Add an API entry to calculate stress

index.js

@@ -18,3 +18,6 @@ export { default as forceLinkTweaked}
 
 export { default as hybridSimulation}
   from "./src/hybridSimulation";
+
+export { getStress as calculateStress }
+  from "./src/stress";

src/stress.js

@@ -1,19 +1,20 @@
 /**
-  * Calculates the stress. Basically, it computes the difference between
-  * high dimensional distance and real distance. The lower the stress is,
-  * the better layout.
+  * 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 totalDiffSq = 0, totalHighDistSq = 0;
-  for (let j = 0; j < nodes.length; j++) {
+  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);
-      totalDiffSq += Math.pow(highDDist - lowDDist, 2);
-      totalLowDDistSq += lowDDist * lowDDist;
+      sumDiffSq += Math.pow(highDDist - lowDDist, 2);
+      sumLowDDistSq += lowDDist * lowDDist;
     }
   }
-  return Math.sqrt(totalDiffSq / totalLowDDistSq);
+  return Math.sqrt(sumDiffSq / sumLowDDistSq);
 }