Expose mju_sparse2dense to public API.

PiperOrigin-RevId: 688985153
Change-Id: I9559f1d47363d0afada80c86abdc729cd1910b63

python/mujoco/bindings_test.py

@@ -1337,6 +1337,16 @@ Euler integrator, semi-implicit in velocity.
     mat = np.array([[1., 2., 3.], [4., 5., 6.], [7., 8., 9.]])
     self.assertEqual(mujoco.mju_mulVecMatVec(vec1, mat, vec2), 204.)
 
+  def test_mju_sparse_to_dense(self):
+    expected = np.array([[0., 1., 0.], [2., 0., 3.]])
+    mat = np.array((1., 2., 3.))
+    rownnz = np.array([1, 2])
+    rowadr = np.array([0, 1])
+    colind = np.array([1, 0, 2])
+    res = np.zeros((2, 3))
+    mujoco.mju_sparse2dense(res, mat, rownnz, rowadr, colind)
+    np.testing.assert_array_equal(res, expected)
+
   def test_mju_euler_to_quat(self):
     quat = np.zeros(4)
     euler = np.array([0, np.pi/2, 0])

python/mujoco/functions.cc

@@ -1021,6 +1021,25 @@ PYBIND11_MODULE(_functions, pymodule) {
       });
   Def<traits::mju_transformSpatial>(pymodule);
 
+  // Sparse math
+  DEF_WITH_OMITTED_PY_ARGS(traits::mju_sparse2dense, "nr", "nc")(
+      pymodule,
+      [](Eigen::Ref<EigenArrayXX> res,
+         Eigen::Ref<const EigenVectorX> mat,
+         Eigen::Ref<const EigenVectorI> rownnz,
+         Eigen::Ref<const EigenVectorI> rowadr,
+         Eigen::Ref<const EigenVectorI> colind) {
+        if (res.rows() != rownnz.size()) {
+          throw py::type_error("#rows in res should equal size of rownnz");
+        }
+        if (res.rows() != rowadr.size()) {
+          throw py::type_error("#rows in res should equal size of rowadr");
+        }
+        return ::mju_sparse2dense(res.data(), mat.data(), res.rows(),
+                                  res.cols(), rownnz.data(), rowadr.data(),
+                                  colind.data());
+      });
+
   // Quaternions
   Def<traits::mju_rotVecQuat>(pymodule);
   Def<traits::mju_negQuat>(pymodule);