Clean up Shell validation code

crates/fj-core/src/objects/kinds/edge.rs

@@ -67,16 +67,6 @@ impl HalfEdge {
         self.boundary
     }
 
-    /// Compute the surface position where the edge starts
-    pub fn start_position(&self) -> Point<2> {
-        // Computing the surface position from the curve position is fine.
-        // `Edge` "owns" its start position. There is no competing code that
-        // could compute the surface position from slightly different data.
-
-        let [start, _] = self.boundary.inner;
-        self.path.point_from_path_coords(start)
-    }
-
     /// Access the curve of the edge
     pub fn curve(&self) -> &Handle<Curve> {
         &self.curve
@@ -86,4 +76,14 @@ impl HalfEdge {
     pub fn start_vertex(&self) -> &Handle<Vertex> {
         &self.start_vertex
     }
+
+    /// Compute the surface position where the edge starts
+    pub fn start_position(&self) -> Point<2> {
+        // Computing the surface position from the curve position is fine.
+        // `Edge` "owns" its start position. There is no competing code that
+        // could compute the surface position from slightly different data.
+
+        let [start, _] = self.boundary.inner;
+        self.path.point_from_path_coords(start)
+    }
 }

crates/fj-core/src/objects/kinds/shell.rs

@@ -1,5 +1,6 @@
 use crate::{
-    objects::{handles::Handles, Face},
+    objects::{handles::Handles, Face, HalfEdge},
+    queries::BoundingVerticesOfHalfEdge,
     storage::Handle,
 };
 
@@ -21,4 +22,28 @@ impl Shell {
     pub fn faces(&self) -> &Handles<Face> {
         &self.faces
     }
+
+    /// Indicate whether the provided half-edges are siblings
+    pub fn are_siblings(
+        &self,
+        a: &Handle<HalfEdge>,
+        b: &Handle<HalfEdge>,
+    ) -> bool {
+        let same_curve = a.curve().id() == b.curve().id();
+        let same_boundary = a.boundary() == b.boundary().reverse();
+        let same_vertices = {
+            let Some(a_vertices) = self.bounding_vertices_of_half_edge(a)
+            else {
+                return false;
+            };
+            let Some(b_vertices) = self.bounding_vertices_of_half_edge(b)
+            else {
+                return false;
+            };
+
+            a_vertices == b_vertices.reverse()
+        };
+
+        same_curve && same_boundary && same_vertices
+    }
 }