null).eval() {
+            Value::Dimension(n, Unit::None)
+            | Value::Dimension(n, Unit::Percent) => Some(n / Number::from(100)),
+            Value::Null => None,
+            _ => todo!("expected either unitless or % number for luminance"),
+        };
+
+        if !hue.is_none() || !saturation.is_none() || !luminance.is_none() {
+            // Color::as_hsla() returns more exact values than Color::hue(), etc.
+            let (this_hue, this_saturation, this_luminance, this_alpha) = color.as_hsla();
+            return Some(Value::Color(Color::from_hsla(hue.unwrap_or(this_hue), saturation.unwrap_or(this_saturation), luminance.unwrap_or(this_luminance), alpha.unwrap_or(this_alpha))))
+        }
+
+        Some(Value::Color(if !alpha.is_none() {
+            color.with_alpha(alpha.unwrap())
+        } else {
+            color
+        }))
+    });
 }

src/color/mod.rs

@@ -179,7 +179,7 @@ impl Color {
         (((min + max) / Number::from(2)) * Number::from(100)).round()
     }
 
-    fn as_hsla(&self) -> (Number, Number, Number, Number) {
+    pub fn as_hsla(&self) -> (Number, Number, Number, Number) {
         let red = self.red.clone() / Number::from(255);
         let green = self.green.clone() / Number::from(255);
         let blue = self.blue.clone() / Number::from(255);

tests/color.rs

@@ -376,3 +376,18 @@ test!(
     "a {\n  color: mix(black, white);\n}\n",
     "a {\n  color: gray;\n}\n"
 );
+test!(
+    change_color_blue,
+    "a {\n  color: change-color(#102030, $blue: 5);\n}\n",
+    "a {\n  color: #102005;\n}\n"
+);
+test!(
+    change_color_red_blue,
+    "a {\n  color: change-color(#102030, $red: 120, $blue: 5);\n}\n",
+    "a {\n  color: #782005;\n}\n"
+);
+test!(
+    change_color_lum_alpha,
+    "a {\n  color: change-color(hsl(25, 100%, 80%), $lightness: 40%, $alpha: 0.8);\n}\n",
+    "a {\n  color: rgba(204, 85, 0, 0.8);\n}\n"
+);