mesh: report a kernel failure to partition as the kernel's, not a plane that misses (#283)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/datum.rs

@@ -94,7 +94,7 @@ pub const DATUM_DELETE: CommandSpec = CommandSpec {
 pub const PARTITION: CommandSpec = CommandSpec {
     id: "geometry.partition",
     label: "Partition",
-    params: r#"{"name": "Half", "datum": "Mid", "bodies"?: ["Body1"] (default: every body the plane divides)} — cuts bodies of the open design into cells by a datum plane, in memory (the design file is not changed); the model keeps the partition and applies it again whenever the design is opened, after the partitions before it. A cut body `B` becomes the cells `B/a` (on the plane's negative side) and `B/b` (several pieces on a side: `B/a1`, `B/a2`, …). The two faces of the cut are paired by name: `partition:<name>:a` on the cells on the negative side, `partition:<name>:b` on the positive side (`…#0`, `…#1` where a cell has several); every other face keeps the persistent name of the face it is a piece of (`<face>#k` for several pieces on one cell), so sets by face name still select it. Reports the cells made and the cut faces named; a plane that divides no body, or a listed body it does not divide, is an error"#,
+    params: r#"{"name": "Half", "datum": "Mid", "bodies"?: ["Body1"] (default: every body the plane divides)} — cuts bodies of the open design into cells by a datum plane, in memory (the design file is not changed); the model keeps the partition and applies it again whenever the design is opened, after the partitions before it. A cut body `B` becomes the cells `B/a` (on the plane's negative side) and `B/b` (several pieces on a side: `B/a1`, `B/a2`, …). The two faces of the cut are paired by name: `partition:<name>:a` on the cells on the negative side, `partition:<name>:b` on the positive side (`…#0`, `…#1` where a cell has several); every other face keeps the persistent name of the face it is a piece of (`<face>#k` for several pieces on one cell), so sets by face name still select it. Reports the cells made and the cut faces named; a plane that divides no body, a listed body it does not divide, or a body the kernel fails to cut (its message) is an error"#,
     form: Some(super::forms::PARTITION),
     kind: CommandKind::Model,
     requires: &[Requirement::Design, Requirement::DatumPlane],

crates/engine/src/sample.rs

@@ -708,6 +708,32 @@ mod tests {
         assert!(r["energy_balance"].as_f64().unwrap() < 0.05, "{r}");
     }
 
+    /// A partition the kernel fails to make is reported as the kernel's failure, never as a plane
+    /// that does not divide the body (#283): x = 148 plainly crosses the crank's half y > 0, whose
+    /// cut fails in the kernel's boolean, and the same plane cuts the half y < 0.
+    #[test]
+    fn a_kernel_failure_to_partition_is_reported_as_such() {
+        use fieldcraft_geometry::Cut;
+        let design = fieldcraft_geometry::open_design(&fieldcraft_geometry::script_design(CRANK_ARM).unwrap()).unwrap();
+        let cut = |name: &str, point: [f64; 3], normal: [f64; 3], bodies: &[&str]| Cut {
+            name: name.into(),
+            point,
+            normal,
+            bodies: bodies.iter().map(|b| b.to_string()).collect(),
+        };
+        let half = cut("Half", [0.0; 3], [0.0, 1.0, 0.0], &[]);
+        let p = design.partitioned(&[half.clone(), cut("X148", [148.0, 0.0, 0.0], [1.0, 0.0, 0.0], &[CRANK_HALF])]).unwrap();
+        assert!(p.errors.is_empty(), "{:?}", p.errors);
+        let p = design.partitioned(&[half, cut("X148", [148.0, 0.0, 0.0], [1.0, 0.0, 0.0], &["CrankArm/b"])]).unwrap();
+        // Should the kernel learn to make this cut, the cut is all there is to check.
+        if p.cuts[1].split.is_empty() {
+            let errors: Vec<&str> = p.errors.iter().filter(|e| e.feature == "partition X148").map(|e| e.message.as_str()).collect();
+            assert!(matches!(errors.as_slice(), [m] if m.starts_with("the kernel could not cut `CrankArm/b`")), "{errors:?}");
+        } else {
+            assert_eq!(p.cuts[1].split, ["CrankArm/b"]);
+        }
+    }
+
     /// `file.sample NAME`, checked clean: ready, no errors or warnings.
     fn open(name: &str) -> Session {
         let mut s = Session::default();

crates/geometry/src/partition.rs

@@ -89,8 +89,8 @@ fn plane_of(cut: &Cut) -> Result<(Plane, [f64; 3])> {
 
 impl Design {
     /// The design as opened, cut by `cuts` in order (the partitions replace any applied before).
-    /// A cut that names a body the design does not have, or that does not divide a body it
-    /// names, is listed in [`Design::errors`] (feature `partition <name>`), as a feature of the
+    /// A cut that names a body the design does not have, that does not divide a body it names,
+    /// or that the kernel fails to make on any body it applies to, is listed in [`Design::errors`] (feature `partition <name>`), as a feature of the
     /// design that failed to rebuild; the rest still apply.
     pub fn partitioned(&self, cuts: &[Cut]) -> Result<Design> {
         if cuts.len() > MAX_CUTS {
@@ -116,12 +116,12 @@ impl Design {
                     continue;
                 }
                 match split(&body, plane, *n, cut)? {
-                    Some((cells, faces)) => {
+                    Split::Cells(cells, faces) => {
                         report.split.push(body.name.clone());
                         report.faces += faces;
                         next.extend(cells);
                     }
-                    None => {
+                    Split::Misses => {
                         if listed {
                             errors.push(FeatureError {
                                 feature: feature.clone(),
@@ -131,6 +131,16 @@ impl Design {
                         }
                         next.push(body);
                     }
+                    // Listed or not: the plane may well divide the body, and a cut that silently
+                    // leaves it whole would be mistaken for one that does not reach it (#283).
+                    Split::Failed(e) => {
+                        errors.push(FeatureError {
+                            feature: feature.clone(),
+                            message: format!("the kernel could not cut `{}` ({e})", body.name),
+                            bodies: vec![body.name.clone()],
+                        });
+                        next.push(body);
+                    }
                 }
                 if next.len() > MAX_CELLS {
                     return Err(Error::Invalid(format!("the partitions make at most {MAX_CELLS} cells")));
@@ -143,12 +153,22 @@ impl Design {
     }
 }
 
-/// Cut one body: its cells, named, and how many cut faces they have; `None` when the plane does
-/// not divide it.
-fn split(body: &Shape, plane: &Plane, n: [f64; 3], cut: &Cut) -> Result<Option<(Vec<Shape>, usize)>> {
+/// What cutting one body gave.
+enum Split {
+    /// Its cells, named, and how many cut faces they have.
+    Cells(Vec<Shape>, usize),
+    /// The plane does not divide it.
+    Misses,
+    /// The kernel failed to cut it: its message.
+    Failed(String),
+}
+
+/// Cut one body.
+fn split(body: &Shape, plane: &Plane, n: [f64; 3], cut: &Cut) -> Result<Split> {
     let parts = match guard("partition", || solidcraft_kernel::split_by_plane(&body.body, plane)) {
         Ok(p) if p.len() >= 2 => p,
-        _ => return Ok(None),
+        Ok(_) => return Ok(Split::Misses),
+        Err(e) => return Ok(Split::Failed(e.to_string())),
     };
     let size = body.size()?;
     let tol = (MATCH_TOLERANCE * size).max(1e-4);
@@ -180,7 +200,7 @@ fn split(body: &Shape, plane: &Plane, n: [f64; 3], cut: &Cut) -> Result<Option<(
         faces += name_faces(&mut shape, &parent, cut, n, size, tol)?;
         out.push(shape);
     }
-    Ok(Some((out, faces)))
+    Ok(Split::Cells(out, faces))
 }
 
 /// A body's faces as matching needs them: each face's analytic surface, name and triangles.