up-down circular boundaries (pipe-surface)

var speed=80;  
var Xreflectionrate=0.5; 

What?

function checkY(y,Num){
   height=parseIntohnepx(StyleOfTheSquare[Num].height);
   edge=doc_height-height
   if (y > edge)  {
      y=0 + y - edge;
   }
   if (y<0) {
       y=doc_height+y;
   }
   return y;
} 

How?

What's next??