Model Editing

The API augments the traditional workflow of creating and editing models using XML files, breaking up the *parse* and
*compile* steps. As summarized in the :ref:`Overview chapter<Instance>`, the traditional workflow is:

 1. Create an XML model description file (MJCF or URDF) and associated assets. |br|
 2. Call :ref:`mj_loadXML`, obtain an :ref:`mjModel` instance.

The workflow using :ref:`mjSpec` is:

 1. Create an empty :ref:`mjSpec` using :ref:`mj_makeSpec` or parse an existing XML file using :ref:`mj_parseXML`.
 2. Programmatically edit the :ref:`mjSpec` data structure by adding, modifying, and removing elements.
 3. Compile the :ref:`mjSpec` to an :ref:`mjModel` instance using :ref:`mj_compile`.

 After compilation, the :ref:`mjSpec` remains editable, so steps 2 and 3 are interchangeable.


.. _meUsage:

Usage
~~~~~

Here we describe the C API for procedural model editing, but it is also exposed in the :ref:`Python
bindings<PyModelEdit>`. Advanced users can refer to `user_api_test.cc
<https://github.com/google-deepmind/mujoco/blob/main/test/user/user_api_test.cc>`__ and the MJCF parser in
`xml_native_reader.cc <https://github.com/google-deepmind/mujoco/blob/main/src/xml/xml_native_reader.cc>`__ for
more usage examples. After creating a new :ref:`mjSpec` or parsing an existing XML file to an :ref:`mjSpec`,
procedural editing corresponds to setting attributes. For example, in order to change the timestep, one can do:

.. code-block:: C

   mjSpec* spec = mj_makeSpec();
   spec->opt.timestep = 0.01;
   ...
   mjModel* model = mj_compile(spec, NULL);

Attributes which have variable length are C++ vectors and strings, :ref:`exposed to C as opaque types<ArrayHandles>`.
In C one uses the provided :ref:`getters<AttributeGetters>` and :ref:`setters<AttributeSetters>`:

.. code-block:: C

   mjs_setString(spec->modelname, "my_model");

In C++, one can use vectors and strings directly:

.. code-block:: C++

   std::string modelname = "my_model";
   *spec->modelname = modelname;

Loading a spec from XML can be done as follows:

.. code-block:: C

   std::array<char, 1000> error;
   mjSpec* s = mj_parseXML(filename, vfs, error.data(), error.size());

.. _meMjsElements:

Model elements
^^^^^^^^^^^^^^
Model elements corresponding to MJCF are exposed to the user as C structs with the ``mjs`` prefix. The definitions are
listed under the :ref:`Model Editing<tySpecStructure>` section of the struct reference. For example, an MJCF
:ref:`geom<body-geom>` corresponds to an :ref:`mjsGeom`.

Global defaults for all elements are set by :ref:`initializers<ElementInitialization>` like :ref:`mjs_defaultGeom`.
These functions are defined in `user_init.c
<https://github.com/google-deepmind/mujoco/blob/main/src/user/user_init.c>`__ and are the source of truth for all
default values.

Elements cannot be created directly; they are returned to the user by the corresponding constructor function, e.g.
:ref:`mjs_addGeom`. For example, to add a box geom to the world body, one would do

.. code-block:: C

   mjSpec* spec = mj_makeSpec();                                  // make an empty spec
   mjsBody* world = mjs_findBody(spec, "world");                  // find the world body
   mjsGeom* my_geom = mjs_addGeom(world, NULL);                   // add a geom to the world
   my_geom->type = mjGEOM_BOX;                                    // set geom type
   my_geom->size[0] = my_geom->size[1] = my_geom->size[2] = 0.5;  // set box size
   mjModel* model = mj_compile(spec, NULL);                       // compile to mjModel
   ...
   mj_deleteModel(model);                                         // free model
   mj_deleteSpec(spec);                                           // free spec

The ``NULL`` second argument to :ref:`mjs_addGeom` is the optional default class pointer. When using defaults
procedurally, default classes are passed in explicitly to element constructors. The global defaults of all elements
(used when no default class is passed in) can be inspected in
`user_init.c <https://github.com/google-deepmind/mujoco/blob/main/src/user/user_init.c>`__.

.. _meMemory:

Memory management
^^^^^^^^^^^^^^^^^

As seen in the examples above, model elements are never allocated by the user directly, but rather returned by a
constructor. The library takes ownership of all elements and frees them when the parent :ref:`mjSpec` is deleted using
:ref:`mj_deleteSpec`. The user is only responsible for freeing :ref:`mjSpec` structs.

.. _meAttachment:

Attachment
^^^^^^^^^^

This framework introduces a powerful new feature: attaching and deleting model subtrees. This feature is already used to
power the :ref:`attach<body-attach>` and :ref:`replicate<replicate>` meta-elements in MJCF. Attachment allows the user to
move or copy a subtree from one model into another, while also copying or moving related referenced assets and
referencing elements from outside the kinematic tree (e.g., actuators and sensors). Similarly, deleting a subtree will
remove all associated elements from the model. The default behavior ("shallow copy") is to move the child into the
parent while attaching, so subsequent changes to the child will also change the parent. Alternatively, the user can
choose to make an entirely new copy during attach using :ref:`mjs_setDeepCopy`. This flag is temporarily set to true
while parsing XMLs. It is possible to :ref:`attach a body or an mjSpec to a frame<mjs_attach>`:

.. code-block:: C

   mjSpec* parent = mj_makeSpec();
   mjSpec* child = mj_makeSpec();
   parent->compiler.degree = 0;
   child->compiler.degree = 1;
   mjsElement* frame = mjs_addFrame(mjs_findBody(parent, "world"), NULL)->element;
   mjsElement* body = mjs_addBody(mjs_findBody(child, "world"), NULL)->element;
   mjsBody* attached_body_1 = mjs_asBody(mjs_attach(frame, body, "attached-", "-1"));

or :ref:`attach a body or an mjSpec to a site<mjs_attach>`:

.. code-block:: C

   mjSpec* parent = mj_makeSpec();
   mjSpec* child = mj_makeSpec();
   mjsElement* site = mjs_addSite(mjs_findBody(parent, "world"), NULL)->element;
   mjsElement* body = mjs_addBody(mjs_findBody(child, "world"), NULL)->element;
   mjsBody* attached_body_2 = mjs_asBody(mjs_attach(site, body, "attached-", "-2"));

or :ref:`attach a frame or an mjSpec to a body<mjs_attach>`:

.. code-block:: C

   mjSpec* parent = mj_makeSpec();
   mjSpec* child = mj_makeSpec();
   mjsElement* body = mjs_addBody(mjs_findBody(parent, "world"), NULL)->element;
   mjsElement* frame = mjs_addFrame(mjs_findBody(child, "world"), NULL)->element;
   mjsFrame* attached_frame = mjs_asFrame(mjs_attach(body, frame, "attached-", "-1"));

Note that in the above examples, the parent and child models have different values for ``compiler.degree``,
corresponding to the :ref:`compiler/angle<compiler-angle>` attribute, specifying the units in which angles are
interpreted. Compiler flags are carried over during attachment, so the child model will be compiled using the child
flags, while the parent will be compiled using the parent flags.

Note also that once a child is attached by reference to a parent, the child cannot be compiled on its own.

.. admonition:: Known issues
   :class: note

   The following known limitations exist:

   - All assets from the child model will be copied in, whether they are referenced or not, if the parent and the child
     are not the same mjSpec.
   - Circular references are not checked for and will lead to infinite loops.
   - When attaching a model with :ref:`keyframes<keyframe>`, model compilation is required for the re-indexing to be
     finalized. If a second attachment is performed without compilation, the keyframes from the first attachment will be
     lost.

.. _meDefault:

Default classes
^^^^^^^^^^^^^^^
Default classes are fully supported in the new API, however using them requires an understanding of how defaults
are implemented. As explained in the :ref:`Default settings <CDefault>` section, default classes are first loaded as a
tree of dummy elements, which are then used to initialize elements which reference them. When editing models with
defaults, this initialization is explicit:

.. code-block:: C

   mjSpec* spec = mj_makeSpec();
   mjsDefault* main = mjs_getSpecDefault(spec);
   main->geom.type = mjGEOM_BOX;
   mjsGeom* geom = mjs_addGeom(mjs_findBody(spec, "world"), main);

Importantly, changing a default class after it has been used to initialize elements will not change the properties of
already initialized elements.

.. admonition:: Possible future change
   :class: note

   The behavior described above, where defaults are only applied at initialization, is a remnant of the old, XML-only
   loading pipeline. A future API change could allow defaults to be changed and applied after initialization. If you
   think this feature is important to you, please let us know on GitHub.

.. _meSaving:

XML saving
^^^^^^^^^^
Specs can be saved to an XML file or string using :ref:`mj_saveXML` or :ref:`mj_saveXMLString`, respectively.
Saving requires that the spec first be compiled.
Importantly, the saved XML will take into account any defined defaults. This is useful when a model has many repeated
values, for example if loaded from URDF, which does not support defaults. In such a case one can add default classes,
set the class of the relevant elements, and save; the resulting XML will use the defaults and be more human-readable.

.. _meRecompilation:

In-place recompilation
^^^^^^^^^^^^^^^^^^^^^^

Compilation with :ref:`mj_compile` can be called at any point to obtain a new mjModel instance. In contrast,
:ref:`mj_recompile` updates an existing mjModel and mjData pair in-place, while preserving the simulation state. This
allows model editing to occur **during simulation**, for example adding or removing bodies.