Modify mjListKeyMap to be a fixed-size array, and add internal object_lists variable to mjCModel to remove redundant code.

PiperOrigin-RevId: 590997348
Change-Id: I5f4d7de605f44d40a75eb1e9ceec208325fa3be5

src/user/user_model.h

@@ -32,9 +32,8 @@ typedef enum _mjtInertiaFromGeom {
   mjINERTIAFROMGEOM_AUTO          // use only if inertial element is missing
 } mjtInertiaFromGeom;
 
-// TODO: convert mjListKeyMap to mjKeyMap[mjNOBJECT] by adding mjNOBJECT to mjtObj
 typedef std::map<std::string, int, std::less<> > mjKeyMap;
-typedef std::map<std::string, mjKeyMap, std::less<> > mjListKeyMap;
+typedef std::array<mjKeyMap, mjNOBJECT> mjListKeyMap;
 
 
 
@@ -294,6 +293,9 @@ class mjCModel {
 
   //------------------------ internal variables
 
+  // array of pointers to each object list (enumerated by type)
+  std::array<std::vector<mjCBase*>*, mjNOBJECT> object_lists;
+
   // statistics, as computed by mj_setConst
   double meaninertia_auto;        // mean diagonal inertia, as computed by mj_setConst
   double meanmass_auto;           // mean body mass, as computed by mj_setConst