static PT_THREAD (protothread_compute_balls(struct pt *pt))
{
  PT_BEGIN(pt);

  while(1) {
    
    begin_time = PT_GET_TIME();

    For each ball i from 1 to n
	  For each ball j from i+1 to n
		Compute approximate rij by checking:
		if Δx and Δy are both less than 2
		 if (||rij||2 less than (2*(ballRadius))2 and hitCounter is zero)
			Compute vij
			Compute Δvi
			Add Δvi to vi
			Subtract Δvi from vj
			Set hitCounter big enough to avoid particle capture
		 elseif (hitCounter>0)
			decrement hitCounter
		 endif
		endif
	  endfor
	endfor
	
	For each ball i from 1 to n
		erase ball i
		update ball i velocity using drag and wall collisions
		update position
		draw (or recycle) ball i
	endfor
	
	Inject new balls into playing field (if any)
	
	Check for user interaction by reading ADC and buttons (if any)
	
	// 15 fps => frame time of 67 mSec
	PT_YIELD_TIME_msec(67 - (PT_GET_TIME() - begin_time)) ;
  } // end while

   PT_END(pt);
} // end thread