Fix panic on empty parent shapes in bind/solver.rs

iOverlay/src/bind/segment.rs

@@ -12,6 +12,15 @@ pub(crate) struct ContourIndex {
 impl ContourIndex {
     pub(crate) const EMPTY: ContourIndex = ContourIndex { data: usize::MAX };
 
+    /// True when this is the sentinel returned by `ScanHoleStore::first_less`
+    /// when no segment to the left of the query was found. Must be checked
+    /// before calling `is_hole()` / `index()`, whose outputs are bogus for
+    /// the sentinel.
+    #[inline]
+    pub(crate) fn is_empty(&self) -> bool {
+        self.data == usize::MAX
+    }
+
     #[inline]
     pub(crate) fn is_hole(&self) -> bool {
         self.data & 1 == 1

iOverlay/src/bind/solver.rs

@@ -53,6 +53,17 @@ impl ShapeBinder {
         segments: Vec<IdSegment>,
     ) -> BindSolution {
         let children_count = anchors.len();
+
+        // With no parent shapes there is nothing to bind children to. Return
+        // a trivially empty solution rather than indexing `children_count_for_parent`
+        // below (which would be zero-length and OOB on every child).
+        if shape_count == 0 {
+            return BindSolution {
+                parent_for_child: vec![0; children_count],
+                children_count_for_parent: Vec::new(),
+            };
+        }
+
         let mut parent_for_child = {
             #[cfg(debug_assertions)]
             {
@@ -84,6 +95,24 @@ impl ShapeBinder {
             }
 
             let target_id = scan_list.first_less(anchor.v_segment.a.x, ContourIndex::EMPTY, anchor.v_segment);
+            let child_index = anchor.contour_index.index();
+
+            // `first_less` returns `ContourIndex::EMPTY` (data == usize::MAX) when
+            // nothing lies to the left of the anchor. Its low bit is 1, so
+            // `is_hole()` would return true and `index()` would return
+            // usize::MAX >> 1 — both meaningless. Without this guard the
+            // `is_hole` branch below would index `parent_for_child[usize::MAX >> 1]`,
+            // which panics on large, topologically-tangled inputs. When the
+            // anchor has no enclosing parent (malformed / ambiguous input),
+            // fall back to attaching it to shape 0 rather than crashing.
+            // `shape_count > 0` is guaranteed by the early return at function
+            // entry, so `children_count_for_parent[0]` is in-bounds.
+            if target_id.is_empty() {
+                parent_for_child[child_index] = 0;
+                children_count_for_parent[0] += 1;
+                continue;
+            }
+
             let parent_index = if target_id.is_hole() {
                 // index is a hole index
                 // at this moment this hole parent is known
@@ -92,8 +121,6 @@ impl ShapeBinder {
                 target_id.index()
             };
 
-            let child_index = anchor.contour_index.index();
-
             parent_for_child[child_index] = parent_index;
             children_count_for_parent[parent_index] += 1;
         }
@@ -268,12 +295,71 @@ impl SortByAngle for [IdSegment] {
 
 #[cfg(test)]
 mod tests {
-    use crate::bind::solver::JoinHoles;
+    use crate::bind::segment::{ContourIndex, IdSegment};
+    use crate::bind::solver::{JoinHoles, ShapeBinder};
     use crate::geom::v_segment::VSegment;
     use alloc::vec;
     use core::cmp::Ordering;
     use i_float::int::point::IntPoint;
 
+    #[test]
+    fn test_bind_zero_shapes_no_children() {
+        // shape_count == 0, no anchors: early-return path must produce an empty solution.
+        let solution = ShapeBinder::bind(0, vec![], vec![]);
+        assert!(solution.children_count_for_parent.is_empty());
+        assert!(solution.parent_for_child.is_empty());
+    }
+
+    #[test]
+    fn test_bind_zero_shapes_with_children() {
+        // shape_count == 0 with one hole anchor: previously panicked with an OOB index
+        // into the zero-length children_count_for_parent vec.
+        let anchor = IdSegment {
+            contour_index: ContourIndex::new_hole(0),
+            v_segment: VSegment {
+                a: IntPoint::new(0, 0),
+                b: IntPoint::new(5, 0),
+            },
+        };
+        let solution = ShapeBinder::bind(0, vec![anchor], vec![]);
+        assert!(solution.children_count_for_parent.is_empty());
+        assert_eq!(solution.parent_for_child, vec![0]);
+    }
+
+    #[test]
+    fn test_hole_left_of_all_parents_does_not_panic() {
+        // Both parent shapes start at x >= 10, so when the sweep processes
+        // the hole anchor at x == 1 the scan list is empty and first_less
+        // returns ContourIndex::EMPTY.  Before the fix, the is_hole() branch
+        // would index parent_for_child[usize::MAX >> 1] and panic.
+        let mut shapes = vec![
+            vec![vec![
+                IntPoint::new(10, 0),
+                IntPoint::new(20, 0),
+                IntPoint::new(20, 10),
+                IntPoint::new(10, 10),
+            ]],
+            vec![vec![
+                IntPoint::new(30, 0),
+                IntPoint::new(40, 0),
+                IntPoint::new(40, 10),
+                IntPoint::new(30, 10),
+            ]],
+        ];
+        let holes = vec![vec![
+            IntPoint::new(1, 2),
+            IntPoint::new(5, 2),
+            IntPoint::new(5, 8),
+            IntPoint::new(1, 8),
+        ]];
+
+        // Must not panic — the stray hole falls back to shape 0.
+        shapes.join_unsorted_holes(holes, false);
+
+        assert_eq!(shapes[0].len(), 2); // outer contour + fallback hole
+        assert_eq!(shapes[1].len(), 1);
+    }
+
     #[test]
     fn test_0() {
         let mut shapes = vec![