fix: join merges every body the tool meets

A Join unioned its tools into the first target only, leaving other target
bodies the tool overlapped or touched as separate, interpenetrating bodies.
Each tool now merges with every target it meets (shared material or face
contact, tested on the operands' boundaries) into one body under the first
one's name; absorbed bodies are removed and their threads follow. A tool that
meets no target still joins the first target as a separate lump. Sheet metal
and mesh bodies are never absorbed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J53sR6gEaVt56soFDpTck8

crates/doc/src/eval.rs

@@ -924,13 +924,40 @@ fn apply_op(state: &mut ModelState, f: &Feature, tools: Vec<Body>, op: Operation
     }
     match op {
         Operation::Join => {
-            let Some(&first) = target_idx.first() else { return Ok(()) };
-            let Some(mut acc) = state.bodies.get(first).map(|b| b.body.clone()) else { return Ok(()) };
-            for t in &tools {
-                acc = kernel::boolean(&acc, t, BoolOp::Union)?.ok_or_else(|| DocError::Invalid("join produced nothing".into()))?;
-            }
-            if let Some(mb) = state.bodies.get_mut(first) {
-                *mb = ModelBody::new(mb.name.clone(), acc, mb.feature);
+            // Each tool joins every target it meets into one body, kept under the first one's
+            // name; a tool that meets none joins the first target as a separate lump. A sheet
+            // metal body keeps its own identity: only the first body met may be one.
+            let mut live: Vec<String> = target_idx.iter().filter_map(|i| state.bodies.get(*i).map(|b| b.name.clone())).collect();
+            let union = |a: &Body, b: &Body| kernel::boolean(a, b, BoolOp::Union)?.ok_or_else(|| DocError::Invalid("join produced nothing".into()));
+            for tool in &tools {
+                let mut met: Vec<String> = Vec::new();
+                for name in &live {
+                    let Some(mb) = state.body(name) else { continue };
+                    let sheet = state.sheets.iter().any(|s| &s.body == name);
+                    if mb.body.is_mesh() || (!met.is_empty() && sheet) {
+                        continue;
+                    }
+                    if mb.body.meets(tool)? {
+                        met.push(name.clone());
+                    }
+                }
+                let Some(keep) = met.first().or(live.first()).cloned() else { return Ok(()) };
+                let Some(base) = state.body(&keep).map(|b| b.body.clone()) else { continue };
+                let mut acc = union(&base, tool)?;
+                for name in met.iter().skip(1) {
+                    if let Some(b) = state.body(name) {
+                        acc = union(&acc, &b.body)?;
+                    }
+                }
+                let absorbed: std::collections::BTreeSet<String> = met.into_iter().skip(1).collect();
+                if let Some(mb) = state.bodies.iter_mut().find(|b| b.name == keep) {
+                    *mb = ModelBody::new(mb.name.clone(), acc, mb.feature);
+                }
+                state.bodies.retain(|b| !absorbed.contains(&b.name));
+                for t in state.threads.iter_mut().filter(|t| absorbed.contains(&t.body)) {
+                    t.body = keep.clone();
+                }
+                live.retain(|n| !absorbed.contains(n));
             }
         }
         Operation::Cut | Operation::Intersect => {

crates/engine/src/tests.rs

@@ -47,6 +47,50 @@ fn join_extrude_from_a_chamfered_top_face() {
     assert!(rel(volume(&mut s), base + PI * 64.0 * 5.0) < 5e-4);
 }
 
+/// A join that bridges two bodies makes them one body, not an overlap of the first and the second.
+#[test]
+fn join_bridging_two_bodies_merges_them() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10, "corner": [20, 0, 0], "operation": "new_body"}));
+    run(&mut s, "solid.box", json!({"length": 20, "width": 4, "height": 4, "corner": [5, 3, 3], "operation": "join"}));
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 1, "both boxes and the bar are one body");
+    assert_eq!(st.bodies[0].name, "Body1");
+    assert_eq!(st.bodies[0].body.lumps().unwrap().len(), 1);
+    // 2 boxes + the bar's 10 mm outside them.
+    assert!(rel(volume(&mut s), 2000.0 + 10.0 * 16.0) < 1e-6);
+}
+
+/// A join touching only the second body merges into that body and leaves the first alone.
+#[test]
+fn join_merges_into_the_body_it_meets() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10, "corner": [20, 0, 0], "operation": "new_body"}));
+    // Sits on Body2's top face: a face contact, no shared volume.
+    run(&mut s, "solid.box", json!({"length": 4, "width": 4, "height": 4, "corner": [23, 3, 10], "operation": "join"}));
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 2);
+    let b1 = st.bodies.iter().find(|b| b.name == "Body1").unwrap();
+    let b2 = st.bodies.iter().find(|b| b.name == "Body2").unwrap();
+    assert_eq!(b1.body.face_count(), 6, "Body1 is untouched");
+    assert_eq!(b2.body.lumps().unwrap().len(), 1);
+    assert!(rel(volume(&mut s), 2064.0) < 1e-6);
+}
+
+/// A join that meets no body still joins the first body, as a separate lump.
+#[test]
+fn join_meeting_nothing_keeps_the_first_body() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    run(&mut s, "solid.box", json!({"length": 4, "width": 4, "height": 4, "corner": [40, 0, 0], "operation": "join"}));
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 1);
+    assert_eq!(st.bodies[0].body.lumps().unwrap().len(), 2);
+    assert!(rel(volume(&mut s), 1064.0) < 1e-6);
+}
+
 mod component_frames;
 mod mirror_planes;
 mod primitive_points;

crates/kernel/src/body.rs

@@ -883,6 +883,15 @@ impl Body {
         self.lumps_with_clearance(0.0)
     }
 
+    /// Whether this body and `other` share material or touch (within a fine mesh tolerance).
+    pub fn meets(&self, other: &Body) -> Result<bool> {
+        self.require_brep("contact test")?;
+        other.require_brep("contact test")?;
+        let tol = (self.size().max(other.size()) * 1e-4).max(1e-6);
+        let (a, b) = (self.tessellate(tol)?, other.tessellate(tol)?);
+        shell_groups::meshes_meet(&a, &b, 2.0 * tol)
+    }
+
     /// A boolean shortcut needs clearance larger than both shell mesh deflections.
     pub(crate) fn separated_lumps(&self) -> Result<Vec<Body>> {
         self.lumps_with_clearance(4.0 * (self.size() * 1e-3).max(1e-3))

crates/kernel/src/shell_groups.rs

@@ -79,6 +79,44 @@ fn triangles_meet(a: &[Vec3; 3], b: &[Vec3; 3], gap: f64) -> bool {
     true
 }
 
+/// Whether two closed meshes share material or touch: a boundary triangle of one comes within
+/// `gap` of one of the other's, or one encloses the other.
+pub(super) fn meshes_meet(a: &Mesh, b: &Mesh, gap: f64) -> Result<bool> {
+    if !gap.is_finite() || gap <= 0.0 {
+        return Err(failed("invalid contact clearance"));
+    }
+    let (ba, bb) = (a.bounds(), b.bounds());
+    if ba.is_empty() || bb.is_empty() || boxes_separate(ba, bb, gap) {
+        return Ok(false);
+    }
+    // Only triangles near the other body can touch it.
+    let near = |m: &Mesh, other: Aabb3| -> Result<Vec<([Vec3; 3], Aabb3)>> {
+        let mut out = Vec::new();
+        for t in &m.triangles {
+            let tri = m.tri(t).ok_or_else(|| failed("invalid contact triangle"))?;
+            let bounds = box_of(&tri);
+            if !boxes_separate(bounds, other, gap) {
+                out.push((tri, bounds));
+            }
+        }
+        Ok(out)
+    };
+    let (ta, tb) = (near(a, bb)?, near(b, ba)?);
+    let work = ta.len().checked_mul(tb.len()).ok_or_else(|| failed("contact work exceeds the limit"))?;
+    if work > MAX_PAIR_WORK {
+        return Err(failed("contact work exceeds the limit"));
+    }
+    for (x, xb) in &ta {
+        for (y, yb) in &tb {
+            if !boxes_separate(*xb, *yb, gap) && triangles_meet(x, y, gap) {
+                return Ok(true);
+            }
+        }
+    }
+    // No boundary contact: they meet only if one lies wholly inside the other.
+    Ok(b.positions.first().is_some_and(|p| a.contains(*p)) || a.positions.first().is_some_and(|p| b.contains(*p)))
+}
+
 pub(super) fn groups(meshes: &[Mesh], volumes: &[f64], gap: f64) -> Result<Vec<Vec<usize>>> {
     if meshes.is_empty() || meshes.len() != volumes.len() || meshes.len() > MAX_SHELLS {
         return Err(failed("shell count exceeds the grouping limit or is invalid"));