Refactor: Extract getStress

src/stress.js

@@ -0,0 +1,19 @@
+/**
+  * 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);
+}