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// +-----------------------------------------------------------------------------+
//
/**
* 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
* @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.
*
* @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('acyclic-test-visited')) && $graphNodes[$key]->connectsTo($node)) $result++;
}
return $result;
}
/* }}} */
/* _isAcyclic {{{ */
/**
* Check if the graph is acyclic
*/
protected static function _isAcyclic(&$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 $result;
}
/* }}} */
/* isAcyclic {{{ */
/**
*
* isAcyclic returns true if a graph contains no cycles, false otherwise.
*
* @return boolean true iff graph is acyclic
*/
public static function isAcyclic(&$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);
}
/* }}} */
}
/* }}} */
?>