Add Arc::flipped (#367)

CHANGELOG.md

@@ -20,6 +20,7 @@ This release has an [MSRV][] of 1.65.
 - Add `From (f32, f32)` for `Point`. ([#339] by [@rsheeter])
 - Add `Rect::overlaps` and `Rect::contains_rect`. ([#347] by [@nils-mathieu])
 - Add `CubicBez::tangents` ([#288] by [@raphlinus])
+- Add `Arc::flipped`. ([#367] by [@waywardmonkeys])
 
 ### Changed
 
@@ -55,6 +56,7 @@ Note: A changelog was not kept for or before this release
 [#354]: https://github.com/linebender/kurbo/pull/354
 [#356]: https://github.com/linebender/kurbo/pull/356
 [#361]: https://github.com/linebender/kurbo/pull/361
+[#367]: https://github.com/linebender/kurbo/pull/367
 
 [Unreleased]: https://github.com/linebender/kurbo/compare/v0.11.0...HEAD
 [0.11.0]: https://github.com/linebender/kurbo/releases/tag/v0.11.0

src/arc.rs

@@ -49,6 +49,22 @@ impl Arc {
         }
     }
 
+    /// Return a copy of this `Arc` in the opposite direction.
+    ///
+    /// The new `Arc` will sweep towards the original `Arc`s
+    /// start angle.
+    #[must_use]
+    #[inline]
+    pub fn flipped(&self) -> Arc {
+        Self {
+            center: self.center,
+            radii: self.radii,
+            start_angle: self.start_angle + self.sweep_angle,
+            sweep_angle: -self.sweep_angle,
+            x_rotation: self.x_rotation,
+        }
+    }
+
     /// Create an iterator generating Bezier path elements.
     ///
     /// The generated elements can be appended to an existing bezier path.
@@ -206,3 +222,24 @@ impl Mul<Arc> for Affine {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn flipped_arc() {
+        let a = Arc::new((0., 0.), (1., 0.), 0., PI, 0.);
+        let f = a.flipped();
+
+        // Most fields should be unchanged:
+        assert_eq!(a.center, f.center);
+        assert_eq!(a.radii, f.radii);
+        assert_eq!(a.x_rotation, f.x_rotation);
+
+        // Sweep angle should be in reverse
+        assert_eq!(a.sweep_angle, -f.sweep_angle);
+
+        // Flipping it again should result in the original arc
+        assert_eq!(a, f.flipped());
+    }
+}