Stu Black
fbe8c57872
This code identified cycles by marking edges on a root-node path R, ..., A, ..., X with the Target::Cycle variant when there was already some other path R, ..., B, ..., X. This successfully induced an acyclic graph structure while maintaining the topology of which vertices are reachable along a given path, but it did not serve the general interest of constructing a graph for MCTS for a couple of reasons: 1) Parent-child edges with a Target::Cycle target did not have a corresponding child-parent edge, so it became harder to do the right thing in backpropagation. 2) Without more concrete control over which paths are considered the canonical owner of a vertex, it is possible that a suboptimal path (which goes through some nodes that admit very strong opponent play) would be the canonical path. This could prevent discovery of a very good move. |
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Overview
This package provide a rollout-based, DAG-like data structure. Its intended use is building game state graphs for general game-playing or similar AI tasks.
Vertices correspond to game states. They are addressed by unique game states
(which may be de-duplicated as if by transposition table by implementing
std::hash::Hash and std::cmp::Eq appropriately for your game state type).
Edges correspond to game-modifying moves. They are bidirectional, so paths may be traced up and down the graph.
The structure is rollout-based in that edges are expanded lazily. This should make it easy to implement planning algorithms like A* and Monte Carlo tree search on top of the DAG structure.
It is DAG-like in that it nominally maintains a cycle-free topology by
checking for cycles at edge expansion time and noting which edges would create
cycles when they are expanded. Such edges are added to the graph as half-edges,
which are only present in one direction. Unfortunately, checking for cycles
leads to O(|V|) running time when adding vertices.
At present, the graph topology may be grown but not pruned. Vertex and edge deletion may be supported in a future version.
Copyright
Copyright 2015-2016, Donald S. Black.
Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0.
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.