diff --git a/src/bin/flat.rs b/src/bin/flat.rs
index 4c8fb38..58bfc74 100644
--- a/src/bin/flat.rs
+++ b/src/bin/flat.rs
@@ -124,6 +124,46 @@ impl Metric for Rect {
 	}
 }
 
+mod boundary {
+	use glam::*;
+
+	struct Id(u8);
+
+	trait Boundary {
+		fn next(&self, base: Vec2, dir: Vec2, limit: f32) -> Option<(Id, Vec2, Vec2)>;
+	}
+
+	struct Loop(Vec<Vec2>);
+
+	impl Loop {
+		fn hit(&self, base: Vec2, dir: Vec2) -> Option<(usize, f32)> {
+			self.0.iter().enumerate().filter_map(|(k, &a)| {
+				let b = self.0[(k + 1) % self.0.len()];
+				let u = mat2(a - base, dir).determinant();
+				let v = mat2(b - base, dir).determinant();
+				if u < 0.0 && v > 0.0 {
+					let dist = mat2(a - base, b - a).determinant() / mat2(dir, b - a).determinant();
+					if dist >= 0.0 { Some((k, dist)) } else { None }
+				} else {
+					None
+				}
+			}).min_by(|(k1, dist1), (k2, dist2)| dist1.total_cmp(dist2))
+		}
+	}
+
+	#[test]
+	fn test_loop() {
+		let tri = Loop(vec![vec2(-1.0, -1.0), vec2(1.0, -1.0), vec2(0.0, 1.0)]);
+		assert_eq!(tri.hit(vec2(0.0, -2.0), vec2(0.0, 1.0)), Some((0, 1.0)));
+		assert_eq!(tri.hit(vec2(0.0, -2.0), vec2(0.0, 0.5)), Some((0, 2.0)));
+		assert_eq!(tri.hit(vec2(0.0, -2.0), vec2(1.0, 1.0)), None);
+		assert_eq!(tri.hit(vec2(0.0, 0.0), vec2(0.0, 1.0)), None);
+		assert_eq!(tri.hit(vec2(0.0, 0.0), vec2(0.0, -1.0)), None);
+		assert_eq!(tri.hit(vec2(-1.5, 0.5), vec2(2.0, -1.0)), Some((2, 0.5)));
+		assert_eq!(tri.hit(vec2(-1.5, 0.5), vec2(-2.0, 1.0)), None);
+	}
+}
+
 mod riemann {
 	use glam::*;