Sparse ten_J and ten_J_colind

PiperOrigin-RevId: 869712136
Change-Id: Id2979684aa0c39552cb8453852483cbac55f0ea3

src/user/user_model.cc

@@ -3226,7 +3226,16 @@ int mjCModel::CountNJmom(const mjModel* m) {
   return count;
 }
 
+// compute non-zeros in ten_J matrix
+int mjCModel::CountNJten(const mjModel* m) {
+  int nv = m->nv;
+  int ntendon = m->ntendon;
 
+  // conservative upper bound: each tendon can have at most nv non-zeros
+  // TODO(taylorhowell): compute tighter bound
+  int count = ntendon * nv;
+  return count;
+}
 
 // copy objects outside kinematic tree
 void mjCModel::CopyObjects(mjModel* m) {
@@ -5037,6 +5046,9 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
   // compute non-zeros in actuator_moment
   m->nJmom = nJmom = CountNJmom(m);
 
+  // compute non-zeros in ten_J
+  m->nJten = nJten = CountNJten(m);
+
   // scale mass
   if (compiler.settotalmass > 0) {
     mj_setTotalmass(m, compiler.settotalmass);

src/user/user_model.h

@@ -130,6 +130,7 @@ class mjCModel_ : public mjsElement {
   mjtSize nC;              // number of non-zeros in reduced sparse dof-dof matrix
   mjtSize nD;              // number of non-zeros in sparse dof-dof matrix
   mjtSize nJmom;           // number of non-zeros in sparse actuator_moment matrix
+  mjtSize nJten;           // number of non-zeros in sparse ten_J matrix
 
   // statistics, as computed by mj_setConst
   double meaninertia_auto;  // mean diagonal inertia, as computed by mj_setConst
@@ -368,6 +369,7 @@ class mjCModel : public mjCModel_, private mjSpec {
   void FinalizeSimple(mjModel* m);      // finalize simple bodies/dofs including tendon information
   void CopyPlugins(mjModel*);           // copy plugin data
   int CountNJmom(const mjModel* m);     // compute number of non-zeros in actuator_moment matrix
+  int CountNJten(const mjModel* m);     // compute number of non-zeros in ten_J matrix
 
   // remove plugins that are not referenced by any object
   void RemovePlugins();