/**
  * Calculates the stress. Basically, it computes the difference between
  * high dimensional distance and real distance. The lower the stress is,
  * the better layout.
  * @return {number} - stress of the layout.
  */
export function getStress(nodes, distance) {
  let totalDiffSq = 0, totalHighDistSq = 0;
  for (let j = 0; j < nodes.length; j++) {
    for (let i = 0; i < j; i++) {
      let source = nodes[i], target = nodes[j];
      let lowDDist = Math.hypot(target.x - source.x, target.y - source.y);
      let highDDist = distance(source, target);
      totalDiffSq += Math.pow(highDDist - lowDDist, 2);
      totalHighDistSq += highDDist * highDDist;
    }
  }
  return Math.sqrt(totalDiffSq / totalHighDistSq);
}