Use nested config structure

common/filter.js

@@ -3,6 +3,7 @@
 const filterParser = require("./filter-parser");
 const filterCnf = require("./filter-cnf");
 const satSolver = require("./sat-solver");
+const funcCache = require("./func-cache");
 
 function* permute(arr) {
   if (arr.length <= 1) {
@@ -24,6 +25,10 @@ class Filter {
     else this.ast = filterParser.parse(str);
   }
 
+  static parse(str) {
+    return new Filter(str);
+  }
+
   toString() {
     if (!this.string)
       if (this.ast) this.string = filterParser.stringify(this.ast);

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;