mujoco

// 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 "xml/xml_native_reader.h"

#include <array>
#include <cstddef>
#include <cstdio>
#include <cstring>
#include <functional>
#include <limits>
#include <map>
#include <optional>
#include <sstream>
#include <string>
#include <string_view>
#include <type_traits>
#include <utility>
#include <vector>

#include <mujoco/mujoco.h>
#include <mujoco/mjmodel.h>
#include <mujoco/mjtype.h>
#include <mujoco/mjvisualize.h>
#include "engine/engine_util_misc.h"
#include <mujoco/mjspec.h>
#include "user/user_api.h"
#include "user/user_composite.h"
#include "user/user_flexcomp.h"
#include "user/user_util.h"
#include "xml/xml_base.h"
#include "xml/xml_util.h"
#include "tinyxml2.h"

namespace {
using std::string;
using std::string_view;
using std::vector;
using mujoco::user::FilePath;
using tinyxml2::XMLElement;



//---------------------------------- helper utilities ----------------------------------------------

// GetAttrPtr: overload for scalar and pointer fields
template <typename T>
inline auto GetAttrPtr(T& val) -> std::enable_if_t<!std::is_array_v<T>, decltype(&val)> {
  if constexpr (std::is_pointer_v<T>) {
    return val;
  } else {
    return &val;
  }
}

// GetAttrPtr: overload for array fields
template <typename T, size_t N>
inline T* GetAttrPtr(T (&arr)[N]) { return arr; }


// helper class for reading attributes while recording authored bits
struct Reader {
  Reader(XMLElement* xml_node, const void* elem) : xml_node_(xml_node), elem_(elem) {}

  template <typename T>
  int operator()(const char* attr, int len, T& data, bool required = false, bool exact = true) {
    int res = mjXReader::ReadAttr(xml_node_, attr, len, GetAttrPtr(data), text_, required, exact);
    if (res) mjs_setAuthored(elem_, &data, 1);
    return res;
  }

  template <typename T>
  bool operator()(const char* attr, T& data, const mjMap* map, int mapsz, bool required = false) {
    int map_val_temp;
    bool res = mjXReader::MapValue(xml_node_, attr, &map_val_temp, map, mapsz, required);
    if (res) {
      data = static_cast<T>(map_val_temp);
      mjs_setAuthored(elem_, &data, 1);
    }
    return res;
  }

  bool operator()(const char* attr, int& data, bool required = false) {
    bool res = mjXUtil::ReadAttrInt(xml_node_, attr, &data, required);
    if (res) mjs_setAuthored(elem_, &data, 1);
    return res;
  }

  bool operator()(const char* attr, mjString* target) {
    std::string txt_temp;
    bool res = mjXUtil::ReadAttrTxt(xml_node_, attr, txt_temp);
    if (res) {
      mjs_setString(target, txt_temp.c_str());
      mjs_setAuthored(elem_, target, 1);
    }
    return res;
  }

  template <typename T>
  bool txt(const char* attr, T& target, void (&set_func)(T&, const char*)) {
    std::string txt_temp;
    bool res = mjXUtil::ReadAttrTxt(xml_node_, attr, txt_temp);
    if (res) {
      set_func(target, txt_temp.c_str());
      mjs_setAuthored(elem_, &target, 1);
    }
    return res;
  }

  void set_node(XMLElement* node) { xml_node_ = node; }

  XMLElement* xml_node_;
  const void* elem_;
  std::string text_;
};

void ReadPluginConfigs(tinyxml2::XMLElement* elem, mjsPlugin* p) {
  std::map<string, string, std::less<> > config_attribs;
  XMLElement* child = FirstChildElement(elem);
  while (child) {
    string_view name = child->Value();
    if (name == "config") {
      string key, value;
      mjXUtil::ReadAttrTxt(child, "key", key, /* required = */ true);
      if (config_attribs.find(key) != config_attribs.end()) {
        string err = "duplicate config key: " + key;
        throw mjXError(child, "%s", err.c_str());
      }
      mjXUtil::ReadAttrTxt(child, "value", value, /* required = */ true);
      config_attribs[key] = value;
    }
    child = NextSiblingElement(child);
  }

  if (!p && !config_attribs.empty()) {
    throw mjXError(elem,
                   "plugin configuration attributes cannot be used in an "
                   "element that references a predefined plugin instance");
  } else if (p) {
    mjs_setPluginAttributes(p, &config_attribs);
  }
}

static void UpdateString(string& psuffix, int count, int i) {
  int ndigits = std::to_string(count).length();
  string i_string = std::to_string(i);
  string prefix = "";
  while (ndigits-- > i_string.length()) {
    prefix += '0';
  }
  psuffix += prefix + i_string;
}
}  // namespace


//---------------------------------- MJCF schema ---------------------------------------------------
#include "xml/generated/mjcf_table.inc"

//---------------------------------- class mjXReader implementation --------------------------------

// typed attribute rows, generated from mjcf.schema; the keyword maps the
// rows reference are generated into mjcf_map.h
#include "xml/generated/mjcf_read_table.inc"


mjXReader::mjXReader()
    : schema(MJCF, nMJCF, MJCF_constraints, nMJCF_constraints) {
  readingdefaults = false;
}



// print schema
void mjXReader::PrintSchema(std::stringstream& str, bool html, bool pad) {
  if (html) {
    schema.PrintHTML(str, 0, pad);
  } else {
    schema.Print(str, 0);
  }
}



// main entry point for XML parser
//  mjCModel is allocated here; caller is responsible for deallocation
void mjXReader::Parse(XMLElement* root, const mjVFS* vfs) {
  // check schema
  if (!schema.GetError().empty()) {
    throw mjXError(0, "XML Schema Construction Error: %s", schema.GetError().c_str());
  }

  // validate
  XMLElement* bad = 0;
  if ((bad = schema.Check(root, 0))) {
    throw mjXError(bad, "Schema violation: %s", schema.GetError().c_str());
  }

  // get model name
  string modelname;
  if (ReadAttrTxt(root, "model", modelname)) {
    mjs_setString(spec->modelname, modelname.c_str());
  }

  // get comment
  if (root->FirstChild() && root->FirstChild()->ToComment()) {
    mjs_setString(spec->comment, root->FirstChild()->Value());
  } else {
    mjs_setString(spec->comment, "");
  }

  //------------------- parse MuJoCo sections embedded in all XML formats

  for (XMLElement* section = FirstChildElement(root, "compiler"); section;
       section = NextSiblingElement(section, "compiler")) {
    Compiler(section, spec);
  }

  for (XMLElement* section = FirstChildElement(root, "option"); section;
       section = NextSiblingElement(section, "option")) {
    Option(section, spec, &spec->option);
  }

  for (XMLElement* section = FirstChildElement(root, "size"); section;
       section = NextSiblingElement(section, "size")) {
    Size(section, spec);
  }

  //------------------ parse MJCF-specific sections

  for (XMLElement* section = FirstChildElement(root, "visual"); section;
       section = NextSiblingElement(section, "visual")) {
    Visual(section);
  }

  for (XMLElement* section = FirstChildElement(root, "statistic"); section;
       section = NextSiblingElement(section, "statistic")) {
    Statistic(section);
  }

  readingdefaults = true;
  for (XMLElement* section = FirstChildElement(root, "default"); section;
       section = NextSiblingElement(section, "default")) {
    Default(section, nullptr, vfs);
  }
  readingdefaults = false;

  for (XMLElement* section = FirstChildElement(root, "extension"); section;
       section = NextSiblingElement(section, "extension")) {
    Extension(section);
  }

  for (XMLElement* section = FirstChildElement(root, "custom"); section;
       section = NextSiblingElement(section, "custom")) {
    Custom(section);
  }

  for (XMLElement* section = FirstChildElement(root, "asset"); section;
       section = NextSiblingElement(section, "asset")) {
    Asset(section, vfs);
  }

  for (XMLElement* section = FirstChildElement(root, "contact"); section;
       section = NextSiblingElement(section, "contact")) {
    Contact(section);
  }

  for (XMLElement* section = FirstChildElement(root, "deformable"); section;
       section = NextSiblingElement(section, "deformable")) {
    Deformable(section, vfs);
  }

  for (XMLElement* section = FirstChildElement(root, "equality"); section;
       section = NextSiblingElement(section, "equality")) {
    Equality(section);
  }

  for (XMLElement* section = FirstChildElement(root, "tendon"); section;
       section = NextSiblingElement(section, "tendon")) {
    Tendon(section);
  }

  for (XMLElement* section = FirstChildElement(root, "actuator"); section;
       section = NextSiblingElement(section, "actuator")) {
    Actuator(section);
  }

  for (XMLElement* section = FirstChildElement(root, "sensor"); section;
       section = NextSiblingElement(section, "sensor")) {
    Sensor(section);
  }

  for (XMLElement* section = FirstChildElement(root, "keyframe"); section;
       section = NextSiblingElement(section, "keyframe")) {
    Keyframe(section);
  }

  // set deepcopy flag to true to copy child specs during attach calls
  mjs_setDeepCopy(spec, true);

  for (XMLElement* section = FirstChildElement(root, "worldbody"); section;
       section = NextSiblingElement(section, "worldbody")) {
    Body(section, mjs_findBody(spec, "world"), nullptr, vfs);
  }

  // set deepcopy flag to false to disable copying during attach in all future calls
  mjs_setDeepCopy(spec, false);
}



// compiler section parser
void mjXReader::Compiler(XMLElement* section, mjSpec* s) {
  Reader read(section, s);

  // strippath is stored on the spec, not the compiler options
  read("strippath", s->strippath, bool_map, 2);

  // global coordinates are no longer supported
  if (int n = 0; MapValue(section, "coordinate", &n, coordinate_map, 2)) {
    if (n == 1) {
      throw mjXError(section, "global coordinates no longer supported. To convert existing models, "
                     "load and save them in MuJoCo 2.3.3 or older");
    }
  }
  if (ReadAttrTxt(section, "assetdir", read.text_)) {
    mjs_setString(s->compiler.meshdir, read.text_.c_str());
    mjs_setString(s->compiler.texturedir, read.text_.c_str());
  }
  // mechanical attributes; meshdir/texturedir override assetdir above
  ReadAttrTableCore(section, &s->compiler, kCompilerAttrs, kCompilerAttrsN,
                    /*skipnodefault=*/false, /*authored=*/s);

  // lengthrange subelement
  XMLElement* elem = FindSubElem(section, "lengthrange");
  if (elem) {
    ReadAttrTableCore(elem, &(s->compiler.LRopt), kLengthrangeAttrs,
                      kLengthrangeAttrsN, /*skipnodefault=*/false);
  }
}



// option section parser
void mjXReader::Option(XMLElement* section, mjSpec* s, mjOption* opt) {
  Reader read(section, s);

  // mechanical attributes
  ReadAttrTableCore(section, opt, kOptionAttrs, kOptionAttrsN,
                    /*skipnodefault=*/false, /*authored=*/s);

  // actuatorgroupdisable
  constexpr int num_bitflags = 31;
  int disabled_act_groups[num_bitflags];
  int num_found = read("actuatorgroupdisable", num_bitflags,
                            disabled_act_groups, false, false);
  for (int i=0; i < num_found; i++) {
    int group = disabled_act_groups[i];
    if (group < 0) {
      throw mjXError(section, "disabled actuator group value must be non-negative");
    }
    if (group > num_bitflags - 1) {
      throw mjXError(section, "disabled actuator group value cannot exceed 30");
    }
    opt->disableactuator |= (1 << group);
    s->authored.disableactuator |= (1 << group);
  }

  // read disable sub-element
  XMLElement* elem = FindSubElem(section, "flag");
  if (elem) {
    int n = 0;
#define READDSBL(NAME, MASK)                         \
  if (MapValue(elem, NAME, &n, enable_map, 2)) {     \
    opt->disableflags ^= (opt->disableflags & MASK); \
    opt->disableflags |= (n ? 0 : MASK);             \
    s->authored.disableflags |= MASK;                \
  }

    READDSBL("constraint",   mjDSBL_CONSTRAINT)
    READDSBL("equality",     mjDSBL_EQUALITY)
    READDSBL("frictionloss", mjDSBL_FRICTIONLOSS)
    READDSBL("limit",        mjDSBL_LIMIT)
    READDSBL("contact",      mjDSBL_CONTACT)
    READDSBL("spring",       mjDSBL_SPRING)
    READDSBL("damper",       mjDSBL_DAMPER)
    READDSBL("gravity",      mjDSBL_GRAVITY)
    READDSBL("clampctrl",    mjDSBL_CLAMPCTRL)
    READDSBL("warmstart",    mjDSBL_WARMSTART)
    READDSBL("filterparent", mjDSBL_FILTERPARENT)
    READDSBL("actuation",    mjDSBL_ACTUATION)
    READDSBL("refsafe",      mjDSBL_REFSAFE)
    READDSBL("sensor",       mjDSBL_SENSOR)
    READDSBL("midphase",     mjDSBL_MIDPHASE)
    READDSBL("eulerdamp",    mjDSBL_EULERDAMP)
    READDSBL("autoreset",    mjDSBL_AUTORESET)
    READDSBL("nativeccd",    mjDSBL_NATIVECCD)
    READDSBL("island",       mjDSBL_ISLAND)
    READDSBL("multiccd",     mjDSBL_MULTICCD)
#undef READDSBL

#define READENBL(NAME, MASK)                       \
  if (MapValue(elem, NAME, &n, enable_map, 2)) {   \
    opt->enableflags ^= (opt->enableflags & MASK); \
    opt->enableflags |= (n ? MASK : 0);            \
    s->authored.enableflags |= MASK;               \
  }

    READENBL("override",    mjENBL_OVERRIDE)
    READENBL("energy",      mjENBL_ENERGY)
    READENBL("fwdinv",      mjENBL_FWDINV)
    READENBL("invdiscrete", mjENBL_INVDISCRETE)
    READENBL("sleep",       mjENBL_SLEEP)
    READENBL("diagexact",   mjENBL_DIAGEXACT)
#undef READENBL
  }
}



// size section parser
void mjXReader::Size(XMLElement* section, mjSpec* s) {
  // read memory bytes
  {
    constexpr char err_msg[] =
      "unsigned integer with an optional suffix {K,M,G,T,P,E} is expected in "
      "attribute 'memory' (or the size specified is too big)";

    auto memory = [&]() -> std::optional<std::size_t> {
      const char* pstr = section->Attribute("memory");
      if (!pstr) {
        return std::nullopt;
      }

      // trim entire string
      string trimmed;
      {
        std::istringstream strm((string(pstr)));
        strm >> trimmed;
        string trailing;
        strm >> trailing;
        if (!trailing.empty() || !strm.eof()) {
          throw mjXError(section, "%s", err_msg);
        }

        // allow explicit specification of the default "-1" value
        if (trimmed == "-1") {
          return std::nullopt;
        }
      }

      std::istringstream strm(trimmed);

      // check that the number is not negative
      if (strm.peek() == '-') {
        throw mjXError(section, "%s", err_msg);
      }

      std::size_t base_size;
      strm >> base_size;
      if (strm.fail()) {
        // either not an integer or the number without the suffix is already bigger than size_t
        throw mjXError(section, "%s", err_msg);
      }

      // parse the multiplier suffix
      int multiplier_bit = 0;
      if (!strm.eof()) {
        char suffix = strm.get();
        if (suffix == 'K' || suffix == 'k') {
          multiplier_bit = 10;
        } else if (suffix == 'M' || suffix == 'm') {
          multiplier_bit = 20;
        } else if (suffix == 'G' || suffix == 'g') {
          multiplier_bit = 30;
        } else if (suffix == 'T' || suffix == 't') {
          multiplier_bit = 40;
        } else if (suffix == 'P' || suffix == 'p') {
          multiplier_bit = 50;
        } else if (suffix == 'E' || suffix == 'e') {
          multiplier_bit = 60;
        }

        // check for invalid suffix, or suffix longer than one character
        strm.get();
        if (!multiplier_bit || !strm.eof()) {
          throw mjXError(section, "%s", err_msg);
        }
      }

      // check that the specified suffix isn't bigger than size_t
      if (multiplier_bit + 1 > std::numeric_limits<std::size_t>::digits) {
        throw mjXError(section, "%s", err_msg);
      }

      // check that the suffix won't take the total size beyond size_t
      const std::size_t max_base_size =
          (std::numeric_limits<std::size_t>::max() << multiplier_bit) >> multiplier_bit;
      if (base_size > max_base_size) {
        throw mjXError(section, "%s", err_msg);
      }

      const std::size_t total_size = base_size << multiplier_bit;
      return total_size;
    }();

    if (memory.has_value()) {
      if (*memory / sizeof(mjtNum) > std::numeric_limits<std::size_t>::max()) {
        throw mjXError(section, "%s", err_msg);
      }
      s->memory = *memory;
    }
  }

  // mechanical attributes
  ReadAttrTableCore(section, s, kSizeAttrs, kSizeAttrsN,
                    /*skipnodefault=*/false);

  ReadAttrInt(section, "nconmax", &s->nconmax);
  if (s->nconmax < -1) throw mjXError(section, "nconmax must be >= -1");

  // memory/nstack and memory/njmax exclusivity is enforced by the schema
  {
    int nstack = -1;
    if (ReadAttrInt(section, "nstack", &nstack)) {
      if (nstack < -1) throw mjXError(section, "nstack must be >= -1");
      s->nstack = nstack;
    }
  }
  if (ReadAttrInt(section, "njmax", &s->njmax)) {
    if (s->njmax < -1) throw mjXError(section, "njmax must be >= -1");
  }

  if (s->nuser_body < -1) throw mjXError(section, "nuser_body must be >= -1");

  if (s->nuser_jnt < -1) throw mjXError(section, "nuser_jnt must be >= -1");

  if (s->nuser_geom < -1) throw mjXError(section, "nuser_geom must be >= -1");

  if (s->nuser_site < -1) throw mjXError(section, "nuser_site must be >= -1");

  if (s->nuser_cam < -1) throw mjXError(section, "nuser_cam must be >= -1");

  if (s->nuser_tendon < -1) throw mjXError(section, "nuser_tendon must be >= -1");

  if (s->nuser_actuator < -1) throw mjXError(section, "nuser_actuator must be >= -1");

  if (s->nuser_sensor < -1) throw mjXError(section, "nuser_sensor must be >= -1");
}



// statistic section parser
void mjXReader::Statistic(XMLElement* section) {
  // mechanical attributes
  ReadAttrTableCore(section, &spec->stat, kStatisticAttrs, kStatisticAttrsN,
                    /*skipnodefault=*/false);
  if (mjuu_defined(spec->stat.extent) && spec->stat.extent <= 0) {
    throw mjXError(section, "extent must be strictly positive");
  }
}



//---------------------------------- one-element parsers -------------------------------------------

// flex element parser
void mjXReader::OneFlex(XMLElement* elem, mjsFlex* flex) {
  string text;
  int n;

  // mechanical attributes
  ReadAttrTable(elem, flex, flex->element, kFlexAttrs,
                kFlexAttrsN);

  // cellcount is seeded before reading
  flex->cellcount[0] = 1;
  flex->cellcount[1] = 1;
  flex->cellcount[2] = 1;
  ReadAttr(elem, "cellcount", 3, flex->cellcount, text);

  // dof lowers to interpolation order
  flex->order = 0;
  if (MapValue(elem, "dof", &n, fdof_map, mjNFCOMPDOFS)) {
    flex->order = (n == mjFCOMPDOF_QUADRATIC) ? 2 : (n == mjFCOMPDOF_TRILINEAR ? 1 : 0);
  }

  // sub-elements project into the flex
  XMLElement* cont = FirstChildElement(elem, "contact");
  if (cont) {
    ReadAttrTable(cont, flex, flex->element, kFlexcomp_contactAttrs,
                  kFlexcomp_contactAttrsN);
  }
  XMLElement* edge = FirstChildElement(elem, "edge");
  if (edge) {
    ReadAttrTable(edge, flex, flex->element, kFlex_edgeAttrs,
                  kFlex_edgeAttrsN);
  }
  XMLElement* elasticity = FirstChildElement(elem, "elasticity");
  if (elasticity) {
    ReadAttrTable(elasticity, flex, flex->element, kElasticityAttrs,
                  kElasticityAttrsN);
  }

  // write error info
  mjs_setString(flex->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// mesh element parser
void mjXReader::OneMesh(XMLElement* elem, mjsMesh* mesh, const mjVFS* vfs) {
  int n;
  string text;

  // mechanical attributes
  ReadAttrTable(elem, mesh, mesh->element, kMeshAttrs, kMeshAttrsN);

  // file, resolved against the mesh directory
  auto file = ReadAttrFile(elem, "file", vfs, MeshDir());
  if (file) {
    mjs_setString(mesh->file, file->c_str());
  }

  // plugin sub-element
  XMLElement* eplugin = FirstChildElement(elem, "plugin");
  if (eplugin) {
    OnePlugin(eplugin, &mesh->plugin);
  }

  // maxhullvert with validation
  if (ReadAttrInt(elem, "maxhullvert", &n)) {
    if (n != -1 && n < 4) throw mjXError(elem, "maxhullvert must be larger than 3");
    mesh->maxhullvert = n;
  }

  // builtin options
  if (MapValue(elem, "builtin", &n, meshbuiltin_map, meshbuiltin_sz)) {
    std::vector<double> params;
    int nparams = ReadVector(elem, "params", params, text, /*required*/ true);
    if (mjs_makeMesh(mesh, (mjtMeshBuiltin)n, params.data(), nparams)) {
      throw mjXError(elem, "%s", mjs_getError(spec));
    }
  }

  // write error info
  mjs_setString(mesh->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// skin element parser
void mjXReader::OneSkin(XMLElement* elem, mjsSkin* skin, const mjVFS* vfs) {
  string text;
  float data[4];

  // mechanical attributes
  ReadAttrTable(elem, skin, skin->element, kSkinAttrs, kSkinAttrsN);

  // file, resolved against the asset directory
  auto file = ReadAttrFile(elem, "file", vfs, AssetDir());
  if (file.has_value()) {
    mjs_setString(skin->file, file->c_str());
  }

  // group with range validation
  ReadAttrInt(elem, "group", &skin->group);
  if (skin->group < 0 || skin->group >= mjNGROUP) {
    throw mjXError(elem, "skin group must be between 0 and 5");
  }

  // read bones
  XMLElement* bone = FirstChildElement(elem, "bone");
  std::vector<float> bindpos;
  std::vector<float> bindquat;

  while (bone) {
    // read body
    ReadAttrTxt(bone, "body", text, true);
    mjs_appendString(skin->bodyname, text.c_str());

    // read bindpos
    ReadAttr(bone, "bindpos", 3, data, text, true);
    bindpos.push_back(data[0]);
    bindpos.push_back(data[1]);
    bindpos.push_back(data[2]);

    // read bindquat
    ReadAttr(bone, "bindquat", 4, data, text, true);
    bindquat.push_back(data[0]);
    bindquat.push_back(data[1]);
    bindquat.push_back(data[2]);
    bindquat.push_back(data[3]);

    // read vertid
    auto tempid = ReadAttrVec<int>(bone, "vertid", true);
    if (tempid.has_value()) {
      mjs_appendIntVec(skin->vertid, tempid->data(), tempid->size());
    }

    // read vertweight
    auto tempweight = ReadAttrVec<float>(bone, "vertweight", true);
    if (tempweight.has_value()) {
      mjs_appendFloatVec(skin->vertweight, tempweight->data(), tempweight->size());
    }

    // advance to next bone
    bone = NextSiblingElement(bone, "bone");
  }

  // set bind vectors
  mjs_setFloat(skin->bindpos, bindpos.data(), bindpos.size());
  mjs_setFloat(skin->bindquat, bindquat.data(), bindquat.size());

  // write error info
  mjs_setString(skin->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// material element parser
void mjXReader::OneMaterial(XMLElement* elem, mjsMaterial* material) {
  string text, texture;

  // mechanical attributes
  ReadAttrTable(elem, material, material->element, kMaterialAttrs,
                kMaterialAttrsN);

  // the texture attribute and layer sub-elements are mutually exclusive
  bool tex_attributes_found = false;
  if (ReadAttrTxt(elem, "texture", texture)) {
    mjs_setInStringVec(material->textures, mjTEXROLE_RGB, texture.c_str());
    tex_attributes_found = true;
  }
  XMLElement* layer = FirstChildElement(elem);
  while (layer) {
    if (tex_attributes_found) {
      throw mjXError(layer, "A material with a texture attribute cannot have layer sub-elements");
    }

    // layer sub-element
    ReadAttrTxt(layer, "role", text, true);
    int role = FindKey(texrole_map, texrole_sz, text);
    ReadAttrTxt(layer, "texture", text, true);
    mjs_setInStringVec(material->textures, role, text.c_str());
    layer = NextSiblingElement(layer);
  }

  // write error info
  mjs_setString(material->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// joint element parser
void mjXReader::OneJoint(XMLElement* elem, mjsJoint* joint) {
  // mechanical attributes
  ReadAttrTable(elem, joint, joint->element, kJointAttrs, kJointAttrsN);

  // write error info
  mjs_setString(joint->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// geom element parser
void mjXReader::OneGeom(XMLElement* elem, mjsGeom* geom) {
  string text;
  int n;

  // mechanical attributes
  ReadAttrTable(elem, geom, geom->element, kGeomAttrs, kGeomAttrsN);

  // fluid interaction model: keyword lowers to on/off
  if (MapValue(elem, "fluidshape", &n, fluidshape_map, 2)) {
    geom->fluid_ellipsoid = (n == 1);
  }

  // orientation alternatives
  ReadQuat(elem, "quat", geom->quat, text);
  ReadAlternative(elem, geom->alt);

  // plugin sub-element
  XMLElement* eplugin = FirstChildElement(elem, "plugin");
  if (eplugin) {
    OnePlugin(eplugin, &geom->plugin);
  }

  // write error info
  mjs_setString(geom->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// site element parser
void mjXReader::OneSite(XMLElement* elem, mjsSite* site) {
  string text;

  // mechanical attributes
  ReadAttrTable(elem, site, site->element, kSiteAttrs, kSiteAttrsN);

  // orientation alternatives
  ReadQuat(elem, "quat", site->quat, text);
  ReadAlternative(elem, site->alt);

  // write error info
  mjs_setString(site->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// camera element parser
void mjXReader::OneCamera(XMLElement* elem, mjsCamera* camera) {
  string text;

  // mechanical attributes
  ReadAttrTable(elem, camera, camera->element, kCameraAttrs, kCameraAttrsN);

  // orientation alternatives
  ReadQuat(elem, "quat", camera->quat, text);
  ReadAlternative(elem, camera->alt);

  // write error info
  mjs_setString(camera->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// light element parser
void mjXReader::OneLight(XMLElement* elem, mjsLight* light) {
  int n;

  // mechanical attributes
  ReadAttrTable(elem, light, light->element, kLightAttrs, kLightAttrsN);

  // directional is a legacy alias for type; the pair is mutually exclusive
  if (MapValue(elem, "directional", &n, bool_map, 2)) {
    light->type = (n == 1) ? mjLIGHT_DIRECTIONAL : mjLIGHT_SPOT;
  }
  if (MapValue(elem, "type", &n, lighttype_map, lighttype_sz)) {
    light->type = (mjtLightType)n;
  }

  // write error info
  mjs_setString(light->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// pair element parser
// read the mechanical attributes of an element, driven by its generated rows
void mjXReader::ReadAttrTable(XMLElement* elem, void* obj, mjsElement* el,
                              const mjXAttr* rows, int nrow) {
  // element names need the spec for error context; the core handles the rest
  string text;
  for (int i = 0; i < nrow; i++) {
    const mjXAttr& row = rows[i];
    if (row.kind != mjXAttr::kName || (readingdefaults && row.nodefault)) {
      continue;
    }
    if (ReadAttrTxt(elem, row.attr, text, row.required) &&
        mjs_setName(el, text.c_str())) {
      throw mjXError(elem, "%s", mjs_getError(spec));
    }
  }
  ReadAttrTableCore(elem, obj, rows, nrow, readingdefaults);
}



// static core: every row kind except element names
void mjXReader::ReadAttrTableCore(XMLElement* elem, void* obj,
                                  const mjXAttr* rows, int nrow,
                                  bool skipnodefault, const void* authored) {
  string text;
  for (int i = 0; i < nrow; i++) {
    const mjXAttr& row = rows[i];
    if (skipnodefault && row.nodefault) {
      continue;
    }
    char* base = (char*)obj + row.offset;
    bool got = false;
    switch (row.kind) {
      case mjXAttr::kName:
        break;  // handled by the member wrapper
      case mjXAttr::kString:
        if (ReadAttrTxt(elem, row.attr, text, row.required)) {
          mjs_setString(*(mjString**)base, text.c_str());
          got = true;
        }
        break;
      case mjXAttr::kStringVec:
        if (ReadAttrTxt(elem, row.attr, text, row.required)) {
          mjs_setStringVec(*(mjStringVec**)base, text.c_str());
          got = true;
        }
        break;
      case mjXAttr::kInt:
        got = ReadAttr(elem, row.attr, row.len, (int*)base, text, row.required,
                       row.exact) > 0;
        break;
      case mjXAttr::kDouble:
        got = ReadAttr(elem, row.attr, row.len, (double*)base, text,
                       row.required, row.exact) > 0;
        break;
      case mjXAttr::kNum:
        got = ReadAttr(elem, row.attr, row.len, (mjtNum*)base, text,
                       row.required, row.exact) > 0;
        break;
      case mjXAttr::kFloat:
        got = ReadAttr(elem, row.attr, row.len, (float*)base, text,
                       row.required, row.exact) > 0;
        break;
      case mjXAttr::kEnum: {
        int value;
        if (MapValue(elem, row.attr, &value, row.map, row.mapsz,
                     row.required)) {
          *(int*)base = value;  // enum fields are int-sized
          got = true;
        }
        break;
      }
      case mjXAttr::kFlags: {
        std::vector<int> values(row.mapsz);
        int nvalues = MapValues(elem, row.attr, values.data(), row.map,
                                row.mapsz);
        if (nvalues) {
          int combined = 0;
          for (int j = 0; j < nvalues; j++) {
            combined |= values[j];
          }
          *(int*)base = combined;
          got = true;
        }
        break;
      }
      case mjXAttr::kEnumByte: {
        int value;
        if (MapValue(elem, row.attr, &value, row.map, row.mapsz,
                     row.required)) {
          *(mjtByte*)base = (mjtByte)value;
          got = true;
        }
        break;
      }
      case mjXAttr::kBool: {
        int value;
        if (MapValue(elem, row.attr, &value, bool_map, 2, row.required)) {
          *(mjtBool*)base = (value == 1);
          got = true;
        }
        break;
      }
      case mjXAttr::kConst:
        *(int*)base = row.value;  // enum fields are int-sized
        break;
      case mjXAttr::kDoubleVec: {
        std::vector<double> values;
        if (ReadVector(elem, row.attr, values, text)) {
          mjs_setDouble(*(mjDoubleVec**)base, values.data(), values.size());
          got = true;
        }
        break;
      }
      case mjXAttr::kFloatVec: {
        auto values = ReadAttrVec<float>(elem, row.attr, row.required);
        if (values.has_value()) {
          mjs_setFloat(*(mjFloatVec**)base, values->data(), values->size());
          got = true;
        }
        break;
      }
      case mjXAttr::kIntVec: {
        auto values = ReadAttrVec<int>(elem, row.attr, row.required);
        if (values.has_value()) {
          mjs_setInt(*(mjIntVec**)base, values->data(), values->size());
          got = true;
        }
        break;
      }
      case mjXAttr::kChars:
        if (ReadAttrTxt(elem, row.attr, text, row.required)) {
          if (row.exact && (int)text.size() != row.len) {
            throw mjXError(elem, "attribute '%s' must have exactly %d "
                           "characters", row.attr, row.len);
          }
          if ((int)text.size() > row.len) {
            throw mjXError(elem, "attribute '%s' may have at most %d "
                           "characters", row.attr, row.len);
          }
          std::memcpy(base, text.data(), text.size());
          got = true;
        }
        break;
    }

    // record XML-authored fields for attach conflict resolution
    if (got && authored) {
      mjs_setAuthored(authored, base, 1);
    }
  }
}



void mjXReader::OnePair(XMLElement* elem, mjsPair* pair) {
  ReadAttrTable(elem, pair, pair->element, kPairAttrs, kPairAttrsN);

  // write error info
  mjs_setString(pair->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// equality element parser
void mjXReader::OneEquality(XMLElement* elem, mjsEquality* equality) {
  string text, name1, name2;

  // read type (bad keywords already detected by schema)
  text = elem->Value();
  equality->type = (mjtEq)FindKey(equality_map, equality_sz, text);

  // common attributes
  ReadAttrTable(elem, equality, equality->element, kEqualityBaseAttrs,
                kEqualityBaseAttrsN);

  // regular only
  if (!readingdefaults) {
    switch (equality->type) {
      case mjEQ_CONNECT: {
        auto maybe_site1 = ReadAttrStr(elem, "site1");
        auto maybe_site2 = ReadAttrStr(elem, "site2");
        auto maybe_body1 = ReadAttrStr(elem, "body1");
        auto maybe_body2 = ReadAttrStr(elem, "body2");
        bool has_anchor = ReadAttr(elem, "anchor", 3, equality->data, text);

        bool body_semantic = maybe_body1.has_value() && has_anchor;

        if (body_semantic) {
          name1 = maybe_body1.value();
          if (maybe_body2.has_value()) {
            name2 = maybe_body2.value();
          }
          equality->objtype = mjOBJ_BODY;
        } else {
          name1 = maybe_site1.value();
          name2 = maybe_site2.value();
          equality->objtype = mjOBJ_SITE;
        }
      }
      break;

      case mjEQ_WELD: {
        auto maybe_site1 = ReadAttrStr(elem, "site1");
        auto maybe_site2 = ReadAttrStr(elem, "site2");
        auto maybe_body1 = ReadAttrStr(elem, "body1");
        auto maybe_body2 = ReadAttrStr(elem, "body2");
        bool has_anchor = ReadAttr(elem, "anchor", 3, equality->data, text);
        ReadAttr(elem, "relpose", 7, equality->data+3, text);

        bool body_semantic = maybe_body1.has_value();

        if (body_semantic) {
          name1 = maybe_body1.value();
          if (maybe_body2.has_value()) {
            name2 = maybe_body2.value();
          }
          equality->objtype = mjOBJ_BODY;
          if (!has_anchor) {
            mjuu_zerovec(equality->data, 3);
          }
        } else {
          name1 = maybe_site1.value();
          name2 = maybe_site2.value();
          equality->objtype = mjOBJ_SITE;
        }

        ReadAttr(elem, "torquescale", 1, equality->data+10, text);
      }
      break;

      case mjEQ_JOINT:
        ReadAttrTxt(elem, "joint1", name1, true);
        ReadAttrTxt(elem, "joint2", name2);
        ReadAttr(elem, "polycoef", 5, equality->data, text, false, false);
        break;

      case mjEQ_TENDON:
        ReadAttrTxt(elem, "tendon1", name1, true);
        ReadAttrTxt(elem, "tendon2", name2);
        ReadAttr(elem, "polycoef", 5, equality->data, text, false, false);
        break;

      case mjEQ_FLEX:
      case mjEQ_FLEXVERT:
        ReadAttrTxt(elem, "flex", name1, true);
        break;

      case mjEQ_FLEXSTRAIN:
        ReadAttrTxt(elem, "flex", name1, true);
        ReadAttr(elem, "cell", 3, equality->data, text);
        break;

      case mjEQ_DISTANCE:
        throw mjXError(elem, "support for distance equality constraints was removed in MuJoCo 2.2.2");
        break;

      default:                  // SHOULD NOT OCCUR
        throw mjXError(elem, "unrecognized equality constraint type");
    }

    mjs_setString(equality->name1, name1.c_str());
    if (!name2.empty()) {
      mjs_setString(equality->name2, name2.c_str());
    }
  }

  // write error info
  mjs_setString(equality->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// tendon element parser
void mjXReader::OneTendon(XMLElement* elem, mjsTendon* tendon) {
  string text;

  // mechanical attributes; fixed tendons read a subset of the spatial rows
  ReadAttrTable(elem, tendon, tendon->element, kSpatialAttrs, kSpatialAttrsN);

  // read springlength, either one or two values; if one, copy to second value
  if (ReadAttr(elem, "springlength", 2, tendon->springlength, text, false, false) == 1) {
    tendon->springlength[1] = tendon->springlength[0];
  }

  // write error info
  mjs_setString(tendon->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// read the "input" attribute: so3 chart keyword, the "none" keyword (empty signature),
// or a servo input token list, required to be in canonical order [pos, vel, ff, voltage]
static bool ReadInputSpec(tinyxml2::XMLElement* elem, int* ctrlspec) {
  std::string text;
  if (!mjXUtil::ReadAttrTxt(elem, "input", text)) {
    return false;
  }

  // so3 chart keyword
  int chart = mjXUtil::FindKey(inputchart_map, inputchart_sz, text);
  if (chart >= 0) {
    *ctrlspec = chart;
    return true;
  }

  // empty-signature keyword
  int keyword = mjXUtil::FindKey(inputkeyword_map, inputkeyword_sz, text);
  if (keyword >= 0) {
    *ctrlspec = keyword;
    return true;
  }

  // servo input tokens; strictly ascending bits = canonical order, no duplicates
  int bits[inputbit_sz];
  int nbit = mjXUtil::MapValues(elem, "input", bits, inputbit_map, inputbit_sz);
  int spec = 0;
  for (int k=0; k < nbit; k++) {
    if (bits[k] <= (k ? bits[k-1] : 0)) {
      throw mjXError(elem, "inputs must be listed in canonical order [pos, vel, ff, voltage]");
    }
    spec |= bits[k];
  }
  *ctrlspec = spec;
  return true;
}


// actuator element parser
void mjXReader::OneActuator(XMLElement* elem, mjsActuator* actuator) {
  string text, type, target, slidersite, refsite;

  // mechanical attributes; per-tag legality is enforced by the schema check
  ReadAttrTable(elem, actuator, actuator->element, kGeneralAttrs,
                kGeneralAttrsN);

  // transmission target and type
  if (ReadAttrTxt(elem, "joint", target)) {
    mjs_setString(actuator->target, target.c_str());
    actuator->trntype = mjTRN_JOINT;
  }
  if (ReadAttrTxt(elem, "jointinparent", target)) {
    mjs_setString(actuator->target, target.c_str());
    actuator->trntype = mjTRN_JOINTINPARENT;
  }
  if (ReadAttrTxt(elem, "tendon", target)) {
    mjs_setString(actuator->target, target.c_str());
    actuator->trntype = mjTRN_TENDON;
  }
  if (ReadAttrTxt(elem, "cranksite", target)) {
    mjs_setString(actuator->target, target.c_str());
    actuator->trntype = mjTRN_SLIDERCRANK;
  }
  if (ReadAttrTxt(elem, "site", target)) {
    mjs_setString(actuator->target, target.c_str());
    actuator->trntype = mjTRN_SITE;
  }
  if (ReadAttrTxt(elem, "body", target)) {
    mjs_setString(actuator->target, target.c_str());
    actuator->trntype = mjTRN_BODY;
  }
  // slidercrank-specific parameters
  int r1 = ReadAttr(elem, "cranklength", 1, &actuator->cranklength, text);
  int r2 = ReadAttrTxt(elem, "slidersite", slidersite);
  if (r2) {
    mjs_setString(actuator->slidersite, slidersite.c_str());
  }
  if ((r1 || r2) &&
      actuator->trntype != mjTRN_SLIDERCRANK &&
      actuator->trntype != mjTRN_UNDEFINED) {
    throw mjXError(elem, "cranklength and slidersite can only be used in slidercrank transmission");
  }

  // site-specific parameters (refsite)
  int r3 = ReadAttrTxt(elem, "refsite", refsite);
  if (r3) {
    mjs_setString(actuator->refsite, refsite.c_str());
  }
  if (r3 && actuator->trntype != mjTRN_SITE && actuator->trntype != mjTRN_UNDEFINED) {
    throw mjXError(elem, "refsite can only be used with site transmission");
  }

  // get predefined type
  type = elem->Value();

  // explicit attributes
  string err;
  if (type == "general") {
    // so3 chart keyword or servo token subset; dcmotor accepts the voltage keyword
    ReadInputSpec(elem, &actuator->ctrlspec);
  }

  // direct drive motor
  else if (type == "motor") {
    err = mjs_setToMotor(actuator);
  }

  // position or integrated velocity servo
  else if (type == "position" || type == "intvelocity") {
    double kp = actuator->gainprm[0];
    ReadAttr(elem, "kp", 1, &kp, text);

    // read kv
    double kv_data;
    double *kv = &kv_data;
    if (!ReadAttr(elem, "kv", 1, kv, text)) {
      kv = nullptr;
    }

    // read dampratio
    double dampratio_data;
    double *dampratio = &dampratio_data;
    if (!ReadAttr(elem, "dampratio", 1, dampratio, text)) {
      dampratio = nullptr;
    }

    // read timeconst, set dyntype
    double timeconst_data;
    double *timeconst = &timeconst_data;
    if (!ReadAttr(elem, "timeconst", 1, timeconst, text)) {
      timeconst = nullptr;
    }

    // handle inheritrange
    double inheritrange = actuator->inheritrange;
    ReadAttr(elem, "inheritrange", 1, &inheritrange, text);

    if (type == "position") {
      err = mjs_setToPosition(actuator, kp, kv, dampratio, timeconst, inheritrange);
    } else {
      err = mjs_setToIntVelocity(actuator, kp, kv, dampratio, timeconst, inheritrange);
    }
  }

  // orientation servo: geodesic PD on an SO3 transmission
  else if (type == "orientation") {
    double kp = actuator->gainprm[0];
    ReadAttr(elem, "kp", 1, &kp, text);

    double kv_data;
    double *kv = &kv_data;
    if (!ReadAttr(elem, "kv", 1, kv, text)) {
      kv = nullptr;
    }

    double dampratio_data;
    double *dampratio = &dampratio_data;
    if (!ReadAttr(elem, "dampratio", 1, dampratio, text)) {
      dampratio = nullptr;
    }

    // input chart: expmap (default) or quat
    ReadInputSpec(elem, &actuator->ctrlspec);

    err = mjs_setToOrientation(actuator, kp, kv, dampratio, actuator->ctrlspec);
  }

  // PID servo: inputs are position and velocity setpoints
  else if (type == "pid") {
    // kp: default inherited via -biasprm[1]
    double kp = -actuator->biasprm[1];
    ReadAttr(elem, "kp", 1, &kp, text);

    double kv_data;
    double *kv = &kv_data;
    if (!ReadAttr(elem, "kv", 1, kv, text)) {
      kv = nullptr;
    }

    double dampratio_data;
    double *dampratio = &dampratio_data;
    if (!ReadAttr(elem, "dampratio", 1, dampratio, text)) {
      dampratio = nullptr;
    }

    // controller parameters: ki (gainprm[0]), imax (dynprm[0]), slewmax (dynprm[1]); inherited
    double ki = actuator->gainprm[0] * (actuator->dyntype == mjDYN_PID);
    ReadAttr(elem, "ki", 1, &ki, text);
    double imax = actuator->dynprm[0] * (actuator->dyntype == mjDYN_PID);
    ReadAttr(elem, "imax", 1, &imax, text);
    double slewmax = actuator->dynprm[1] * (actuator->dyntype == mjDYN_PID);
    ReadAttr(elem, "slewmax", 1, &slewmax, text);

    // input subset selection
    ReadInputSpec(elem, &actuator->ctrlspec);

    // posrange is an alias of ctrlrange (the position-setpoint input);
    // velrange and ffrange are read by the shared rows
    ReadAttr(elem, "posrange", 2, actuator->ctrlrange, text);

    // handle inheritrange
    double inheritrange = actuator->inheritrange;
    ReadAttr(elem, "inheritrange", 1, &inheritrange, text);

    err = mjs_setToPID(actuator, kp, kv, dampratio, &ki, &imax, &slewmax, inheritrange,
                      actuator->ctrlspec);
  }

  // velocity servo
  else if (type == "velocity") {
    double kv = actuator->gainprm[0];
    ReadAttr(elem, "kv", 1, &kv, text);
    err = mjs_setToVelocity(actuator, kv);
  }

  // damper
  else if (type == "damper") {
    bool inherited = (actuator->gaintype == mjGAIN_AFFINE);
    double kv = inherited ? -actuator->gainprm[2] : 0;
    ReadAttr(elem, "kv", 1, &kv, text);
    err = mjs_setToDamper(actuator, kv);
  }

  // cylinder
  else if (type == "cylinder") {
    double timeconst = actuator->dynprm[0];
    double bias[3] = {actuator->biasprm[0], actuator->biasprm[1], actuator->biasprm[2]};
    double area = actuator->gainprm[0];
    double diameter = -1;
    ReadAttr(elem, "timeconst", 1, &timeconst, text);
    ReadAttr(elem, "bias", 3, bias, text);
    ReadAttr(elem, "area", 1, &area, text);
    ReadAttr(elem, "diameter", 1, &diameter, text);
    err = mjs_setToCylinder(actuator, timeconst, bias[0], area, diameter);
    actuator->biasprm[1] = bias[1];
    actuator->biasprm[2] = bias[2];
  }

  // muscle
  else if (type == "muscle") {
    double tausmooth = actuator->dynprm[2];
    double force = -1, scale = -1, lmin = -1, lmax = -1, vmax = -1, fpmax = -1, fvmax = -1;
    double range[2] = {-1, -1}, timeconst[2] = {-1, -1};
    ReadAttr(elem, "timeconst", 2, timeconst, text);
    ReadAttr(elem, "tausmooth", 1, &tausmooth, text);
    ReadAttr(elem, "range", 2, range, text);
    ReadAttr(elem, "force", 1, &force, text);
    ReadAttr(elem, "scale", 1, &scale, text);
    ReadAttr(elem, "lmin", 1, &lmin, text);
    ReadAttr(elem, "lmax", 1, &lmax, text);
    ReadAttr(elem, "vmax", 1, &vmax, text);
    ReadAttr(elem, "fpmax", 1, &fpmax, text);
    ReadAttr(elem, "fvmax", 1, &fvmax, text);
    err = mjs_setToMuscle(actuator, timeconst, tausmooth, range, force, scale,
                          lmin, lmax, vmax, fpmax, fvmax);
  }

  // adhesion
  else if (type == "adhesion") {
    double gain = actuator->gainprm[0];
    ReadAttr(elem, "gain", 1, &gain, text);
    ReadAttr(elem, "ctrlrange", 2, actuator->ctrlrange, text);
    err = mjs_setToAdhesion(actuator, gain);
  }

  // DC motor
  else if (type == "dcmotor") {
    bool inherited = (actuator->gaintype == mjGAIN_DCMOTOR);
    double motorconst[2] = {inherited ? actuator->gainprm[1] : 0, 0};
    double resistance = inherited ? actuator->gainprm[0] : 0;
    double nominal[3] = {0, 0, 0};
    double saturation[3] = {0, 0,
                            inherited ? actuator->dynprm[1] : 0};
    double controller[6] = {inherited ? actuator->gainprm[4] : 0,
                            inherited ? actuator->gainprm[5] : 0,
                            inherited ? actuator->gainprm[6] : 0,
                            inherited ? actuator->dynprm[7] : 0,
                            inherited ? actuator->dynprm[8] : 0,
                            inherited ? actuator->gainprm[7] : 0};
    double inductance[2] = {0, inherited ? actuator->dynprm[0] : 0};
    double cogging[3] = {inherited ? actuator->biasprm[0] : 0,
                         inherited ? actuator->biasprm[1] : 0,
                         inherited ? actuator->biasprm[2] : 0};
    double thermal[6] = {inherited ? actuator->dynprm[2] : 0,
                         inherited ? actuator->dynprm[3] : 0,
                         0,
                         inherited ? actuator->gainprm[2] : 0,
                         inherited ? actuator->gainprm[3] : 0,
                         inherited ? actuator->dynprm[4] : 0};
    double lugre[5] = {inherited ? actuator->dynprm[5] : 0,
                       inherited ? actuator->dynprm[6] : 0,
                       inherited ? actuator->biasprm[3] : 0,
                       inherited ? actuator->biasprm[4] : 0,
                       inherited ? actuator->biasprm[5] : 0};
    int ctrlspec = inherited ? actuator->ctrlspec : 0;
    ReadAttr(elem, "motorconst", 2, motorconst, text, false, false);
    ReadAttr(elem, "resistance", 1, &resistance, text);
    ReadAttr(elem, "nominal", 3, nominal, text, false, false);
    ReadAttr(elem, "saturation", 3, saturation, text, false, false);
    ReadAttr(elem, "inductance", 2, inductance, text, false, false);
    ReadAttr(elem, "cogging", 3, cogging, text, false, false);
    ReadAttr(elem, "controller", 6, controller, text, false, false);
    ReadAttr(elem, "thermal", 6, thermal, text, false, false);
    ReadAttr(elem, "lugre", 5, lugre, text, false, false);
    ReadInputSpec(elem, &ctrlspec);
    err = mjs_setToDCMotor(actuator, motorconst, resistance,
                           nominal, saturation, inductance,
                           cogging, controller, thermal, lugre, ctrlspec);
  }

  else if (type == "plugin") {
    OnePlugin(elem, &actuator->plugin);
  }

  else {          // SHOULD NOT OCCUR
    throw mjXError(elem, "unrecognized actuator type: %s", type.c_str());
  }

  // throw error if any of the above failed
  if (!err.empty()) {
    throw mjXError(elem, err.c_str());
  }

  // write info
  mjs_setString(actuator->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
}



// make composite
void mjXReader::OneComposite(XMLElement* elem, mjsBody* body, mjsFrame* frame, const mjsDefault* def) {
  string text;
  int n;

  // create out-of-DOM element
  mjCComposite comp;

  // common properties
  ReadAttrTxt(elem, "prefix", comp.prefix);
  if (MapValue(elem, "type", &n, comp_map, mjNCOMPTYPES, true)) {
    comp.type = (mjtCompType)n;
  }
  ReadAttr(elem, "count", 3, comp.count, text, false, false);
  ReadAttr(elem, "offset", 3, comp.offset, text);
  ReadAttr(elem, "quat", 4, comp.quat, text);
  comp.frame = frame;

  // plugin
  XMLElement* eplugin = FirstChildElement(elem, "plugin");
  if (eplugin) {
    OnePlugin(eplugin, &comp.plugin);
  }

  // cable
  string curves;
  ReadAttrTxt(elem, "curve", curves);
  ReadAttrTxt(elem, "initial", comp.initial);
  ReadAttr(elem, "size", 3, comp.size, text, false, false);
  auto uservert = ReadAttrVec<float>(elem, "vertex");
  if (uservert.has_value()) {
    comp.uservert = std::move(uservert.value());
  }

  // process curve string
  std::istringstream iss(curves);
  int i = 0;
  while (iss) {
    if (curves.empty()) {
      break;
    }
    iss >> text;
    if (i > 2) {
      throw mjXError(elem, "The curve array must have a maximum of 3 components");
    }
    comp.curve[i++] = (mjtCompShape)FindKey(shape_map, mjNCOMPSHAPES, text);
    if (comp.curve[i-1] == -1) {
      throw mjXError(elem, "The curve array contains an invalid shape");
    }
    if (iss.eof()){
      break;
    }
  };

  // skin
  XMLElement* eskin = FirstChildElement(elem, "skin");
  if (eskin) {
    comp.skin = true;
    if (MapValue(eskin, "texcoord", &n, bool_map, 2)) {
      comp.skintexcoord = (n == 1);
    }
    ReadAttrTxt(eskin, "material", comp.skinmaterial);
    ReadAttr(eskin, "rgba", 4, comp.skinrgba, text);
    ReadAttr(eskin, "inflate", 1, &comp.skininflate, text);
    ReadAttrInt(eskin, "subgrid", &comp.skinsubgrid);
    ReadAttrInt(eskin, "group", &comp.skingroup, 0);
    if (comp.skingroup < 0 || comp.skingroup >= mjNGROUP) {
      throw mjXError(eskin, "skin group must be between 0 and 5");
    }
  }

  // set type-specific defaults
  comp.SetDefault();

  // geom
  XMLElement* egeom = FirstChildElement(elem, "geom");
  if (egeom) {
    string material;
    mjsGeom& dgeom = *comp.def[0].spec.geom;
    if (MapValue(egeom, "type", &n, geomtype_map, mjNGEOMTYPES)) {
      dgeom.type = (mjtGeom)n;
    }
    ReadAttr(egeom, "size", 3, dgeom.size, text, false, false);
    ReadAttrInt(egeom, "contype", &dgeom.contype);
    ReadAttrInt(egeom, "conaffinity", &dgeom.conaffinity);
    ReadAttrInt(egeom, "condim", &dgeom.condim);
    ReadAttrInt(egeom, "group", &dgeom.group);
    ReadAttrInt(egeom, "priority", &dgeom.priority);
    ReadAttr(egeom, "friction", 3, dgeom.friction, text, false, false);
    ReadAttr(egeom, "solmix", 1, &dgeom.solmix, text);
    ReadAttr(egeom, "solref", mjNREF, dgeom.solref, text, false, false);
    ReadAttr(egeom, "solimp", mjNIMP, dgeom.solimp, text, false, false);
    ReadAttr(egeom, "margin", 1, &dgeom.margin, text);
    ReadAttr(egeom, "gap", 1, &dgeom.gap, text);
    ReadAttr(egeom, "surfacevel", 6, dgeom.surfacevel, text, false, false);
    ReadAttr(egeom, "adhesion", 1, &dgeom.adhesion, text);
    if (ReadAttrTxt(egeom, "material", material)) {
      mjs_setString(dgeom.material, material.c_str());
    }
    ReadAttr(egeom, "rgba", 4, dgeom.rgba, text);
    ReadAttr(egeom, "mass", 1, &dgeom.mass, text);
    ReadAttr(egeom, "density", 1, &dgeom.density, text);
  }

  // site
  XMLElement* esite = FirstChildElement(elem, "site");
  if (esite) {
    string material;
    mjsSite& dsite = *comp.def[0].spec.site;
    ReadAttr(esite, "size", 3, dsite.size, text, false, false);
    ReadAttrInt(esite, "group", &dsite.group);
    ReadAttrTxt(esite, "material", material);
    ReadAttr(esite, "rgba", 4, dsite.rgba, text);
    mjs_setString(dsite.material, material.c_str());
  }

  // joint
  XMLElement* ejnt = FirstChildElement(elem, "joint");
  while (ejnt) {
    // kind
    int kind;
    MapValue(ejnt, "kind", &kind, jkind_map, 1, true);

    // create a new element if this kind already exists
    if (comp.add[kind]) {
      char error[200];
      if (!comp.AddDefaultJoint(error, 200)) {
        throw mjXError(elem, "%s", error);
      }
    }
    comp.add[kind] = true;

    // get element
    mjsDefault* dspec = &comp.defjoint[(mjtCompKind)kind].back().spec;
    mjsJoint& djoint = *dspec->joint;
    mjsEquality& dequality = *dspec->equality;

    // particle joint
    if (MapValue(ejnt, "type", &n, jointtype_map, jointtype_sz)) {
      djoint.type = (mjtJoint)n;
    }
    ReadAttr(ejnt, "axis", 3, djoint.axis, text);

    // solreffix, solimpfix
    ReadAttr(ejnt, "solreffix", mjNREF, dequality.solref, text, false, false);
    ReadAttr(ejnt, "solimpfix", mjNIMP, dequality.solimp, text, false, false);

    // joint attributes
    MapValue(elem, "limited", &djoint.limited, FalseTrueAuto_map, 3);
    ReadAttrInt(ejnt, "group", &djoint.group);
    ReadAttr(ejnt, "solreflimit", mjNREF, djoint.solref_limit, text, false, false);
    ReadAttr(ejnt, "solimplimit", mjNIMP, djoint.solimp_limit, text, false, false);
    ReadAttr(ejnt,
             "solreffriction", mjNREF, djoint.solref_friction, text, false, false);
    ReadAttr(ejnt,
             "solimpfriction", mjNIMP, djoint.solimp_friction, text, false, false);
    ReadAttr(ejnt, "stiffness", 1, djoint.stiffness, text);
    ReadAttr(ejnt, "range", 2, djoint.range, text);
    ReadAttr(ejnt, "margin", 1, &djoint.margin, text);
    ReadAttr(ejnt, "armature", 1, &djoint.armature, text);
    ReadAttr(ejnt, "damping", 1, djoint.damping, text);
    ReadAttr(ejnt, "frictionloss", 1, &djoint.frictionloss, text);

    // advance
    ejnt = NextSiblingElement(ejnt, "joint");
  }


  // make composite
  char error[200];
  bool res = comp.Make(spec, body, error, 200);

  // throw error
  if (!res) {
    throw mjXError(elem, "%s", error);
  }
}



// make flexcomp
void mjXReader::OneFlexcomp(XMLElement* elem, mjsBody* body, const mjVFS* vfs) {
  string text, material;
  int n;

  // create out-of-DOM element
  mjCFlexcomp fcomp;
  mjsFlex& dflex = *fcomp.def.spec.flex;

  // common properties
  ReadAttrTxt(elem, "name", fcomp.name, true);
  if (MapValue(elem, "type", &n, fcomp_map, mjNFCOMPTYPES)) {
    fcomp.type = (mjtFcompType)n;
  }
  ReadAttr(elem, "count", 3, fcomp.count, text);
  ReadAttr(elem, "cellcount", 3, fcomp.cellcount, text);
  ReadAttr(elem, "spacing", 3, fcomp.spacing, text);
  ReadAttr(elem, "scale", 3, fcomp.scale, text);
  ReadAttr(elem, "mass", 1, &fcomp.mass, text);
  ReadAttr(elem, "inertiabox", 1, &fcomp.inertiabox, text);
  auto maybe_file = ReadAttrFile(elem, "file", vfs, modelfiledir_);
  if (maybe_file.has_value()) {
    fcomp.file = std::move(maybe_file.value().Str());
  } else {
    fcomp.file = "";
  }
  if (ReadAttrTxt(elem, "material", material)) {
    mjs_setString(dflex.material, material.c_str());
  }
  ReadAttr(elem, "rgba", 4, dflex.rgba, text);
  if (MapValue(elem, "flatskin", &n, bool_map, 2)) {
    dflex.flatskin = (n == 1);
  }
  ReadAttrInt(elem, "dim", &dflex.dim);
  ReadAttr(elem, "radius", 1, &dflex.radius, text);
  ReadAttrInt(elem, "group", &dflex.group);
  if (!ReadAttr(elem, "origin", 3, fcomp.origin, text) &&
      fcomp.type == mjFCOMPTYPE_MESH && dflex.dim == 3) {
    throw mjXError(elem, "origin must be specified for mesh flexcomps if dim=3");
  }

  // pose
  ReadAttr(elem, "pos", 3, fcomp.pos, text);
  ReadAttr(elem, "quat", 4, fcomp.quat, text);
  ReadAlternative(elem, fcomp.alt);

  // user or internal
  if (MapValue(elem, "rigid", &n, bool_map, 2)) {
    fcomp.rigid = (n == 1);
  }
  auto point = ReadAttrVec<double>(elem, "point");
  if (point.has_value()) {
    fcomp.point = std::move(point.value());
  }
  auto element = ReadAttrVec<int>(elem, "element");
  if (element.has_value()) {
    fcomp.element = std::move(element.value());
  }
  auto texcoord = ReadAttrVec<float>(elem, "texcoord");
  if (texcoord.has_value()) {
    fcomp.texcoord = std::move(texcoord.value());
  }

  // dof type
  if (MapValue(elem, "dof", &n, fdof_map, mjNFCOMPDOFS)) {
    fcomp.doftype = (mjtDof)n;
  }

  // edge
  XMLElement* edge = FirstChildElement(elem, "edge");
  if (edge) {
    MapValue(edge, "equality", &fcomp.equality, flexeq_map, 4);
    ReadAttr(edge, "solref", mjNREF, fcomp.def.spec.equality->solref, text, false, false);
    ReadAttr(edge, "solimp", mjNIMP, fcomp.def.spec.equality->solimp, text, false, false);
    ReadAttr(edge, "stiffness", 1, &dflex.edgestiffness, text);
    ReadAttr(edge, "damping", 1, &dflex.edgedamping, text);
  }

  // elasticity
  XMLElement* elasticity = FirstChildElement(elem, "elasticity");
  if (elasticity) {
    ReadAttr(elasticity, "young", 1, &dflex.young, text);
    ReadAttr(elasticity, "poisson", 1, &dflex.poisson, text);
    ReadAttr(elasticity, "damping", 1, &dflex.damping, text);
    ReadAttr(elasticity, "thickness", 1, &dflex.thickness, text);
    MapValue(elasticity, "elastic2d", &dflex.elastic2d, elastic2d_map, 4);
  }

  // check errors
  if (dflex.elastic2d != 1 && fcomp.equality && dflex.young > 0) {
    throw mjXError(elem, "flex constraints and elasticity (young) cannot both be present");
  }

  // contact
  XMLElement* cont = FirstChildElement(elem, "contact");
  if (cont) {
    ReadAttrInt(cont, "contype", &dflex.contype);
    ReadAttrInt(cont, "conaffinity", &dflex.conaffinity);
    ReadAttrInt(cont, "condim", &dflex.condim);
    ReadAttrInt(cont, "priority", &dflex.priority);
    ReadAttr(cont, "friction", 3, dflex.friction, text, false, false);
    ReadAttr(cont, "solmix", 1, &dflex.solmix, text);
    ReadAttr(cont, "solref", mjNREF, dflex.solref, text, false, false);
    ReadAttr(cont, "solimp", mjNIMP, dflex.solimp, text, false, false);
    ReadAttr(cont, "margin", 1, &dflex.margin, text);
    ReadAttr(cont, "gap", 1, &dflex.gap, text);
    if (MapValue(cont, "internal", &n, bool_map, 2)) {
      dflex.internal = (n == 1);
    }
    MapValue(cont, "selfcollide", &dflex.selfcollide, flexself_map, 5);
    if (MapValue(cont, "passive", &n, bool_map, 2)) {
      dflex.passive = (n == 1);
    }
    ReadAttrInt(cont, "activelayers", &dflex.activelayers);
  }

  // pin
  XMLElement* epin = FirstChildElement(elem, "pin");
  while (epin) {
    auto id = ReadAttrVec<int>(epin, "id");
    if (id.has_value()) {
      fcomp.pinid.insert(fcomp.pinid.end(), id->begin(), id->end());
    }
    auto range = ReadAttrVec<int>(epin, "range");
    if (range.has_value()) {
      fcomp.pinrange.insert(fcomp.pinrange.end(), range->begin(), range->end());
    }
    auto grid = ReadAttrVec<int>(epin, "grid");
    if (grid.has_value()) {
      fcomp.pingrid.insert(fcomp.pingrid.end(), grid->begin(), grid->end());
    }
    auto gridrange = ReadAttrVec<int>(epin, "gridrange");
    if (gridrange.has_value()) {
      fcomp.pingridrange.insert(fcomp.pingridrange.end(),
                                gridrange->begin(), gridrange->end());
    }

    // advance
    epin = NextSiblingElement(epin, "pin");
  }

  // plugin
  XMLElement* eplugin = FirstChildElement(elem, "plugin");
  if (eplugin) {
    OnePlugin(eplugin, &fcomp.plugin);
  }

  // make flexcomp
  char error[200];
  bool res = fcomp.Make(body, error, 200, vfs);

  // throw error
  if (!res) {
    throw mjXError(elem, "%s", error);
  }
}



// add plugin
void mjXReader::OnePlugin(XMLElement* elem, mjsPlugin* plugin) {
  plugin->active = true;
  string name = "";
  string instance_name = "";
  ReadAttrTxt(elem, "plugin", name);
  ReadAttrTxt(elem, "instance", instance_name);
  mjs_setString(plugin->plugin_name, name.c_str());
  mjs_setString(plugin->name, instance_name.c_str());
  if (instance_name.empty()) {
    plugin->element = mjs_addPlugin(spec)->element;
    ReadPluginConfigs(elem, plugin);
  } else {
    spec->hasImplicitPluginElem = true;
  }
}



//------------------ MJCF-specific sections --------------------------------------------------------

// default section parser
void mjXReader::Default(XMLElement* section, const mjsDefault* def, const mjVFS* vfs) {
  XMLElement* elem;
  string text, name;

  // create new default, except at top level (already added in mjCModel constructor)
  text.clear();
  ReadAttrTxt(section, "class", text);
  if (text.empty()) {
    if (def) {
      throw mjXError(section, "empty class name");
    }
  }
  if (def) {
    def = mjs_addDefault(spec, text.c_str(), def);
    if (!def) {
      throw mjXError(section, "repeated default class name");
    }
  } else {
    def = mjs_getSpecDefault(spec);
    if (!text.empty() && text != "main") {
      throw mjXError(section, "top-level default class 'main' cannot be renamed");
    }
  }

  // iterate over elements other than nested defaults
  elem = FirstChildElement(section);
  while (elem) {
    // get element name
    name = elem->Value();

    // read mesh
    if (name == "mesh")OneMesh(elem, def->mesh, vfs);

    // read material
    else if (name == "material")OneMaterial(elem, def->material);

    // read joint
    else if (name == "joint")OneJoint(elem, def->joint);

    // read geom
    else if (name == "geom")OneGeom(elem, def->geom);

    // read site
    else if (name == "site")OneSite(elem, def->site);

    // read camera
    else if (name == "camera")OneCamera(elem, def->camera);

    // read light
    else if (name == "light")OneLight(elem, def->light);

    // read pair
    else if (name == "pair")OnePair(elem, def->pair);

    // read equality
    else if (name == "equality")OneEquality(elem, def->equality);

    // read tendon
    else if (name == "tendon")OneTendon(elem, def->tendon);

    // read actuator
    else if (name == "general"     ||
             name == "motor"       ||
             name == "position"    ||
             name == "velocity"    ||
             name == "damper"      ||
             name == "intvelocity" ||
             name == "orientation" ||
             name == "pid"         ||
             name == "cylinder"    ||
             name == "muscle"      ||
             name == "adhesion"    ||
             name == "dcmotor") {
      OneActuator(elem, def->actuator);
    }

    // advance
    elem = NextSiblingElement(elem);
  }

  // iterate over nested defaults
  elem = FirstChildElement(section);
  while (elem) {
    // get element name
    name = elem->Value();

    // read default
    if (name == "default") {
      Default(elem, def, vfs);
    }

    // advance
    elem = NextSiblingElement(elem);
  }
}



// extension section parser
void mjXReader::Extension(XMLElement* section) {
  XMLElement* elem = FirstChildElement(section);

  while (elem) {
    // get sub-element name
    string_view name = elem->Value();

    if (name == "plugin") {
      string plugin_name;
      ReadAttrTxt(elem, "plugin", plugin_name, /* required = */ true);
      if (mjs_activatePlugin(spec, plugin_name.c_str())) {
        throw mjXError(elem, "plugin %s not found", plugin_name.c_str());
      }

      XMLElement* child = FirstChildElement(elem);
      while (child) {
        if (string(child->Value()) == "instance") {
          if (spec->hasImplicitPluginElem) {
            throw mjXError(
                    child, "explicit plugin instance must appear before implicit plugin elements");
          }
          string name;
          mjsPlugin* p = mjs_addPlugin(spec);
          mjs_setString(p->plugin_name, plugin_name.c_str());
          mjs_setString(p->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
          ReadAttrTxt(child, "name", name, /* required = */ true);
          mjs_setString(p->name, name.c_str());
          if (!p->name) {
            throw mjXError(child, "plugin instance must have a name");
          }
          ReadPluginConfigs(child, p);
        }
        child = NextSiblingElement(child);
      }
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// custom section parser
void mjXReader::Custom(XMLElement* section) {
  string str, name;
  XMLElement* elem;
  double data[500];

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get sub-element name
    name = elem->Value();
    string elname;

    // numeric
    if (name == "numeric") {
      // create custom
      mjsNumeric* numeric = mjs_addNumeric(spec);

      // write error info
      mjs_setString(numeric->info, ("line " + std::to_string(elem->GetLineNum())).c_str());

      // read attributes
      ReadAttrTxt(elem, "name", elname, true);
      if (mjs_setName(numeric->element, elname.c_str())) {
        throw mjXError(elem, "%s", mjs_getError(spec));
      }
      if (ReadAttrInt(elem, "size", &numeric->size)) {
        int sz = numeric->size < 500 ? numeric->size : 500;
        for (int i=0; i < sz; i++) {
          data[i] = 0;
        }
      } else {
        numeric->size = 501;
      }
      int len = ReadAttr(elem, "data", numeric->size, data, str, false, false);
      if (numeric->size == 501) {
        numeric->size = len;
      }
      if (numeric->size < 1 || numeric->size > 500) {
        throw mjXError(elem, "custom field size must be between 1 and 500");
      }

      // copy data
      mjs_setDouble(numeric->data, data, numeric->size);
    }

    // text
    else if (name == "text") {
      // create custom
      mjsText* text = mjs_addText(spec);

      // write error info
      mjs_setString(text->info, ("line " + std::to_string(elem->GetLineNum())).c_str());

      // read attributes
      ReadAttrTxt(elem, "name", elname, true);
      if (mjs_setName(text->element, elname.c_str())) {
        throw mjXError(elem, "%s", mjs_getError(spec));
      }
      ReadAttrTxt(elem, "data", str, true);
      if (str.empty()) {
        throw mjXError(elem, "text field cannot be empty");
      }

      // copy data
      mjs_setString(text->data, str.c_str());
    }

    // tuple
    else if (name == "tuple") {
      // create custom
      mjsTuple* tuple = mjs_addTuple(spec);

      // write error info
      mjs_setString(tuple->info, ("line " + std::to_string(elem->GetLineNum())).c_str());

      // read attributes
      ReadAttrTxt(elem, "name", elname, true);
      if (mjs_setName(tuple->element, elname.c_str())) {
        throw mjXError(elem, "%s", mjs_getError(spec));
      }

      // read objects and add
      XMLElement* obj = FirstChildElement(elem);
      std::vector<int> objtype;
      string objname = "";
      std::vector<double> objprm;

      while (obj) {
        // get sub-element name
        name = obj->Value();

        // new object
        if (name == "element") {
          // read type, check and assign
          ReadAttrTxt(obj, "objtype", str, true);
          mjtObj otype = (mjtObj)mju_str2Type(str.c_str());
          if (otype == mjOBJ_UNKNOWN) {
            throw mjXError(obj, "unknown object type");
          }
          objtype.push_back(otype);

          // read name and assign
          ReadAttrTxt(obj, "objname", str, true);
          objname += " " + str;

          // read parameter and assign
          double oprm = 0;
          ReadAttr(obj, "prm", 1, &oprm, str);
          objprm.push_back(oprm);
        }

        // advance to next object
        obj = NextSiblingElement(obj);
      }

      mjs_setInt(tuple->objtype, objtype.data(), objtype.size());
      mjs_setStringVec(tuple->objname, objname.c_str());
      mjs_setDouble(tuple->objprm, objprm.data(), objprm.size());
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// visual section parser
void mjXReader::Visual(XMLElement* section) {
  string name;
  XMLElement* elem;
  mjVisual* vis = &spec->visual;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get sub-element name
    name = elem->Value();

    // global sub-element
    if (name == "global") {
      ReadAttrTableCore(elem, vis, kGlobalAttrs, kGlobalAttrsN,
                        /*skipnodefault=*/false,
                        /*authored=*/spec);
      if (vis->global.realtime <= 0) {
        throw mjXError(elem, "realtime must be greater than 0");
      }
    }

    // quality sub-element
    else if (name == "quality") {
      ReadAttrTableCore(elem, vis, kQualityAttrs, kQualityAttrsN, false,
                        /*authored=*/spec);
    }

    // headlight sub-element
    else if (name == "headlight") {
      ReadAttrTableCore(elem, vis, kHeadlightAttrs, kHeadlightAttrsN, false,
                        /*authored=*/spec);
    }

    // map sub-element
    else if (name == "map") {
      ReadAttrTableCore(elem, vis, kMapAttrs, kMapAttrsN, false,
                        /*authored=*/spec);
      if (vis->map.znear <= 0) {
        throw mjXError(elem, "znear must be strictly positive");
      }
    }

    // scale sub-element
    else if (name == "scale") {
      ReadAttrTableCore(elem, vis, kScaleAttrs, kScaleAttrsN, false,
                        /*authored=*/spec);
    }

    // rgba sub-element
    else if (name == "rgba") {
      ReadAttrTableCore(elem, vis, kRgbaAttrs, kRgbaAttrsN, false,
                        /*authored=*/spec);
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}




// asset section parser
void mjXReader::Asset(XMLElement* section, const mjVFS* vfs) {
  string text, name;
  XMLElement* elem;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get sub-element name
    name = elem->Value();

    // get class if specified, otherwise use default0
    const mjsDefault* def = GetClass(elem);
    if (!def) {
      def = mjs_getSpecDefault(spec);
    }

    // texture sub-element
    if (name == "texture") {
      // create texture
      mjsTexture* texture = mjs_addTexture(spec);

      // write error info
      mjs_setString(texture->info, ("line " + std::to_string(elem->GetLineNum())).c_str());

      // mechanical attributes
      ReadAttrTable(elem, texture, texture->element, kTextureAttrs,
                    kTextureAttrsN);

      // file, resolved against the texture directory
      auto file = ReadAttrFile(elem, "file", vfs, TextureDir());
      if (file.has_value()) {
        mjs_setString(texture->file, file->c_str());
      }

      // gridlayout length must equal the gridsize product (value-conditional)
      if (ReadAttrTxt(elem, "gridlayout", text) &&
          text.length() != texture->gridsize[0]*texture->gridsize[1]) {
        throw mjXError(elem, "gridlayout length must match gridsize");
      }

      // separate files
      std::vector<string> cubefiles(6);
      std::vector<string> cubefile_names = {"fileright", "fileleft",
                                            "fileup", "filedown",
                                            "filefront", "fileback"};
      for (int i = 0; i < cubefiles.size(); i++) {
        auto maybe_file = ReadAttrFile(elem, cubefile_names[i].c_str(), vfs,
                                       TextureDir());
        if (maybe_file.has_value()) {
          cubefiles[i] = maybe_file.value().Str();
        } else {
          cubefiles[i] = "";
        }
        mjs_setInStringVec(texture->cubefiles, i, cubefiles[i].c_str());
      }
    }

    // material sub-element
    else if (name == "material") {
      // create material and parse
      mjsMaterial* material = mjs_addMaterial(spec, def);
      OneMaterial(elem, material);
    }

    // mesh sub-element
    else if (name == "mesh") {
      // create mesh and parse
      mjsMesh* mesh = mjs_addMesh(spec, def);
      OneMesh(elem, mesh, vfs);
    }

    // skin sub-element... deprecate ???
    else if (name == "skin") {
      // create skin and parse
      mjsSkin* skin = mjs_addSkin(spec);
      OneSkin(elem, skin, vfs);
    }

    // hfield sub-element
    else if (name == "hfield") {
      // create hfield
      mjsHField* hfield = mjs_addHField(spec);

      // write error info
      mjs_setString(hfield->info, ("line " + std::to_string(elem->GetLineNum())).c_str());

      // mechanical attributes
      ReadAttrTable(elem, hfield, hfield->element, kHfieldAttrs,
                    kHfieldAttrsN);

      // file, resolved against the asset directory
      auto file = ReadAttrFile(elem, "file", vfs, AssetDir());
      if (file.has_value()) {
        mjs_setString(hfield->file, file->c_str());
      }

      // allocate buffer for dynamic hfield, copy user data if given
      if (!file.has_value() && hfield->nrow > 0 && hfield->ncol > 0) {
        int nrow = hfield->nrow;
        int ncol = hfield->ncol;

        // read user data
        auto userdata = ReadAttrVec<float>(elem, "elevation");

        // user data given, copy into data
        if (userdata.has_value()) {
          if (userdata->size() != nrow*ncol) {
            throw mjXError(elem, "elevation data length must match nrow*ncol");
          }

          // copy in reverse row order, so XML string is top-to-bottom
          std::vector<float> flipped(nrow*ncol);
          for (int i = 0; i < nrow; i++) {
            int flip = nrow-1-i;
            for (int j = 0; j < ncol; j++) {
              flipped[flip*ncol + j] = userdata->data()[i*ncol + j];
            }
          }

          mjs_setFloat(hfield->userdata, flipped.data(), flipped.size());
        }

        // user data not given, set to 0
        else {
          std::vector<float> zero(nrow*ncol);
          mjs_setFloat(hfield->userdata, zero.data(), zero.size());
        }
      }
    }

    // model sub-element
    else if (name == "model") {
      std::string content_type;
      ReadAttrTxt(elem, "content_type", content_type);

      // parse the child
      std::array<char, 1024> error;
      auto filename = modelfiledir_ + ReadAttrFile(elem, "file", vfs).value();

      mjSpec* child = mj_parse(filename.c_str(), content_type.c_str(), vfs,
                               error.data(), error.size());
      if (!child) {
        throw mjXError(elem, "could not parse model file with error: %s", error.data());
      }

      // overwrite model name if given
      string modelname = "";
      if (ReadAttrTxt(elem, "name", modelname)) {
        mjs_setString(child->modelname, modelname.c_str());
      }

      // store child spec in model
      mjs_addSpec(spec, child);
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}

// strip the "Error: " prefix from compiler/spec error messages
static const char* stripError(const char* err) {
  if (err && std::strncmp(err, "Error: ", 7) == 0) {
    return err + 7;
  }
  return err;
}

// body/world section parser; recursive
void mjXReader::Body(XMLElement* section, mjsBody* body, mjsFrame* frame,
                     const mjVFS* vfs) {
  string text, name;
  XMLElement* elem;

  // sanity check
  if (!body) {
    throw mjXError(section, "null body pointer");
  }

  // no attributes allowed in world body
  if (mjs_getId(body->element) == 0 && section->FirstAttribute() && !frame) {
    throw mjXError(section, "World body cannot have attributes");
  }

  // iterate over sub-elements; attributes set while parsing parent body
  elem = FirstChildElement(section);
  while (elem) {
    // get sub-element name
    name = elem->Value();

    // get class if specified, otherwise use body
    const mjsDefault* def = GetClass(elem);
    if (!def) {
      def = mjs_getDefault(frame ? frame->element : body->element);
    }

    // inertial sub-element
    if (name == "inertial") {
      // no inertia allowed in world body
      if (mjs_getId(body->element) == 0) {
        throw mjXError(elem, "World body cannot have inertia");
      }
      body->explicitinertial = true;
      ReadAttrTable(elem, body, body->element, kInertialAttrs, kInertialAttrsN);
      ReadQuat(elem, "quat", body->iquat, text);
      ReadAlternative(elem, body->ialt);
      ReadAttr(elem, "fullinertia", 6, body->fullinertia, text);
    }

    // joint sub-element
    else if (name == "joint") {
      // no joints allowed in world body
      if (mjs_getId(body->element) == 0) {
        throw mjXError(elem, "World body cannot have joints");
      }

      // create joint and parse
      mjsJoint* joint = mjs_addJoint(body, def);
      OneJoint(elem, joint);
      mjs_setFrame(joint->element, frame);
    }

    // freejoint sub-element
    else if (name == "freejoint") {
      // no joints allowed in world body
      if (mjs_getId(body->element) == 0) {
        throw mjXError(elem, "World body cannot have joints");
      }

      // create free joint without defaults
      mjsJoint* joint = mjs_addFreeJoint(body);
      mjs_setFrame(joint->element, frame);

      // save defaults after creation, to make sure writing is ok
      mjs_setDefault(joint->element, def);

      // read attributes
      string name;
      if (ReadAttrTxt(elem, "name", name)) {
        if (mjs_setName(joint->element, name.c_str())) {
          throw mjXError(elem, "%s", mjs_getError(spec));
        }
      }
      ReadAttrInt(elem, "group", &joint->group);
      MapValue(elem, "align", &joint->align, FalseTrueAuto_map, 3);
    }

    // geom sub-element
    else if (name == "geom") {
      // create geom and parse
      mjsGeom* geom = mjs_addGeom(body, def);
      OneGeom(elem, geom);
      mjs_setFrame(geom->element, frame);
    }

    // site sub-element
    else if (name == "site") {
      // create site and parse
      mjsSite* site = mjs_addSite(body,  def);
      OneSite(elem, site);
      mjs_setFrame(site->element, frame);
    }

    // camera sub-element
    else if (name == "camera") {
      // create camera and parse
      mjsCamera* camera = mjs_addCamera(body, def);
      OneCamera(elem, camera);
      mjs_setFrame(camera->element, frame);
    }

    // light sub-element
    else if (name == "light") {
      // create light and parse
      mjsLight* light = mjs_addLight(body, def);
      OneLight(elem, light);
      mjs_setFrame(light->element, frame);
    }

    // plugin sub-element
    else if (name == "plugin") {
      OnePlugin(elem, &(body->plugin));
    }

    // composite sub-element
    else if (name == "composite") {
      // parse composite
      OneComposite(elem, body, frame, def);
    }

    // flexcomp sub-element
    else if (name == "flexcomp") {
      // parse flexcomp
      OneFlexcomp(elem, body, vfs);
    }

    // frame sub-element
    else if (name == "frame") {
      // read childdef
      bool has_childclass = ReadAttrTxt(elem, "childclass", text);
      const mjsDefault* childdef = has_childclass ? mjs_findDefault(spec, text.c_str()) : nullptr;
      if (has_childclass && !childdef) {
        throw mjXError(elem, "unknown default childclass");
      }

      // create frame
      mjsFrame* pframe = mjs_addFrame(body, frame);
      mjs_setString(pframe->info, ("line " + std::to_string(elem->GetLineNum())).c_str());
      mjs_setDefault(pframe->element, childdef ? childdef : def);

      // read attributes
      string name, childclass;
      if (ReadAttrTxt(elem, "name", name)) {
        if (mjs_setName(pframe->element, name.c_str())) {
          throw mjXError(elem, "%s", mjs_getError(spec));
        }
      }
      if (ReadAttrTxt(elem, "childclass", childclass)) {
        mjs_setString(pframe->childclass, childclass.c_str());
      }
      ReadAttr(elem, "pos", 3, pframe->pos, text);
      ReadQuat(elem, "quat", pframe->quat, text);
      ReadAlternative(elem, pframe->alt);

      Body(elem, body, pframe, vfs);
    }

    // replicate sub-element
    else if (name == "replicate") {
      int count;
      double offset[3] = {0, 0, 0};
      double euler[3] = {0, 0, 0};
      string separator = "";
      ReadAttr(elem, "count", 1, &count, text, true);
      ReadAttr(elem, "offset", 3, offset, text);
      ReadAttr(elem, "euler", 3, euler, text);
      ReadAttrTxt(elem, "sep", separator);

      // store rotation difference
      mjsOrientation alt;
      mjs_defaultOrientation(&alt);
      alt.type = mjORIENTATION_EULER;
      mjuu_copyvec(alt.euler, euler, 3);
      double rotation[4] = {1, 0, 0, 0};
      mjs_resolveOrientation(rotation, spec->compiler.degree, spec->compiler.eulerseq, &alt);

      // read childdef
      bool has_childclass = ReadAttrTxt(elem, "childclass", text);
      const mjsDefault* childdef = has_childclass ? mjs_findDefault(spec, text.c_str()) : nullptr;
      if (has_childclass && !childdef) {
        throw mjXError(elem, "unknown default childclass");
      }

      // create subtree
      mjsBody* subtree = mjs_addBody(body, childdef);
      double pos[3] = {0, 0, 0};
      double quat[4] = {1, 0, 0, 0};

      // parent frame that will be used to attach the subtree
      mjsFrame* pframe = mjs_addFrame(subtree, frame);
      mjs_setDefault(pframe->element, childdef ? childdef : def);
      mjs_setString(pframe->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());

      // parse subtree
      Body(elem, subtree, pframe, vfs);

      // update pframe and attach
      for (int i = 0; i < count; i++) {
        // overwrite orientation to increase precision
        alt.euler[0] = i*euler[0];
        alt.euler[1] = i*euler[1];
        alt.euler[2] = i*euler[2];
        mjs_resolveOrientation(quat, spec->compiler.degree, spec->compiler.eulerseq, &alt);

        // set position and orientation
        mjuu_setvec(pframe->pos, pos[0], pos[1], pos[2]);
        mjuu_setvec(pframe->quat, quat[0], quat[1], quat[2], quat[3]);

        // accumulate rotation
        mjuu_frameaccum(pos, quat, offset, rotation);

        // process suffix
        string suffix = separator;
        UpdateString(suffix, count, i);

        // attach to parent
        if (!mjs_attach(body->element, pframe->element, /*prefix=*/"", suffix.c_str())) {
          throw mjXError(elem, "%s", stripError(mjs_getError(spec)));
        }
      }

      // delete subtree
      if (mjs_delete(spec, subtree->element)) {
        throw mjXError(elem, "%s", stripError(mjs_getError(spec)));
      }
    }

    // body sub-element
    else if (name == "body") {
      // read childdef
      bool has_childclass = ReadAttrTxt(elem, "childclass", text);
      const mjsDefault* childdef = has_childclass ? mjs_findDefault(spec, text.c_str()) : nullptr;
      if (has_childclass && !childdef) {
        throw mjXError(elem, "unknown default childclass");
      }

      // create child body
      mjsBody* child = mjs_addBody(body, childdef);
      mjs_setString(child->info, string("line " + std::to_string(elem->GetLineNum())).c_str());

      // set default from class or childclass
      mjs_setDefault(child->element, childdef ? childdef : def);

      // mechanical attributes
      ReadAttrTable(elem, child, child->element, kBodyAttrs, kBodyAttrsN);
      string childclass;
      if (ReadAttrTxt(elem, "childclass", childclass)) {
        mjs_setString(child->childclass, childclass.c_str());
      }

      // orientation alternatives
      ReadQuat(elem, "quat", child->quat, text);
      ReadAlternative(elem, child->alt);

      // add frame
      mjs_setFrame(child->element, frame);

      // make recursive call
      Body(elem, child, nullptr, vfs);
    }

    // attachment
    else if (name == "attach") {
      string model_name, child_name, prefix;
      bool has_model = ReadAttrTxt(elem, "model", model_name, /*required=*/false);
      bool has_body = ReadAttrTxt(elem, "body", child_name, /*required=*/false);
      bool has_frame = ReadAttrTxt(elem, "frame", child_name, /*required=*/false);
      ReadAttrTxt(elem, "prefix", prefix, /*required=*/true);

      mjtObj type = mjOBJ_UNKNOWN;
      if (has_body) type = mjOBJ_BODY;
      else if (has_frame) type = mjOBJ_FRAME;

      mjsElement* source_elem = nullptr;
      if (!has_model) { // Self-attach
        if (type == mjOBJ_UNKNOWN) {
          throw mjXError(elem, "either 'body' or 'frame' attribute must be specified for self-attach");
        }

        // check for name collision in the current spec
        string full_name = prefix + child_name;
        if (mjs_findElement(spec, type, full_name.c_str())) {
          throw mjXError(elem, "cannot self-attach: element %s already exists", full_name.c_str());
        }
        source_elem = mjs_findElement(spec, type, child_name.c_str());
        if (!source_elem) {
          throw mjXError(elem, "%s",
                         (string("could not find ") + mju_type2Str(type) + " '" + child_name +
                          "' in the current model for self-attachment").c_str());
        }
      } else { // Attach from external model asset
        // Check for name collision in the current spec
        if (!child_name.empty()) {
            string full_name = prefix + child_name;
            if (mjs_findElement(spec, type, full_name.c_str())) {
                throw mjXError(elem, "%s",
                               (string("cannot attach: element ") + child_name +
                                " already exists with prefix " + prefix).c_str());
            }
        }

        mjSpec* asset = mjs_findSpec(spec, model_name.c_str());
        if (!asset) {
          throw mjXError(elem, "could not find model '%s'", model_name.c_str());
        }

        if (type == mjOBJ_UNKNOWN) { // Attach world body contents
          source_elem = asset->element;
        } else { // Attach specific body or frame
          source_elem = mjs_findElement(asset, type, child_name.c_str());
          if (!source_elem) {
            throw mjXError(elem, "%s",
                           (string("could not find ") + mju_type2Str(type) + " '" + child_name +
                            "' in model asset '" + model_name + "'").c_str());
          }
        }
      }

      mjsFrame* pframe = frame ? frame : mjs_addFrame(body, nullptr);
      // Set default for the new frame from the current context
      mjs_setDefault(pframe->element, mjs_getDefault(frame ? frame->element : body->element));
      mjs_setString(pframe->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());

      if (!mjs_attach(pframe->element, source_elem, prefix.c_str(), "")) {
        throw mjXError(elem, "%s", stripError(mjs_getError(spec)));
      }
    }

    // no match
    else {
      throw mjXError(elem, "unrecognized model element '%s'", name.c_str());
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// contact section parser
void mjXReader::Contact(XMLElement* section) {
  string text, name;
  XMLElement* elem;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get sub-element name
    name = elem->Value();

    // get class if specified, otherwise use default0
    const mjsDefault* def = GetClass(elem);
    if (!def) {
      def = mjs_getSpecDefault(spec);
    }

    // geom pair to include
    if (name == "pair") {
      // create pair and parse
      mjsPair* pair = mjs_addPair(spec, def);
      OnePair(elem, pair);
    }

    // body pair to exclude
    else if (name == "exclude") {
      mjsExclude* exclude = mjs_addExclude(spec);

      // write error info
      mjs_setString(exclude->info, ("line " + std::to_string(elem->GetLineNum())).c_str());

      // read attributes
      ReadAttrTable(elem, exclude, exclude->element, kExcludeAttrs,
                    kExcludeAttrsN);
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// constraint section parser
void mjXReader::Equality(XMLElement* section) {
  XMLElement* elem;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get class if specified, otherwise use default0
    const mjsDefault* def = GetClass(elem);
    if (!def) {
      def = mjs_getSpecDefault(spec);
    }

    // create equality constraint and parse
    mjsEquality* equality = mjs_addEquality(spec, def);
    OneEquality(elem, equality);

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// deformable section parser
void mjXReader::Deformable(XMLElement* section, const mjVFS* vfs) {
  string name;
  XMLElement* elem;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get sub-element name
    name = elem->Value();

    // get class if specified, otherwise use default0
    const mjsDefault* def = GetClass(elem);
    if (!def) {
      def = mjs_getSpecDefault(spec);
    }

    // flex sub-element
    if (name == "flex") {
      // create flex and parse
      mjsFlex* flex = mjs_addFlex(spec);
      OneFlex(elem, flex);
    }

    // skin sub-element
    else if (name == "skin") {
      // create skin and parse
      mjsSkin* skin = mjs_addSkin(spec);
      OneSkin(elem, skin, vfs);
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// tendon section parser
void mjXReader::Tendon(XMLElement* section) {
  string text, text1;
  XMLElement* elem;
  double data;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get class if specified, otherwise use default0
    const mjsDefault* def = GetClass(elem);
    if (!def) {
      def = mjs_getSpecDefault(spec);
    }

    // create tendon and parse
    mjsTendon* tendon = mjs_addTendon(spec, def);
    OneTendon(elem, tendon);

    // process wrap sub-elements
    XMLElement* sub = FirstChildElement(elem);
    while (sub) {
      // get wrap type
      string type = sub->Value();
      mjsWrap* wrap;;

      // read attributes depending on type
      if (type == "site") {
        ReadAttrTxt(sub, "site", text, true);
        wrap = mjs_wrapSite(tendon, text.c_str());
      }

      else if (type == "geom") {
        ReadAttrTxt(sub, "geom", text, true);
        if (!ReadAttrTxt(sub, "sidesite", text1)) {
          text1.clear();
        }
        wrap = mjs_wrapGeom(tendon, text.c_str(), text1.c_str());
      }

      else if (type == "pulley") {
        ReadAttr(sub, "divisor", 1, &data, text, true);
        wrap = mjs_wrapPulley(tendon, data);
      }

      else if (type == "joint") {
        ReadAttrTxt(sub, "joint", text, true);
        ReadAttr(sub, "coef", 1, &data, text1, true);
        wrap = mjs_wrapJoint(tendon, text.c_str(), data);
      }

      else {
        throw mjXError(sub, "unknown wrap type");  // SHOULD NOT OCCUR
      }

      mjs_setString(wrap->info, ("line " + std::to_string(sub->GetLineNum())).c_str());

      // advance to next sub-element
      sub = NextSiblingElement(sub);
    }

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// actuator section parser
void mjXReader::Actuator(XMLElement* section) {
  XMLElement* elem;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    // get class if specified, otherwise use default0
    const mjsDefault* def = GetClass(elem);
    if (!def) {
      def = mjs_getSpecDefault(spec);
    }

    // create actuator and parse
    mjsActuator* actuator = mjs_addActuator(spec, def);
    OneActuator(elem, actuator);

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// sensor section parser
void mjXReader::Sensor(XMLElement* section) {
  int n;
  XMLElement* elem = FirstChildElement(section);
  while (elem) {
    // create sensor, get string type
    mjsSensor* sensor = mjs_addSensor(spec);
    string type = elem->Value();
    string text, objname, refname;

    // mechanical attributes shared by all sensor types
    ReadAttrTable(elem, sensor, sensor->element, kSensorBaseAttrs,
                  kSensorBaseAttrsN);

    // common robotic sensors, attached to a site
    // sensors fully described by the schema: constants and references
    bool dispatched = false;
    for (int i = 0; i < kSensorDispatchN; i++) {
      if (type == kSensorDispatch[i].tag) {
        ReadAttrTable(elem, sensor, sensor->element, kSensorDispatch[i].rows,
                      kSensorDispatch[i].n);
        dispatched = true;
        break;
      }
    }
    if (dispatched) {}
    else if (type == "rangefinder") {
      sensor->type = mjSENS_RANGEFINDER;
      bool use_site = ReadAttrTxt(elem, "site", objname, false);
      ReadAttrTxt(elem, "camera", objname, false);
      sensor->objtype = use_site ? mjOBJ_SITE : mjOBJ_CAMERA;

      // process data specification (intprm[0])
      int dataspec = 1 << mjRAYDATA_DIST;
      std::vector<int> raydata(mjNRAYDATA);
      int nkeys = MapValues(elem, "data", raydata.data(), raydata_map, mjNRAYDATA);
      if (nkeys) {
        dataspec = 1 << raydata[0];

        // check ordering while adding bits to dataspec
        for (int i = 1; i < nkeys; ++i) {
          if (raydata[i] <= raydata[i-1]) {
            std::string correct_order;
            for (int j = 0; j < mjNRAYDATA; ++j) {
              correct_order += raydata_map[j].key;
              if (j < mjNRAYDATA - 1) correct_order += ", ";
            }
            throw mjXError(elem, "data attributes must be in order: %s", correct_order.c_str());
          }
          dataspec |= 1 << raydata[i];
        }
      }
      sensor->intprm[0] = dataspec;
    }
    else if (type == "distance" || type == "normal" || type == "fromto") {
      bool has_body1 = ReadAttrTxt(elem, "body1", objname);
      ReadAttrTxt(elem, "geom1", objname);
      sensor->objtype = has_body1 ? mjOBJ_BODY : mjOBJ_GEOM;
      bool has_body2 = ReadAttrTxt(elem, "body2", refname);
      ReadAttrTxt(elem, "geom2", refname);
      sensor->reftype = has_body2 ? mjOBJ_BODY : mjOBJ_GEOM;
      if (type == "distance") {
        sensor->type = mjSENS_GEOMDIST;
      } else if (type == "normal") {
        sensor->type = mjSENS_GEOMNORMAL;
      } else {
        sensor->type = mjSENS_GEOMFROMTO;
      }
    }
    else if (type == "contact") {
      // first matching criterion
      bool has_site = ReadAttrTxt(elem, "site", objname);
      bool has_body1 = ReadAttrTxt(elem, "body1", objname);
      bool has_subtree1 = ReadAttrTxt(elem, "subtree1", objname);
      bool has_geom1 = ReadAttrTxt(elem, "geom1", objname);
      if (has_site)          { sensor->objtype = mjOBJ_SITE; }
      else if (has_body1)    { sensor->objtype = mjOBJ_BODY; }
      else if (has_subtree1) { sensor->objtype = mjOBJ_XBODY; }
      else if (has_geom1)    { sensor->objtype = mjOBJ_GEOM; }
      else                   { sensor->objtype = mjOBJ_UNKNOWN; }

      // second matching criterion
      bool has_body2 = ReadAttrTxt(elem, "body2", refname);
      bool has_subtree2 = ReadAttrTxt(elem, "subtree2", refname);
      bool has_geom2 = ReadAttrTxt(elem, "geom2", refname);
      if (has_body2)         { sensor->reftype = mjOBJ_BODY; }
      else if (has_subtree2) { sensor->reftype = mjOBJ_XBODY; }
      else if (has_geom2)    { sensor->reftype = mjOBJ_GEOM; }
      else                   { sensor->reftype = mjOBJ_UNKNOWN; }

      // process data specification (intprm[0])
      int dataspec = 1 << mjCONDATA_FOUND;
      std::vector<int> condata(mjNCONDATA);
      int nkeys = MapValues(elem, "data", condata.data(), condata_map, mjNCONDATA);
      if (nkeys) {
        dataspec = 1 << condata[0];

        // check ordering while adding bits to dataspec
        for (int i = 1; i < nkeys; ++i) {
          if (condata[i] <= condata[i-1]) {
            std::string correct_order;
            for (int j = 0; j < mjNCONDATA; ++j) {
              correct_order += condata_map[j].key;
              if (j < mjNCONDATA - 1) correct_order += ", ";
            }
            throw mjXError(elem, "data attributes must be in order: %s", correct_order.c_str());
          }
          dataspec |= 1 << condata[i];
        }
      }
      sensor->intprm[0] = dataspec;

      // reduction type (intprm[1])
      sensor->intprm[1] = 0;
      if (MapValue(elem, "reduce", &n, reduce_map, reduce_sz)) {
        sensor->intprm[1] = n;
      }

      // number of contacts (intprm[2])
      sensor->intprm[2] = 1;
      ReadAttrInt(elem, "num", &sensor->intprm[2]);
      if (sensor->intprm[2] <= 0) {
        throw mjXError(elem, "'num' must be positive in sensor");
      }

      // sensor type
      sensor->type = mjSENS_CONTACT;
    }
    else if (type == "user") {
      sensor->type = mjSENS_USER;
      ReadAttrTxt(elem, "objname", objname);
      if (ReadAttrTxt(elem, "objtype", text)) {
        sensor->objtype = (mjtObj)mju_str2Type(text.c_str());
      }
      ReadAttrInt(elem, "dim", &sensor->dim, true);

      // keywords
      if (MapValue(elem, "needstage", &n, stage_map, stage_sz)) {
        sensor->needstage = (mjtStage)n;
      }
      if (MapValue(elem, "datatype", &n, datatype_map, datatype_sz)) {
        sensor->datatype = (mjtDataType)n;
      }
    }

    // tactile sensor
    if (type == "tactile") {
      sensor->type = mjSENS_TACTILE;
      sensor->reftype = mjOBJ_GEOM;
      ReadAttrTxt(elem, "geom", refname, /*required=*/true);

      // associate the sensor with a mesh
      sensor->objtype = mjOBJ_MESH;
      ReadAttrTxt(elem, "mesh", objname, /*required=*/true);
      mjs_setString(sensor->objname, objname.c_str());
    }

    else if (type == "plugin") {
      sensor->type = mjSENS_PLUGIN;
      OnePlugin(elem, &sensor->plugin);
      ReadAttrTxt(elem, "objtype", text);
      sensor->objtype = (mjtObj)mju_str2Type(text.c_str());
      ReadAttrTxt(elem, "objname", objname);
      if (sensor->objtype != mjOBJ_UNKNOWN && objname.empty()) {
        throw mjXError(elem, "objtype is specified but objname is not");
      }
      if (sensor->objtype == mjOBJ_UNKNOWN && !objname.empty()) {
        throw mjXError(elem, "objname is specified but objtype is not");
      }
      if (ReadAttrTxt(elem, "reftype", text)) {
        sensor->reftype = (mjtObj)mju_str2Type(text.c_str());
      }
      ReadAttrTxt(elem, "refname", refname);
      if (sensor->reftype != mjOBJ_UNKNOWN && refname.empty()) {
        throw mjXError(elem, "reftype is specified but refname is not");
      }
      if (sensor->reftype == mjOBJ_UNKNOWN && !refname.empty()) {
        throw mjXError(elem, "refname is specified but reftype is not");
      }
    }

    if (!objname.empty()) {
      mjs_setString(sensor->objname, objname.c_str());
    }

    if (!refname.empty()) {
      mjs_setString(sensor->refname, refname.c_str());
    }

    // write info
    mjs_setString(sensor->info, ("line " + std::to_string(elem->GetLineNum())).c_str());

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// keyframe section parser
void mjXReader::Keyframe(XMLElement* section) {
  XMLElement* elem;

  // iterate over child elements
  elem = FirstChildElement(section);
  while (elem) {
    string name = "";

    // add keyframe
    mjsKey* key = mjs_addKey(spec);

    // read name: set even when the attribute is absent
    ReadAttrTxt(elem, "name", name);
    if (mjs_setName(key->element, name.c_str())) {
      throw mjXError(elem, "%s", mjs_getError(spec));
    }

    // mechanical attributes
    ReadAttrTable(elem, key, key->element, kKeyAttrs, kKeyAttrsN);

    // advance to next element
    elem = NextSiblingElement(elem);
  }
}



// get defaults class
const mjsDefault* mjXReader::GetClass(XMLElement* section) {
  string text;

  if (!ReadAttrTxt(section, "class", text)) {
    return nullptr;
  }

  const mjsDefault* def = mjs_findDefault(spec, text.c_str());
  if (!def) {
    throw mjXError(
            section,
            string("unknown default class name '" + text + "'").c_str());
  }
  return def;
}

void mjXReader::SetModelFileDir(const string& modelfiledir) {
  modelfiledir_ = FilePath(modelfiledir);
}

void mjXReader::SetAssetDir(const string& assetdir) {
  assetdir_ = FilePath(assetdir);
}

void mjXReader::SetMeshDir(const string& meshdir) {
  meshdir_ = FilePath(meshdir);
}

void mjXReader::SetTextureDir(const string& texturedir) {
  texturedir_ = FilePath(texturedir);
}

FilePath mjXReader::AssetDir() const {
  return modelfiledir_ + assetdir_;
}

FilePath mjXReader::MeshDir() const {
  if (meshdir_.empty()) {
    return AssetDir();
  }
  return modelfiledir_ + meshdir_;
}
FilePath mjXReader::TextureDir() const {
  if (texturedir_.empty()) {
    return AssetDir();
  }
  return modelfiledir_ + texturedir_;
}