Raw space (no optimisation, no grid, no quadtree, no ...) every particles are check against every particles (inter Particles and intra Particles group) and check against every Segment

class Space_QuadtreeXY implements Space{
 final QuadTreeXYAabb _grid;
 final Checker checker;
 final Resolver resolver;
 final Vector4 _scol = new Vector4.zero();

 final _pool = new _ProviderStack<_SQE>()
   ..createE = _SQE.ctor
   ..resetE =  _SQE.reset
   ;

 Space_QuadtreeXY(this.checker, this.resolver, {QuadTreeXYAabb grid}) : _grid = (grid == null)? new QuadTreeXYAabb(-500.0, -500.0, 1000.0, 1000.0, 10) : grid;

 void reset() {
   _grid.reset();
   _pool.reset();
 }

 /// return true if at least one [ps].collide != 0 (the Particles [ps] is included in the space)
 bool addParticles(Particles ps) {
   if (ps.length < 1) return false;
   var b = false;
   for(int i = ps.length - 1; i >= 0; --i) {
     b  = b || ps.collide[i] != 0;
   }
   if (b) {
     for(int i = ps.length - 1; i >= 0; --i) {
       ps.collide[i] = 1;
       var e = _pool.provide();
       e.ps = ps;
       e.i = i;
       extractAabbDisc(ps.position3d[i], ps.radius[i], e.aabb);
       _grid.insert(e.aabb, e);
     }
   }
   return b;
 }

 /// return true if [seg].collide != 0 (the Segment [seg] is include in the space)
 bool addSegment(Segment seg) {
   if (seg.collide != 0) {
     seg.collide = 1;
     var e = _pool.provide();
     e.segment = seg;
     extractAabbDisc2(seg.ps.position3d[seg.i1], seg.ps.position3d[seg.i2], 0.0, e.aabb);
     _grid.insert(e.aabb, e);
     return true;
   }
   return false;
 }

 void handleCollision() {
   _grid.scan(_handle0);
 }

 _handle0(_SQE e1, _SQE e2) {
   if (e1.ps != null && e2.ps != null) _handlePP(e1, e2);
   else if (e1.ps != null && e2.segment.ps != null) _handlePS(e1, e2);
   else if (e2.ps != null && e1.segment.ps != null) _handlePS(e2, e1);
   else {
     // segment vs segment collision not managed
   }
 }

 _handlePP(_SQE e1, _SQE e2) {
   if (e1.ps != e2.ps  || e1.ps.intraCollide) {
     if(checker.collideParticleParticle(e1.ps, e1.i, e2.ps, e2.i, _scol)){
       resolver.notifyCollisionParticleParticle(e1.ps, e1.i, e2.ps, e2.i, _scol.w);
     }
   }
 }

 _handlePS(_SQE e1, _SQE e2) {
   if (e1.ps != e2.segment.ps  || e1.ps.intraCollide) {
     if (checker.collideParticleSegment(e1.ps, e1.i, e2.segment, _scol)) {
       resolver.notifyCollisionParticleSegment(e1.ps, e1.i, e2.segment, _scol.w);
     }
   }
 }

}