fix: fuse bodies that touch back to back over part of a face (#679, #674) (#704)

Two L's extruded up and down from one 2 x 2 grid now join into one solid (the planar rebuild gives each fan of faces around a pinched vertex its own vertex copy), and a pad whose end lies half past the edge of the face it stands on fuses exactly: contact faces are imprinted with each other's outline and the common faces glued away, still held to the retry guard. Face splitting handles a line crossing an edge twice; glue matches differently parametrised curves and keeps several pieces apart. Robustness seed 16 now fuses with the exact volume.

Fixes #679, fixes #674.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/boolean_regression_tests.rs

@@ -153,3 +153,36 @@ fn ribs_flush_with_a_window_change_nothing_away_from_them() {
     let bounds = s.model.state().body("Lid").unwrap().body.tessellate(0.01).unwrap().bounds();
     assert!((bounds.max.z - 14.1).abs() < 1e-9, "{}", bounds.max.z);
 }
+
+/// #679: one sketch of a 2 × 2 grid of 10 mm squares; three cells extruded up 10 mm (XO/XX) and
+/// three down 10 mm, joined (XX/OX). The two L's meet back to back over two whole cells, along
+/// an edge and at the centre: one solid of 6 cells × 10 mm × 100 mm².
+#[test]
+fn ls_extruded_up_and_down_from_one_grid_join() {
+    let mut s = Session::default();
+    let run = |s: &mut Session, id: &str, p: Value| s.execute(id, &p).unwrap_or_else(|e| panic!("{id}: {e}"));
+    run(&mut s, "sketch.create", json!({"plane": "XY", "name": "Grid"}));
+    for (p0, p1) in [([-10, -10], [0, 0]), ([0, -10], [10, 0]), ([-10, 0], [0, 10]), ([0, 0], [10, 10])] {
+        run(&mut s, "sketch.rectangle.two_point", json!({"p0": p0, "p1": p1}));
+    }
+    run(&mut s, "sketch.finish", json!({}));
+    let cells = |pts: &[[f64; 2]]| Value::Array(pts.iter().map(|p| json!({"point": p})).collect());
+    run(
+        &mut s,
+        "solid.extrude",
+        json!({"sketch": "Grid", "profiles": cells(&[[-5., 5.], [-5., -5.], [5., -5.]]), "distance": 10, "body_name": "Logo"}),
+    );
+    let reply = run(
+        &mut s,
+        "solid.extrude",
+        json!({"sketch": "Grid", "profiles": cells(&[[-5., 5.], [5., 5.], [5., -5.]]), "distance": "10 mm", "direction": "negative", "operation": "join"}),
+    );
+    assert!(reply["warning"].as_str().is_none_or(|w| !w.contains("tool displacement")), "{reply}");
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 1);
+    let body = &st.body("Logo").unwrap().body;
+    assert_eq!(body.shell_count(), 1);
+    assert!(body.validity().is_empty(), "{:?}", body.validity());
+    let v = volume(&s, "Logo");
+    assert!((v - 6000.0).abs() < 1e-6, "{v}");
+}

crates/engine/src/tests/robustness.rs

@@ -202,8 +202,8 @@ fn fillet_into_an_earlier_round() {
     }
 }
 
-/// Clean seeds, and the original seed 5 edge contact and seed 16 partial face contact, now
-/// rejected without distortion.
+/// Clean seeds (seed 16: a partial face contact, fused), and the original seed 5 edge contact,
+/// now rejected without distortion.
 #[test]
 fn fixed_seeds_run_clean() {
     for seed in FIXED_SEEDS {
@@ -228,12 +228,10 @@ fn fixed_seeds_run_clean() {
             continue;
         }
         if *seed == 16 {
-            // Seed 16 patterns a short cylinder around a block: one copy's cap lies on a side of
-            // the block but half of it runs past the block's bottom edge. That contact can't be
-            // fused yet (#674). Before #577 the join came back as two shells touching over the
-            // half disc (#396); with #577 alone, the cap was pushed into the block over its whole
-            // area, which added 72 mm³ past the edge (45290.6 mm³ for 45218.4). The retry guard (#644) refuses that,
-            // so the pattern is refused and the part stays as it was.
+            // Seed 16 patterns a short cylinder around a block: one copy's end lies on a side of
+            // the block with half of it past the block's bottom edge (#674). The end and the side
+            // are split along each other's outline and the half disc they share goes: the block,
+            // half of the first cylinder (the other half is inside the block) and three whole ones.
             let mut s = Session::default();
             run(&mut s, "solid.box", json!({"length": 25.0, "width": 47.5, "height": 37.5}));
             run(
@@ -241,13 +239,13 @@ fn fixed_seeds_run_clean() {
                 "solid.cylinder",
                 json!({"axis": [1.0, 0.0, 0.0], "base": [0.0, 7.5, 0.0], "height": 2.5, "operation": "join", "radius": 5.0}),
             );
-            let before = total_volume(&s);
-            // The block, and the half of the cylinder outside it.
-            assert!((before - (25.0 * 47.5 * 37.5 + PI * 25.0 * 2.5 / 2.0)).abs() < 0.05, "{before}");
-            let error = s.execute("solid.pattern.circular", &json!({"axis": "Z", "count": 4, "features": ["Cylinder1"]})).unwrap_err();
-            assert!(error.to_string().contains("touch along faces that could not be fused"), "{error}");
-            assert!((total_volume(&s) - before).abs() < 1e-6);
+            run(&mut s, "solid.pattern.circular", json!({"axis": "Z", "count": 4, "features": ["Cylinder1"]}));
+            // (Within the measured round sides' mesh error, about 0.03 mm³ a cylinder.)
+            let want = 25.0 * 47.5 * 37.5 + 3.5 * PI * 25.0 * 2.5;
+            assert!((total_volume(&s) - want).abs() < 0.2, "{} vs {want}", total_volume(&s));
             assert!(s.model.state().bodies.iter().all(|b| b.body.validity().is_empty()));
+            let r = fuzz::run(*seed, 8);
+            assert!(!r.outcome.is_bug(), "seed {seed}: {:?}\n{}", r.outcome, r.script());
             continue;
         }
         let r = fuzz::run(*seed, 8);