BDSUtilities.cc

/* 
Beam Delivery Simulation (BDSIM) Copyright (C) Royal Holloway, 
University of London 2001 - 2023.
 
This file is part of BDSIM.
 
BDSIM is free software: you can redistribute it and/or modify 
it under the terms of the GNU General Public License as published 
by the Free Software Foundation version 3 of the License.
 
BDSIM is distributed in the hope that it will be useful, but 
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.
 
You should have received a copy of the GNU General Public License
along with BDSIM.  If not, see <http://www.gnu.org/licenses/>.
*/
#include "BDSDebug.hh"
#include "BDSException.hh"
#include "BDSExtent.hh"
#include "BDSGlobalConstants.hh"
#include "BDSPhysicalConstants.hh"
#include "BDSRunManager.hh"
#include "BDSUtilities.hh"
 
#include "globals.hh" // geant4 types / globals
#include "G4String.hh"
#include "G4ThreeVector.hh"
#include "G4Track.hh"
#include "G4TwoVector.hh"
#include "G4UserLimits.hh"
#include "G4Version.hh"
 
#include "CLHEP/Units/SystemOfUnits.h"
 
#include <algorithm>
#include <cmath>
#include <cctype>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <signal.h>
#include <sstream>
#include <string>
#include <unistd.h>
#include <utility>
#include <vector>
 
#include <sys/resource.h>
#include <sys/stat.h>
 
#ifdef __APPLE__
#include <mach-o/dyld.h> // for executable path
#endif
 
 
G4bool BDS::non_alpha::operator()(char c)
{
  return !isalpha(c);
}
 
G4bool BDS::StrContains(const G4String& str, const G4String& test)
{
#if G4VERSION_NUMBER > 1099
  return G4StrUtil::contains(str, test);
#else
  return str.contains(test);
#endif
}
#if G4VERSION_NUMBER > 1099
G4int BDS::StrCompare(const G4String& str, const G4String& test, G4String::caseCompare)
#else
G4int BDS::StrCompare(const G4String& str, const G4String& test, G4String::caseCompare mode)
#endif
{
#if G4VERSION_NUMBER > 1099
  return G4StrUtil::icompare(str, test);
#else
  return str.compareTo(test, mode);
#endif
}
 
G4String BDS::LowerCase(const G4String& str)
{
  G4String result = str;
#if G4VERSION_NUMBER > 1099
  G4StrUtil::to_lower(result);
#else
  result.toLower();
#endif
  return result;
}
 
G4String BDS::StrStrip(const G4String& str,
                       char ch,
                       StringStripType stripType)
{
  G4String result = str;
  switch (stripType)
    {
#if G4VERSION_NUMBER > 1099
    case StringStripType::leading:
      {G4StrUtil::lstrip(result, ch); break;}
    case StringStripType::trailing:
      {G4StrUtil::rstrip(result, ch); break;}
    case StringStripType::both:
      {G4StrUtil::strip(result, ch); break;}
#else
    case StringStripType::leading:
      {result.strip(G4String::stripType::leading, ch); break;}
    case StringStripType::trailing:
      {result.strip(G4String::stripType::trailing, ch); break;}
    case StringStripType::both:
      {result.strip(G4String::stripType::both, ch); break;}
#endif
    }
  return result;
}
 
G4String BDS::PrepareSafeName(G4String name)
{
  //remove white space
  name.erase(std::remove_if(name.begin(),name.end(),isspace),name.end());
  //remove non alpha-numeric characters
  std::replace_if(name.begin(),name.end(),BDS::non_alpha(),'_');
  
  return name;
}
 
G4int BDS::CalculateOrientation(G4double angle)
{
  return angle < 0 ? 1 : -1;
}
 
std::pair<G4ThreeVector,G4ThreeVector> BDS::CalculateFaces(G4double angleIn,
                                                           G4double angleOut)
{
  G4int orientationIn  = BDS::CalculateOrientation(angleIn);
  G4int orientationOut = BDS::CalculateOrientation(angleOut);
  
  G4double in_z  = std::cos(std::abs(angleIn)); // calculate components of normal vectors (in the end mag(normal) = 1)
  G4double in_x  = std::sin(std::abs(angleIn)); // note full angle here as it's the exit angle
  G4double out_z = std::cos(std::abs(angleOut));
  G4double out_x = std::sin(std::abs(angleOut));
  G4ThreeVector inputface  = G4ThreeVector(orientationIn*in_x, 0.0, -1.0*in_z); //-1 as pointing down in z for normal
  G4ThreeVector outputface = G4ThreeVector(orientationOut*out_x, 0.0, out_z);   // no output face angle
  return std::make_pair(inputface,outputface);
}
 
void BDS::EnsureInLimits(G4double& value, G4double lowerLimit, G4double upperLimit)
{
  if (value < lowerLimit)
    {value = lowerLimit;}
  if (value > upperLimit)
    {value = upperLimit;}
}
 
G4bool BDS::FileExists(const G4String& fileName)
{
  std::ifstream infile(fileName.c_str());
  return infile.good();
  // note the destructor of ifstream will close the stream
}
 
G4bool BDS::DirectoryExists(const G4String& path)
{
  struct stat sb;
  bool result = (stat(path.c_str(), &sb) == 0) && S_ISDIR(sb.st_mode);
  return G4bool(result);
}
 
std::string BDS::GetCurrentDir()
{ 
  char currentPath[PATH_MAX]; // defined in <limits>
  std::string currentPathString;
 
  if (getcwd(currentPath, sizeof(currentPath)) != nullptr)
    {currentPathString = std::string(currentPath);}
  else
    {throw BDSException(__METHOD_NAME__, "Cannot determine current working directory");}
 
  return currentPathString;
}
 
std::string BDS::GetBDSIMExecPath()
{
  // get path to executable (platform dependent)
  char path[1024];
#ifdef __linux__
  // get path from /proc/self/exe
  ssize_t len = ::readlink("/proc/self/exe", path, sizeof(path) - 1);
  if (len != -1)
    {path[len] = '\0';}
#elif __APPLE__
  uint32_t size = sizeof(path);
  if (_NSGetExecutablePath(path, &size) != 0)
    {std::cout << "buffer too small; need size " << size << std::endl;}
#endif
  std::string bdsimPath(path);
  // remove executable from path
  std::string::size_type found = bdsimPath.rfind('/'); // find the last '/'
  if (found != std::string::npos)
    {bdsimPath = bdsimPath.substr(0,found+1);} // the path is the bit before that, including the '/'
  return bdsimPath;
}
 
G4String BDS::GetFullPath(G4String fileName, bool excludeNameFromPath, bool useCWDForPrefix)
{
#ifdef BDSDEBUG
  G4cout << __METHOD_NAME__ << fileName << " strip name off?: " << excludeNameFromPath << G4endl;
#endif
  //Return fullPath of a file:
  //mirror what is done in parser.l (i.e. if no environment variable set, assume base filename path is that of the gmad file).
  // 1) if absolute path (starting with a slash) return that
  // 2) if relative path, then
  // 2a) return path relative to environment variable BDSIMPATH (if set)
  // 2b) return path relative to main BDSIM input gmad file
 
  // return value
  G4String fullPath;
 
  // protect against unneeded ./ at beginning of filename - strip it off
  if (fileName.substr(0,2) == "./")
    {fileName = fileName.substr(2);}
 
  // split input into path and filename
  G4String inputFilepath, inputFilename;
  BDS::SplitPathAndFileName(fileName, inputFilepath, inputFilename);
  
  // need to know whether it's an absolute or relative path
  if ((fileName.substr(0,1)) == "/")
    {fullPath = inputFilepath;}
  else // the main file has a relative path or just the file name, add bdsimpath
    {
      G4String prefixPath = useCWDForPrefix ? BDS::GetCurrentDir() : BDSGlobalConstants::Instance()->BDSIMPath();
      fullPath = prefixPath;
      if (inputFilepath != "./") // don't insert a ./ in path
        {fullPath += "/" + inputFilepath;}
    }
  
  if (fullPath.back() != '/') // ensure ends in '/'
    {fullPath += "/";} // only add if needed
 
  // add filename if not excluded
  if (!excludeNameFromPath)
    {fullPath += inputFilename;}
#ifdef BDSDEBUG
  G4cout << __METHOD_NAME__ << "determined full path to be: " << fullPath << G4endl;
#endif
  return fullPath;
}
 
void BDS::SplitPathAndFileName(const G4String& filePath,
                               G4String&       path,
                               G4String&       fileName)
{
  G4String::size_type found = filePath.rfind('/'); // find the last '/'
  if (found != G4String::npos)
    {
      path     = filePath.substr(0,found) + "/"; // the path is the bit before that
      fileName = filePath.substr(found+1); // the rest
    }
  else
    {// no slash, only file name
      path     = "./";     // use current directory
      fileName = filePath;
    }
}
 
void BDS::SplitFileAndExtension(const G4String& fileName,
                                G4String&       file,
                                G4String&       extension)
{
  G4String::size_type found = fileName.rfind("."); // fine the last '.'
  if (found != G4String::npos)
    {
      file      = fileName.substr(0, found);
      extension = fileName.substr(found); // the rest
    }
  else
    {
      file      = fileName;
      extension = "";
    }
}
 
void BDS::HandleAborts(int signal_number)
{
  // prevent recursive calling
  static int nrOfCalls=0;
  if (nrOfCalls>0)
    {exit(1);}
  nrOfCalls++;
  std::cout << std::endl << "BDSIM is about to crash or was interrupted! " << std::endl;
  std::cout << "With signal: " << strsignal(signal_number) << std::endl;
  
  BDSRunManager::GetRunManager()->AbortRun();
  std::cout << "Ave, Imperator, morituri te salutant!" << std::endl;
}
 
G4bool BDS::IsFinite(G4double value,
                     G4double tolerance)
{
  return std::abs(value) > tolerance;
}
 
G4bool BDS::NonZero(G4double value)
{
  return std::abs(value) > std::numeric_limits<double>::min();
}
 
G4bool BDS::IsFiniteStrength(G4double value)
{
  return IsFinite(value, 1e-50);
}
 
G4bool BDS::IsFinite(const G4ThreeVector& value,
                     G4double tolerance)
{
  G4bool resultX = BDS::IsFinite(value.x(), tolerance);
  G4bool resultY = BDS::IsFinite(value.y(), tolerance);
  G4bool resultZ = BDS::IsFinite(value.z(), tolerance);
  return resultX || resultY || resultZ;
}
 
G4bool BDS::IsInteger(const char* ch, int& convertedInteger)
{
  // from http://stackoverflow.com/questions/2844817/how-do-i-check-if-a-c-string-is-an-int
  // convert to string
  std::string s(ch);
  if(s.empty() || ((!std::isdigit(s[0])) && (s[0] != '-') && (s[0] != '+')))
    {return false;}
  
  char * p;
  convertedInteger = std::strtol(ch, &p, 10);
  
  return (*p == 0);
}
 
G4bool BDS::IsNumber(const char* ch, double& convertedNumber)
{
  // from http://stackoverflow.com/questions/2844817/how-do-i-check-if-a-c-string-is-an-int
  // convert to string
  std::string s(ch);
  if(s.empty() || ((!std::isdigit(s[0])) && (s[0] != '-') && (s[0] != '+')))
    {return false;}
  
  char * p;
  convertedNumber = std::strtod(ch, &p);
  
  return (*p == 0);
}
 
void BDS::PrintRotationMatrix(G4RotationMatrix* rm, G4String keyName)
{
  G4cout << "Rotation matrix - reference: \"" << keyName << "\"" << *rm << G4endl;
  G4cout << "unit x -> " << G4ThreeVector(1,0,0).transform(*rm) << G4endl;
  G4cout << "unit y -> " << G4ThreeVector(0,1,0).transform(*rm) << G4endl;
  G4cout << "unit z -> " << G4ThreeVector(0,0,1).transform(*rm) << G4endl;
}
 
G4bool BDS::Geant4EnvironmentIsSet()
{
#if G4VERSION_NUMBER > 1102
  // Since V4.11.p03, there is just 1 environmental variable to check for
  std::string entireDataDir = "GEANT4_DATA_DIR";
  const char* envVar = std::getenv( entireDataDir.c_str() );
  if (envVar)
    {return true;}
#endif
 
  std::vector<G4String> variables = {//"G4ABLADATA",
                                     "G4NEUTRONHPDATA",
                                     "G4RADIOACTIVEDATA",
                                     "G4LEDATA",
#if G4VERSION_NUMBER < 1049
                                     "G4NEUTRONXSDATA",
#else
                                     "G4PARTICLEXSDATA",
#endif
                                     "G4REALSURFACEDATA",
                                     "G4LEVELGAMMADATA",
                                     "G4PIIDATA",
                                     "G4SAIDXSDATA"};
 
  // here we loop through all variables - ie don't break the loop
  G4bool result = true;
  for (const auto& variable : variables)
    {
      const char* env_p = std::getenv( variable.c_str() );
      if (!env_p)
        {
          result = false;
          G4cerr << "Variable: \"" << variable << "\" not found." << G4endl;
        }
    }
  return result;
}
 
void BDS::CheckHighPrecisionDataExists(const G4String& physicsListName)
{
  const char* envHPData = std::getenv("G4PARTICLEHPDATA");
  if (!envHPData)
    {
      std::string msg = "The G4TENDL low energy data is not available!\n";
      msg += "G4TENDL data is required through the environmental variable \"G4PARTICLEHPDATA\"\n;";
      msg += "This is required for the \"" + physicsListName + "\" physics list.\n";
      msg += "This data is an optional download from the Geant4 website. Please\n";
      msg += "download from the Geant4 website and export the environmental variable.";
      throw BDSException(__METHOD_NAME__, msg);
    }
}
 
void BDS::CheckLowEnergyNeutronDataExists(const G4String& physicsListName)
{
  const char* envHPData = std::getenv("G4LENDDATA");
  if (!envHPData)
    {
      std::string msg = "The Low Energy Neutron Data ('LEND') is not available!\n";
      msg += "Data is required through the environmental variable \"G4LENDDATA\"\n";
      msg += "This is required for the \"" + physicsListName + "\" physics list.\n";
      msg += "This data is an optional download from the Geant4 website. Please\n";
      msg += "download from the Geant4 website and export the environmental variable.";
      throw BDSException(__METHOD_NAME__, msg);
    }
}
 
G4double BDS::GetParameterValueDouble(G4String spec, G4String name)
{
  try
    {return (G4double)std::stol(GetParameterValueString(spec,name).c_str());}
  catch(const std::invalid_argument& e)
    {throw;}
}
 
G4int BDS::GetParameterValueInt(G4String spec, G4String name)
{
  try
    {return (G4int)std::stoi(GetParameterValueString(spec,name).c_str());}
  catch (const std::invalid_argument& e)
    {throw;}
}
 
G4String BDS::GetParameterValueString(G4String spec, G4String name)
{
  G4String value;
  std::string param = name + "=";
 
  int pos = spec.find(param);
  if (pos >= 0)
    {   
      int pos2 = spec.find("&",pos);
      int pos3 = spec.length();
      int tend = pos2 < 0 ? pos3 : pos2; 
      int llen = tend - pos - param.length();
      
      value = spec.substr(pos + param.length(), llen);
    }
  return value;
}
 
std::vector<G4String> BDS::SplitOnWhiteSpace(const G4String& input)
{
  std::vector<G4String> result;
  if (input.empty())
    {return result;}
  
  std::istringstream ss(input);
 
  G4String word;
  while (ss >> word)
    {result.push_back(word);}
  return result;
}
 
G4TwoVector BDS::Rotate(const G4TwoVector& vec, const G4double& angle)
{
  G4double xp = vec.x()*std::cos(angle) - vec.y()*std::sin(angle);
  G4double yp = vec.x()*std::sin(angle) + vec.y()*std::cos(angle);
  return G4TwoVector(xp,yp);
}
 
G4bool BDS::WillIntersect(const G4ThreeVector& incomingNormal,
                          const G4ThreeVector& outgoingNormal,
                          const G4double&      zSeparation,
                          const BDSExtent&     incomingExtent,
                          const BDSExtent&     outgoingExtent)
{  
  // for any two normal vectors of planes - if their cross
  // product is zero, then they're (anti) parallel and will
  // never intersect
  G4ThreeVector cross = incomingNormal.cross(outgoingNormal);
  G4double det = cross.mag2();
  if (!BDS::IsFinite(det))
    {return false;}
 
  // shortcuts / copies - OG for outgoing and IC for incoming
  const BDSExtent& eog = outgoingExtent;
  const BDSExtent& eic = incomingExtent;
 
  // z coordinate of points at maximum extents in x,y
  G4double xNegYNegOG = BDS::GetZOfPointOnPlane(outgoingNormal, eog.XNeg(), eog.YNeg());
  G4double xNegYPosOG = BDS::GetZOfPointOnPlane(outgoingNormal, eog.XNeg(), eog.YPos());
  G4double xPosYPosOG = BDS::GetZOfPointOnPlane(outgoingNormal, eog.XPos(), eog.YPos());
  G4double xPosYNegOG = BDS::GetZOfPointOnPlane(outgoingNormal, eog.XPos(), eog.YNeg());
  G4double xNegYNegIC = BDS::GetZOfPointOnPlane(incomingNormal, eic.XNeg(), eic.YNeg());
  G4double xNegYPosIC = BDS::GetZOfPointOnPlane(incomingNormal, eic.XNeg(), eic.YPos());
  G4double xPosYPosIC = BDS::GetZOfPointOnPlane(incomingNormal, eic.XPos(), eic.YPos());
  G4double xPosYNegIC = BDS::GetZOfPointOnPlane(incomingNormal, eic.XPos(), eic.YNeg());
 
  // test of they'd overlap
  G4bool xNegYNegFail = xNegYNegIC > (zSeparation + xNegYNegOG);
  G4bool xNegYPosFail = xNegYPosIC > (zSeparation + xNegYPosOG);
  G4bool xPosYPosFail = xPosYPosIC > (zSeparation + xPosYPosOG);
  G4bool xPosYNegFail = xPosYNegIC > (zSeparation + xPosYNegOG);
 
  if (xNegYNegFail || xNegYPosFail || xPosYPosFail || xPosYNegFail)
    {return true;}
  else // shouldn't happen
    {return false;}
}
 
G4bool BDS::WillIntersect(const G4double& angleIn,
                          const G4double& angleOut,
                          const G4double& horizontalWidth,
                          const G4double& length)
{
  // Calculate the z component of triangle with each angle and
  // axis along length.
  G4double dzIn  = horizontalWidth * std::tan(angleIn);
  G4double dzOut = horizontalWidth * std::tan(angleOut);
  if (dzIn > length - dzOut)
    {return true;}
  else
    {return false;}
}
 
G4double BDS::GetZOfPointOnPlane(const G4ThreeVector& normal, G4double x, G4double y)
{
  // equation of a plane with offset v_0, normal unit n and any point on plane v
  // n.(v-v_0) = 0
  // for equation of plane that intercepts 0,0,0
  // n.v = 0;
  // exanding dot product
  // n.x()*v.x() + n.y()*v.y() + n.z()*v.z() = 0;
  // for given x and y can solve for z
  // v.z = (-n.x*v.x - n.y*v.y ) / n.z;
 
  return (-normal.x()*x - normal.y()*y) / normal.z();
}
 
G4ThreeVector BDS::RotateToReferenceFrame(G4ThreeVector faceNormal, G4double fullAngle)
{
  if (!BDS::IsFinite(fullAngle))
    {return faceNormal;} // no angle -> no rotation
  G4RotationMatrix rm = G4RotationMatrix();
  rm.rotateY(fullAngle*0.5);
  return faceNormal.transform(rm);
}
 
std::pair<G4String, G4String> BDS::SplitOnColon(const G4String& formatAndPath)
{
  if(!formatAndPath.empty())
    {
      std::size_t found = formatAndPath.find(":");
      if (found == std::string::npos)
        {
          throw BDSException(__METHOD_NAME__, "invalid specifier \"" + formatAndPath + "\"\n"
                             + "Missing \":\" to separate format and file path");
        }
      else
        {
          G4String format   = formatAndPath.substr(0,found);
          G4String filePath = formatAndPath.substr(found+1); // get everything after ":"
          return std::make_pair(format,filePath);
        }
    }
  return std::make_pair("","");
}
 
G4UserLimits* BDS::CreateUserLimits(G4UserLimits*  defaultUL,
                                    G4double length,
                                    G4double fraction)
{
  G4UserLimits* result = defaultUL;
  // construct a dummy G4Track that typically isn't used for the check
  G4Track t = G4Track();
  if (defaultUL->GetMaxAllowedStep(t) > length)
    {// copy and change length in UL
      result = new G4UserLimits(*defaultUL);
      G4double lengthScale = length * fraction;
      lengthScale = std::max(lengthScale, 1.0*CLHEP::um); // no smaller than 1um limit
      if (!BDS::IsFinite(lengthScale))
        {lengthScale = 1*CLHEP::mm;} // if identically 0 due to some upstream issue, set a small default
      result->SetMaxAllowedStep(lengthScale);
    }
  return result;
}
 
G4double BDS::GetMemoryUsage()
{
  struct rusage r_usage;
  int itWorked = getrusage(RUSAGE_SELF, &r_usage);
  if (itWorked != 0)
    {return 0;} // failed
  else
    {
      G4double maxMemory = (G4double)r_usage.ru_maxrss;
#ifdef __APPLE__
      maxMemory /= 1048*1048;
#else
      maxMemory /= 1048;
#endif
      return maxMemory;
    }
}
 
std::map<G4String, G4String> BDS::GetUserParametersMap(const G4String& userParameters,
                                                       char delimiter)
{
  // split by white space then by colon
  std::istringstream iss(userParameters);
  std::vector<std::string> paramaterPairs(std::istream_iterator<std::string>{iss},
                                          std::istream_iterator<std::string>{});
 
  std::map<G4String, G4String> result;
  for (auto& pair : paramaterPairs)
    {
      auto index = pair.find(delimiter);
      std::string key = pair.substr(0, index);
      std::string value = pair.substr(index+1);
      result[G4String(key)] = G4String(value);
    }
  return result;
}
 
G4int BDS::VerboseEventStop(G4int verboseEventStart,
                            G4int verboseEventContinueFor)
{
  G4int verboseEventStop = 0;
  if (verboseEventContinueFor < 1)
    {verboseEventStop = std::numeric_limits<int>::max();}
  else
    {verboseEventStop = verboseEventStart + verboseEventContinueFor;}
  return verboseEventStop;
}
 
G4bool BDS::VerboseThisEvent(G4int eventIndex,
                             G4int eventStart,
                             G4int eventStop)
{
  return eventIndex >= eventStart && eventIndex < eventStop;
}
 
G4double BDS::Rigidity(G4double momentumMagnitude,
                       G4double charge)
{
  return momentumMagnitude / CLHEP::GeV / BDS::cOverGeV / charge;
}
 
G4double BDS::CalculateSafeAngledVolumeLength(G4ThreeVector inputfaceIn,
                                              G4ThreeVector outputfaceIn,
                                              G4double length,
                                              G4double containerWidth,
                                              G4double containerHeight)
{
  G4double angleIn = inputfaceIn.angle();
  G4double angleOut = outputfaceIn.angle();
  return BDS::CalculateSafeAngledVolumeLength(angleIn, angleOut, length, containerWidth, containerHeight);
}
 
G4double BDS::CalculateSafeAngledVolumeLength(G4double angleIn,
                                              G4double angleOut,
                                              G4double length,
                                              G4double containerWidth,
                                              G4double containerHeight)
{
  G4double sLength = length;
  if (!BDS::IsFinite(containerHeight))
    {containerHeight = containerWidth;}
 
  if (BDS::IsFinite(angleIn) || BDS::IsFinite(angleOut))
    {
      // In the case of angled faces, calculate a length so that the straight solids
      // used in intersection are long enough to reach the edges of the angled faces.
      // Could simply do 2x length, but for short dipole sections with strongly angled
      // faces this doesn't work. Calculate extent along z for each angled face. This
      // is called the 'safe' length -> sLength
      G4double hypotenuse = std::hypot(containerWidth, containerHeight);
      G4double dzIn = std::tan(std::abs(angleIn)) * 1.2 * hypotenuse; // 20% over estimation for safety
      G4double dzOut = std::tan(std::abs(angleOut)) * 1.2 * hypotenuse;
      // take the longest of different estimations (2x and 1.5x + dZs)
      sLength = std::max(2 * length, 1.5 * length + dzIn + dzOut);
    }
  return sLength;
}
 
G4double BDS::ArcLengthFromChordLength(G4double chordLength, G4double angle)
{
  if (!BDS::IsFinite(angle))
    {return chordLength;} // avoid division by zero in the following lines
  G4double radiusOfCurvature = 0.5*chordLength / std::sin(0.5*std::abs(angle));
  G4double arcLength = radiusOfCurvature * std::abs(angle);
  return arcLength;
}