class Resolver_Backward implements Resolver {
 backward(Vector3 vc, Vector3 vp, double t) {
   vc.sub(vp).scale(t).add(vp);
   vp.setValues(vc.x, vc.y, vc.z); //HACK to reset valocity// inertie
 }

 void notifyCollisionParticleSegment(Particles psA, int iA, Segment s, double tcoll) {
   //var t = math.max(0.0, tcoll - 0.5); //small repulsion should not override
   var t = 0.5 * tcoll;
   //ci.psA.position3d[ci.iA].setValues(ci.collA.x, ci.collA.y, ci.collA.z);
   backward(psA.position3d[iA], psA.position3dPrevious[iA], t);
   backward(s.ps.position3d[s.i1], s.ps.position3dPrevious[s.i1], t);
   backward(s.ps.position3d[s.i2], s.ps.position3dPrevious[s.i2], t);
   //print("${ci.psA.hashCode}  ... ${ci.s.ps.hashCode} ${ci.psA == ci.s.ps}");
 }

 void notifyCollisionParticleParticle(Particles psA, int iA, Particles psB, int iB, double tcoll) {
   var body1 = psA.position3d[iA];
   var body2 = psB.position3d[iB];
   var x = body1.x - body2.x;
   var y = body1.y - body2.y;
   var slength = x*x+y*y;
   var length = math.sqrt(slength);
   var target = psA.radius[iA] + psB.radius[iB];

   // if the spheres are closer
   // then their radii combined
   if(length < target){
     var factor = (length-target)/length;
     // move the spheres away from each other
     // by half the conflicting length
     body1.x -= x*factor*0.5;
     body1.y -= y*factor*0.5;
     body2.x += x*factor*0.5;
     body2.y += y*factor*0.5;
   }
 }

}