maven-resolver-util/src/main/java/org/eclipse/aether/util/graph/visitor/LevelOrderDependencyNodeConsumerVisitor.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */
package org.eclipse.aether.util.graph.visitor;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.function.Consumer;

import org.eclipse.aether.ConfigurationProperties;
import org.eclipse.aether.graph.DependencyNode;

/**
 * Processes dependency graph by traversing the graph in level order. This visitor visits each node exactly once
 * regardless how many paths within the dependency graph lead to the node such that the resulting node sequence is
 * free of duplicates.
 *
 * @since 2.0.0
 *
 * @see NodeListGenerator
 */
public final class LevelOrderDependencyNodeConsumerVisitor extends AbstractDependencyNodeConsumerVisitor {

    public static final String NAME = ConfigurationProperties.REPOSITORY_SYSTEM_DEPENDENCY_VISITOR_LEVELORDER;

    private final HashMap<Integer, ArrayList<DependencyNode>> nodesPerLevel;

    private final Stack<Boolean> visits;

    /**
     * Creates a new level order list generator.
     */
    public LevelOrderDependencyNodeConsumerVisitor(Consumer<DependencyNode> nodeConsumer) {
        super(nodeConsumer);
        nodesPerLevel = new HashMap<>(16);
        visits = new Stack<>();
    }

    @Override
    public boolean visitEnter(DependencyNode node) {
        boolean visited = !setVisited(node);
        visits.push(visited);
        if (!visited) {
            nodesPerLevel.computeIfAbsent(visits.size(), k -> new ArrayList<>()).add(node);
        }
        return !visited;
    }

    @Override
    public boolean visitLeave(DependencyNode node) {
        Boolean visited = visits.pop();
        if (visited) {
            return true;
        }
        if (visits.isEmpty()) {
            for (int l = 1; nodesPerLevel.containsKey(l); l++) {
                nodesPerLevel.get(l).forEach(nodeConsumer);
            }
        }
        return true;
    }
}