[librsvg: 15/35] Add a Length.to_points function

[Marge Bot <marge-bot src gnome org>]

commit ba635fb087804b774fc4237b892943c61a210891
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon Jun 14 18:15:43 2021 -0500

    Add a Length.to_points function
    
    Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/547>

 src/length.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)
---
diff --git a/src/length.rs b/src/length.rs
index 42b9a072..21e0d8e9 100644
--- a/src/length.rs
+++ b/src/length.rs
@@ -425,6 +425,41 @@ impl<N: Normalize, V: Validate> CssLength<N, V> {
             }
         }
     }
+
+    /// Converts a Length to points.  Pixels are taken to be respect with the DPI.
+    ///
+    /// # Panics
+    ///
+    /// Will panic if the length is in Percent, Em, or Ex units.
+    pub fn to_points(&self, params: &NormalizeParams) -> f64 {
+        match self.unit {
+            LengthUnit::Px => {
+                self.length / <N as Normalize>::normalize(params.dpi.x, params.dpi.y) * 72.0
+            }
+
+            LengthUnit::Percent => {
+                panic!("Cannot convert a percentage length into points");
+            }
+
+            LengthUnit::Em => {
+                panic!("Cannot convert an Em length into points");
+            }
+
+            LengthUnit::Ex => {
+                panic!("Cannot convert an Ex length into points");
+            }
+
+            LengthUnit::In => self.length * POINTS_PER_INCH,
+
+            LengthUnit::Cm => self.length / CM_PER_INCH * POINTS_PER_INCH,
+
+            LengthUnit::Mm => self.length / MM_PER_INCH * POINTS_PER_INCH,
+
+            LengthUnit::Pt => self.length,
+
+            LengthUnit::Pc => self.length / PICA_PER_INCH * POINTS_PER_INCH,
+        }
+    }
 }
 
 fn font_size_from_values(values: &ComputedValues, dpi: Dpi) -> f64 {
@@ -639,4 +674,18 @@ mod tests {
             6.0
         );
     }
+
+    #[test]
+    fn to_points_works() {
+        let params = NormalizeParams::from_dpi(Dpi::new(40.0, 96.0));
+
+        assert_approx_eq_cairo!(
+            Length::<Horizontal>::new(80.0, LengthUnit::Px).to_points(&params),
+            2.0 * 72.0
+        );
+        assert_approx_eq_cairo!(
+            Length::<Vertical>::new(192.0, LengthUnit::Px).to_points(&params),
+            2.0 * 72.0
+        );
+    }
 }