/**
 * Initialize the link force algorithm and start simulation.
 */
function startLinkSimulation() {
  console.log("startLinkSimulation")
  springForce = false;
  alreadyRanIterations = 0;
  simulation.stop();
  p1 = performance.now();
  let links = [];

  // Initialize link array.
  nodes = simulation.nodes();
  for (i = nodes.length-1; i >= 1; i--) {
    for (j = i-1; j >= 0; j--) {
      links.push({
        source: nodes[i],
        target: nodes[j],
      });
    }
  }

  /* Add force
   * Please add the distance function before feeding the force
   *   it to the simulation or adding links.
   *
   * On setting the distance fn and being initialized by the simulation, the
   *   force will pre-calculate high-dimensional distances of every link and
   *   store that as cache.
   * Adding distance fn before links means that the first pre-calculation will
   *   calculate noting as there was no link.
   * The full pre-calculation will then occur once when the force is being
   *   initialized by the simulation.
   */
  simulation.force(forceName,
    d3.forceLinkOptimized()
      .distance(function (n) {
         return distanceFunction(n.source, n.target, props, norm);
       })
    .links(links)
  ).alphaDecay(0)
    //.velocityDecay(0.8)
   .alpha(1)
   .restart();
}