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SchemaCrawler is an open-source Java API that makes working with database metadata as easy as working with plain old Java objects. SchemaCrawler is also a database schema discovery and comprehension, and schema documentation tool. You can search for database schema objects using regular expressions, and output the schema and data in a readable text format. The output is designed to be diff-ed against other database schemas.

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/*
 *
 * SchemaCrawler
 * http://www.schemacrawler.com
 * Copyright (c) 2000-2016, Sualeh Fatehi.
 *
 * This library is free software; you can redistribute it and/or modify it under the terms
 * of the GNU Lesser General Public License as published by the Free Software Foundation;
 * either version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
 * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License along with this
 * library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
 * Boston, MA 02111-1307, USA.
 *
 */
package sf.util.graph;


import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Objects;

public class SimpleTopologicalSort>
{

  private final DirectedGraph graph;

  public SimpleTopologicalSort(final DirectedGraph graph)
  {
    this.graph = Objects.requireNonNull(graph);
  }

  public List topologicalSort()
    throws GraphException
  {
    if (containsCycle())
    {
      throw new GraphException("Graph contains a cycle, so cannot be topologically sorted");
    }

    final Collection> vertices = graph.vertexSet();
    final int collectionSize = vertices.size();

    final Collection> edges = new ArrayList<>(graph.edgeSet());
    final List sortedValues = new ArrayList<>(collectionSize);

    while (!vertices.isEmpty())
    {

      final List nodesAtLevel = new ArrayList<>(collectionSize);

      // Remove unattached nodes
      for (final Iterator> iterator = vertices.iterator(); iterator
        .hasNext();)
      {
        final Vertex vertex = iterator.next();
        if (isUnattachedNode(vertex, edges))
        {
          nodesAtLevel.add(vertex.getValue());
          iterator.remove();
        }
      }

      // Find all nodes at the current level
      final List> startNodes = new ArrayList<>(collectionSize);
      for (final Vertex vertex: vertices)
      {
        if (isStartNode(vertex, edges))
        {
          startNodes.add(vertex);
        }
      }

      for (final Vertex vertex: startNodes)
      {
        // Save the vertex value
        nodesAtLevel.add(vertex.getValue());
        // Remove all out edges
        dropOutEdges(vertex, edges);
        // Remove the vertex itself
        vertices.remove(vertex);
      }

      Collections.sort(nodesAtLevel);
      sortedValues.addAll(nodesAtLevel);
    }

    return sortedValues;
  }

  private boolean containsCycle()
  {
    final SimpleCycleDetector cycleDetector = new SimpleCycleDetector<>(graph);
    return cycleDetector.containsCycle();
  }

  private void dropOutEdges(final Vertex vertex,
                            final Collection> edges)
  {
    for (final Iterator> iterator = edges.iterator(); iterator
      .hasNext();)
    {
      final DirectedEdge edge = iterator.next();
      if (edge.isFrom(vertex))
      {
        iterator.remove();
      }
    }
  }

  private boolean isStartNode(final Vertex vertex,
                              final Collection> edges)
  {
    for (final DirectedEdge edge: edges)
    {
      if (edge.isTo(vertex))
      {
        return false;
      }
    }
    return true;
  }

  private boolean isUnattachedNode(final Vertex vertex,
                                   final Collection> edges)
  {
    for (final DirectedEdge edge: edges)
    {
      if (edge.isTo(vertex) || edge.isFrom(vertex))
      {
        return false;
      }
    }
    return true;
  }

}