From cbf64d599f85f32751b4e9b1901f8e2fbc4165b2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jochen=20G=C3=B6rtler?= <me@jgoertler.com>
Date: Fri, 29 Nov 2024 12:35:25 +0100
Subject: [PATCH] Add new `Rect::intersects_ray_from_center` method

---
 crates/emath/src/rect.rs | 77 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/crates/emath/src/rect.rs b/crates/emath/src/rect.rs
index 6c0677ad55e..f4217ed89fb 100644
--- a/crates/emath/src/rect.rs
+++ b/crates/emath/src/rect.rs
@@ -671,6 +671,30 @@ impl Rect {
 
         0.0 <= tmax && tmin <= tmax
     }
+
+    /// Where does a ray from the center intersect the rectangle?
+    ///
+    /// `d` is the direction of the ray and assumed to be normalized.
+    pub fn intersects_ray_from_center(&self, d: Vec2) -> Pos2 {
+        let mut tmin = f32::NEG_INFINITY;
+        let mut tmax = f32::INFINITY;
+
+        for i in 0..2 {
+            let inv_d = 1.0 / -d[i];
+            let mut t0 = (self.min[i] - self.center()[i]) * inv_d;
+            let mut t1 = (self.max[i] - self.center()[i]) * inv_d;
+
+            if inv_d < 0.0 {
+                std::mem::swap(&mut t0, &mut t1);
+            }
+
+            tmin = tmin.max(t0);
+            tmax = tmax.min(t1);
+        }
+
+        let t = tmax.min(tmin); // Pick the first intersection
+        self.center() + t * -d
+    }
 }
 
 impl fmt::Debug for Rect {
@@ -793,4 +817,57 @@ mod tests {
         println!("Leftward ray from right:");
         assert!(rect.intersects_ray(pos2(4.0, 2.0), Vec2::LEFT));
     }
+
+    #[test]
+    fn test_ray_from_center_intersection() {
+        let rect = Rect::from_min_max(pos2(1.0, 1.0), pos2(3.0, 3.0));
+
+        assert_eq!(
+            rect.intersects_ray_from_center(Vec2::RIGHT),
+            pos2(3.0, 2.0),
+            "rightward ray"
+        );
+
+        assert_eq!(
+            rect.intersects_ray_from_center(Vec2::UP),
+            pos2(2.0, 1.0),
+            "upward ray"
+        );
+
+        assert_eq!(
+            rect.intersects_ray_from_center(Vec2::LEFT),
+            pos2(1.0, 2.0),
+            "leftward ray"
+        );
+
+        assert_eq!(
+            rect.intersects_ray_from_center(Vec2::DOWN),
+            pos2(2.0, 3.0),
+            "downward ray"
+        );
+
+        assert_eq!(
+            rect.intersects_ray_from_center((Vec2::LEFT + Vec2::DOWN).normalized()),
+            pos2(1.0, 3.0),
+            "bottom-left corner ray"
+        );
+
+        assert_eq!(
+            rect.intersects_ray_from_center((Vec2::LEFT + Vec2::UP).normalized()),
+            pos2(1.0, 1.0),
+            "top-left corner ray"
+        );
+
+        assert_eq!(
+            rect.intersects_ray_from_center((Vec2::RIGHT + Vec2::DOWN).normalized()),
+            pos2(3.0, 3.0),
+            "bottom-right corner ray"
+        );
+
+        assert_eq!(
+            rect.intersects_ray_from_center((Vec2::RIGHT + Vec2::UP).normalized()),
+            pos2(3.0, 1.0),
+            "top-right corner ray"
+        );
+    }
 }