- Added passive forces plugins

- Added new `cable` composite type:
  * The `initial` parameter specifies the joint at the starting boundary: `free`, `ball`, or `none`.
  * The boundary bodies are exposed with the names:`B_left` and `B_right`.
  * The vertex initial positions can be specified directly in the XML with the parameter `vertex`.
  * The orientation of the body frame **is** the orientation of the material frame of the curve.

- Added new `cable` passive force plugin:
  * Twist and bending stiffness can be set separately with the parameters `twist` and `bend`.
  * The stress-free configuration can be set to be the initial one or flat with the flag `flat`.
  * New cable example showing the formation of plectoneme.
  * New coil example.
  * New belt example showing interaction between twist and anisotropy.
  * Added test using cantilever exact solution.

PiperOrigin-RevId: 480033694
Change-Id: I491271bce8fccb185961477e903e5a72d172c8a3

include/mujoco/mjmodel.h

@@ -632,6 +632,7 @@ struct mjModel_ {
   mjtNum*   body_inertia;         // diagonal inertia in ipos/iquat frame     (nbody x 3)
   mjtNum*   body_invweight0;      // mean inv inert in qpos0 (trn, rot)       (nbody x 2)
   mjtNum*   body_user;            // user data                                (nbody x nuser_body)
+  int*      body_plugin;          // plugin instance id (-1 if not in use)    (nbody x 1)
 
   // joints
   int*      jnt_type;             // type of joint (mjtJoint)                 (njnt x 1)
@@ -864,7 +865,7 @@ struct mjModel_ {
   mjtNum*   actuator_length0;     // actuator length in qpos0                 (nu x 1)
   mjtNum*   actuator_lengthrange; // feasible actuator length range           (nu x 2)
   mjtNum*   actuator_user;        // user data                                (nu x nuser_actuator)
-  int*      actuator_plugin;      // plugin instance id; -1: not a plugin actuator  (nu x 1)
+  int*      actuator_plugin;      // plugin instance id; -1: not a plugin     (nu x 1)
 
   // sensors
   int*      sensor_type;          // sensor type (mjtSensor)                  (nsensor x 1)
@@ -879,11 +880,12 @@ struct mjModel_ {
   mjtNum*   sensor_cutoff;        // cutoff for real and positive; 0: ignore  (nsensor x 1)
   mjtNum*   sensor_noise;         // noise standard deviation                 (nsensor x 1)
   mjtNum*   sensor_user;          // user data                                (nsensor x nuser_sensor)
-  int*      sensor_plugin;        // plugin instance id; -1: not a plugin sensor  (nsensor x 1)
+  int*      sensor_plugin;        // plugin instance id; -1: not a plugin     (nsensor x 1)
 
   // plugin instances
   int*      plugin;               // globally registered plugin slot number   (nplugin x 1)
   int*      plugin_stateadr;      // address in the plugin state array        (nplugin x 1)
+  int*      plugin_statenum;      // number of states in the plugin instance  (nplugin x 1)
   char*     plugin_attr;          // config attributes of plugin instances    (npluginattr x 1)
   int*      plugin_attradr;       // address to each instance's config attrib (nplugin x 1)
 

include/mujoco/mjplugin.h

@@ -20,7 +20,8 @@
 
 typedef enum mjtPluginTypeBit_ {
   mjPLUGIN_ACTUATOR = 1<<0,
-  mjPLUGIN_SENSOR   = 1<<1
+  mjPLUGIN_SENSOR   = 1<<1,
+  mjPLUGIN_PASSIVE  = 1<<2,
 } mjtPluginTypeBit;
 
 struct mjpPlugin_ {
@@ -38,8 +39,8 @@ struct mjpPlugin_ {
   // dimension of the specified sensor's output (required only for sensor plugins)
   int (*nsensordata)(const mjModel* m, int instance, int sensor_id);
 
-  // called when a new mjData is being created (required)
-  void (*init)(const mjModel* m, mjData* d, int instance);
+  // called when a new mjData is being created (required), returns 0 on success or -1 on failure
+  int (*init)(const mjModel* m, mjData* d, int instance);
 
   // called when an mjData is being freed (optional)
   void (*destroy)(mjData* d, int instance);

include/mujoco/mjxmacro.h

@@ -171,6 +171,7 @@
     X( mjtNum,  body_inertia,          nbody,         3                    ) \
     X( mjtNum,  body_invweight0,       nbody,         2                    ) \
     X( mjtNum,  body_user,             nbody,         MJ_M(nuser_body)     ) \
+    X( int,     body_plugin,           nbody,         1                    ) \
     X( int,     jnt_type,              njnt,          1                    ) \
     X( int,     jnt_qposadr,           njnt,          1                    ) \
     X( int,     jnt_dofadr,            njnt,          1                    ) \
@@ -385,6 +386,7 @@
     X( int,     sensor_plugin,         nsensor,       1                    ) \
     X( int,     plugin,                nplugin,       1                    ) \
     X( int,     plugin_stateadr,       nplugin,       1                    ) \
+    X( int,     plugin_statenum,       nplugin,       1                    ) \
     X( char,    plugin_attr,           npluginattr,   1                    ) \
     X( int,     plugin_attradr,        nplugin,       1                    ) \
     X( int,     numeric_adr,           nnumeric,      1                    ) \