图论-拓扑排序

最近又写到有向图的拓扑排序，这段排序代码在几年前也写过。做个简单记录，本次用到了google guava的Graph类。
主要是用方法可以参考：https://www.jianshu.com/p/78786a4f2cf1

相关排序可以参考： https://juejin.cn/post/7251844608774209592

图1

拓扑排序：
Visited node: A
Visited node: D
Visited node: E
Visited node: B
Visited node: C
Visited node: F

但在实际的工作中，并不是单纯的对节点进行排序或者遍历，比如说常用的就是A有三条下游节点时，则选择其中一条路径执行，其余路径均不可执行。

图2

在拓扑排序的基础上，我们增加不执行节点的判断与处理。具体处理逻辑如下：

public class UnReachableTopologyTraversal {


    public static void topologyTraverse(MutableGraph<String> graph, Function<String, Boolean> function) {
        // 1.1 拓扑排序需要的辅助队列。
        Queue<String> queue = Queues.newLinkedBlockingQueue();
        //1.2 初始化本次不执行节点集合
        List<String> unReachNodeList = Lists.newArrayList();


        // 2. 将root节点(入度为0的节点)入队列。
        for (String node : graph.nodes()) {
            if (graph.inDegree(node) == 0) {
                queue.add(node);
            }
        }


        // 3. 非root节点(入度非0的节点)对应的入度记录表。
        Map<String, Integer> nodeIdInDegreeMap = graph.nodes().stream()
                .filter(task -> graph.inDegree(task) != 0)
                .collect(Collectors.toMap(node -> node, node -> graph.inDegree(node)));


        // 4. 拓扑遍历
        while (!queue.isEmpty()) {
            //5. 随机选取其中一个可执行节点
            List<String> allNodes = new ArrayList<>(queue);
            Collections.shuffle(allNodes);
            queue = new LinkedList<>(allNodes);
            String current = queue.poll();




            // 6. 判断是不是不可执行节点，如果是不可执行节点，则跳过
            if (unReachNodeList.contains(current)) {
                continue;
            }


            //7.1 执行本次节点函数
            function.apply(current);


            //7.2 后续节点入度减一
            subSuccessorInDegree(current, graph, nodeIdInDegreeMap);


            //8.1 标记不可执行节点
            for (String node : allNodes) {
                if (!unReachNodeList.contains(node) && !StringUtils.equals(node,
                        current)) {
                    unReachNodeList.add(node);
                    subSuccessorInDegree(node, graph, nodeIdInDegreeMap);
                    markSuccessorUnReachable(graph, node, unReachNodeList, nodeIdInDegreeMap);
                }
            }


            //9. 拓扑排序, 将当前节点的后继节点入度减一, 如果入度为0, 则加入队列, 等待调度。
            for (String node : graph.successors(current)) {
                if (nodeIdInDegreeMap.get(node) == 0) {
                    queue.add(node);
                }
            }
        }
    }


    /**
     * 将当前节点的后续节点入度-1
     * @param currentNode
     * @param graph
     * @param nodeIdInDegreeMap
     */
    private static void subSuccessorInDegree(String currentNode,
                                             Graph<String> graph, Map<String, Integer> nodeIdInDegreeMap) {


        Set<String> successorNodes = graph.successors(currentNode);
        if (successorNodes == null || successorNodes.size() == 0) {
            return;
        }


        for (String successor : successorNodes) {
            Integer inDegree = nodeIdInDegreeMap.get(successor);
            nodeIdInDegreeMap.put(successor, inDegree - 1);
        }
    }




    /**
     * 标记当前节点的后续节点为不可执行节点
     * @param graph
     * @param currentNode
     * @param unReachNodeList
     * @param nodeIdInDegreeMap
     */
    private static void markSuccessorUnReachable(Graph<String> graph, String currentNode,
                                                 List<String> unReachNodeList, Map<String, Integer> nodeIdInDegreeMap) {
        Set<String> afterNodeList = graph.successors(currentNode);
        for (String afterNode : afterNodeList) {
            boolean unReachable = true;
            
            //当前节点的前置节点如果存在不可执行节点，则不需要递归不可执行
            Set<String> preAfterNodeList = graph.predecessors(afterNode);
            for (String preAfterNode : preAfterNodeList) {
                if (!unReachNodeList.contains(preAfterNode)) {
                    unReachable = false;
                }
            }
            
            //后续节点为不可执行节点，继续递归不可执行
            if (unReachable) {
                unReachNodeList.add(afterNode);
                subSuccessorInDegree(afterNode, graph, nodeIdInDegreeMap);
                markSuccessorUnReachable(graph, afterNode, unReachNodeList, nodeIdInDegreeMap);
            }
        }
    }




    public static void main(String[] args) {
        MutableGraph<String> graph = GraphBuilder.directed() //指定为有向图
                .nodeOrder(ElementOrder.<String>insertion()) //节点按插入顺序输出
                //(还可以取值无序unordered()、节点类型的自然顺序natural())
                .expectedNodeCount(6) //预期节点数
                .allowsSelfLoops(false) //不允许自环
                .build();


        graph.putEdge("A", "B");
        graph.putEdge("A", "D");
        graph.putEdge("A", "E");
        graph.putEdge("B", "C");
        graph.putEdge("C", "F");
        graph.putEdge("D", "F");


        graph.addNode("A");
        graph.addNode("B");
        graph.addNode("C");
        graph.addNode("D");
        graph.addNode("E");


        System.out.println("选择分支执行：");
        UnReachableTopologyTraversal.topologyTraverse(graph, node -> {
            System.out.println("Visited node: " + node);
        });


    }
}

结果1
结果2
结果3