From 8d99501918393ea0c046d721e6fa6015fe43b70f Mon Sep 17 00:00:00 2001
From: Arne Dußin
Date: Wed, 18 Nov 2020 21:54:41 +0100
Subject: Implement bounding box function

---
 src/math/line_segment.rs | 74 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 72 insertions(+), 2 deletions(-)

(limited to 'src/math/line_segment.rs')

diff --git a/src/math/line_segment.rs b/src/math/line_segment.rs
index 2b67783..e6ca70f 100644
--- a/src/math/line_segment.rs
+++ b/src/math/line_segment.rs
@@ -192,15 +192,57 @@ where
      * they are the same it follows that the three points are collinear.
      */
     match (b.y - a.y) * (c.x - b.x) - (b.x - a.x) * (c.y - b.y) {
-        q if q > T::zero() => TripletOrientation::Clockwise,
-        q if q < T::zero() => TripletOrientation::Counterclockwise,
+        q if q > T::zero() => TripletOrientation::Counterclockwise,
+        q if q < T::zero() => TripletOrientation::Clockwise,
         _ => TripletOrientation::Collinear,
     }
 }
 
+/// Calculate the angle between three points. Guaranteed to have 0 <= angle < 2Pi. The angle is
+/// calculated as the angle between a line from a to b and then from b to c, increasing
+/// counterclockwise.
+///
+/// # Panics
+/// If the length from a to b or the length from b to c is zero.
+pub(crate) fn triplet_angle<T>(a: Vec2<T>, b: Vec2<T>, c: Vec2<T>) -> T
+where
+    T: Scalar + Copy + ClosedSub + RealField + Zero,
+{
+    assert!(a != b);
+    assert!(b != c);
+
+    // Handle the extreme 0 and 180 degree cases
+    let orientation = triplet_orientation(a, b, c);
+    if orientation == TripletOrientation::Collinear {
+        if LineSegment::new(a, b).contains_collinear(c)
+            || LineSegment::new(b, c).contains_collinear(a)
+        {
+            return T::zero();
+        } else {
+            return T::pi();
+        }
+    }
+
+    // Calculate the vectors out of the points
+    let ba = a - b;
+    let bc = c - b;
+
+    // Calculate the angle between 0 and Pi.
+    let angle = ((ba * bc) / (ba.length() * bc.length())).acos();
+
+    // Make angle into a full circle angle by looking at the orientation of the triplet.
+    let angle = match orientation {
+        TripletOrientation::Counterclockwise => T::pi() + (T::pi() - angle),
+        _ => angle,
+    };
+
+    angle
+}
+
 #[cfg(test)]
 mod test {
     use super::*;
+    use std::f64::consts::PI;
 
     #[test]
     fn contains_collinear() {
@@ -214,4 +256,32 @@ mod test {
         assert!(!segment.contains_collinear(Vec2::new(3., 3.)));
         assert!(!segment.contains_collinear(Vec2::new(-1., 0.)));
     }
+
+    #[test]
+    fn triplet_angle() {
+        assert_eq!(
+            super::triplet_angle(Vec2::new(0., 0.), Vec2::new(0., -1.), Vec2::new(-1., -1.)),
+            1.5 * PI
+        );
+        assert_eq!(
+            super::triplet_angle(Vec2::new(2., 2.), Vec2::new(0., 0.), Vec2::new(-1., -1.)),
+            PI
+        );
+        assert_eq!(
+            super::triplet_angle(Vec2::new(3., 1.), Vec2::new(0., 0.), Vec2::new(6., 2.)),
+            0.
+        );
+        assert_eq!(
+            super::triplet_angle(Vec2::new(3., 1.), Vec2::new(0., 0.), Vec2::new(-6., -2.)),
+            PI
+        );
+        assert_eq!(
+            super::triplet_angle(Vec2::new(0., 1.), Vec2::new(0., 2.), Vec2::new(-3., 2.)),
+            0.5 * PI
+        );
+        assert_eq!(
+            super::triplet_angle(Vec2::new(3., 1.), Vec2::new(2., 2.), Vec2::new(0., 2.)),
+            0.75 * PI
+        );
+    }
 }
-- 
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