STEP: bound assembly traversal, name imports after a lone body

A visit budget stops shared sub-assemblies from growing exponentially (with
a test); the loose-solid pass gets its own budget. An Import feature is
named after the root product, else its only body, else the file. Restores
a test function closed off by the rebase.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/step_in/mod.rs

@@ -26,6 +26,9 @@ use p21::{Entity, Exchange, Param};
 const MAX_ASSEMBLY_DEPTH: usize = 32;
 /// Most body instances in one import.
 const MAX_BODIES: usize = 10_000;
+/// Most product occurrences and mapped representations visited (a shared sub-assembly used
+/// twice at every level would otherwise grow exponentially).
+const MAX_VISITS: usize = 100_000;
 
 /// A body read from a STEP file.
 #[derive(Clone, Debug)]
@@ -228,6 +231,8 @@ struct Reader<'a> {
     used_items: HashSet<u64>,
     /// Items already instanced for the product being read.
     node_items: HashSet<u64>,
+    /// Occurrences and mapped representations visited so far.
+    visits: usize,
     bodies: Vec<ImportedBody>,
 }
 
@@ -417,6 +422,11 @@ impl<'a> Reader<'a> {
             self.warn("mapped items nested too deeply");
             return;
         }
+        self.visits += 1;
+        if self.visits > MAX_VISITS {
+            self.warn(format!("more than {MAX_VISITS} component instances: the rest are left out"));
+            return;
+        }
         let u = self.units_of(rep);
         let items = self.reps.get(&rep).map(|r| r.0.clone()).unwrap_or_default();
         for item in items {
@@ -542,6 +552,11 @@ impl<'a> Reader<'a> {
             self.warn("assembly nested too deeply or recursive: some components were left out");
             return node;
         }
+        self.visits += 1;
+        if self.visits > MAX_VISITS {
+            self.warn(format!("more than {MAX_VISITS} component instances: the rest are left out"));
+            return node;
+        }
         stack.push(pd);
         let mut path = path.to_vec();
         path.push(name.clone());
@@ -609,6 +624,7 @@ pub fn step_import(text: &str) -> Result<StepImport> {
         solids: HashMap::new(),
         used_items: HashSet::new(),
         node_items: HashSet::new(),
+        visits: 0,
         bodies: Vec::new(),
     };
     rd.index();
@@ -642,6 +658,7 @@ pub fn step_import(text: &str) -> Result<StepImport> {
     if !loose.is_empty() {
         let mut bodies = Vec::new();
         rd.node_items = rd.used_items.clone();
+        rd.visits = 0;
         for r in loose {
             rd.rep_bodies(r, &identity, &[], "Import", &mut bodies, 0);
         }

crates/kernel/src/step_in/tests.rs

@@ -283,3 +283,26 @@ fn assembly_cycles_are_bounded() {
     let imp = r.unwrap();
     assert_eq!(imp.bodies.len(), 1);
 }
+
+#[test]
+fn exponential_assemblies_are_bounded() {
+    // P0 uses P1 twice, P1 uses P2 twice, … 30 levels: 2^30 occurrences if followed blindly.
+    let mut t = String::from(HEAD);
+    t += "#8=PRODUCT_DEFINITION_CONTEXT('part definition',#9,'design');\n#9=APPLICATION_CONTEXT('');\n";
+    t += &cube_entities();
+    t += &context(20, "(LENGTH_UNIT()NAMED_UNIT(*)SI_UNIT(.MILLI.,.METRE.))");
+    let levels = 30u64;
+    for l in 0..=levels {
+        t += &product(1000 + l * 10, &format!("P{l}"));
+        if l < levels {
+            for k in 0..2 {
+                t += &format!("#{}=NEXT_ASSEMBLY_USAGE_OCCURRENCE('','','',#{},#{},$);\n", 2000 + l * 10 + k, 1000 + l * 10, 1000 + (l + 1) * 10);
+            }
+        }
+    }
+    t += &format!("#60=ADVANCED_BREP_SHAPE_REPRESENTATION('',(#101),#20);\n#61=SHAPE_DEFINITION_REPRESENTATION(#{},#60);\n", 1000 + levels * 10 + 4);
+    t += TAIL;
+    let imp = step_import(&t).unwrap();
+    assert!(!imp.bodies.is_empty() && imp.bodies.len() <= MAX_BODIES);
+    assert!(imp.warnings.iter().any(|w| w.contains("left out")), "{:?}", imp.warnings);
+}