Clean up Arc

crates/fj-math/src/arc.rs

@@ -1,23 +1,25 @@
-use crate::{Point, Scalar};
+use num_traits::Float;
+
+use crate::{Point, Scalar, Vector};
 
 /// Calculated geometry that is useful when dealing with an arc
 pub struct Arc {
-    /// Start point of the arc
-    pub start: Point<2>,
-    /// End point of the arc
-    pub end: Point<2>,
     /// Center of the circle the arc is constructed on
     pub center: Point<2>,
+
     /// Radius of the circle the arc is constructed on
     pub radius: Scalar,
+
     /// Angle of `start` relative to `center`, in radians
     ///
     /// Guaranteed to be less than `end_angle`.
     pub start_angle: Scalar,
+
     /// Angle of `end` relative to `center`, in radians
     ///
     /// Guaranteed to be greater than `end_angle`.
     pub end_angle: Scalar,
+
     /// True if `start` and `end` were switched to ensure `end_angle` > `start_angle`
     pub flipped_construction: bool,
 }
@@ -27,14 +29,22 @@ impl Arc {
     pub fn from_endpoints_and_angle(
         p0: impl Into<Point<2>>,
         p1: impl Into<Point<2>>,
-        angle: Scalar,
+        angle_rad: Scalar,
     ) -> Self {
-        use num_traits::Float;
+        let p0 = p0.into();
+        let p1 = p1.into();
+
+        // This is an implementation of this solution:
+        // https://math.stackexchange.com/a/87374
 
-        let (p0, p1) = (p0.into(), p1.into());
+        let distance_between_endpoints = (p1 - p0).magnitude();
+        let radius = distance_between_endpoints
+            / (2. * (angle_rad.abs().into_f64() / 2.).sin());
+        let distance_center_to_midpoint =
+            (radius.powi(2) - (distance_between_endpoints.powi(2) / 4.)).sqrt();
 
-        let flipped_construction = angle <= Scalar::ZERO;
-        let angle_rad = angle.abs();
+        let flipped_construction = angle_rad <= Scalar::ZERO;
+        let angle_rad = angle_rad.abs();
 
         let [p0, p1] = if flipped_construction {
             [p1, p0]
@@ -47,27 +57,25 @@ impl Arc {
         } else {
             (Scalar::ONE, Scalar::ZERO)
         };
-        let [[x0, y0], [x1, y1]] = [p0, p1].map(|p| p.coords.components);
-        // https://math.stackexchange.com/questions/27535/how-to-find-center-of-an-arc-given-start-point-end-point-radius-and-arc-direc
-        // distance between endpoints
-        let d = ((x1 - x0).powi(2) + (y1 - y0).powi(2)).sqrt();
-        // radius
-        let r = d / (2. * (angle_rad.into_f64() / 2.).sin());
-        // distance from center to midpoint between endpoints
-        let h = (r.powi(2) - (d.powi(2) / 4.)).sqrt();
-        // (u, v) is the unit normal in the direction of p1 - p0
-        let u = (x1 - x0) / d * uv_factor;
-        let v = (y1 - y0) / d * uv_factor;
-        // (cx, cy) is the center of the circle
-        let cx = ((x0 + x1) / 2.) - h * v;
-        let cy = ((y0 + y1) / 2.) + h * u;
-        let start_angle = (y0 - cy).atan2(x0 - cx);
-        let end_angle = (y1 - cy).atan2(x1 - cx) + end_angle_offset;
+        let unit_vector_p0_to_p1 =
+            (p1 - p0) / distance_between_endpoints * uv_factor;
+        let unit_vector_midpoint_to_center =
+            Vector::from([-unit_vector_p0_to_p1.v, unit_vector_p0_to_p1.u]);
+        let center = Point {
+            coords: (p0.coords + p1.coords) / 2.
+                + unit_vector_midpoint_to_center * distance_center_to_midpoint,
+        };
+        let start_angle = {
+            let center_to_start = p0 - center;
+            center_to_start.v.atan2(center_to_start.u)
+        };
+        let end_angle = {
+            let center_to_end = p1 - center;
+            center_to_end.v.atan2(center_to_end.u) + end_angle_offset
+        };
         Self {
-            start: p0,
-            end: p1,
-            center: Point::from([cx, cy]),
-            radius: r,
+            center,
+            radius,
             start_angle,
             end_angle,
             flipped_construction,
@@ -77,39 +85,11 @@ impl Arc {
 
 #[cfg(test)]
 mod tests {
-    use crate::{Point, Scalar};
+    use crate::{Point, Scalar, Vector};
 
     use super::Arc;
 
-    use approx::AbsDiffEq;
-
-    fn check_arc_calculation(center: [f64; 2], radius: f64, a0: f64, a1: f64) {
-        let angle = a1 - a0;
-
-        let p0 = [center[0] + radius * a0.cos(), center[1] + radius * a0.sin()];
-        let p1 = [center[0] + radius * a1.cos(), center[1] + radius * a1.sin()];
-
-        let arc = Arc::from_endpoints_and_angle(p0, p1, Scalar::from(angle));
-
-        let epsilon = Scalar::default_epsilon() * 10.;
-
-        dbg!(center, arc.center);
-        dbg!(arc.start_angle);
-        dbg!(arc.end_angle);
-        dbg!(arc.flipped_construction);
-        assert!(arc.center.abs_diff_eq(&Point::from(center), epsilon));
-        assert!(arc.radius.abs_diff_eq(&Scalar::from(radius), epsilon));
-
-        if a0 < a1 {
-            assert!(!arc.flipped_construction);
-            assert!(arc.start_angle.abs_diff_eq(&Scalar::from(a0), epsilon));
-            assert!(arc.end_angle.abs_diff_eq(&Scalar::from(a1), epsilon));
-        } else {
-            assert!(arc.flipped_construction);
-            assert!(arc.end_angle.abs_diff_eq(&Scalar::from(a0), epsilon));
-            assert!(arc.start_angle.abs_diff_eq(&Scalar::from(a1), epsilon));
-        }
-    }
+    use approx::{assert_abs_diff_eq, AbsDiffEq};
 
     #[test]
     fn arc_construction() {
@@ -144,4 +124,57 @@ mod tests {
             270_f64.to_radians(),
         );
     }
+
+    fn check_arc_calculation(
+        center: impl Into<Point<2>>,
+        radius: f64,
+        a0: f64,
+        a1: f64,
+    ) {
+        let center = center.into();
+        let angle = a1 - a0;
+
+        let p0 = center + Vector::from([a0.cos(), a0.sin()]) * radius;
+        let p1 = center + Vector::from([a1.cos(), a1.sin()]) * radius;
+
+        let arc = Arc::from_endpoints_and_angle(p0, p1, Scalar::from(angle));
+
+        let epsilon = Scalar::default_epsilon() * 10.;
+
+        dbg!(arc.start_angle);
+        dbg!(arc.end_angle);
+        dbg!(arc.flipped_construction);
+        assert_abs_diff_eq!(arc.center, center, epsilon = epsilon);
+        assert_abs_diff_eq!(
+            arc.radius,
+            Scalar::from(radius),
+            epsilon = epsilon
+        );
+
+        if a0 < a1 {
+            assert!(!arc.flipped_construction);
+            assert_abs_diff_eq!(
+                arc.start_angle,
+                Scalar::from(a0),
+                epsilon = epsilon
+            );
+            assert_abs_diff_eq!(
+                arc.end_angle,
+                Scalar::from(a1),
+                epsilon = epsilon
+            );
+        } else {
+            assert!(arc.flipped_construction);
+            assert_abs_diff_eq!(
+                arc.end_angle,
+                Scalar::from(a0),
+                epsilon = epsilon
+            );
+            assert_abs_diff_eq!(
+                arc.start_angle,
+                Scalar::from(a1),
+                epsilon = epsilon
+            );
+        }
+    }
 }