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 * 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
 * Unless required by applicable law or agreed to in writing,
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package org.apache.axiom.core.util;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

 * Implements topological sorting.
public final class TopologicalSort {
    private TopologicalSort() {}
    private static <T> void visit(Collection<T> vertices, EdgeRelation<? super T> edgeRelation, List<T> result, Set<T> visited, T vertex) {
        if (visited.add(vertex)) {
            for (T vertex2 : vertices) {
                if (vertex2 != vertex && edgeRelation.isEdge(vertex, vertex2)) {
                    visit(vertices, edgeRelation, result, visited, vertex2);
     * Apply topological ordering to a given collection.
     * @param <T>
     *            the vertex type
     * @param vertices
     *            the vertices of the directed acyclic graph
     * @param edgeRelation
     *            the relation defining the edges of the graph
     * @return the topologically sorted list of vertices, where the vertices with no incoming edges
     *         come first
    public static <T> List<T> sort(Collection<T> vertices, EdgeRelation<? super T> edgeRelation) {
        List<T> result = new ArrayList<T>(vertices.size());
        Set<T> visited = new HashSet<T>();
        for (T vertex : vertices) {
            visit(vertices, edgeRelation, result, visited, vertex);
        return result;