fix(kernel): preserve intentional Split Face lines in contact joins (#782)

Intentional Split Face lines survive a join with partial planar contact, so the far face stays in two pieces. Follow-up to #704 / #674.

crates/engine/src/boolean_regression_tests.rs

@@ -5,6 +5,35 @@ use serde_json::{Value, json};
 
 use crate::Session;
 
+/// Runs in PR CI as well as the heavier fixed-seed suite. Two pattern copies are
+/// disconnected, and one cylinder overlaps the block by half its volume.
+#[test]
+fn partial_face_contact_pattern_has_analytic_material() {
+    // Repeated construction exposes re-imprinting a cylinder through both of its
+    // side faces, which can create a redundant arc along the first imprint.
+    for _ in 0..8 {
+        check_partial_face_contact_pattern();
+    }
+}
+
+fn check_partial_face_contact_pattern() {
+    let s = replay(include_str!("../../../examples/regressions/partial_face_contact_pattern.json"));
+    let (mut volume, mut area, mut shells) = (0.0, 0.0, 0);
+    for b in &s.model.state().bodies {
+        assert!(b.body.validity().is_empty(), "{:?}", b.body.validity());
+        let m = b.body.tessellate(1e-5).unwrap().measure();
+        volume += m.volume;
+        area += m.area;
+        shells += b.body.shell_count();
+    }
+    let expected_volume = 25.0 * 47.5 * 37.5 + 3.5 * std::f64::consts::PI * 25.0 * 2.5;
+    let expected_area = 2.0 * (25.0 * 47.5 + 25.0 * 37.5 + 47.5 * 37.5) + 237.5 * std::f64::consts::PI - 25.0;
+    assert!((volume - expected_volume).abs() < 0.02, "{volume} expected {expected_volume}");
+    assert!((area - expected_area).abs() < 0.02, "{area} expected {expected_area}");
+    assert_eq!(shells, 3);
+    println!("#674 volume {volume:.6} / {expected_volume:.6} mm³; area {area:.6} / {expected_area:.6} mm²; {shells} shells");
+}
+
 fn replay(script: &str) -> Session {
     let mut s = Session::default();
     let script: Value = serde_json::from_str(script).unwrap();

crates/kernel/src/coplanar.rs

@@ -716,7 +716,9 @@ pub(crate) fn glue(a: &Body, b: &Body) -> Option<Result<Body>> {
         let from: Vec<FaceProv> = solid.face_iter().filter_map(|f| prov.get(&f.id()).cloned()).collect();
         // Each edge's origins in the solid's edge order.
         let origins = crate::body::edge_order(solid.face_iter()).0.iter().map(|e| origins.get(&e.id()).cloned()).collect();
-        crate::boolean_record::heal_build(solid, size, &[], Some(from), origins, &record::counts(&[a, b]))
+        let keep: Vec<_> = a.split_keep().iter().chain(b.split_keep()).cloned().collect();
+        crate::boolean_record::heal_build(solid, size, &keep, Some(from), origins, &record::counts(&[a, b]))
+            .map(|body| body.with_split_lines(Some(std::sync::Arc::new(keep))))
     }))
 }
 

crates/kernel/src/issue_674_tests.rs

@@ -0,0 +1,70 @@
+//! Partial planar contact must sew a solid without displacing or inventing material.
+use crate::*;
+use solidcraft_geom::Vec3;
+use std::f64::consts::PI;
+
+#[test]
+fn split_contact_arc_crossing_one_edge_twice_preserves_the_body() {
+    use truck_modeling::Surface;
+    let a = box_solid(Vec3::ZERO, Vec3::new(25.0, 47.5, 37.5)).unwrap();
+    let b = cylinder(Vec3::new(0.0, 7.5, 0.0), -Vec3::X, 5.0, 2.5).unwrap();
+    let face = a.faces(0.01).unwrap().iter().position(|f| f.plane_normal.is_some_and(|n| n.dot(-Vec3::X) > 0.99)).unwrap();
+    let side = b.solid.face_iter().position(|f| !matches!(f.surface(), Surface::Plane(_))).unwrap();
+    let r = split_faces(&a, &[face], &SplitTool::Face { body: &b, face: side }).unwrap();
+    assert_eq!(r.face_count(), a.face_count() + 1);
+    assert!(r.validity().is_empty(), "{:?}", r.validity());
+    assert!(r.history().is_some());
+    let (before, after) = (measure(&a).unwrap(), measure(&r).unwrap());
+    assert!((before.volume - after.volume).abs() < 1e-7);
+    assert!((before.area - after.area).abs() < 1e-7);
+}
+
+#[test]
+fn partial_contact_does_not_join_tangent_or_separated_faces() {
+    let a = box_solid(Vec3::ZERO, Vec3::new(25.0, 47.5, 37.5)).unwrap();
+    for z in [-5.0, -6.0] {
+        let b = cylinder(Vec3::new(0.0, 7.5, z), -Vec3::X, 5.0, 2.5).unwrap();
+        assert!(crate::coplanar::imprint_union(&a, &b, &|_| true).is_none());
+    }
+}
+
+#[test]
+fn partial_contact_retains_an_authored_split_away_from_the_contact() {
+    let a = box_solid(Vec3::ZERO, Vec3::new(25.0, 47.5, 37.5)).unwrap();
+    let face = a.faces(0.01).unwrap().iter().position(|f| f.plane_normal.is_some_and(|n| n.dot(Vec3::X) > 0.99)).unwrap();
+    let a = split_faces(&a, &[face], &SplitTool::Plane(solidcraft_geom::Plane { origin: Vec3::new(0.0, 0.0, 15.0), ..solidcraft_geom::Plane::XY }))
+        .unwrap();
+    let b = cylinder(Vec3::new(0.0, 7.5, 0.0), -Vec3::X, 5.0, 2.5).unwrap();
+    let r = boolean(&a, &b, BoolOp::Union).unwrap().unwrap();
+    assert_eq!(r.split_lines(), a.split_lines());
+    assert!(r.regularization_protected);
+    assert!(r.validity().is_empty());
+    let remote_faces =
+        r.faces(0.01).unwrap().iter().filter(|f| f.plane_normal.is_some_and(|n| n.dot(Vec3::X) > 0.99) && (f.centroid.x - 25.0).abs() < 1e-7).count();
+    assert_eq!(remote_faces, 2);
+}
+
+#[test]
+fn partial_circular_contact_preserves_material_and_history() {
+    for z in [-3.0_f64, 0.0, 3.0] {
+        for angle in [0.0, 0.37] {
+            let a = box_solid(Vec3::ZERO, Vec3::new(25.0, 47.5, 37.5)).unwrap();
+            let b = cylinder(Vec3::new(0.0, 7.5, z), -Vec3::X, 5.0, 2.5).unwrap();
+            let place = |body: &Body| transform(body, Vec3::new(4.0, -8.0, 2.0), Vec3::ZERO, Vec3::new(1.0, 2.0, 3.0), angle).unwrap();
+            let (a, b) = (place(&a), place(&b));
+            for reversed in [false, true] {
+                let (a, b) = if reversed { (&b, &a) } else { (&a, &b) };
+                let r = boolean(a, b, BoolOp::Union).unwrap().unwrap();
+                assert!(r.validity().is_empty(), "z={z} angle={angle}: {:?}", r.validity());
+                assert_eq!(r.shell_count(), 1);
+                assert!(r.history().is_some(), "missing history z={z} angle={angle}");
+                let m = measure(&r).unwrap();
+                let volume = 25.0 * 47.5 * 37.5 + PI * 25.0 * 2.5;
+                let contact = 25.0 * (-z / 5.0).acos() + z * (25.0 - z * z).sqrt();
+                let area = 2.0 * (25.0 * 47.5 + 25.0 * 37.5 + 47.5 * 37.5) + 2.0 * PI * 5.0 * (5.0 + 2.5) - 2.0 * contact;
+                assert!((m.volume - volume).abs() < 0.1, "z={z} angle={angle}: volume {} expected {volume}", m.volume);
+                assert!((m.area - area).abs() < 0.2, "z={z} angle={angle}: area {} expected {area}", m.area);
+            }
+        }
+    }
+}

crates/kernel/src/lib.rs

@@ -40,6 +40,8 @@ mod intersection_curve_tests;
 #[cfg(test)]
 mod issue_616_tests;
 #[cfg(test)]
+mod issue_674_tests;
+#[cfg(test)]
 mod issue_679_tests;
 mod lip;
 mod loopblend;

docs/robustness.md

@@ -97,6 +97,14 @@ fixes below (sound face pushes, no panics in the boolean's final assembly).
 
 ### Fixed
 
+- Back-to-back contact joins preserve intentional Split Face boundaries, including on
+  faces away from the contact. The glue pass retains their lines during healing and in
+  the resulting body for subsequent operations. Partial circular-contact regressions
+  cover offsets, rotation, operand order, volume, area and history; the seed-16 pattern
+  also runs in the ordinary engine suite with analytic volume and area checks.
+  Imprinting visits a shared analytic support only once: the two side faces of a
+  cylinder must not recut the same outline into redundant arcs.
+
 - **Shell corners where a curved face meets another at an angle** (the biggest cause, 215 of
   the 363 failed designs before). Moving a vertex took one linear step, exact only for planes;
   a cylinder meeting a plane at an angle (a shallow arc side) landed off the moved surface.

examples/regressions/partial_face_contact_pattern.json

@@ -0,0 +1,8 @@
+{
+  "description": "Issue #674: a patterned cylinder cap touches only part of a block face; fuse the shared region without adding material beyond the edge.",
+  "commands": [
+    {"command": "solid.box", "params": {"height": 37.5, "length": 25.0, "width": 47.5}},
+    {"command": "solid.cylinder", "params": {"axis": [1.0, 0.0, 0.0], "base": [0.0, 7.5, 0.0], "height": 2.5, "operation": "join", "radius": 5.0}},
+    {"command": "solid.pattern.circular", "params": {"axis": "Z", "count": 4, "features": ["Cylinder1"]}}
+  ]
+}