fixpoint algorithm: parameterize the Map to use

lib/util/rbtree_wrapper.js

@@ -29,3 +29,46 @@ export class RBTreeWrapper {
     return this.tree.keys.map(key => [key, this.tree.get(key)]);
   }
 }
+
+// Can be used as drop-in replacement for Map
+// Why create a mutable adapter for a purely-functional data structure?
+// Because the builtin Map does not allow custom comparison functions and this one does.
+export class MutableRBTree {
+  constructor(tree, printf = defaultPrintf) {
+    this.tree = tree;
+    this[inspect.custom] = printf;
+  }
+
+  static new(cmp) {
+    return new MutableRBTree(createRBTree(cmp));
+  }
+
+  set(key, value) {
+    this.tree = this.tree.remove(key).insert(key, value);
+  }
+
+  delete(key) {
+    this.tree = this.tree.remove(key);
+  }
+
+  get(key) {
+    return this.tree.get(key);
+  }
+
+  has(key) {
+    return this.tree.get(key) !== undefined;
+  }
+
+  clear() {
+    this.tree = createRBTree(this.tree._compare);
+  }
+
+  *[Symbol.iterator]() {
+    const iter = this.tree.begin;
+    while (iter !== undefined && iter.valid) {
+      yield [iter.key, iter.value];
+      iter.next();
+    }
+  }
+}
+