DynamicDAG.java

/*
 * Copyright (c) 2014 David Sowerby
 *
 * 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.
 */

package uk.q3c.util;

import java.util.Collection;
import java.util.Stack;

public class DynamicDAG<V> extends BasicForest<V> {

    public DynamicDAG() {
        super();
    }

    @Override
    public void addChild(V parentNode, V childNode) {
        if (!detectCycle(parentNode, childNode)) {
            super.addChild(parentNode, childNode);
        } else {
            throw new CycleDetectedException();
        }
    }

    /**
     * Checks the proposed connection between parent and child nodes, and returns true if a cycle would be created by
     * adding the child to the parent, or false if not
     *
     * @param parentNode
     * @param childNode
     *
     * @return
     */
    protected boolean detectCycle(V parentNode, V childNode) {
        if (parentNode == childNode) {
            return true;
        }
        Stack<V> stack = new Stack<>();
        stack.push(parentNode);
        while (!stack.isEmpty()) {
            V node = stack.pop();
            Collection<V> predecessors = getGraph().getPredecessors(node);
            if (predecessors != null) {
                for (V pred : predecessors) {
                    if (pred == childNode) {
                        return true;
                    }
                }
                stack.addAll(predecessors);
            }
        }
        return false;

    }

}