From 44167beff5cbb8005751aee26498e55adb146895 Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Fri, 31 May 2019 16:01:53 -0700
Subject: [PATCH] Implement `Path2D::arc_to`.

---
 canvas/src/lib.rs       | 19 +++++++++++++++++++
 geometry/src/outline.rs | 10 +++++-----
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/canvas/src/lib.rs b/canvas/src/lib.rs
index 1a0163a6..674bc2d6 100644
--- a/canvas/src/lib.rs
+++ b/canvas/src/lib.rs
@@ -14,6 +14,7 @@ use font_kit::family_name::FamilyName;
 use font_kit::hinting::HintingOptions;
 use font_kit::properties::Properties;
 use font_kit::source::SystemSource;
+use pathfinder_geometry::basic::line_segment::LineSegmentF;
 use pathfinder_geometry::basic::point::Point2DF;
 use pathfinder_geometry::basic::rect::RectF;
 use pathfinder_geometry::basic::transform2d::Transform2DF;
@@ -341,6 +342,24 @@ impl Path2D {
         self.current_contour.push_arc(&transform, start_angle, end_angle);
     }
 
+    #[inline]
+    pub fn arc_to(&mut self, ctrl: Point2DF, to: Point2DF, radius: f32) {
+        // FIXME(pcwalton): What should we do if there's no initial point?
+        let from = self.current_contour.last_position().unwrap_or_default();
+        let (v0, v1) = (from - ctrl, to - ctrl);
+        let (vu0, vu1) = (v0.normalize(), v1.normalize());
+        let hypot = radius / f32::sqrt(0.5 * (1.0 - vu0.dot(vu1)));
+        let bisector = vu0 + vu1;
+        let center = ctrl + bisector.scale(hypot / bisector.length());
+
+        let mut transform = Transform2DF::from_scale(Point2DF::splat(radius));
+        transform = transform.post_mul(&Transform2DF::from_translation(center));
+
+        let chord = LineSegmentF::new(vu0.yx().scale_xy(Point2DF::new(-1.0, 1.0)),
+                                      vu1.yx().scale_xy(Point2DF::new(1.0, -1.0)));
+        self.current_contour.push_arc_from_unit_chord(&transform, chord);
+    }
+
     pub fn rect(&mut self, rect: RectF) {
         self.flush_current_contour();
         self.current_contour.push_endpoint(rect.origin());
diff --git a/geometry/src/outline.rs b/geometry/src/outline.rs
index 2cb65a13..a0e1c374 100644
--- a/geometry/src/outline.rs
+++ b/geometry/src/outline.rs
@@ -269,13 +269,13 @@ impl Contour {
     }
 
     #[inline]
-    pub(crate) fn position_of_last(&self, index: u32) -> Point2DF {
-        self.points[self.points.len() - index as usize]
+    pub fn last_position(&self) -> Option<Point2DF> {
+        self.points.last().cloned()
     }
 
     #[inline]
-    pub(crate) fn last_position(&self) -> Option<Point2DF> {
-        self.points.last().cloned()
+    pub(crate) fn position_of_last(&self, index: u32) -> Point2DF {
+        self.points[self.points.len() - index as usize]
     }
 
     #[inline]
@@ -409,7 +409,7 @@ impl Contour {
                 break;
             }
 
-            vector = sweep_vector.rotate_by(vector);
+            vector = vector.rotate_by(sweep_vector);
         }
 
         const EPSILON: f32 = 0.001;