From d7c4dea2cb96742ac71d6ffd20f2b82ad2007ffc Mon Sep 17 00:00:00 2001
From: Lily Iliana Luna Ylva Anderson Grigaitis <lilylanderson@zoho.com>
Date: Wed, 27 Aug 2025 17:16:17 -0500
Subject: [PATCH] Code to extract the point pair from a bivector and get the
 intersection points of two circles.

---
 src/lib.rs | 52 +++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 41 insertions(+), 11 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 64f103e..0754f87 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -348,20 +348,42 @@ fn _get_hyperbolic_translation(vec: &CGA) -> CGA {
     t
 }
 
-enum _CircleIntersection {
+enum CircleIntersection {
     TwoPoints(CGA, CGA),
-    OnePoint(CGA),
+    //OnePoint(CGA),
     None,
 }
 
-/*fn get_circle_intersection(circle_1: &CGA, circle_2: &CGA) -> CircleIntersection {
-    if !circle_intersection(;//circle_1, circle_2) {
+fn extract_point_pair(pair: &CGA) -> (CGA, CGA) {
+    let n_vec = CGA::e4() + CGA::e5();
+    let pair = grade_selection(&pair, 2); //Examine only the bivector part
+    let f = pair.normalized();
+    let p = CGA::new(0.5, SCALAR) + 0.5*&f; //Projection Operator
+    let p_rev = CGA::new(0.5, SCALAR) - 0.5*&f; //Projection Operator
+    let diff = &pair | &n_vec; //point A - point B (or a scalar multiple)
+    let a = -1. * &p_rev * &diff;
+    let b = 1. * &p * &diff;
+    (a,b)
+}
+
+fn get_circle_intersection(circle_1: &CGA, circle_2: &CGA) -> CircleIntersection {
+    if !circle_intersection(circle_1, circle_2) {
         return CircleIntersection::None;
     }
-    let n = CGA::e4() + CGA::e5();
-}*/
+    //Assumes circles intersect in two places.
+    let circle_1 = grade_selection(circle_1, 3); //Explicitly check only trivector part
+    let circle_2 = grade_selection(circle_2, 3);
+    let i4 = CGA::e1245(); //xy plane in 3D CGA.
+    let ab = &circle_1 * &circle_2;
+    let ba = &circle_2 * &circle_1;
+    let comm = &ab - &ba; //Twice the commutator of circle_1 and circle_2. Unnormalized, but that isn't important.
+    let inter = &i4 * &comm; //The bivector of the pair of intersection points.
+    let (a,b) = extract_point_pair(&inter);
+    CircleIntersection::TwoPoints(a,b)
+}
 
-fn _circle_intersection(circle_1: &CGA, circle_2: &CGA) -> bool {
+
+fn circle_intersection(circle_1: &CGA, circle_2: &CGA) -> bool {
     let circle_1 = grade_selection(circle_1, 4);
     let circle_2 = grade_selection(circle_2, 4);
     let meet = circle_1 & circle_2;
@@ -472,16 +494,16 @@ fn circle_from_coords(x: f64, y: f64, r: f64) -> CGA {
     let a = point_to_cga(a_x, a_y);
     let b = point_to_cga(b_x, b_y);
     let c = point_to_cga(c_x, c_y);
-    let circle = a^b^c^CGA::e3();
+    let circle = a^b^c;
     circle
 }
 
 impl Asteroid {
     fn new() -> Asteroid {
-        let v = 0.1;
+        let v = 0.;
         let (v_x, v_y) = (v * 1.0_f64.cos(), v*1.0_f64.sin());
         let vel = v_x*CGA::e15() + v_y*CGA::e25();
-        let circle = circle_from_coords(0., 0., 0.);
+        let circle = circle_from_coords(0.9, 0., 0.1);
         Asteroid { circle, vel }
     }
 
@@ -539,7 +561,7 @@ impl Game {
             orientation: CGA::new(1., SCALAR),
         };
 
-        let boundary = circle_from_coords(0., 0., 0.99);
+        let boundary = circle_from_coords(0., 0., 0.9);
         Game { 
             context: context,
             ship,
@@ -561,6 +583,14 @@ impl Game {
         draw_line(&self.context, &self.boundary);
         self.ship.draw(&self.context);
         for asteroid in &self.asteroids {
+            let pair = get_circle_intersection(&asteroid.circle, &self.boundary);
+            match pair {
+                CircleIntersection::TwoPoints(a,b) => {
+                    draw_point(&self.context, &a);
+                    draw_point(&self.context, &b);
+                },
+                _ => {}
+            }
             asteroid.draw(&self.context);
         }
     }