Problem 1052

from http://hypernews.slac.stanford.edu/HyperNews/geant4/get/geometry/956.html

The next code leads Geant4 to the infinite loop. 

//___________________________________________________________________________________________________________________________
void test(){
     const char* nms[]={"kOutside","kSurface","kInside"};

     double t_dz=0.3392505000;
     double t_rad=85.94;
     double p_rmz=85.9400000000;
     double p_rpz=99.2300000000;
     double p_hz=0.3392505000;

     G4Paraboloid* par = new G4Paraboloid("par",p_hz,p_rmz,p_rpz);
     G4DisplacedSolid* pard=new G4DisplacedSolid("disp_par",par,new G4RotationMatrix(0.,180.*deg,0.),G4ThreeVector(0.,0.,0.));
     G4Tubs* t=new G4Tubs("absorber",0.,t_rad,t_dz,0.*deg,360.*deg);
     G4SubtractionSolid* seg1 = new G4SubtractionSolid("par1_i",pard,t,0,0);

     G4ThreeVector v(-82.1262056167, 40.0738142048, 0.0732052691);
     G4ThreeVector dir(-0.00013143732014871606, 0.00006413537198823375,-0.99999998930544242715);
     dir=dir.unit();

     printf("%20.10f %20s %20.10f\n",v.x(),nms[t->Inside(v)],t->DistanceToOut(v,dir));
     printf("%20.10f %20s %20.10f\n",v.x(),nms[pard->Inside(v)],pard->DistanceToIn(v,dir));

     G4ThreeVector v0(-81.97424905536260553163, 39.99966638606156266178, 1156.18746249999981046130);
     printf("infinite_loop...\n");
     seg1->DistanceToIn(v0,dir);
     printf("finished...\n");
}
//___________________________________________________________________________________________________________________________

The reason is printed to the output: both constituents solid say that point "v" is outside, and both of them return 0.0 as distance to in/out. 

G4SubtractionSolid::DistanceToIn() uses the "while" loop:
//___________________________________________________________________________________________________________________________
while( Inside(p+dist*v) == kOutside )  // pushing loop
{
  disTmp = fPtrSolidB->DistanceToOut(p+dist*v,v) ;
  dist += disTmp ;

  if( Inside(p+dist*v) == kOutside ){ 
    disTmp = fPtrSolidA->DistanceToIn(p+dist*v,v) ;
    if(disTmp == kInfinity){return kInfinity;}                 
    dist += disTmp ;
  }
}
//___________________________________________________________________________________________________________________________

as "disTmp" is always 0., this loop is infinite. I suggest to add a simple check: if both "disTmp" are zeroes, move them manually by a small number, like 1.e-9. 

It also seems that other G4BooleanSolids are also "dangerous", but I do not have exact examples for them. 

best regards, Svetlana Biktemerova