fix: booleans with whole-turn round faces read from STEP are exact instead of failing (#1892)

Thanks @NOWN-DEV.

crates/kernel/src/seam_faces.rs

@@ -0,0 +1,92 @@
+//! Faces that run along a seam edge twice: a whole turn of a cylinder or a cone bounded by one
+//! wire, as STEP files write them (the wire goes round one end, along the seam, round the other
+//! end and back along the seam). SolidCraft builds its own round faces in pieces with no such
+//! seam, and a boolean that has to divide one of these faces can fail where the same bodies with
+//! the face in two halves work: an intersection loop lying wholly inside the face is a hole in a
+//! piece whose outer wire visits the seam's vertices twice, which the boolean refuses. So when the
+//! first tries at the boolean of the bodies as they are fail, each such face is split in two along
+//! a plane through its axis, a quarter turn either side of the seam (the solids are unchanged), and
+//! the boolean is asked again; the result's history is composed back over the operands as they
+//! were given.
+
+use solidcraft_geom::{Plane, Vec3};
+
+use crate::body::{Body, from_p3};
+use crate::boolean_record as recorded;
+use crate::history::History;
+use crate::splitface::{SplitTool, split_any};
+use crate::{BoolOp, FaceSurface, Result, face_surface};
+
+/// Most faces split before the boolean.
+const MAX_SPLITS: usize = 64;
+
+thread_local! {
+    /// Set while the boolean of the split bodies runs (no second round of splitting).
+    static SPLITTING: std::cell::Cell<bool> = const { std::cell::Cell::new(false) };
+}
+
+/// The first face of `body` whose wire runs along a seam edge twice, on a cylinder or a cone,
+/// with the plane that splits it in two: through its axis, square to the seam.
+fn next_split(body: &Body) -> Option<(usize, Plane)> {
+    body.solid.face_iter().enumerate().find_map(|(i, face)| {
+        let mut seen = std::collections::HashSet::new();
+        let seam = face.absolute_boundaries().iter().flat_map(|w| w.edge_iter()).find(|e| !seen.insert(e.id()))?;
+        let (origin, axis) = match face_surface(body, i)? {
+            FaceSurface::Cylinder { origin, axis, .. } => (origin, axis),
+            FaceSurface::Cone { apex, axis, .. } => (apex, axis),
+            _ => return None,
+        };
+        let axis = axis.normalized()?;
+        let p = from_p3(seam.front().point());
+        let off = p - origin;
+        let radial: Vec3 = (off - axis * off.dot(axis)).normalized()?;
+        Some((i, Plane::from_normal(origin, radial)?))
+    })
+}
+
+/// Whether either body has a face for [`boolean`] to split.
+pub(crate) fn needed(a: &Body, b: &Body) -> bool {
+    !a.is_mesh() && !b.is_mesh() && !SPLITTING.with(|c| c.get()) && (next_split(a).is_some() || next_split(b).is_some())
+}
+
+/// `body` with every face that runs along a seam edge twice split in two, and its history over
+/// `body`. `None` when a split fails.
+fn halved(body: &Body) -> Option<(Body, Option<History>)> {
+    let (mut x, mut h) = (body.clone(), Some(History::identity(body)));
+    for _ in 0..MAX_SPLITS {
+        let Some((face, plane)) = next_split(&x) else { return Some((x, h)) };
+        let (mut done, _) = split_any(&x, &[face], &SplitTool::Plane(plane)).ok()?;
+        // The split is no authored boundary: a later feature may still merge what it separates.
+        done.regularization_protected = x.regularization_protected;
+        let through = recorded::through(&done, 0, h.as_ref());
+        (x, h) = (done, through);
+    }
+    next_split(&x).is_none().then_some((x, h))
+}
+
+/// The boolean `op` of `a` and `b` with their faces that run along a seam edge twice split in
+/// two, `run` doing the boolean of the split bodies. `None` when neither body has such a face, a
+/// split fails, or the boolean of the split bodies gave no (checked) result: the caller then
+/// keeps what it had.
+pub(crate) fn boolean(a: &Body, b: &Body, op: BoolOp, run: &dyn Fn(&Body, &Body, BoolOp) -> Result<Option<Body>>) -> Option<Result<Option<Body>>> {
+    if !needed(a, b) {
+        return None;
+    }
+    let ((x, hx), (y, hy)) = (halved(a)?, halved(b)?);
+    SPLITTING.with(|c| c.set(true));
+    let result = crate::guard("boolean of faces split off their seams", || run(&x, &y, op));
+    SPLITTING.with(|c| c.set(false));
+    let Ok(result) = result else { return None };
+    // Held to what a boolean can do, as a retry's result is (see `boolean_guard`).
+    crate::boolean_guard::check_retry(a, b, op, result.as_ref()).ok()?;
+    let counts = crate::record::counts(&[a, b]);
+    Some(Ok(result.map(|body| {
+        // Each step's history over the operands `[a, b]`.
+        let history = recorded::of_pair(&body, &recorded::placed(hx.clone(), 0, 2), &recorded::placed(hy.clone(), 1, 2));
+        body.with_checked_history(history, &counts)
+    })))
+}
+
+#[cfg(test)]
+#[path = "seam_faces_tests.rs"]
+mod tests;