It is time.

ui/accessibility/tree_generator.h

-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-namespace ui {
-
-class AXTree;
-
-// A class to create all possible trees with <n> nodes and the ids [1...n].
-//
-// There are two parts to the algorithm:
-//
-// The tree structure is formed as follows: without loss of generality,
-// the first node becomes the root and the second node becomes its
-// child. Thereafter, choose every possible parent for every other node.
-//
-// So for node i in (3...n), there are (i - 1) possible choices for its
-// parent, for a total of (n-1)! (n minus 1 factorial) possible trees.
-//
-// The second part is the assignment of ids to the nodes in the tree.
-// There are exactly n! (n factorial) permutations of the sequence 1...n,
-// and each of these is assigned to every node in every possible tree.
-//
-// The total number of trees returned for a given <n>, then, is
-// n! * (n-1)!
-//
-// n = 2: 2 trees
-// n = 3: 12 trees
-// n = 4: 144 trees
-// n = 5: 2880 trees
-//
-// This grows really fast! Luckily it's very unlikely that there'd be
-// bugs that affect trees with >4 nodes that wouldn't affect a smaller tree
-// too.
-class TreeGenerator {
- public:
-  TreeGenerator(int node_count);
-
-  int UniqueTreeCount() const;
-
-  void BuildUniqueTree(int tree_index, AXTree* out_tree) const;
-
- private:
-  int node_count_;
-  int unique_tree_count_;
-};
-
-}  // namespace ui