Fix #4151: verify continuous angular direction for cubic arc conversion (#4151)

crates/cad/src/scene.rs

@@ -127,9 +127,34 @@ fn circular_subpath(sub: &Sub) -> Option<CircularSub> {
         return None;
     }
     // Verify that the curve progresses monotonically around the circle without reversing,
+    // both along each cubic segment's angular derivative and across sampled intervals,
     // and that closed subpaths make exactly one full turn while open subpaths do not exceed one turn.
     let mut total_turn = 0.0;
     let mut dir: Option<bool> = None;
+    let ang_threshold = radius * radius * 1e-9;
+    let mut seg_from = sub.start;
+    for seg in &sub.segs {
+        let Seg::Cubic(a, b, to) = *seg else { return None };
+        for step in 0..=32 {
+            let t = f64::from(step) / 32.0;
+            let p = cubic_point(seg_from, a, b, to, t);
+            let u = 1.0 - t;
+            let dx = 3.0 * u * u * (a.x - seg_from.x) + 6.0 * u * t * (b.x - a.x) + 3.0 * t * t * (to.x - b.x);
+            let dy = 3.0 * u * u * (a.y - seg_from.y) + 6.0 * u * t * (b.y - a.y) + 3.0 * t * t * (to.y - b.y);
+            let cross = (p.x - centre.x) * dy - (p.y - centre.y) * dx;
+            if cross.abs() > ang_threshold {
+                let is_ccw = cross > 0.0;
+                if let Some(expected_ccw) = dir {
+                    if is_ccw != expected_ccw {
+                        return None;
+                    }
+                } else {
+                    dir = Some(is_ccw);
+                }
+            }
+        }
+        seg_from = to;
+    }
     for w in samples.windows(2) {
         let (p0, p1) = (w[0], w[1]);
         let (v0x, v0y) = (p0.x - centre.x, p0.y - centre.y);

crates/cad/src/tests.rs

@@ -279,6 +279,19 @@ fn a_closed_sliver_along_a_circle_is_not_a_circle() {
     assert_eq!(kinds(&modern), ["SPLINE"]);
 }
 
+#[test]
+fn cubic_with_hidden_angular_reversal_is_not_promoted_to_arc() {
+    let d = doc_with(|d| {
+        let mut b = vectorcraft_geom::BezPath::new();
+        b.move_to(Point::new(-8.404028665134, 62.061475842818));
+        b.curve_to(Point::new(-13.285253116737, 62.103123153545), Point::new(52.811163576591, 52.471253544459), Point::new(60.0, 50.0));
+        vec![path(d, PathData::from_bezpath(&b), Paint::None, Paint::solid(Color::BLACK), 1.0)]
+    });
+    let dxf = export(&d, &opts(&d)).unwrap();
+    assert_ne!(kinds(&dxf), ["ARC"]);
+    assert_eq!(kinds(&dxf), ["SPLINE"]);
+}
+
 #[test]
 fn spline_closed_flags_follow_path_intent_across_versions_without_changing_geometry() {
     let cases = [(true, true), (true, false), (false, false), (false, true)];