1 files changed, 112 insertions, 0 deletions
diff --git a/src/geom/em.rs b/src/geom/em.rs
new file mode 100644
index 00000000..45e5ba60
--- /dev/null
+++ b/src/geom/em.rs
@@ -0,0 +1,112 @@
+use super::*;
+
+/// A length that is relative to the font size.
+///
+/// `1em` is the same as the font size.
+#[derive(Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[derive(Serialize, Deserialize)]
+pub struct Em(N64);
+
+impl Em {
+    /// The zero length.
+    pub fn zero() -> Self {
+        Self(N64::from(0.0))
+    }
+
+    /// The font size.
+    pub fn one() -> Self {
+        Self(N64::from(1.0))
+    }
+
+    /// Create an font-relative length.
+    pub fn new(em: f64) -> Self {
+        Self(N64::from(em))
+    }
+
+    /// Create font units at the given units per em.
+    pub fn from_units(units: impl Into<f64>, units_per_em: f64) -> Self {
+        Self(N64::from(units.into() / units_per_em))
+    }
+
+    /// Convert to a length at the given font size.
+    pub fn to_length(self, font_size: Length) -> Length {
+        self.get() * font_size
+    }
+
+    /// The number of em units.
+    pub fn get(self) -> f64 {
+        self.0.into()
+    }
+
+    /// Whether the length is zero.
+    pub fn is_zero(self) -> bool {
+        self.0 == 0.0
+    }
+}
+
+impl Debug for Em {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        Display::fmt(self, f)
+    }
+}
+
+impl Display for Em {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{}em", self.get())
+    }
+}
+
+impl Neg for Em {
+    type Output = Self;
+
+    fn neg(self) -> Self {
+        Self(-self.0)
+    }
+}
+
+impl Add for Em {
+    type Output = Self;
+
+    fn add(self, other: Self) -> Self {
+        Self(self.0 + other.0)
+    }
+}
+
+sub_impl!(Em - Em -> Em);
+
+impl Mul<f64> for Em {
+    type Output = Self;
+
+    fn mul(self, other: f64) -> Self {
+        Self(self.0 * other)
+    }
+}
+
+impl Mul<Em> for f64 {
+    type Output = Em;
+
+    fn mul(self, other: Em) -> Em {
+        other * self
+    }
+}
+
+impl Div<f64> for Em {
+    type Output = Self;
+
+    fn div(self, other: f64) -> Self {
+        Self(self.0 / other)
+    }
+}
+
+impl Div for Em {
+    type Output = f64;
+
+    fn div(self, other: Self) -> f64 {
+        self.get() / other.get()
+    }
+}
+
+assign_impl!(Em += Em);
+assign_impl!(Em -= Em);
+assign_impl!(Em *= f64);
+assign_impl!(Em /= f64);