function tick() {
const {x:x1,y:y1,r:r1} = circles[0];
const {x:x2,y:y2,r:r2} = circles[1];
const [lx1,ly1,lx2,ly2] = f(x1,y1,r1+2*strokeWidth,x2,y2,r2+2*strokeWidth);
svg.selectAll("line")
.data(links)
.join("line")
.attr("x1", lx1)
.attr("y1", ly1)
.attr("x2", lx2)
.attr("y2", ly2)
}