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License: GNU General Public License v3.0
PHP implementation of Tarjan's strongly connected components algorithm
License: GNU General Public License v3.0
Hello. Nice one :) I used your algorithm and converted it into a class and it seems to fork well. I would like to suggest to put your code into a class which would be more handy for real life usage I think. See my example below (I am not sure why $G is modified so hopefully my class is working correctly)
Update: I also modified how the $G array is used - You expect indices to be continous 0-N while my situation was different. My indices are random IDs from DB. This is the only difference I forgot to mention.
Update2: I also converted internal variable $maxlooplength and hardcoded string '|' into private attributes with setters and getters so they can be configured. See example below.
PS2: I would also like to ask for enhancement of linked article - I was missing graphical representation of the graph whose "Adjacency Matrix" is used.
https://www.vacilando.org/article/php-implementation-tarjans-cycle-detection-algorithm
Thank you for your contribution.
// Class usage:
// $tarjanLoopDetector = new TarjanLoopDetector($adjacencyMatrix);
// $tarjanLoopDetector->setLoopVertexSeparator('-'); // unnecessary
// $tarjanLoopDetector->setMaxLoopLength(3); // unnecessary
// $loops = $tarjanLoopDetector->php_tarjan_entry();
// $result = [];
// foreach ($loops as $loop) {
// $result []= explode($tarjanLoopDetector->getLoopVertexSeparator(), $loop);
// }
<?php
/**
* Code origin:
* https://www.vacilando.org/article/php-implementation-tarjans-cycle-detection-algorithm
* https://github.com/Vacilando/php-tarjan
*
* Adjacency Matrix:
* https://www.geeksforgeeks.org/graph-and-its-representations/
*/
class TarjanLoopDetector
{
private $cycles = [];
private $marked = [];
private $marked_stack = [];
private $point_stack = [];
private $G = [];
private $loopVertexSeparator = '|';
private $maxLoopLength = null; // For example 3
public function __construct($adjacencyMatrix)
{
$this->G = $adjacencyMatrix;
}
public function setLoopVertexSeparator($loopVertexSeparator)
{
$this->loopVertexSeparator = $loopVertexSeparator;
}
public function getLoopVertexSeparator()
{
return $this->loopVertexSeparator;
}
/**
* Set a value to Limit the length of loops to keep in the results
* @param $maxLoopLength
* @return void
*/
public function setMaxLoopLength($maxLoopLength)
{
$this->maxLoopLength = (int)$maxLoopLength;
}
public function getMaxLoopLength()
{
return $this->maxLoopLength;
}
public function php_tarjan_entry()
{
foreach ($this->G as $x => $data) {
$this->marked[$x] = FALSE;
}
foreach ($this->G as $i => $data) {
$this->php_tarjan($i, $i);
while (!empty($this->marked_stack)) {
$this->marked[array_pop($this->marked_stack)] = FALSE;
}
//echo '<br>'.($i+1).' / '.count($this->G); // Enable if you wish to follow progression through the array rows.
}
$this->cycles = array_keys($this->cycles);
return $this->cycles;
}
/*
* Recursive function to detect strongly connected components (cycles, loops).
*/
private function php_tarjan($s, $v)
{
$f = FALSE;
$this->point_stack[] = $v;
$this->marked[$v] = TRUE;
$this->marked_stack[] = $v;
foreach ($this->G[$v] as $w) {
if ($w < $s) {
$this->G[$w] = array(); // Users note: Why is this array modified?
} else if ($w == $s) {
if ($this->maxLoopLength == null || count($this->point_stack) == $this->maxLoopLength) {
// Collect cycles of a given length only.
// Add new cycles as array keys to avoid duplication. Way faster than using array_search.
$this->cycles[implode($this->loopVertexSeparator, $this->point_stack)] = TRUE;
}
$f = TRUE;
} else if ($this->marked[$w] === FALSE) {
if ($this->maxLoopLength == null || count($this->point_stack) < $this->maxLoopLength) {
// Collect cycles up to $this->maxLoopLength.
$g = $this->php_tarjan($s, $w);
}
if (!empty($f) or !empty($g)) {
$f = TRUE;
}
}
}
if ($f === TRUE) {
while (end($this->marked_stack) != $v) {
$this->marked[array_pop($this->marked_stack)] = FALSE;
}
array_pop($this->marked_stack);
$this->marked[$v] = FALSE;
}
array_pop($this->point_stack);
return $f;
}
}
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