Add mj_copyBack for copying real-valued arrays from mjModel back to mjSpec.

Also add `SaveAndReadXML` function to test fixture using `mjSpec` as input.

PiperOrigin-RevId: 765393821
Change-Id: Ic257addd2fc89678fc11b226c168c077626cc51e

doc/APIreference/functions.rst

@@ -51,6 +51,15 @@ Compile :ref:`mjSpec` to :ref:`mjModel`. A spec can be edited and compiled multi
 :ref:`mjModel` instance that takes the edits into account.
 If compilation fails, :ref:`mj_compile` returns ``NULL``; the error can be read with :ref:`mjs_getError`.
 
+.. _mj_copyBack:
+
+`mj_copyBack <#mj_copyBack>`__
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mj_copyBack
+
+Copy real-valued arrays from model to spec, returns 1 on success.
+
 .. _mj_recompile:
 
 `mj_recompile <#mj_recompile>`__

doc/changelog.rst

@@ -17,6 +17,7 @@ General
 - Added new sub-component :ref:`mj_makeM` which combines the :ref:`mj_crb` call with additional logic to support the
   introduction in 3.3.1 of :ref:`tendon armature<tendon-spatial-armature>`. In addition to the traditional
   ``mjData.qM``, :ref:`mj_makeM` also computes ``mjData.M``, a CSR representation of the same matrix.
+- Added a new function :ref:`mj_copyBack` to copy real-valued arrays in an mjModel to a compatible mjSpec.
 
 Simulate
 ^^^^^^^^

doc/includes/references.h

@@ -2992,6 +2992,7 @@ mjModel* mj_loadXML(const char* filename, const mjVFS* vfs, char* error, int err
 mjSpec* mj_parseXML(const char* filename, const mjVFS* vfs, char* error, int error_sz);
 mjSpec* mj_parseXMLString(const char* xml, const mjVFS* vfs, char* error, int error_sz);
 mjModel* mj_compile(mjSpec* s, const mjVFS* vfs);
+int mj_copyBack(mjSpec* s, const mjModel* m);
 int mj_recompile(mjSpec* s, const mjVFS* vfs, mjModel* m, mjData* d);
 int mj_saveLastXML(const char* filename, const mjModel* m, char* error, int error_sz);
 void mj_freeLastXML(void);