Implement builtin function comparable()

src/builtin/math.rs

@@ -36,4 +36,16 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             _ => todo!("expected number in builtin function `abs()`")
         }
     });
+    decl!(f "comparable", |args, _| {
+        let unit1 = match arg!(args, 0, "number1").eval() {
+            Value::Dimension(_, u) => u,
+            _ => todo!("$number1: ___ is not a number.")
+        };
+        let unit2 = match arg!(args, 1, "number2").eval() {
+            Value::Dimension(_, u) => u,
+            _ => todo!("$number2: ____ is not a number.")
+        };
+
+        Some(Value::bool(unit1.comparable(&unit2)))
+    });
 }

src/units.rs

@@ -25,7 +25,6 @@ pub(crate) enum Unit {
     Rem,
     /// Line height of the element
     Lh,
-    Percent,
     /// x-height of the element's font
     Ex,
     /// The advance measure (width) of the glyph "0" of the element's font
@@ -86,6 +85,7 @@ pub(crate) enum Unit {
     // Other units
     /// Represents a fraction of the available space in the grid container
     Fr,
+    Percent,
 
     /// Unknown unit
     Unknown(String),
@@ -93,6 +93,69 @@ pub(crate) enum Unit {
     None,
 }
 
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub(crate) enum UnitKind {
+    Absolute,
+    FontRelative,
+    ViewportRelative,
+    Angle,
+    Time,
+    Frequency,
+    Resolution,
+    Other,
+    None,
+}
+
+impl Unit {
+    pub fn comparable(&self, other: &Unit) -> bool {
+        match self.kind() {
+            UnitKind::FontRelative | UnitKind::ViewportRelative | UnitKind::Other => self == other,
+            UnitKind::None => true,
+            u => other.kind() == u || other.kind() == UnitKind::None,
+        }
+    }
+
+    pub fn kind(&self) -> UnitKind {
+        match self {
+            Unit::Px
+            | Unit::Mm
+            | Unit::In
+            | Unit::Cm
+            | Unit::Q
+            | Unit::Pt
+            | Unit::Pc => UnitKind::Absolute,
+            Unit::Em
+            | Unit::Rem
+            | Unit::Lh
+            | Unit::Ex
+            | Unit::Ch
+            | Unit::Cap
+            | Unit::Ic
+            | Unit::Rlh => UnitKind::FontRelative,
+            Unit::Vw
+            | Unit::Vh
+            | Unit::Vmin
+            | Unit::Vmax
+            | Unit::Vi
+            | Unit::Vb => UnitKind::ViewportRelative,
+            Unit::Deg
+            | Unit::Grad
+            | Unit::Rad
+            | Unit::Turn => UnitKind::Angle,
+            Unit::S
+            | Unit::Ms => UnitKind::Time,
+            Unit::Hz
+            | Unit::Khz => UnitKind::Frequency,
+            Unit::Dpi
+            | Unit::Dpcm
+            | Unit::Dppx
+            | Unit::X => UnitKind::Resolution,
+            Unit::None => UnitKind::None,
+            _ => UnitKind::Other,
+        }
+    }
+}
+
 impl From<&String> for Unit {
     fn from(unit: &String) -> Self {
         match unit.to_ascii_lowercase().as_bytes() {

tests/math.rs

@@ -63,3 +63,33 @@ test!(
     "a {\n  color: abs(-10px);\n}\n",
     "a {\n  color: 10px;\n}\n"
 );
+test!(
+    comparable_unitless,
+    "a {\n  color: comparable(1, 2);\n}\n",
+    "a {\n  color: true;\n}\n"
+);
+test!(
+    comparable_none_px,
+    "a {\n  color: comparable(1, 2px);\n}\n",
+    "a {\n  color: true;\n}\n"
+);
+test!(
+    comparable_px_px,
+    "a {\n  color: comparable(1px, 2px);\n}\n",
+    "a {\n  color: true;\n}\n"
+);
+test!(
+    comparable_absolute,
+    "a {\n  color: comparable(1px, 2in);\n}\n",
+    "a {\n  color: true;\n}\n"
+);
+test!(
+    comparable_absolute_font_relative,
+    "a {\n  color: comparable(1px, 2em);\n}\n",
+    "a {\n  color: false;\n}\n"
+);
+test!(
+    comparable_named,
+    "a {\n  color: comparable($number1: 1, $number2: 2);\n}\n",
+    "a {\n  color: true;\n}\n"
+);