Skip missing segments when starting a region loop walk

Fixes #449

Co-Authored-By: Claude Haiku 5.5 <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/doc/src/vector_regions.rs

@@ -18,8 +18,9 @@ impl VectorNetwork {
     /// second; a segment that doesn't touch the point reached so far (a malformed loop) is taken
     /// as stored. Missing segments are skipped.
     pub fn loop_walk(&self, lp: &[usize]) -> Vec<(usize, bool)> {
-        let Some(first) = lp.first().and_then(|i| self.segments.get(*i)) else { return Vec::new() };
-        let forward_first = match lp.get(1).and_then(|i| self.segments.get(*i)) {
+        let mut existing = lp.iter().filter_map(|i| self.segments.get(*i));
+        let Some(first) = existing.next() else { return Vec::new() };
+        let forward_first = match existing.next() {
             Some(second) => first.b == second.a || first.b == second.b,
             None => true,
         };

crates/doc/src/vector_regions_tests.rs

@@ -148,6 +148,28 @@ fn a_loop_that_does_not_close_is_closed_implicitly() {
     assert_eq!(n.region_at(p(300.0, 100.0)), None);
 }
 
+#[test]
+fn missing_segments_are_skipped_wherever_they_sit_in_a_loop() {
+    use figcraft_geom::Shape;
+    let mut n = VectorNetwork::default();
+    square(&mut n, 0.0, 0.0, 200.0);
+    n.regions = vec![region(vec![vec![0, 1, 2]], WindingRule::NonZero)];
+    let control = n.region_path(0).unwrap();
+    let control_walk = n.loop_walk(&[0, 1, 2]);
+    for lp in [vec![99, 0, 1, 2], vec![0, 99, 1, 2], vec![99, 98, 0, 1, 2]] {
+        n.regions = vec![region(vec![lp.clone()], WindingRule::NonZero)];
+        assert_eq!(n.loop_walk(&lp), control_walk, "{lp:?}");
+        assert_eq!(n.region_at(p(100.0, 100.0)), Some(0), "{lp:?}");
+        assert!(n.region_contains(0, p(100.0, 100.0)), "{lp:?}");
+        let path = n.region_path(0).unwrap();
+        assert_eq!(path.elements().len(), control.elements().len(), "{lp:?}");
+        assert!((path.area().abs() - control.area().abs()).abs() < 1e-6, "{lp:?}");
+    }
+    n.regions = vec![region(vec![vec![99]], WindingRule::NonZero)];
+    assert!(n.region_path(0).is_none());
+    assert!(!n.region_contains(0, p(100.0, 100.0)));
+}
+
 #[test]
 fn a_regions_points_are_listed_once() {
     let mut n = nested();