solidcraft-smlib: a horn torus to rounding is no ring torus

A revolved circle whose centre is as far from the axis as its radius (a horn torus, its
tube touching the axis) measured its major radius a few ulps above its minor from the
profile's control points, became SMLib's ring torus, and SMLib refused it
(SM_ERR_INVALID_INPUT: no ring torus of that NURBS). A ring torus now needs its tube
clear of the axis by more than rounding (1e-9 of the radius, the slack the same test
already takes for the circle's plane); a horn torus goes as its rational B-spline.

Test: a revolved unit circle 1.5 from the axis is a ring torus; at 1, 1 + 4 ulp and 0.5
it stays a revolution (develop calls the first two ring tori).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LUXeKMoxpM7Vuv8Rb6E3K2

crates/smlib/src/import/analytic.rs

@@ -331,8 +331,10 @@ impl Revolution {
             let v = [v[0].max(-FRAC_PI_2), v[1].min(FRAC_PI_2)];
             return Ok((b::Surface::Sphere { frame: frame(foot), radius }, [angle, v]));
         }
+        // A ring torus only (SMLib has no other): a tube that reaches the axis to rounding (a
+        // horn torus, major = minor) is none.
         let major = off.dot(x);
-        if !(major > radius) {
+        if !(major > radius + slack) {
             return Err(NOT_ANALYTIC);
         }
         let torus = b::Surface::Torus { frame: frame(foot), major_radius: major, minor_radius: radius };

crates/smlib/src/import/tests.rs

@@ -237,3 +237,33 @@ fn arcs_past_a_turn_come_a_turn_back() {
     let line = Curve::Line { start: [0.0; 3], end: [1.0, 0.0, 0.0] };
     assert_eq!(turned(&line, [7.0, 9.0]), ([7.0, 9.0], 0.0));
 }
+
+/// A revolved circle is SMLib's ring torus only where its tube stays off the axis: a tube that
+/// reaches it to rounding (a horn torus, its centre as far from the axis as its radius, give or
+/// take an ulp) and one across it (a spindle) go as their rational B-splines.
+#[test]
+fn only_a_tube_clear_of_its_axis_is_a_ring_torus() {
+    let ring = |major: f64| {
+        let frame = solidcraft_geom::brep::Frame { origin: [major, 0.0, 0.0], x_axis: [1.0, 0.0, 0.0], y_axis: [0.0, 0.0, 1.0] };
+        let (profile, _) = solidcraft_geom::brep::form::curve(&Curve::Circle { frame, radius: 1.0 }, [-1.0, 1.0]).unwrap();
+        let identity = [[1.0, 0.0, 0.0, 0.0], [0.0, 1.0, 0.0, 0.0], [0.0, 0.0, 1.0, 0.0], [0.0, 0.0, 0.0, 1.0]];
+        let surface =
+            Surface::PlacedRevolution { profile: Box::new(profile), origin: [0.0; 3], axis: [0.0, 0.0, 1.0], matrix: identity, forward: false };
+        let face = Face { surface, nurbs: None, domain: [[0.0, 1.0], [-0.5, 0.5]], loops: Vec::new() };
+        let record = Record {
+            version: b::VERSION,
+            tolerance: 1e-6,
+            regions: Vec::new(),
+            shells: Vec::new(),
+            faces: vec![face],
+            edges: Vec::new(),
+            vertices: Vec::new(),
+        };
+        let made = analytic::analytic(&record).unwrap();
+        made.faces[0].surface.clone()
+    };
+    assert!(matches!(ring(1.5), Surface::Torus { .. }));
+    for major in [1.0, 1.0 + 4.0 * f64::EPSILON, 0.5] {
+        assert!(matches!(ring(major), Surface::PlacedRevolution { .. }), "major {major}");
+    }
+}