Check by intersection of polygon particles are converted into rectangle : * width : start/end point x 2 radius (normal of displacement), * longer : displacement Segment : previouss.i1, previouss.i2, currents.i2, currents.i1 (ignore radius)

class Checker_MvtAsPoly4 implements Checker{
 final IntersectionFinder _intf = new IntersectionFinderXY();
 var _v0 = new Vector3.zero();
 var _poly0 = new List.generate(4, (i) => new Vector3.zero(), growable: false);
 var _poly1 = new List.generate(4, (i) => new Vector3.zero(), growable: false);

 /// Returns whether two particles A ([psA] + [iA]) and B [psB] + [iB]) intersect
 /// [psA.collide[iA]] and [psB.collide[iB]] are set to true if collision.
 /// Doesn't check if provided particles are the same or part of same group,... should be done before calling.
 /// take care of radius of each particle
 collideParticleParticle(Particles psA, int iA, Particles psB, int iB, Vector4 acol) {
   var b = _intf.poly_poly(makePoly4(psA, iA, _poly0), makePoly4(psB, iB, _poly1));
   if (b) {
     psA.collide[iA] = -1;
     psB.collide[iB] = -1;
     var v = psA.position3d[iA];
     acol.setValues(v.x, v.y, v.z, 1.0);
   }
   return b;
 }

 /// Returns whether the provided particle A and the segment [s]
 /// Doesn't check if provided particles are the same or part of same group,... should be done before calling.
 collideParticleSegment(Particles psA, int iA, Segment s, Vector4 scol) {
   _poly1[0].setFrom(s.ps.position3dPrevious[s.i1]);
   _poly1[1].setFrom(s.ps.position3dPrevious[s.i2]);
   _poly1[2].setFrom(s.ps.position3d[s.i2]);
   _poly1[3].setFrom(s.ps.position3d[s.i1]);
   var b = _intf.poly_poly(makePoly4(psA, iA, _poly0), _poly1);
   if (b) {
     psA.collide[iA] = -1;
     s.collide = -1;
     //TODO project scol on segment (final position)
     var v = psA.position3d[iA];
     scol.setValues(v.x, v.y, v.z, 1.0);
   }
   return b;
 }

 // if no displacement (< r/10) then an outside square is return;
 makePoly4(Particles ps, int i, List<Vector3> out) {
   var pn = ps.position3d[i];
   var pp = ps.position3dPrevious[i];
   // _v0.setFrom(pn).sub(pp);
   var vx = pn.x - pp.x;
   var vy = pn.y - pp.y;
   //var l = _v0.length;
   var l2 = vx * vx + vy * vy;
   var r = ps.radius[i];
   if ((l2 * 10) < r) {
     out[0].x = pn.x + r;
     out[0].y = pn.y + r;
     out[1].x = pn.x + r;
     out[1].x = pn.y - r;
     out[2].x = pn.x - r;
     out[2].y = pn.y - r;
     out[3].x = pn.x - r;
     out[3].y = pn.y + r;
   } else {
     //var t = _v0.x;
     //_v0.scale(r / l);
     var rl = r / math.sqrt(l2);
     var t = vx;
     vx = -vy * rl;
     vy = t *rl;
     //out[0].setFrom(pp).add(_v0);
     out[0].x = pp.x + vx;
     out[0].y = pp.y + vy;
     //out[1].setFrom(ps.position3dPrevious[i]).sub(_v0);
     out[1].x = pp.x - vx;
     out[1].y = pp.y - vy;
     //out[2].setFrom(ps.position3d[i]).sub(_v0);
     out[2].x = pn.x - vx;
     out[2].y = pn.y - vy;
     //out[3].setFrom(ps.position3d[i]).add(_v0);
     out[3].x = pn.x + vx;
     out[3].y = pn.y + vy;
   }
   return out;
 }
}