Move (copy, so far) all the digraph code in Graph

utils.js

@@ -6,3 +6,21 @@ function randStr32() {
 function randStr64() {
   return randStr32() + randStr32();
 }
+
+function getOrSet(container, key, newValue) {
+  let val = container.get(key);
+  if (!val) {
+    val = newValue;
+    container.set(key, val);
+  }
+  return val;
+}
+
+function intersects(set1, set2) {
+  for (let item of set1) {
+    if (set2.has(item)) {
+      return true;
+    }
+  }
+  return false;
+}