data/src/engine/engine_setconst.c

// Copyright 2021 DeepMind Technologies Limited
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "engine/engine_setconst.h"

#include <stdio.h>
#include <string.h>

#include <mujoco/mjdata.h>
#include <mujoco/mjmacro.h>
#include <mujoco/mjmodel.h>
#include <mujoco/mjsan.h>  // IWYU pragma: keep
#include "engine/engine_core_constraint.h"
#include "engine/engine_core_smooth.h"
#include "engine/engine_forward.h"
#include "engine/engine_io.h"
#include "engine/engine_support.h"
#include "engine/engine_util_blas.h"
#include "engine/engine_util_errmem.h"
#include "engine/engine_util_misc.h"
#include "engine/engine_util_sparse.h"
#include "engine/engine_util_spatial.h"


// compute dof_M0 via composite rigid body algorithm
static void mj_setM0(mjModel* m, mjData* d) {
  mjtNum buf[6];
  mjtNum* crb = d->crb;
  int last_body = m->nbody - 1, nv = m->nv;

  // copy cinert into crb
  mju_copy(crb, d->cinert, 10*m->nbody);

  // backward pass over bodies, accumulate composite inertias
  for (int i=last_body; i > 0; i--) {
    if (m->body_parentid[i] > 0) {
      mju_addTo(crb+10*m->body_parentid[i], crb+10*i, 10);
    }
  }

  for (int i=0; i < nv; i++) {
    // precomute buf = crb_body_i * cdof_i
    mju_mulInertVec(buf, crb+10*m->dof_bodyid[i], d->cdof+6*i);

    // dof_M0(i) = armature inertia + cdof_i * (crb_body_i * cdof_i)
    m->dof_M0[i] = m->dof_armature[i] + mju_dot(d->cdof+6*i, buf, 6);
  }
}



// set quantities that depend on qpos0
static void set0(mjModel* m, mjData* d) {
  int nv = m->nv;
  mjtNum A[36] = {0}, pos[3], quat[4];
  mj_markStack(d);
  mjtNum* jac = mjSTACKALLOC(d, 6*nv, mjtNum);
  mjtNum* tmp = mjSTACKALLOC(d, 6*nv, mjtNum);
  mjtNum* moment = mjSTACKALLOC(d, nv, mjtNum);
  int* cammode = 0;
  int* lightmode = 0;

  // save camera and light mode, set to fixed
  if (m->ncam) {
    cammode = mjSTACKALLOC(d, m->ncam, int);
    for (int i=0; i < m->ncam; i++) {
      cammode[i] = m->cam_mode[i];
      m->cam_mode[i] = mjCAMLIGHT_FIXED;
    }
  }
  if (m->nlight) {
    lightmode = mjSTACKALLOC(d, m->nlight, int);
    for (int i=0; i < m->nlight; i++) {
      lightmode[i] = m->light_mode[i];
      m->light_mode[i] = mjCAMLIGHT_FIXED;
    }
  }

  // run computations in qpos0
  mju_copy(d->qpos, m->qpos0, m->nq);
  mj_kinematics(m, d);
  mj_comPos(m, d);
  mj_camlight(m, d);

  // compute dof_M0 for CRB algorithm
  mj_setM0(m, d);

  // save flex_rigid, temporarily make all flexes non-rigid
  mjtByte* rigid = mju_malloc(m->nflex);
  memcpy(rigid, m->flex_rigid, m->nflex);
  memset(m->flex_rigid, 0, m->nflex);

  // run remaining computations
  mj_tendon(m, d);
  mj_makeM(m, d);
  mj_factorM(m, d);
  mj_flex(m, d);
  mj_transmission(m, d);

  // restore flex rigidity
  memcpy(m->flex_rigid, rigid, m->nflex);
  mju_free(rigid);

  // restore camera and light mode
  for (int i=0; i < m->ncam; i++) {
    m->cam_mode[i] = cammode[i];
  }
  for (int i=0; i < m->nlight; i++) {
    m->light_mode[i] = lightmode[i];
  }

  // copy fields
  mju_copy(m->flexedge_length0, d->flexedge_length, m->nflexedge);
  mju_copy(m->tendon_length0, d->ten_length, m->ntendon);
  mju_copy(m->actuator_length0, d->actuator_length, m->nu);

  // compute body_invweight0
  m->body_invweight0[0] = m->body_invweight0[1] = 0.0;
  for (int i=1; i < m->nbody; i++) {
    // static bodies: zero invweight0
    if (m->body_weldid[i] == 0) {
      m->body_invweight0[2*i] = m->body_invweight0[2*i+1] = 0;
    }

    // accelerate simple bodies with no rotations
    else if (m->body_simple[i] == 2) {
      mjtNum mass = m->body_mass[i];
      if (!mass) {  // SHOULD NOT OCCUR
        mjERROR("moving body %d has 0 mass", i);
      }
      m->body_invweight0[2*i+0] = 1/mju_max(mjMINVAL, mass);
      m->body_invweight0[2*i+1] = 0;
    }

    // general body: full inertia
    else {
      if (nv) {
        // inverse spatial inertia: A = J*inv(M)*J'
        mj_jacBodyCom(m, d, jac, jac+3*nv, i);
        mj_solveM(m, d, tmp, jac, 6);
        mju_mulMatMatT(A, jac, tmp, 6, nv, 6);
      }

      // average diagonal and assign
      m->body_invweight0[2*i] = (A[0] + A[7] + A[14])/3;
      m->body_invweight0[2*i+1] = (A[21] + A[28] + A[35])/3;
    }
  }

  // compute dof_invweight0
  for (int i=0; i < m->njnt; i++) {
    // simple body with no rotations: no off-diagonal inertia
    if (m->body_simple[m->jnt_bodyid[i]] == 2) {
      int id = m->jnt_dofadr[i];
      int bi = m->jnt_bodyid[i];
      mjtNum mass = m->body_mass[bi];
      if (!mass) {  // SHOULD NOT OCCUR
        mjERROR("moving body %d has 0 mass", bi);
      }
      m->dof_invweight0[id] = 1/mju_max(mjMINVAL, mass);
    }

    // general joint: full inertia
    else {
      int dnum, id = m->jnt_dofadr[i];

      // get number of components
      if (m->jnt_type[i] == mjJNT_FREE) {
        dnum = 6;
      } else if (m->jnt_type[i] == mjJNT_BALL) {
        dnum = 3;
      } else {
        dnum = 1;
      }

      // inverse joint inertia:  A = J*inv(M)*J'
      if (nv) {
        mju_zero(jac, dnum*nv);
        for (int j=0; j < dnum; j++) {
          jac[j*(nv+1) + id] = 1;
        }
        mj_solveM(m, d, tmp, jac, dnum);
        mju_mulMatMatT(A, jac, tmp, dnum, nv, dnum);
      }

      // average diagonal and assign
      if (dnum == 6) {
        m->dof_invweight0[id] = m->dof_invweight0[id+1] = m->dof_invweight0[id+2] =
          (A[0] + A[7] + A[14])/3;
        m->dof_invweight0[id+3] = m->dof_invweight0[id+4] = m->dof_invweight0[id+5] =
          (A[21] + A[28] + A[35])/3;
      } else if (dnum == 3) {
        m->dof_invweight0[id] = m->dof_invweight0[id+1] = m->dof_invweight0[id+2] =
          (A[0] + A[4] + A[8])/3;
      } else {
        m->dof_invweight0[id] = A[0];
      }
    }
  }

  // compute flexedge_invweight0, tendon_invweight0, actuator_acc0
  if (nv) {
    // compute flexedge_invweight0
    for (int f=0; f < m->nflex; f++) {
      if (m->flex_interp[f]) {
        continue;
      }

      for (int i=m->flex_edgeadr[f]; i < m->flex_edgeadr[f]+m->flex_edgenum[f]; i++) {
        // bodies connected by edge
        int b1 = m->flex_vertbodyid[m->flex_vertadr[f] + m->flex_edge[2*i]];
        int b2 = m->flex_vertbodyid[m->flex_vertadr[f] + m->flex_edge[2*i+1]];

        // rigid edge: set to 0
        if (m->flexedge_rigid[i]) {
          m->flexedge_invweight0[i] = 0;
        }

        // accelerate edges that connect simple bodies with no rotations
        else if (m->body_simple[b1] == 2 && m->body_simple[b2] == 2) {
          m->flexedge_invweight0[i] = (1/m->body_mass[b1] + 1/m->body_mass[b2])/2;
        }

        // handle general edge
        else {
          // make dense vector into tmp
          if (mj_isSparse(m)) {
            mju_zero(tmp, nv);
            int end = d->flexedge_J_rowadr[i] + d->flexedge_J_rownnz[i];
            for (int j=d->flexedge_J_rowadr[i]; j < end; j++) {
              tmp[d->flexedge_J_colind[j]] = d->flexedge_J[j];
            }
          } else {
            mju_copy(tmp, d->flexedge_J+i*nv, nv);
          }

          // solve into tmp+nv
          mj_solveM(m, d, tmp+nv, tmp, 1);
          m->flexedge_invweight0[i] = mju_dot(tmp, tmp+nv, nv);
        }
      }
    }

    // compute tendon_invweight0
    for (int i=0; i < m->ntendon; i++) {
      // make dense vector into tmp
      if (mj_isSparse(m)) {
        mju_zero(tmp, nv);
        int end = d->ten_J_rowadr[i] + d->ten_J_rownnz[i];
        for (int j=d->ten_J_rowadr[i]; j < end; j++) {
          tmp[d->ten_J_colind[j]] = d->ten_J[j];
        }
      } else {
        mju_copy(tmp, d->ten_J+i*nv, nv);
      }

      // solve into tmp+nv
      mj_solveM(m, d, tmp+nv, tmp, 1);
      m->tendon_invweight0[i] = mju_dot(tmp, tmp+nv, nv);
    }

    // compute actuator_acc0
    for (int i=0; i < m->nu; i++) {
      mju_sparse2dense(moment, d->actuator_moment, 1, nv, d->moment_rownnz + i,
                       d->moment_rowadr + i, d->moment_colind);
      mj_solveM(m, d, tmp, moment, 1);
      m->actuator_acc0[i] = mju_norm(tmp, nv);
    }
  } else {
    for (int i=0; i < m->ntendon; i++) {
      m->tendon_invweight0[i] = 0;
    }
    for (int i=0; i < m->nu; i++) {
      m->actuator_acc0[i] = 0;
    }
  }

  // compute missing eq_data for body constraints
  for (int i=0; i < m->neq; i++) {
    // get ids
    int id1 = m->eq_obj1id[i];
    int id2 = m->eq_obj2id[i];

    // connect constraint
    if (m->eq_type[i] == mjEQ_CONNECT) {
      switch ((mjtObj) m->eq_objtype[i]) {
        case mjOBJ_BODY:
          // pos = anchor position in global frame
          mj_local2Global(d, pos, 0, m->eq_data+mjNEQDATA*i, 0, id1, 0);

          // data[3-5] = anchor position in body2 local frame
          mju_subFrom3(pos, d->xpos+3*id2);
          mju_mulMatTVec3(m->eq_data+mjNEQDATA*i+3, d->xmat+9*id2, pos);
          break;
        case mjOBJ_SITE:
          // site-based connect, eq_data is unused
          mju_zero(m->eq_data+mjNEQDATA*i, mjNEQDATA);
          break;
        default:
          mjERROR("invalid objtype in connect constraint %d", i);
      }
    }

    // weld constraint
    else if (m->eq_type[i] == mjEQ_WELD) {
      switch ((mjtObj) m->eq_objtype[i]) {
        case mjOBJ_BODY: {
          // skip if user has set any quaternion data
          if (!mju_isZero(m->eq_data + mjNEQDATA*i + 6, 4)) {
            // normalize quaternion just in case
            mju_normalize4(m->eq_data+mjNEQDATA*i+6);
            continue;
          }

          // anchor position is in body2 local frame
          mj_local2Global(d, pos, 0, m->eq_data+mjNEQDATA*i, 0, id2, 0);

          // data[3-5] = anchor position in body1 local frame
          mju_subFrom3(pos, d->xpos+3*id1);
          mju_mulMatTVec3(m->eq_data+mjNEQDATA*i+3, d->xmat+9*id1, pos);

          // data[6-9] = neg(xquat1)*xquat2 = "xquat2-xquat1" in body1 local frame
          mju_negQuat(quat, d->xquat+4*id1);
          mju_mulQuat(m->eq_data+mjNEQDATA*i+6, quat, d->xquat+4*id2);
          break;
        }
        case mjOBJ_SITE: {
          break;
        }
        default:
          mjERROR("invalid objtype in weld constraint %d", i);
      }
    }
  }

  // camera compos0, pos0, mat0
  for (int i=0; i < m->ncam; i++) {
    // get body ids
    int id = m->cam_bodyid[i];              // camera body
    int id1 = m->cam_targetbodyid[i];       // target body

    // compute positional offsets
    mju_sub3(m->cam_pos0+3*i, d->cam_xpos+3*i, d->xpos+3*id);
    mju_sub3(m->cam_poscom0+3*i, d->cam_xpos+3*i, d->subtree_com+ (id1 >= 0 ? 3*id1 : 3*id));

    // copy mat
    mju_copy(m->cam_mat0+9*i, d->cam_xmat+9*i, 9);
  }

  // light compos0, pos0, dir0
  for (int i=0; i < m->nlight; i++) {
    // get body ids
    int id = m->light_bodyid[i];            // light body
    int id1 = m->light_targetbodyid[i];     // target body

    // compute positional offsets
    mju_sub3(m->light_pos0+3*i, d->light_xpos+3*i, d->xpos+3*id);
    mju_sub3(m->light_poscom0+3*i, d->light_xpos+3*i, d->subtree_com + (id1 >= 0 ? 3*id1 : 3*id));

    // copy dir
    mju_copy3(m->light_dir0+3*i, d->light_xdir+3*i);
  }

  // compute actuator damping from dampratio
  for (int i=0; i < m->nu; i++) {
    // get bias, gain parameters
    mjtNum* biasprm = m->actuator_biasprm + i*mjNBIAS;
    mjtNum* gainprm = m->actuator_gainprm + i*mjNGAIN;

    // not a position-like actuator: skip
    if (gainprm[0] != -biasprm[1]) {
      continue;
    }

    // damping is 0 or negative (interpreted as regular "kv"): skip
    if (biasprm[2] <= 0) {
      continue;
    }

    // === interpret biasprm[2] > 0 as dampratio for position-like actuators

    // "reflected" inertia (inversely scaled by transmission squared)
    int rownnz = d->moment_rownnz[i];
    int rowadr = d->moment_rowadr[i];
    mjtNum* transmission = d->actuator_moment + rowadr;
    mjtNum mass = 0;
    for (int j=0; j < rownnz; j++) {
      mjtNum trn = mju_abs(transmission[j]);
      mjtNum trn2 = trn*trn;  // transmission squared
      if (trn2 > mjMINVAL) {
        int dof = d->moment_colind[rowadr + j];
        mass += m->dof_M0[dof] / trn2;
      }
    }

    // damping = dampratio * 2 * sqrt(kp * mass)
    mjtNum damping = biasprm[2] * 2 * mju_sqrt(gainprm[0] * mass);

    // set biasprm[2] to negative damping
    biasprm[2] = -damping;
  }

  mj_freeStack(d);
}



// accumulate bounding box
static void updateBox(mjtNum* xmin, mjtNum* xmax, mjtNum* pos, mjtNum radius) {
  for (int i=0; i < 3; i++) {
    xmin[i] = mjMIN(xmin[i], pos[i] - radius);
    xmax[i] = mjMAX(xmax[i], pos[i] + radius);
  }
}


// compute stat; assume computations already executed in qpos0
static void setStat(mjModel* m, mjData* d) {
  mjtNum xmin[3] = {1E+10, 1E+10, 1E+10};
  mjtNum xmax[3] = {-1E+10, -1E+10, -1E+10};
  mjtNum rbound;
  mj_markStack(d);
  mjtNum* body = mjSTACKALLOC(d, m->nbody, mjtNum);

  // compute bounding box of bodies, joint centers, geoms and sites
  for (int i=1; i < m->nbody; i++) {
    updateBox(xmin, xmax, d->xpos+3*i, 0);
    updateBox(xmin, xmax, d->xipos+3*i, 0);
  }
  for (int i=0; i < m->njnt; i++) {
    updateBox(xmin, xmax, d->xanchor+3*i, 0);
  }
  for (int i=0; i < m->nsite; i++) {
    updateBox(xmin, xmax, d->site_xpos+3*i, 0);
  }
  for (int i=0; i < m->ngeom; i++) {
    // set rbound: regular geom rbound, or 0.1 of plane or hfield max size
    rbound = 0;
    if (m->geom_rbound[i] > 0) {
      rbound = m->geom_rbound[i];
    } else if (m->geom_type[i] == mjGEOM_PLANE) {
      // finite in at least one direction
      if (m->geom_size[3*i] || m->geom_size[3*i+1]) {
        rbound = mjMAX(m->geom_size[3*i], m->geom_size[3*i+1]) * 0.1;
      }

      // infinite in both directions
      else {
        rbound = 1;
      }
    } else if (m->geom_type[i] == mjGEOM_HFIELD) {
      int j = m->geom_dataid[i];
      rbound = mjMAX(m->hfield_size[4*j],
                     mjMAX(m->hfield_size[4*j+1],
                           mjMAX(m->hfield_size[4*j+2], m->hfield_size[4*j+3]))) * 0.1;
    }

    updateBox(xmin, xmax, d->geom_xpos+3*i, rbound);
  }

  // compute center
  mju_add3(m->stat.center, xmin, xmax);
  mju_scl3(m->stat.center, m->stat.center, 0.5);

  // compute bounding box size
  if (xmax[0] > xmin[0])
    m->stat.extent = mju_max(1E-5,
                             mju_max(xmax[0]-xmin[0], mju_max(xmax[1]-xmin[1], xmax[2]-xmin[2])));

  // set body size to max com-joint distance
  mju_zero(body, m->nbody);
  for (int i=0; i < m->njnt; i++) {
    // handle this body
    int id = m->jnt_bodyid[i];
    body[id] = mju_max(body[id], mju_dist3(d->xipos+3*id, d->xanchor+3*i));

    // handle parent body
    id = m->body_parentid[id];
    body[id] = mju_max(body[id], mju_dist3(d->xipos+3*id, d->xanchor+3*i));
  }
  body[0] = 0;

  // set body size to max of old value, and geom rbound + com-geom dist
  for (int i=1; i < m->nbody; i++) {
    for (int id=m->body_geomadr[i]; id < m->body_geomadr[i]+m->body_geomnum[i]; id++) {
      if (m->geom_rbound[id] > 0) {
        body[i] = mju_max(body[i], m->geom_rbound[id] + mju_dist3(d->xipos+3*i, d->geom_xpos+3*id));
      }
    }
  }

  // adjust body size for flex edges involving body
  for (int f=0; f < m->nflex; f++) {
    if (m->flex_interp[f]) {
      for (int v1=m->flex_nodeadr[f]; v1 < m->flex_nodeadr[f]+m->flex_nodenum[f]; v1++) {
        for (int v2=m->flex_nodeadr[f]; v2 < m->flex_nodeadr[f]+m->flex_nodenum[f]; v2++) {
          mjtNum edge = mju_dist3(d->xpos+3*m->flex_nodebodyid[v1],
                                  d->xpos+3*m->flex_nodebodyid[v2]);
          body[m->flex_nodebodyid[v1]] = mju_max(body[m->flex_nodebodyid[v1]], edge);
        }
      }
      continue;
    }
    for (int e=m->flex_edgeadr[f]; e < m->flex_edgeadr[f]+m->flex_edgenum[f]; e++) {
      int b1 = m->flex_vertbodyid[m->flex_vertadr[f]+m->flex_edge[2*e]];
      int b2 = m->flex_vertbodyid[m->flex_vertadr[f]+m->flex_edge[2*e+1]];

      body[b1] = mju_max(body[b1], m->flexedge_length0[e]);
      body[b2] = mju_max(body[b2], m->flexedge_length0[e]);
    }
  }

  // compute meansize, make sure all sizes are above min
  if (m->nbody > 1) {
    m->stat.meansize = 0;
    for (int i=1; i < m->nbody; i++) {
      body[i] = mju_max(body[i], 1E-5);
      m->stat.meansize += body[i]/(m->nbody-1);
    }
  }

  // fix extent if too small compared to meanbody
  m->stat.extent = mju_max(m->stat.extent, 2 * m->stat.meansize);

  // compute meanmass
  if (m->nbody > 1) {
    m->stat.meanmass = 0;
    for (int i=1; i < m->nbody; i++) {
      m->stat.meanmass += m->body_mass[i];
    }
    m->stat.meanmass /= (m->nbody-1);
  }

  // compute meaninertia
  if (m->nv) {
    m->stat.meaninertia = 0;
    for (int i=0; i < m->nv; i++) {
      m->stat.meaninertia += d->qM[m->dof_Madr[i]];
    }
    m->stat.meaninertia /= m->nv;
  }

  mj_freeStack(d);
}



// set quantities that depend on qpos_spring
static void setSpring(mjModel* m, mjData* d) {
  // run computations in qpos_spring
  mju_copy(d->qpos, m->qpos_spring, m->nq);
  mj_kinematics(m, d);
  mj_comPos(m, d);
  mj_tendon(m, d);
  mj_transmission(m, d);

  // copy if model spring length is -1
  for (int i=0; i < m->ntendon; i++) {
    if (m->tendon_lengthspring[2*i] == -1 && m->tendon_lengthspring[2*i+1] == -1) {
      // explicit springlength unused, set equal to ten_length
      m->tendon_lengthspring[2*i] = m->tendon_lengthspring[2*i+1] = d->ten_length[i];
    }
  }
}



// entry point: set all constant fields of mjModel, except for lengthrange
void mj_setConst(mjModel* m, mjData* d) {
  // compute subtreemass
  for (int i=0; i < m->nbody; i++) {
    m->body_subtreemass[i] = m->body_mass[i];
  }
  for (int i=m->nbody-1; i > 0; i--) {
    m->body_subtreemass[m->body_parentid[i]] += m->body_subtreemass[i];
  }

  // call functions
  set0(m, d);
  setStat(m, d);
  setSpring(m, d);
}



//----------------------------- actuator length range computation ----------------------------------

// evaluate actuator length, advance special dynamics
static mjtNum evalAct(const mjModel* m, mjData* d, int index, int side,
                      const mjLROpt* opt) {
  int nv = m->nv;

  // reduce velocity
  mju_scl(d->qvel, d->qvel, mju_exp(-m->opt.timestep/mjMAX(0.01, opt->timeconst)), nv);

  // step1: compute inertia and actuator moments
  mj_step1(m, d);

  // dense actuator_moment row
  mj_markStack(d);
  mjtNum* moment = mjSTACKALLOC(d, nv, mjtNum);
  mju_sparse2dense(moment, d->actuator_moment, 1, nv, d->moment_rownnz + index,
                   d->moment_rowadr + index, d->moment_colind);

  // set force to generate desired acceleration
  mj_solveM(m, d, d->qfrc_applied, moment, 1);
  mjtNum nrm = mju_norm(d->qfrc_applied, nv);
  mju_scl(d->qfrc_applied, moment, (2*side-1)*opt->accel/mjMAX(mjMINVAL, nrm), nv);

  // impose maxforce
  nrm = mju_norm(d->qfrc_applied, nv);
  if (opt->maxforce > 0 && nrm > opt->maxforce) {
    mju_scl(d->qfrc_applied, d->qfrc_applied, opt->maxforce/mjMAX(mjMINVAL, nrm), nv);
  }

  // step2: apply force
  mj_step2(m, d);

  mj_freeStack(d);

  // return actuator length
  return d->actuator_length[index];
}



// Set length range for specified actuator, return 1 if ok, 0 if error.
int mj_setLengthRange(mjModel* m, mjData* d, int index,
                      const mjLROpt* opt, char* error, int error_sz) {
  // check index
  if (index < 0 || index >= m->nu) {
    mjERROR("invalid actuator index");
  }

  // skip depending on mode and type
  int ismuscle = (m->actuator_gaintype[index] == mjGAIN_MUSCLE ||
                  m->actuator_biastype[index] == mjBIAS_MUSCLE);
  int isuser = (m->actuator_gaintype[index] == mjGAIN_USER ||
                m->actuator_biastype[index] == mjBIAS_USER);
  if ((opt->mode == mjLRMODE_NONE) ||
      (opt->mode == mjLRMODE_MUSCLE && !ismuscle) ||
      (opt->mode == mjLRMODE_MUSCLEUSER && !ismuscle && !isuser)) {
    return 1;
  }

  // use existing length range if available
  if (opt->useexisting && (m->actuator_lengthrange[2*index] < m->actuator_lengthrange[2*index+1])) {
    return 1;
  }

  // get transmission id
  int threadid = m->actuator_trnid[index];

  // use joint and tendon limits if available
  if (opt->uselimit) {
    // joint or jointinparent
    if (m->actuator_trntype[index] == mjTRN_JOINT ||
        m->actuator_trntype[index] == mjTRN_JOINTINPARENT) {
      // make sure joint is limited
      if (m->jnt_limited[threadid]) {
        // copy range
        m->actuator_lengthrange[2*index] = m->jnt_range[2*threadid];
        m->actuator_lengthrange[2*index+1] = m->jnt_range[2*threadid+1];

        // skip optimization
        return 1;
      }
    }

    // tendon
    if (m->actuator_trntype[index] == mjTRN_TENDON) {
      // make sure tendon is limited
      if (m->tendon_limited[threadid]) {
        // copy range
        m->actuator_lengthrange[2*index] = m->tendon_range[2*threadid];
        m->actuator_lengthrange[2*index+1] = m->tendon_range[2*threadid+1];

        // skip optimization
        return 1;
      }
    }
  }

  // optimize in both directions
  mjtNum lmin[2] = {0, 0}, lmax[2] = {0, 0};
  int side;
  for (side=0; side < 2; side++) {
    // init at qpos0
    mj_resetData(m, d);

    // simulate
    int updated = 0;
    while (d->time < opt->inttotal) {
      // advance and get length
      mjtNum len = evalAct(m, d, index, side, opt);

      // reset: cannot proceed
      if (d->time == 0) {
        snprintf(error, error_sz, "Unstable lengthrange simulation in actuator %d", index);
        return 0;
      }

      // update limits
      if (d->time > opt->inttotal-opt->interval) {
        if (len < lmin[side] || !updated) {
          lmin[side] = len;
        }
        if (len > lmax[side] || !updated) {
          lmax[side] = len;
        }

        updated = 1;
      }
    }

    // assign
    m->actuator_lengthrange[2*index+side] = (side == 0 ? lmin[side] : lmax[side]);
  }

  // check range
  mjtNum dif = m->actuator_lengthrange[2*index+1] - m->actuator_lengthrange[2*index];
  if (dif <= 0) {
    snprintf(error, error_sz,
             "Invalid lengthrange (%g, %g) in actuator %d",
             m->actuator_lengthrange[2*index],
             m->actuator_lengthrange[2*index+1], index);
    return 0;
  }

  // check convergence, side 0
  if (lmax[0]-lmin[0] > opt->tolrange*dif) {
    snprintf(error, error_sz,
             "Lengthrange computation did not converge in actuator %d:\n"
             "  eval (%g, %g)\n  range (%g, %g)",
             index, lmin[0], lmax[0],
             m->actuator_lengthrange[2*index],
             m->actuator_lengthrange[2*index+1]);
    return 0;
  }

  // check convergence, side 1
  if (lmax[1]-lmin[1] > opt->tolrange*dif) {
    snprintf(error, error_sz,
             "Lengthrange computation did not converge in actuator %d:\n"
             "  eval (%g, %g)\n range (%g, %g)",
             index, lmin[1], lmax[1],
             m->actuator_lengthrange[2*index],
             m->actuator_lengthrange[2*index+1]);
    return 0;
  }

  return 1;
}