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<?php

/* vim: set expandtab tabstop=4 shiftwidth=4 foldmethod=marker: */

// +-----------------------------------------------------------------------------+

// | Copyright (c) 2003 Sérgio Gonçalves 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: Sérgio Carvalho <sergio.carvalho@portugalmail.com>                  |

// +-----------------------------------------------------------------------------+

//

/**

 * This file contains the definition of the Structures_Graph_Manipulator_AcyclicTest graph manipulator.

 *

 * @see Structures_Graph_Manipulator_AcyclicTest

 * @package Structures_Graph

 */

/* dependencies {{{ */

/** */

require_once 'PEAR.php';

/** */

require_once 'Structures/Graph.php';

/** */

require_once 'Structures/Graph/Node.php';

/* }}} */

/* class Structures_Graph_Manipulator_AcyclicTest {{{ */

/**

 * The Structures_Graph_Manipulator_AcyclicTest is a graph manipulator

 * which tests whether a graph contains a cycle.

 *

 * The definition of an acyclic graph used in this manipulator is that of a

 * DAG. The graph must be directed, or else it is considered cyclic, even when

 * there are no arcs.

 *

 * @author		Sérgio Carvalho <sergio.carvalho@portugalmail.com>

 * @copyright	(c) 2004 by Sérgio Carvalho

 * @package Structures_Graph

 */

class Structures_Graph_Manipulator_AcyclicTest {

    /* _nonVisitedInDegree {{{ */

    /**

    *

    * This is a variant of Structures_Graph::inDegree which does

    * not count nodes marked as visited.

    *

    * @access   private

    * @return	integer	 Number of non-visited nodes that link to this one

    */

    function _nonVisitedInDegree(&$node) {

        $result = 0;

        $graphNodes =& $node->_graph->getNodes();

        foreach (array_keys($graphNodes) as $key) {

            if ((!$graphNodes[$key]->getMetadata('acyclic-test-visited')) && $graphNodes[$key]->connectsTo($node)) $result++;

        }

        return $result;

    }

    /* }}} */

    /* _isAcyclic {{{ */

    /**

    * @access   private

    */

    function _isAcyclic(&$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('acyclic-test-visited', $refGenerator[sizeof($refGenerator) - 1]);

        }

        // Iteratively peel off leaf nodes

        do {

            // Find out which nodes are leafs (excluding visited nodes)

            $leafNodes = array();

            foreach($nodeKeys as $key) {

                if ((!$nodes[$key]->getMetadata('acyclic-test-visited')) && Structures_Graph_Manipulator_AcyclicTest::_nonVisitedInDegree($nodes[$key]) == 0) {

                    $leafNodes[] =& $nodes[$key];

                }

            }

            // Mark leafs as visited

            for ($i=sizeof($leafNodes) - 1; $i>=0; $i--) {

                $visited =& $leafNodes[$i]->getMetadata('acyclic-test-visited');

                $visited = true;

                $leafNodes[$i]->setMetadata('acyclic-test-visited', $visited);

            }

        } while (sizeof($leafNodes) > 0);

        // If graph is a DAG, there should be no non-visited nodes. Let's try to prove otherwise

        $result = true;

        foreach($nodeKeys as $key) if (!$nodes[$key]->getMetadata('acyclic-test-visited')) $result = false;

        // Cleanup visited marks

        foreach($nodeKeys as $key) $nodes[$key]->unsetMetadata('acyclic-test-visited');

        return $result;

    }

    /* }}} */

    /* isAcyclic {{{ */

    /**

    *

    * isAcyclic returns true if a graph contains no cycles, false otherwise.

    *

    * @return	boolean	 true iff graph is acyclic

    * @access	public

    */

    function isAcyclic(&$graph) {

        // We only test graphs

        if (!is_a($graph, 'Structures_Graph')) return Pear::raiseError('Structures_Graph_Manipulator_AcyclicTest::isAcyclic received an object that is not a Structures_Graph', STRUCTURES_GRAPH_ERROR_GENERIC);

        if (!$graph->isDirected()) return false; // Only directed graphs may be acyclic

        return Structures_Graph_Manipulator_AcyclicTest::_isAcyclic($graph);

    }

    /* }}} */

}

/* }}} */

?>