Use nested config structure

common/func-cache.js

@@ -0,0 +1,27 @@
+"use strict";
+
+let cache1 = new WeakMap();
+let cache2 = new WeakMap();
+
+function get(func, k) {
+  let c1 = cache1.get(func);
+  if (!c1) cache1.set(func, (c1 = new Map()));
+  if (c1.has(k)) return c1.get(k);
+  const c2 = cache2.get(func);
+  const v = c2 && c2.has(k) ? c2.get(k) : func(k);
+  c1.set(k, v);
+  return v;
+}
+
+function getter(func) {
+  return k => get(func, k);
+}
+
+function purge() {
+  cache2 = cache1;
+  cache1 = new WeakMap();
+}
+
+exports.get = get;
+exports.getter = getter;
+exports.purge = purge;