/**
  * Calculates the stress of a system by comparing the difference between the
  * high dimensional distance and 2D distance. The lower the stress contributes
  * to the better layout.
  * @return {number} - stress of the layout.
  */
export function getStress(nodes, distance) {
  let sumDiffSq = 0
  let sumHighDistSq = 0;
  for (let j = nodes.length-1; j >= 1; 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);
      sumDiffSq += Math.pow(highDDist - lowDDist, 2);
      sumLowDDistSq += lowDDist * lowDDist;
    }
  }
  return Math.sqrt(sumDiffSq / sumLowDDistSq);
}