G4MultiLevelLocator.cc

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//
// $Id: G4MultiLevelLocator.cc 110832 2018-06-15 15:00:33Z gcosmo $
//
// Class G4MultiLevelLocator implementation
//
// 27.10.08 - Tatiana Nikitina.
// 04.10.11 - John Apostolakis, revised convergence to use Surface Normal
// ---------------------------------------------------------------------------

#include <iomanip>

#include "G4ios.hh"
#include "G4MultiLevelLocator.hh"


G4MultiLevelLocator::G4MultiLevelLocator(G4Navigator *theNavigator)
 : G4VIntersectionLocator(theNavigator),
   fMaxSteps(10000),  //  Very loose - allows many steps (looping will be rare)
   fWarnSteps(1000),  //  
   fNumCalls(0),
   fNumAdvanceFull(0.), fNumAdvanceGood(0), fNumAdvanceTrials(0)
{
  // In case of too slow progress in finding Intersection Point
  // intermediates Points on the Track must be stored.
  // Initialise the array of Pointers [max_depth+1] to do this  
  
  G4ThreeVector zeroV(0.0,0.0,0.0);
  for (G4int idepth=0; idepth<max_depth+1; idepth++ )
  {
    ptrInterMedFT[ idepth ] = new G4FieldTrack( zeroV, zeroV, 0., 0., 0., 0.);
  }

#ifdef G4DEBUG_FIELD
  //  Trial values       Loose         Tight
  //  To happen:         Infrequent    Often
  SetMaxSteps(50);   //  300             25
  SetWarnSteps(40);  //  250             15
#endif  
}      

G4MultiLevelLocator::~G4MultiLevelLocator()
{
  for ( G4int idepth=0; idepth<max_depth+1; idepth++)
  {
    delete ptrInterMedFT[idepth];
  }
#ifdef G4DEBUG_FIELD
  ReportStatistics(); 
#endif  
}


// --------------------------------------------------------------------------
// G4bool G4PropagatorInField::LocateIntersectionPoint( 
//        const G4FieldTrack&       CurveStartPointVelocity,   // A
//        const G4FieldTrack&       CurveEndPointVelocity,     // B
//        const G4ThreeVector&      TrialPoint,                // E
//              G4FieldTrack&       IntersectedOrRecalculated  // Output
//              G4bool&             recalculated )             // Out
// --------------------------------------------------------------------------
//
// Function that returns the intersection of the true path with the surface
// of the current volume (either the external one or the inner one with one
// of the daughters:
//
//     A = Initial point
//     B = another point 
//
// Both A and B are assumed to be on the true path:
//
//     E is the first point of intersection of the chord AB with 
//     a volume other than A  (on the surface of A or of a daughter)
//
// Convention of Use :
//     i) If it returns "true", then IntersectionPointVelocity is set
//       to the approximate intersection point.
//    ii) If it returns "false", no intersection was found.
//        Potential reasons:
//        a) no segment found an intersection
//        b) too many steps were required - after that it abandoned the effort
//           and is returning how far it could go.  (New - 29 Oct 2015)
//           (If so, it must set 'recalculated' to true.) 
//        TODO/idea: add a new flag: 'unfinished' to identify these cases.
//
//        IntersectedOrRecalculated means different things:
//        a) if it is the same curve lenght along, it is a revision of the
//            original enpdoint due to the need for re-integration.
//        b) if it is at a shorter curve length, it is 'end of what it could do'
//           i.e. as far as it could go, because it took too many steps!
//        Note: IntersectedOrRecalculated is valid only if 'recalculated' is
//       'true'.
// --------------------------------------------------------------------------
// NOTE: implementation taken from G4PropagatorInField
//
G4bool G4MultiLevelLocator::EstimateIntersectionPoint( 
         const  G4FieldTrack&       CurveStartPointVelocity,       // A
         const  G4FieldTrack&       CurveEndPointVelocity,         // B
         const  G4ThreeVector&      TrialPoint,                    // E
                G4FieldTrack&       IntersectedOrRecalculatedFT,   // Output
                G4bool&             recalculatedEndPoint,          // Out
                G4double&           previousSafety,                // In/Out
                G4ThreeVector&      previousSftOrigin)             // In/Out
{
  // Find Intersection Point ( A, B, E )  of true path AB - start at E.
  const char* MethodName= "G4MultiLevelLocator::EstimateIntersectionPoint()";
  
  G4bool found_approximate_intersection = false;
  G4bool there_is_no_intersection       = false;
  
  G4FieldTrack  CurrentA_PointVelocity = CurveStartPointVelocity; 
  G4FieldTrack  CurrentB_PointVelocity = CurveEndPointVelocity;
  G4ThreeVector CurrentE_Point = TrialPoint;
  G4bool        validNormalAtE = false;
  G4ThreeVector NormalAtEntry;

  G4FieldTrack  ApproxIntersecPointV(CurveEndPointVelocity); // FT-Def-Construct
  // G4bool     validApproxIntPV= false; // Is it current: valid and up-to-date?
  G4bool        validIntersectP= true;   // Is it current ?
  G4double      NewSafety = 0.0;
  G4bool        last_AF_intersection = false;   

  // G4bool final_section= true;  // Shows whether current section is last
                                  // (i.e. B=full end)
  G4bool first_section = true;

  recalculatedEndPoint = false; 

  G4bool restoredFullEndpoint = false;

  unsigned int substep_no = 0;

  // Statistics for substeps
  //
  static G4ThreadLocal unsigned int max_no_seen= 0; 

  //--------------------------------------------------------------------------  
  //  Algorithm for the case if progress in founding intersection is too slow.
  //  Process is defined too slow if after N=param_substeps advances on the
  //  path, it will be only 'fraction_done' of the total length.
  //  In this case the remaining length is divided in two half and 
  //  the loop is restarted for each half.  
  //  If progress is still too slow, the division in two halfs continue
  //  until 'max_depth'.
  //--------------------------------------------------------------------------

  const G4int param_substeps=5;  // Test value for the maximum number
                                 // of substeps
  const G4double fraction_done=0.3;

  G4bool Second_half = false;    // First half or second half of divided step

  // We need to know this for the 'final_section':
  // real 'final_section' or first half 'final_section'
  // In algorithm it is considered that the 'Second_half' is true
  // and it becomes false only if we are in the first-half of level
  // depthness or if we are in the first section

  unsigned int depth=0; // Depth counts subdivisions of initial step made
  fNumCalls++;
  
#ifdef G4DEBUG_FIELD
  unsigned int trigger_substepno_print=0;
  const G4double tolerance = 1.0e-8 * CLHEP::mm;
  unsigned int biggest_depth= 0;

#if (G4DEBUG_FIELD>1) 
  G4ThreeVector  StartPosition= CurveStartPointVelocity.GetPosition(); 
  if( (TrialPoint - StartPosition).mag2() < tolerance*tolerance) 
  {
     ReportImmediateHit( MethodName, StartPosition, TrialPoint,
                         tolerance, fNumCalls); 
  }
#endif  
#endif

  NormalAtEntry = GetSurfaceNormal(CurrentE_Point, validNormalAtE); 

  // Intermediates Points on the Track = Subdivided Points must be stored.
  // Give the initial values to 'InterMedFt'
  // Important is 'ptrInterMedFT[0]', it saves the 'EndCurvePoint'
  //
  *ptrInterMedFT[0] = CurveEndPointVelocity;
  for (G4int idepth=1; idepth<max_depth+1; idepth++ )
  {
    *ptrInterMedFT[idepth]=CurveStartPointVelocity;
  }

  // Final_section boolean store
  //
  G4bool fin_section_depth[max_depth];
  for (G4int idepth=0; idepth<max_depth; idepth++ )
  {
    fin_section_depth[idepth]=true;
  }
  // 'SubStartPoint' is needed to calculate the length of the divided step
  //
  G4FieldTrack SubStart_PointVelocity = CurveStartPointVelocity;
   
  do  // Loop checking, 07.10.2016, J.Apostolakis
  {
    unsigned int substep_no_p = 0;
    G4bool sub_final_section = false; // the same as final_section,
                                      // but for 'sub_section'
    SubStart_PointVelocity = CurrentA_PointVelocity;
 
    do // Loop checking, 07.10.2016, J.Apostolakis
    { // REPEAT param
      G4ThreeVector Point_A = CurrentA_PointVelocity.GetPosition();  
      G4ThreeVector Point_B = CurrentB_PointVelocity.GetPosition();
       
      // F = a point on true AB path close to point E 
      // (the closest if possible)
      //
      ApproxIntersecPointV = GetChordFinderFor()
                             ->ApproxCurvePointV( CurrentA_PointVelocity, 
                                                  CurrentB_PointVelocity, 
                                                  CurrentE_Point,
                                                  GetEpsilonStepFor());
        // The above method is the key & most intuitive part ...

      // validApproxIntPV = true;

#ifdef G4DEBUG_FIELD
      if( ApproxIntersecPointV.GetCurveLength() > 
          CurrentB_PointVelocity.GetCurveLength() * (1.0 + tolerance) )
      {
        G4Exception(MethodName, "GeomNav0003", FatalException,
                    "Intermediate F point is past end B point" ); 
      }
#endif

      G4ThreeVector CurrentF_Point= ApproxIntersecPointV.GetPosition();

      // First check whether EF is small - then F is a good approx. point 
      // Calculate the length and direction of the chord AF
      //
      G4ThreeVector  ChordEF_Vector = CurrentF_Point - CurrentE_Point;

      G4ThreeVector  NewMomentumDir= ApproxIntersecPointV.GetMomentumDir(); 
      G4double       MomDir_dot_Norm= NewMomentumDir.dot( NormalAtEntry ) ;
      
#ifdef G4DEBUG_FIELD
      if( fVerboseLevel > 3 )
      { 
         G4ThreeVector  ChordAB           = Point_B - Point_A;
         G4double       ABchord_length    = ChordAB.mag(); 
         G4double       MomDir_dot_ABchord;
         MomDir_dot_ABchord = (1.0 / ABchord_length)
                            * NewMomentumDir.dot( ChordAB );
         G4VIntersectionLocator::ReportTrialStep( substep_no, ChordAB,
              ChordEF_Vector, NewMomentumDir, NormalAtEntry, validNormalAtE ); 
         G4cout << " dot( MomentumDir, ABchord_unit ) = "
                << MomDir_dot_ABchord << G4endl;
      }
#endif
      G4bool adequate_angle =
             ( MomDir_dot_Norm >= 0.0 ) // Can use ( > -epsilon) instead
          || (! validNormalAtE );       // Invalid, cannot use this criterion
      G4double EF_dist2 = ChordEF_Vector.mag2();
      if ( ( EF_dist2 <= sqr(fiDeltaIntersection) && ( adequate_angle ) )
        || ( EF_dist2 <= kCarTolerance*kCarTolerance ) )
      { 
        found_approximate_intersection = true;

        // Create the "point" return value
        //
        IntersectedOrRecalculatedFT = ApproxIntersecPointV;
        IntersectedOrRecalculatedFT.SetPosition( CurrentE_Point );

        if ( GetAdjustementOfFoundIntersection() )
        {
          // Try to Get Correction of IntersectionPoint using SurfaceNormal()
          //  
          G4ThreeVector IP;
          G4ThreeVector MomentumDir=ApproxIntersecPointV.GetMomentumDirection();
          G4bool goodCorrection = AdjustmentOfFoundIntersection(Point_A,
                                    CurrentE_Point, CurrentF_Point, MomentumDir,
                                    last_AF_intersection, IP, NewSafety,
                                    previousSafety, previousSftOrigin );
          if ( goodCorrection )
          {
            IntersectedOrRecalculatedFT = ApproxIntersecPointV;
            IntersectedOrRecalculatedFT.SetPosition(IP);
          }
        }
        // Note: in order to return a point on the boundary, 
        //       we must return E. But it is F on the curve.
        //       So we must "cheat": we are using the position at point E
        //       and the velocity at point F !!!
        //
        // This must limit the length we can allow for displacement!
      }
      else  // E is NOT close enough to the curve (ie point F)
      {
        // Check whether any volumes are encountered by the chord AF
        // ---------------------------------------------------------
        // First relocate to restore any Voxel etc information
        // in the Navigator before calling ComputeStep()
        //
        GetNavigatorFor()->LocateGlobalPointWithinVolume( Point_A );

        G4ThreeVector PointG;   // Candidate intersection point
        G4double stepLengthAF; 
        G4bool Intersects_AF = IntersectChord( Point_A,   CurrentF_Point,
                                               NewSafety, previousSafety,
                                               previousSftOrigin,
                                               stepLengthAF,
                                               PointG );
        last_AF_intersection = Intersects_AF;
        if( Intersects_AF )
        {
          // G is our new Candidate for the intersection point.
          // It replaces  "E" and we will repeat the test to see if
          // it is a good enough approximate point for us.
          //       B    <- F
          //       E    <- G
          //
          CurrentB_PointVelocity = ApproxIntersecPointV;
          CurrentE_Point = PointG;  

          validIntersectP= true;    // 'E' has been updated.
          // validApproxIntPV= false;  // 'F' is no longer valid, as B changed

          G4bool validNormalLast; 
          NormalAtEntry  = GetSurfaceNormal( PointG, validNormalLast ); 
          validNormalAtE = validNormalLast; 

          // By moving point B, must take care if current
          // AF has no intersection to try current FB!!
          //
          fin_section_depth[depth]=false;

#ifdef G4VERBOSE
          if( fVerboseLevel > 3 )
          {
            G4cout << "G4PiF::LI> Investigating intermediate point"
                   << " at s=" << ApproxIntersecPointV.GetCurveLength()
                   << " on way to full s="
                   << CurveEndPointVelocity.GetCurveLength() << G4endl;
          }
#endif
        }
        else  // not Intersects_AF
        {  
          // In this case:
          // There is NO intersection of AF with a volume boundary.
          // We must continue the search in the segment FB!
          //
          GetNavigatorFor()->LocateGlobalPointWithinVolume( CurrentF_Point );

          G4double stepLengthFB;
          G4ThreeVector PointH;

          // Check whether any volumes are encountered by the chord FB
          // ---------------------------------------------------------

          G4bool Intersects_FB = IntersectChord( CurrentF_Point, Point_B, 
                                                 NewSafety, previousSafety,
                                                 previousSftOrigin,
                                                 stepLengthFB,
                                                 PointH );
          if( Intersects_FB )
          {
            // There is an intersection of FB with a volume boundary
            // H <- First Intersection of Chord FB 

            // H is our new Candidate for the intersection point.
            // It replaces  "E" and we will repeat the test to see if
            // it is a good enough approximate point for us.

            // Note that F must be in volume volA  (the same as A)
            // (otherwise AF would meet a volume boundary!)
            //   A    <- F 
            //   E    <- H
            //
            CurrentA_PointVelocity = ApproxIntersecPointV;
            CurrentE_Point = PointH;

            validIntersectP = true;    // 'E' has been updated.
            // validApproxIntPV = false; // 'F' is no longer valid, as A changed

            G4bool validNormalH;
            NormalAtEntry  = GetSurfaceNormal( PointH, validNormalH ); 
            validNormalAtE = validNormalH; 
          }
          else  // not Intersects_FB
          {
            if(fin_section_depth[depth])
            {
              // If B is the original endpoint, this means that whatever
              // volume(s) intersected the original chord, none touch the
              // smaller chords we have used.
              // The value of 'IntersectedOrRecalculatedFT' returned is
              // likely not valid 

              // Check on real final_section or SubEndSection
              //
              if( ((Second_half)&&(depth==0)) || (first_section) )
              {
                there_is_no_intersection = true;   // real final_section
              }
              else
              {
                // end of subsection, not real final section 
                // exit from the and go to the depth-1 level 
                substep_no_p = param_substeps+2;  // exit from the loop

                // but 'Second_half' is still true because we need to find
                // the 'CurrentE_point' for the next loop
                Second_half = true; 
                sub_final_section = true;
              }
            }
            else
            {
              CurrentA_PointVelocity = CurrentB_PointVelocity;  // Got to B
              CurrentB_PointVelocity = (depth==0) ? CurveEndPointVelocity 
                                                  : *ptrInterMedFT[depth] ;
              SubStart_PointVelocity = CurrentA_PointVelocity;
              restoredFullEndpoint = true;

              validIntersectP=  false;  // 'E' has NOT been updated.
              // validApproxIntPV= false; // 'F' is no longer valid, A changed
            }
          } // Endif (Intersects_FB)
        } // Endif (Intersects_AF)

        G4FieldTrack RevisedB_FT= CurrentB_PointVelocity;
        G4int  errorEndPt;
        G4bool recalculatedB= CheckAndReEstimateEndpoint(CurrentA_PointVelocity,
                                                         CurrentB_PointVelocity,
                                                         RevisedB_FT,
                                                         errorEndPt );
        if( recalculatedB )
        {
          CurrentB_PointVelocity= RevisedB_FT;  // Use it !
          // Do not invalidate intersection F -- it is still roughly OK.
          //
          // The best course would be to invalidate (reset validIntersectP)
          // BUT if we invalidate it, we must re-estimate it somewhere!

          // validApproxIntPV= false;  // 'F' is no longer valid, as B changed
          // validIntersectP=  false;  // 'E' has NOT been updated.
 
          if ( (fin_section_depth[depth])           // real final section
             &&( first_section || ((Second_half)&&(depth==0)) ) )
          {
            recalculatedEndPoint = true;
            IntersectedOrRecalculatedFT = RevisedB_FT;
              // So that we can return it, if it is the endpoint!
          }
          // else
          //  Move forward the other points 
          //   - or better flag it, so that they are re-computed when next used
          //     [ Implementation: a counter for # of recomputations
          //       => avoids extra work]

        }
        if( errorEndPt > 1 )  // errorEndPt = 1 is milder, just: len(B)=len(A)
        {
           std::ostringstream errmsg; 
           errmsg << "Location: " << MethodName
                  << "- After EndIf(Intersects_AF)" << G4endl;
           ReportReversedPoints(errmsg, 
                     CurveStartPointVelocity, CurveEndPointVelocity,
                     NewSafety, fiEpsilonStep, 
                     CurrentA_PointVelocity, CurrentB_PointVelocity,
                     SubStart_PointVelocity, CurrentE_Point,
                     ApproxIntersecPointV, substep_no, substep_no_p, depth);
           G4Exception(MethodName, "GeomNav0003", FatalException, errmsg);
        }
        if( restoredFullEndpoint )
        {
          fin_section_depth[depth] = restoredFullEndpoint;
          restoredFullEndpoint = false;
        }
      } // EndIf ( E is close enough to the curve, ie point F. )
        // tests ChordAF_Vector.mag() <= maximum_lateral_displacement 

#ifdef G4DEBUG_FIELD
      if( trigger_substepno_print == 0)
      {
        trigger_substepno_print= fWarnSteps - 20;
      }

      if( substep_no >= trigger_substepno_print )
      {
        G4cout << "Difficulty in converging in " << MethodName
               << G4endl
               << "    Substep no = " << substep_no << G4endl;
        if( substep_no == trigger_substepno_print )
        {
          printStatus( CurveStartPointVelocity, CurveEndPointVelocity,
                       -1.0, NewSafety, 0);
        }
        G4cout << " State of point A: "; 
        printStatus( CurrentA_PointVelocity, CurrentA_PointVelocity,
                     -1.0, NewSafety, substep_no-1);
        G4cout << " State of point B: "; 
        printStatus( CurrentA_PointVelocity, CurrentB_PointVelocity,
                     -1.0, NewSafety, substep_no);
      }
#endif
      substep_no++; 
      substep_no_p++;

    } while (  ( ! found_approximate_intersection )
            && ( ! there_is_no_intersection )     
            && ( substep_no_p <= param_substeps) );  // UNTIL found or
                                                     // failed param substep

    if( (!found_approximate_intersection) && (!there_is_no_intersection) )
    {
      G4double did_len = std::abs( CurrentA_PointVelocity.GetCurveLength()
                       - SubStart_PointVelocity.GetCurveLength()); 
      G4double all_len = std::abs( CurrentB_PointVelocity.GetCurveLength()
                       - SubStart_PointVelocity.GetCurveLength());
   
      G4double distAB= -1;
      G4ThreeVector PointGe;
      //
      // Is progress is too slow, and is it possible to go deeper?
      // If so, then *halve the step*
      //              ==============
      if(   (did_len < fraction_done*all_len)
         && (depth<max_depth) && (!sub_final_section) )
      {
#ifdef G4DEBUG_FIELD         
        static G4ThreadLocal unsigned int numSplits=0;   // For debugging only 
        biggest_depth= std::max(depth, biggest_depth);
        numSplits++;
#endif
        Second_half=false;
        depth++;
        first_section = false;

        G4double Sub_len = (all_len-did_len)/(2.);
        G4FieldTrack midPoint = CurrentA_PointVelocity;
        auto integrDriver
                       = GetChordFinderFor()->GetIntegrationDriver();
        G4bool fullAdvance=              
           integrDriver->AccurateAdvance(midPoint, Sub_len, fiEpsilonStep);
                         
        fNumAdvanceTrials++;
        if( fullAdvance )  { fNumAdvanceFull++; }

        G4double lenAchieved=
           midPoint.GetCurveLength()-CurrentA_PointVelocity.GetCurveLength();
        
        const G4double adequateFraction = (1.0-CLHEP::perThousand);
        G4bool goodAdvance = (lenAchieved >= adequateFraction * Sub_len);
        if ( goodAdvance )  { fNumAdvanceGood++; }

#ifdef G4DEBUG_FIELD
        else  //  !goodAdvance
        {
           G4cout << "MLL> AdvanceChordLimited not full at depth=" << depth 
                  << "  total length achieved = " << lenAchieved << " of "
                  << Sub_len << "  fraction= ";
           if (Sub_len != 0.0 ) { G4cout << lenAchieved / Sub_len; }
           else                 { G4cout << "DivByZero"; }
           G4cout << "  Good-enough fraction = " << adequateFraction;
           G4cout << "  Number of call to mll = " << fNumCalls
                  << "   iteration " << substep_no
                  << "  inner =  " << substep_no_p << G4endl;
           G4cout << "  Epsilon of integration = " << fiEpsilonStep << G4endl;
           G4cout << "  State at start is      = " << CurrentA_PointVelocity
                  << G4endl
                  << "        at end (midpoint)= " << midPoint << G4endl;
           G4cout << "  Particle mass = " << midPoint.GetRestMass() << G4endl;

           G4EquationOfMotion *equation = integrDriver->GetEquationOfMotion();
           ReportFieldValue( CurrentA_PointVelocity, "start", equation );
           ReportFieldValue( midPoint, "midPoint", equation );            
           G4cout << "  Original Start = "
                  << CurveStartPointVelocity << G4endl;
           G4cout << "  Original   End = "
                  << CurveEndPointVelocity << G4endl;
           G4cout << "  Original TrialPoint = "
                  << TrialPoint << G4endl;
           G4cout << "  (this is STRICT mode) "
                  << "  num Splits= " << numSplits;           
           G4cout << G4endl;
        }
#endif

        *ptrInterMedFT[depth] =  midPoint;
        CurrentB_PointVelocity = midPoint; 

        // Adjust 'SubStartPoint' to calculate the 'did_length' in next loop
        //
        SubStart_PointVelocity = CurrentA_PointVelocity;

        // Find new trial intersection point needed at start of the loop
        //
        G4ThreeVector Point_A = CurrentA_PointVelocity.GetPosition();
        G4ThreeVector Point_B = CurrentB_PointVelocity.GetPosition();   
     
        GetNavigatorFor()->LocateGlobalPointWithinVolume(Point_A);
        G4bool Intersects_AB = IntersectChord(Point_A, Point_B,
                                              NewSafety, previousSafety,
                                              previousSftOrigin, distAB,
                                              PointGe);
        if( Intersects_AB )
        {
          last_AF_intersection = Intersects_AB;
          CurrentE_Point = PointGe;
          fin_section_depth[depth]=true;

          validIntersectP=  true;  // 'E' has been updated.
          // validApproxIntPV= false;  // 'F' is no longer valid, as E changed

          G4bool validNormalAB; 
          NormalAtEntry  = GetSurfaceNormal( PointGe, validNormalAB ); 
          validNormalAtE = validNormalAB;
        }
        else
        {
          // No intersection found for first part of curve
          // (CurrentA,InterMedPoint[depth]). Go to the second part
          //
          Second_half = true;

          validIntersectP=  false;  // No new 'E' chord intersection found
          // validApproxIntPV= false;  // So also 'F' is invalid
        }
      } // if did_len

      unsigned int levelPops=0;
      
      G4bool unfinished = Second_half;
      while ( unfinished && (depth>0) )  // Loop checking, 07.10.2016, J. Apostolakis
      {
        // Second part of curve (InterMed[depth],Intermed[depth-1])) 
        // On the depth-1 level normally we are on the 'second_half'

        levelPops++;

        //  Find new trial intersection point needed at start of the loop
        //
        SubStart_PointVelocity = *ptrInterMedFT[depth];
        CurrentA_PointVelocity = *ptrInterMedFT[depth];
        CurrentB_PointVelocity = *ptrInterMedFT[depth-1];

        // Ensure that the new endpoints are not further apart in space
        // than on the curve due to different errors in the integration
        //
        G4FieldTrack RevisedEndPointFT= CurrentB_PointVelocity;
        G4int  errorEndPt;
        G4bool recalculatedB= 
           CheckAndReEstimateEndpoint( CurrentA_PointVelocity,  
                                       CurrentB_PointVelocity,
                                       RevisedEndPointFT,
                                       errorEndPt );
        if( recalculatedB )
        {
          CurrentB_PointVelocity= RevisedEndPointFT;  // Use it !

          if (depth==1)
          {
            recalculatedEndPoint = true;
            IntersectedOrRecalculatedFT = RevisedEndPointFT;
            // So that we can return it, if it is the endpoint!
          }
        }
        if( errorEndPt > 1 )  // errorEndPt = 1 is milder, just: len(B)=len(A)
        {
          std::ostringstream errmsg; 
          errmsg << "Location:  " << MethodName << "- Second-Half" << G4endl;
          ReportReversedPoints(errmsg, 
                    CurveStartPointVelocity, CurveEndPointVelocity,
                    NewSafety, fiEpsilonStep, 
                    CurrentA_PointVelocity, CurrentB_PointVelocity,
                    SubStart_PointVelocity, CurrentE_Point,
                    ApproxIntersecPointV, substep_no, substep_no_p, depth);
          G4Exception(MethodName, "GeomNav0003", FatalException, errmsg);
        }
        G4ThreeVector Point_A = CurrentA_PointVelocity.GetPosition();
        G4ThreeVector Point_B = CurrentB_PointVelocity.GetPosition();   
        GetNavigatorFor()->LocateGlobalPointWithinVolume(Point_A);
        G4bool Intersects_AB = IntersectChord(Point_A, Point_B, NewSafety,
                                              previousSafety,
                                              previousSftOrigin, distAB,
                                              PointGe);
        if( Intersects_AB )
        {
          last_AF_intersection = Intersects_AB;
          CurrentE_Point = PointGe;

          validIntersectP=  true;  // 'E' has been updated.
          // validApproxIntPV= false;  // 'F' is no longer valid, as E changed

          G4bool validNormalAB; 
          NormalAtEntry  = GetSurfaceNormal( PointGe, validNormalAB ); 
          validNormalAtE = validNormalAB;
        }
        else
        {
          validIntersectP=  false;  // No new 'E' chord intersection found
          // validApproxIntPV= false;  // So also 'F' is invalid
          if( depth == 1)
          { 
            there_is_no_intersection = true;
          }
        }       
        depth--;
        fin_section_depth[depth]=true;
        unfinished = !validIntersectP;
      }
#ifdef G4DEBUG_FIELD
      if( ! ( validIntersectP || there_is_no_intersection ) )
      {
         // What happened ??
         G4cout << "MLL - WARNING Potential FAILURE: Conditions not met!"
                << G4endl
                << " Depth = " << depth << G4endl
                << " Levels popped = " << levelPops
                << " Num Substeps= " << substep_no << G4endl;
         G4cout << " Found intersection= " << found_approximate_intersection
                << G4endl;
         G4cout << " Progress report: -- " << G4endl;
         ReportProgress(G4cout, 
                        CurveStartPointVelocity, CurveEndPointVelocity,
                        substep_no, CurrentA_PointVelocity,
                        CurrentB_PointVelocity,
                        NewSafety, depth ); 
         G4cout << G4endl; 
      }
#endif      
    }  // if(!found_aproximate_intersection)

    assert( validIntersectP || there_is_no_intersection
         || found_approximate_intersection);

  } while ( ( ! found_approximate_intersection )
            && ( ! there_is_no_intersection )     
            && ( substep_no <= fMaxSteps) ); // UNTIL found or failed

  if( substep_no > max_no_seen )
  {
    max_no_seen = substep_no; 
#ifdef G4DEBUG_FIELD
    if( max_no_seen > fWarnSteps )
    {
      trigger_substepno_print = max_no_seen-20; // Want to see last 20 steps 
    } 
#endif
  }

  if( !there_is_no_intersection && !found_approximate_intersection )
  {
     if( substep_no >= fMaxSteps)
     {
        // Since we cannot go further (yet), we return as far as we have gone

        recalculatedEndPoint = true;
        IntersectedOrRecalculatedFT = CurrentA_PointVelocity;
        found_approximate_intersection = false; 
     
        std::ostringstream message;
        message << G4endl;
        message << "Convergence is requiring too many substeps: "
                << substep_no << "  ( limit = "<<  fMaxSteps << ")"
                << G4endl 
                << "  Abandoning effort to intersect. " << G4endl << G4endl;
        message << "    Number of calls to MLL: " <<   fNumCalls;
        message << "  iteration = " << substep_no <<G4endl << G4endl;
        
        message.precision( 12 ); 
        G4double done_len = CurrentA_PointVelocity.GetCurveLength(); 
        G4double full_len = CurveEndPointVelocity.GetCurveLength();
        message << "        Undertaken only length: " << done_len
                << " out of " << full_len << " required." << G4endl
                << "        Remaining length = " << full_len - done_len; 
        
        message << "     Start and end-point of requested Step:" << G4endl;
        printStatus( CurveStartPointVelocity, CurveEndPointVelocity,
                     -1.0, NewSafety, 0,     message, -1 );
        message << "        Start and end-point of current Sub-Step:" << G4endl;
        printStatus( CurrentA_PointVelocity, CurrentA_PointVelocity,
                     -1.0, NewSafety, substep_no-1, message, -1 );
        printStatus( CurrentA_PointVelocity, CurrentB_PointVelocity,
                     -1.0, NewSafety, substep_no, message, -1 );
        
        G4Exception(MethodName, "GeomNav0003", JustWarning, message);
     }
     else if( substep_no >= fWarnSteps)
     {  
        std::ostringstream message;
        message << "Many substeps while trying to locate intersection."
                << G4endl
                << "          Undertaken length: "  
                << CurrentB_PointVelocity.GetCurveLength()
                << " - Needed: "  << substep_no << " substeps." << G4endl
                << "          Warning number = " << fWarnSteps
                << " and maximum substeps = " << fMaxSteps;
        G4Exception(MethodName, "GeomNav1002", JustWarning, message);
     }
  }

#ifdef G4DEBUG_FIELD  
  if( found_approximate_intersection )
  {
    assert( validApproxIntPV &&
            "Approximate Intersection must not have been invalidated." );
  }
#endif
  
  return  (!there_is_no_intersection) && found_approximate_intersection;
    //  Success or failure
}

void G4MultiLevelLocator::ReportStatistics()
{
   G4cout << " Number of calls = " << fNumCalls << G4endl;
   G4cout << " Number of split level ('advances'):  "
          << fNumAdvanceTrials << G4endl;
   G4cout << " Number of full advances:             "
          << fNumAdvanceGood << G4endl;
   G4cout << " Number of good advances:             "
          << fNumAdvanceFull << G4endl;
}

void G4MultiLevelLocator::ReportFieldValue( const G4FieldTrack& locationPV,
                                            const char* nameLoc,
                                            const G4EquationOfMotion* equation )
{
   enum { maxNumFieldComp= 24 };

   G4ThreeVector position = locationPV.GetPosition();   
   G4double startPoint[4] = { position.x(), position.y(), position.z(),
                              locationPV.GetLabTimeOfFlight() };
   G4double FieldVec[maxNumFieldComp]; // 24 ;
   for (unsigned int i=0; i<maxNumFieldComp; ++i )
   {
     FieldVec[i]= 0.0;
   }
   equation->GetFieldValue( startPoint, FieldVec);
   G4cout << "  B-field value (" << nameLoc << ")=   "
          << FieldVec[0] << " " << FieldVec[1] << " " << FieldVec[2];
   G4double Emag2= G4ThreeVector( FieldVec[3],
                                  FieldVec[4],
                                  FieldVec[5] ).mag2(); 
   if( Emag2 > 0.0 )
   {
      G4cout << " Electric = " << FieldVec[3] << " "
                               << FieldVec[4] << " "
                               << FieldVec[5]<< G4endl;
   }
   return;
}