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Filename : TopologicalSorter.php
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<?php
/* vim: set expandtab tabstop=4 shiftwidth=4 foldmethod=marker: */
// +-----------------------------------------------------------------------------+
// | Copyright (c) 2003 S�rgio Gon�alves Carvalho                                |
// +-----------------------------------------------------------------------------+
// | This file is part of Structures_Graph.                                      |
// |                                                                             |
// | Structures_Graph 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.                                         |
// |                                                                             |
// | Structures_Graph 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 Structures_Graph; if not, write to the Free Software             |
// | Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA                    |
// | 02111-1307 USA                                                              |
// +-----------------------------------------------------------------------------+
// | Author: S�rgio Carvalho <sergio.carvalho@portugalmail.com>                  |
// +-----------------------------------------------------------------------------+
//
/**
 * This file contains the definition of the Structures_Graph_Manipulator_TopologicalSorter class.
 * 
 * @package Structures_Graph
 */

require_once 'PEAR.php';
require_once 'Structures/Graph.php';
require_once 'Structures/Graph/Node.php';
require_once 'Structures/Graph/Manipulator/AcyclicTest.php';

/**
 * The Structures_Graph_Manipulator_TopologicalSorter is a manipulator 
 * which is able to return the set of nodes in a graph, sorted by topological 
 * order.
 *
 * A graph may only be sorted topologically iff it's a DAG. You can test it
 * with the Structures_Graph_Manipulator_AcyclicTest.
 * 
 * @author    S�rgio Carvalho <sergio.carvalho@portugalmail.com> 
 * @copyright (c) 2004 by S�rgio Carvalho
 * @see       Structures_Graph_Manipulator_AcyclicTest
 * @package   Structures_Graph
 */
class Structures_Graph_Manipulator_TopologicalSorter
{
    /**
     * This is a variant of Structures_Graph::inDegree which does 
     * not count nodes marked as visited.
     *
     * @param object $node Node to check
     *
     * @return integer Number of non-visited nodes that link to this one
     */
    protected static function _nonVisitedInDegree(&$node)
    {
        $result = 0;
        $graphNodes =& $node->_graph->getNodes();
        foreach (array_keys($graphNodes) as $key) {
            if ((!$graphNodes[$key]->getMetadata('topological-sort-visited'))
                && $graphNodes[$key]->connectsTo($node)
            ) {
                $result++;
            }
        }
        return $result;
        
    }

    /**
     * Sort implementation
     *
     * @param object $graph Graph to sort
     *
     * @return void
     */
    protected static function _sort(&$graph)
    {
        // Mark every node as not visited
        $nodes =& $graph->getNodes();
        $nodeKeys = array_keys($nodes);
        $refGenerator = array();
        foreach ($nodeKeys as $key) {
            $refGenerator[] = false;
            $nodes[$key]->setMetadata(
                'topological-sort-visited',
                $refGenerator[sizeof($refGenerator) - 1]
            );
        }

        // Iteratively peel off leaf nodes
        $topologicalLevel = 0;
        do {
            // Find out which nodes are leafs (excluding visited nodes)
            $leafNodes = array();
            foreach ($nodeKeys as $key) {
                if ((!$nodes[$key]->getMetadata('topological-sort-visited'))
                    && static::_nonVisitedInDegree($nodes[$key]) == 0
                ) {
                    $leafNodes[] =& $nodes[$key];
                }
            }
            // Mark leafs as visited
            $refGenerator[] = $topologicalLevel;
            for ($i = sizeof($leafNodes) - 1; $i>=0; $i--) {
                $visited =& $leafNodes[$i]->getMetadata('topological-sort-visited');
                $visited = true;
                $leafNodes[$i]->setMetadata('topological-sort-visited', $visited);
                $leafNodes[$i]->setMetadata(
                    'topological-sort-level',
                    $refGenerator[sizeof($refGenerator) - 1]
                );
            }
            $topologicalLevel++;
        } while (sizeof($leafNodes) > 0);

        // Cleanup visited marks
        foreach ($nodeKeys as $key) {
            $nodes[$key]->unsetMetadata('topological-sort-visited');
        }
    }

    /**
     * Sort returns the graph's nodes, sorted by topological order. 
     * 
     * The result is an array with as many entries as topological levels.
     * Each entry in this array is an array of nodes within
     * the given topological level.
     *
     * @param object $graph Graph to sort
     *
     * @return array The graph's nodes, sorted by topological order.
     */
    public static function sort(&$graph)
    {
        // We only sort graphs
        if (!is_a($graph, 'Structures_Graph')) {
            return Pear::raiseError(
                'Structures_Graph_Manipulator_TopologicalSorter::sort received'
                . ' an object that is not a Structures_Graph',
                STRUCTURES_GRAPH_ERROR_GENERIC
            );
        }
        if (!Structures_Graph_Manipulator_AcyclicTest::isAcyclic($graph)) {
            return Pear::raiseError(
                'Structures_Graph_Manipulator_TopologicalSorter::sort'
                . ' received an graph that has cycles',
                STRUCTURES_GRAPH_ERROR_GENERIC
            );
        }

        Structures_Graph_Manipulator_TopologicalSorter::_sort($graph);
        $result = array();

        // Fill out result array
        $nodes =& $graph->getNodes();
        $nodeKeys = array_keys($nodes);
        foreach ($nodeKeys as $key) {
            if (!array_key_exists($nodes[$key]->getMetadata('topological-sort-level'), $result)) {
                $result[$nodes[$key]->getMetadata('topological-sort-level')]
                    = array();
            }
            $result[$nodes[$key]->getMetadata('topological-sort-level')][]
                =& $nodes[$key];
            $nodes[$key]->unsetMetadata('topological-sort-level');
        }

        return $result;
    }
}
?>