From c3c455fbbf4c4babcb8fb4ea7f6ed7f0c76edf6b Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Sun, 25 Oct 2026 17:25:34 +0100
Subject: [PATCH] Region: test very long segments against every segment so no
 crossing is missed (#929) (#975)

---
 crates/geom/src/region.rs | 61 ++++++++++++++++++++++++++++-----------
 1 file changed, 44 insertions(+), 17 deletions(-)

diff --git a/crates/geom/src/region.rs b/crates/geom/src/region.rs
index 98050cfdc..bef60b530 100644
--- a/crates/geom/src/region.rs
+++ b/crates/geom/src/region.rs
@@ -60,6 +60,21 @@ struct Graph {
     edge_src: HashMap<(usize, usize), usize>,
 }
 
+/// Records the crossing of segments `i` and `j`, if any, as split parameters on both.
+fn split_pair(segs: &[(Vec2, Vec2)], splits: &mut [Vec<f64>], i: usize, j: usize) {
+    let (Some(si), Some(sj)) = (segs.get(i), segs.get(j)) else { return };
+    let li = Line::new(si.0, si.1);
+    let lj = Line::new(sj.0, sj.1);
+    if let Some(x) = line_line(&li, &lj) {
+        if let Some(v) = splits.get_mut(i) {
+            v.push(li.param_of(x).clamp(0.0, 1.0));
+        }
+        if let Some(v) = splits.get_mut(j) {
+            v.push(lj.param_of(x).clamp(0.0, 1.0));
+        }
+    }
+}
+
 fn build(polys: &[Vec<Vec2>], tol: f64) -> Option<Graph> {
     let mut segs: Vec<(Vec2, Vec2)> = Vec::new();
     let mut owner: Vec<usize> = Vec::new();
@@ -94,16 +109,13 @@ fn build(polys: &[Vec<Vec2>], tol: f64) -> Option<Graph> {
         )
     };
     let mut grid: HashMap<(usize, usize), Vec<usize>> = HashMap::new();
+    let mut long: Vec<usize> = Vec::new();
     for (i, (a, b)) in segs.iter().enumerate() {
         let (x0, y0) = cell(a.min(*b));
         let (x1, y1) = cell(a.max(*b));
         if (x1 - x0 + 1) * (y1 - y0 + 1) > 4096 {
-            // Very long segment: register in a coarse band.
-            for gx in (x0..=x1).step_by(((x1 - x0) / 64).max(1)) {
-                for gy in (y0..=y1).step_by(((y1 - y0) / 64).max(1)) {
-                    grid.entry((gx, gy)).or_default().push(i);
-                }
-            }
+            // Very long segment: too many cells to register in; it is tested against every segment below.
+            long.push(i);
             continue;
         }
         for gx in x0..=x1 {
@@ -130,23 +142,24 @@ fn build(polys: &[Vec<Vec2>], tol: f64) -> Option<Graph> {
                 if i == j || !tested.insert((i.min(j), i.max(j))) {
                     continue;
                 }
-                let (Some(si), Some(sj)) = (segs.get(i), segs.get(j)) else { continue };
-                let li = Line::new(si.0, si.1);
-                let lj = Line::new(sj.0, sj.1);
-                if let Some(x) = line_line(&li, &lj) {
-                    if let Some(v) = splits.get_mut(i) {
-                        v.push(li.param_of(x).clamp(0.0, 1.0));
-                    }
-                    if let Some(v) = splits.get_mut(j) {
-                        v.push(lj.param_of(x).clamp(0.0, 1.0));
-                    }
-                }
+                split_pair(&segs, &mut splits, i, j);
                 if tested.len() > 20_000_000 {
                     return None;
                 }
             }
         }
     }
+    for &i in &long {
+        for j in 0..n {
+            if i == j || !tested.insert((i.min(j), i.max(j))) {
+                continue;
+            }
+            split_pair(&segs, &mut splits, i, j);
+            if tested.len() > 20_000_000 {
+                return None;
+            }
+        }
+    }
     let mut g = Graph { pts: Vec::new(), adj: Vec::new(), src: Vec::with_capacity(n), edge_src: HashMap::new() };
     let mut index: HashMap<(i64, i64), usize> = HashMap::new();
     let mut vid = |p: Vec2, g: &mut Graph| -> usize {
@@ -609,6 +622,20 @@ mod tests {
         vec![Vec2::new(x0, y0), Vec2::new(x0 + s, y0), Vec2::new(x0 + s, y0 + s), Vec2::new(x0, y0 + s), Vec2::new(x0, y0)]
     }
 
+    #[test]
+    fn long_segment_crossing_in_unsampled_cell_is_split() {
+        // A diagonal long enough to skip the grid, and a short segment crossing it in a cell the old coarse sampling missed.
+        let mut polys = vec![vec![Vec2::new(0.0, 0.0), Vec2::new(1000.0, 1000.0)], vec![Vec2::new(11.0, 12.0), Vec2::new(12.0, 11.0)]];
+        for i in 0..17_000usize {
+            let x = 600.0 + (i % 130) as f64 * 2.0;
+            let y = 100.0 + (i / 130) as f64 * 2.0;
+            polys.push(vec![Vec2::new(x, y), Vec2::new(x + 1.0, y)]);
+        }
+        let g = build(&polys, 1e-6).unwrap();
+        let hit = g.pts.iter().position(|p| p.near(Vec2::new(11.5, 11.5), 1e-6)).unwrap();
+        assert_eq!(g.adj[hit].len(), 4);
+    }
+
     #[test]
     fn finds_square_around_point() {
         let l = enclosing_loop(&[square(0.0, 0.0, 10.0)], Vec2::new(5.0, 5.0), 1e-9).unwrap();