/*
Cohesion steering behavior gives an character the ability to cohere with (approach and form a group with) 
other nearby characters. See Figure 15. 
Steering for cohesion can be computed by finding all characters in the local neighborhood 
(as described above for separation), 
computing the “average position” (or “center of gravity”) of the nearby characters. 
The steering force can applied in the direction of that “average position” 
(subtracting our character position from the average position, as in the original boids model), 
or it can be used as the target for seek steering behavior.
*/

class CohesionVec extends Vec {
  CohesionVec() {
  }
  
  void update(Vector neighbours, Vec pos) {
    
    if (neighbours.size()>0) {
      Vec averagePos=new Vec();
      for (int i=0;i<neighbours.size();i++) {
        averagePos.add(((Bird)neighbours.elementAt(i)).pos);
      }
      averagePos.div((double)neighbours.size());
      setVec(vecSub(averagePos,pos));
    } else {
      setVec(new Vec(0,0,0));
    }
  }
}