Close kinematic loops: driven joints move four-bars and sliders (#1141)

Driven joints now solve closed kinematic loops, so a four-bar or slider-crank moves as a whole when one joint is driven.

crates/engine/src/cmd/joints.rs

@@ -3,6 +3,8 @@
 //! joint graph and moves the occurrences it places; the session re-solves after any edit too,
 //! so joints follow the geometry their origins snap to.
 
+use std::collections::BTreeMap;
+
 use serde_json::{Value, json};
 use solidcraft_doc::expr::Kind;
 use solidcraft_doc::joints::{self, Joint, JointKind, JointOrigin, MotionLink, NamedOrigin, Snap};
@@ -25,7 +27,8 @@ pub static COMMANDS: &[CommandSpec] = &[
     ),
     CommandSpec::new("joint.rigid_group", "Rigid Group", rigid_group).at("SOLID", "ASSEMBLE").icon("rigid_group").params("occurrences: [ids or names] (move together, as they are)"),
     CommandSpec::new("joint.drive", "Drive Joints", drive).at("SOLID", "ASSEMBLE").icon("drive").params(
-        "joint: id|name; value | values: [angles in deg or expressions, distances] (clamped to the limits)",
+        "joint: id|name; value | values: [angles in deg or expressions, distances] (clamped to the limits; the other joints of a \
+         closed loop follow, and the motion stops where the loop can no longer close → stopped_by_loop)",
     ),
     CommandSpec::new("joint.motion_link", "Motion Link", motion_link)
         .at("SOLID", "ASSEMBLE")
@@ -43,8 +46,14 @@ pub static COMMANDS: &[CommandSpec] = &[
 
 /// Re-solve the joints and move the occurrences they place (after any edit).
 pub fn resolve(s: &mut Session) -> joints::Solution {
+    resolve_holding(s, &BTreeMap::new())
+}
+
+/// Re-solve the joints, keeping the `held` values (the ones being driven, mapped to the values
+/// they move from) where loops are closed.
+pub fn resolve_holding(s: &mut Session, held: &joints::Held) -> joints::Solution {
     let st = s.model.state();
-    let sol = joints::solve(&s.doc, &st);
+    let sol = joints::solve_holding(&s.doc, &st, held);
     if !sol.world.is_empty() {
         let mut next = (*s.doc).clone();
         if next.apply_joint_solution(&sol) {
@@ -353,13 +362,19 @@ fn drive(s: &mut Session, p: &Value) -> Result<Value> {
         }
     }
     joints::clamp(j, &mut q);
-    // Moved as far toward `q` as contact sets allow.
-    let r = super::motion::drive_joint(s, id, q)?;
+    // Moved as far toward `q` as contact sets and closed loops allow. Values left out of
+    // `values` are not driven: in a closed loop they follow.
+    let given = match p.get("values") {
+        Some(Value::Array(a)) => a.len(),
+        _ => 1,
+    };
+    let r = super::motion::drive_joint(s, id, q, given)?;
     let sol = joints::solve(&s.doc, &s.model.state());
     let mut out = sol_json(&sol);
     out["joint"] = json!(id);
     out["values"] = json!(s.doc.assembly.joints.iter().find(|j| j.id == id).map(|j| j.values.clone()));
     out["stopped_by_contact"] = r["stopped_by_contact"].clone();
+    out["stopped_by_loop"] = r["stopped_by_loop"].clone();
     Ok(out)
 }