/*
 * 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.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);
                }
            }
            result.add(vertex);
        }
    }

    /**
     * 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;
    }
}
