Zebra_Mptt.php

<?php

/**
 *  A PHP library that provides an implementation of the modified preorder tree traversal algorithm making it easy to
 *  implement the MPTT algorithm in your PHP applications.
 *
 *  Read more {@link https://github.com/stefangabos/Zebra_Mptt/ here}
 *
 *  @author     Stefan Gabos <contact@stefangabos.ro>
 *  @version    2.3.6 (last revision: January 07, 2019)
 *  @copyright  (c) 2009 - 2019 Stefan Gabos
 *  @license    http://www.gnu.org/licenses/lgpl-3.0.txt GNU LESSER GENERAL PUBLIC LICENSE
 *  @package    Zebra_Mptt
 */

class Zebra_Mptt {

    /**
     *  Constructor of the class.
     *
     *  <i>Make sure that before you instantiate the class you import or execute the SQL code found in the in the
     *  "install/mptt.sql" file, using the command line or your preferred MySQL manager.</i>
     *
     *  <code>
     *  // include the php file
     *  require 'path/to/Zebra_Mptt.php';
     *
     *  // instantiate the class
     *  $mptt = new Zebra_Mptt($db_link);
     *  </code>
     *
     *  @param  resource    &$link          An object representing the connection to a MySQL Server, as returned by
     *                                      {@link http://www.php.net/manual/en/mysqli.construct.php mysqli_connect}.
     *
     *                                      If you use {@link http://stefangabos.ro/php-libraries/zebra-database/ Zebra_Database}
     *                                      to connect to the database, you can get the connection to the MySQL server
     *                                      via Zebra_Database's {@link http://stefangabos.ro/wp-content/docs/Zebra_Database/Zebra_Database/Zebra_Database.html#methodget_link get_link}
     *                                      method.
     *
     *  @param  string      $table_name     (Optional) MySQL table name to be used for storing items.
     *
     *                                      Default is <i>mptt</i>
     *
     *  @param  string      $id_column      (Optional) Name of the column that uniquely identifies items in the table
     *
     *                                      Default is <i>id</i>
     *
     *  @param  string      $title_column   (Optional) Name of the column that stores item names
     *
     *                                      Default is <i>title</i>
     *
     *  @param  string      $left_column    (Optional) Name of the column that stores "left" values
     *
     *                                      Default is <i>lft</i> ("left" is a reserved word in MySQL)
     *
     *  @param  string      $right_column   (Optional) Name of the column that stores "right" values
     *
     *                                      Default is <i>rgt</i> ("right" is a reserved word in MySQL)
     *
     *  @param  string      $parent_column  (Optional) Name of the column that stores the IDs of parent items
     *
     *                                      Default is <i>parent</i>
     *
     *  @return void
     */
    public function __construct(&$link, $table_name = 'mptt', $id_column = 'id', $title_column = 'title', $left_column = 'lft', $right_column = 'rgt', $parent_column = 'parent') {

        // stop if required PHP version is not available
        if (version_compare(phpversion(), '5.0.0') < 0) trigger_error('PHP 5.0.0 or greater required', E_USER_ERROR);

        // stop if the mysqli extension is not loaded
        if (!extension_loaded('mysqli')) trigger_error('mysqli extension is required', E_USER_ERROR);

        // store the connection link
        $this->link = $link;

        // continue only if there is an active MySQL connection
        if (@mysqli_ping($this->link))

            // initialize properties
            $this->properties = array(

                'table_name'    =>  $table_name,
                'id_column'     =>  $id_column,
                'title_column'  =>  $title_column,
                'left_column'   =>  $left_column,
                'right_column'  =>  $right_column,
                'parent_column' =>  $parent_column,

            );

        // if no MySQL connections could be found
        // trigger a fatal error message and stop execution
        else trigger_error('no MySQL connection', E_USER_ERROR);

    }

    /**
     *  Adds a new node as the child of a given parent node.
     *
     *  <code>
     *  // add a new topmost node
     *  $node = $mptt->add(0, 'Main');
     *
     *  // add a child node
     *  $mptt->add($node, 'Child 1');
     *
     *  // add another child node
     *  $mptt->add($node, 'Child 2');
     *
     *  // insert a third child node
     *  // notice the "1" as the last argument, instructing the script to insert the child node
     *  // as the second child node, after "Child 1"
     *  // remember that the trees are 0-based, meaning that the first node in a tree has the index 0!
     *  $mptt->add($node, 'Child 3', 1);
     *
     *  // and finally, insert a fourth child node
     *  // notice the "0" as the last argument, instructing the script to insert the child node
     *  // as the very first child node of the parent node
     *  // remember that the trees are 0-based, meaning that the first node in a tree has the index 0!
     *  $mptt->add($node, 'Child 4', 0);
     *  </code>
     *
     *  @param  integer     $parent     The ID of the parent node.
     *
     *                                  Use "0" to add a topmost node.
     *
     *  @param  string      $title      The title of the node.
     *
     *  @param  integer     $position   (Optional) The position the node will have among the parent node's children nodes.
     *
     *                                  When parent node is given as "0", this refers to the position the node will have
     *                                  among the topmost nodes.
     *
     *                                  The values are 0-based, meaning that if you want the node to be inserted as
     *                                  the first child of the target node, you have to use "0", if you want it to
     *                                  be second, use "1", and so on.
     *
     *                                  If not given (or given as boolean FALSE), the node will be inserted as the last
     *                                  of the parent node's children nodes.
     *
     *  @return mixed                   Returns the ID of the newly inserted node or FALSE on error.
     */
    public function add($parent, $title, $position = false) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // make sure parent ID is an integer
        $parent = (int)$parent;

        // continue only if
        if (

            // we are adding a topmost node OR
            $parent == 0 ||

            // parent node exists in the lookup array
            isset($this->lookup[$parent])

        ) {

            // get parent's descendant nodes (no deeper than the first level)
            $descendants = $this->get_descendants($parent);

            // if node is to be inserted in the default position (as the last of the parent node's children)
            // give a numerical value to the position
            if ($position === false) $position = count($descendants);

            // if a custom position was specified
            else {

                // make sure that position is an integer value
                $position = (int)$position;

                // if position is a bogus number
                // use the default position (as the last of the parent node's children)
                if ($position > count($descendants) || $position < 0) $position = count($descendants);

            }

            // if parent has no descendants OR the node is to be inserted as the parent node's first child
            if (empty($descendants) || $position == 0)

                // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
                // the insert, and will need to be updated
                // if parent is not found (meaning that we're inserting a topmost node) set the boundary to 0
                $boundary = isset($this->lookup[$parent]) ? $this->lookup[$parent][$this->properties['left_column']] : 0;

            // if parent node has descendant nodes and/or the node needs to be inserted at a specific position
            else {

                // find the child node that currently exists at the position where the new node needs to be inserted to
                $slice = array_slice($descendants, $position - 1, 1);

                $descendants = array_shift($slice);

                // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
                // the insert, and will need to be updated
                $boundary = $descendants[$this->properties['right_column']];

            }

            // iterate through all the records in the lookup array
            foreach ($this->lookup as $id => $properties) {

                // if the node's "left" value is outside the boundary
                if ($properties[$this->properties['left_column']] > $boundary)

                    // increment it with 2
                    $this->lookup[$id][$this->properties['left_column']] += 2;

                // if the node's "right" value is outside the boundary
                if ($properties[$this->properties['right_column']] > $boundary)

                    // increment it with 2
                    $this->lookup[$id][$this->properties['right_column']] += 2;

            }

            // lock table to prevent other sessions from modifying the data and thus preserving data integrity
            mysqli_query($this->link, 'LOCK TABLE `' . $this->properties['table_name'] . '` WRITE') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // update the nodes in the database having their "left"/"right" values outside the boundary
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['left_column'] . '` = `' . $this->properties['left_column'] . '` + 2
                WHERE
                    `' . $this->properties['left_column'] . '` > ' . $boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['right_column'] . '` = `' . $this->properties['right_column'] . '` + 2
                WHERE
                    `' . $this->properties['right_column'] . '` > ' . $boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // insert the new node into the database
            mysqli_query($this->link, '
                INSERT INTO
                    `' . $this->properties['table_name'] . '`
                    (
                        `' . $this->properties['title_column'] . '`,
                        `' . $this->properties['left_column'] . '`,
                        `' . $this->properties['right_column'] . '`,
                        `' . $this->properties['parent_column'] . '`
                    )
                VALUES
                    (
                        "' . mysqli_real_escape_string($this->link, $title) . '",
                        ' . ($boundary + 1) . ',
                        ' . ($boundary + 2) . ',
                        ' . $parent . '
                    )
            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // get the ID of the newly inserted node
            $node_id = mysqli_insert_id($this->link);

            // release table lock
            mysqli_query($this->link, 'UNLOCK TABLES') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // add the node to the lookup array
            $this->lookup[$node_id] = array(
                $this->properties['id_column']      => $node_id,
                $this->properties['title_column']   => $title,
                $this->properties['left_column']    => $boundary + 1,
                $this->properties['right_column']   => $boundary + 2,
                $this->properties['parent_column']  => $parent,
            );

            // reorder the lookup array
            $this->_reorder_lookup_array();

            // return the ID of the newly inserted node
            return $node_id;

        }

        // if script gets this far, something must've went wrong so we return false
        return false;

    }

    /**
     *  Creates a copy of a node (including its descendant nodes) as the child node of a given node.
     *
     *  <code>
     *  // insert a topmost node
     *  $node = $mptt->add(0, 'Main');
     *
     *  // add a child node
     *  $child1 = $mptt->add($node, 'Child 1');
     *
     *  // add another child node
     *  $child2 = $mptt->add($node, 'Child 2');
     *
     *  // create a copy of "Child 2" node and put it as "Child 1"'s child
     *  $mptt->copy($child2, $child1);
     *  </code>
     *
     *  @param  integer     $source     The ID of a node to copy.
     *
     *                                  <i>Remember that the node will be copied together with all its descendant nodes!</i>
     *
     *  @param  integer     $target     The ID of a node which will become the copy's parent node.
     *
     *                                  Use "0" to make the copy a topmost node.
     *
     *  @param  integer     $position   (Optional) The position the node will have among the target node's children
     *                                  nodes.
     *
     *                                  When target node is "0", this refers to the position the node will have among
     *                                  the topmost nodes.
     *
     *                                  The values are 0-based, meaning that if you want the node to be inserted as
     *                                  the first child of the target node, you have to use "0", if you want it to
     *                                  be second, use "1", and so on.
     *
     *                                  If not given (or given as boolean FALSE), the node will be inserted as the last
     *                                  of the target node's children nodes.
     *
     *  @return mixed                   Returns the ID of the newly created copy, or FALSE on error.
     */
    public function copy($source, $target, $position = false) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // continue only if
        if (

            // source node exists in the lookup array AND
            isset($this->lookup[$source]) &&

            // target node exists in the lookup array OR is 0 (indicating a topmost node)
            (isset($this->lookup[$target]) || $target == 0)

        ) {

            // get the source's children nodes (if any)
            $source_children = $this->get_descendants($source, false);

            // this array will hold the items we need to copy
            // by default we add the source item to it
            $sources = array($this->lookup[$source]);

            // the copy's parent will be the target node
            $sources[0][$this->properties['parent_column']] = $target;

            // iterate through source node's children
            foreach ($source_children as $child)

                // save them for later use
                $sources[] = $this->lookup[$child[$this->properties['id_column']]];

            // the value with which items outside the boundary set below, are to be updated with
            $source_rl_difference =

                $this->lookup[$source][$this->properties['right_column']] -

                $this->lookup[$source][$this->properties['left_column']]

                + 1;

            // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
            // the insert, and will need to be updated
            $source_boundary = $this->lookup[$source][$this->properties['left_column']];

            // get target node's children (no deeper than the first level)
            $target_children = $this->get_descendants($target);

            // if copy is to be inserted in the default position (as the last of the target node's children)
            if ($position === false)

                // give a numerical value to the position
                $position = count($target_children);

            // if a custom position was specified
            else {

                // make sure given position is an integer value
                $position = (int)$position;

                // if position is a bogus number
                if ($position > count($target_children) || $position < 0)

                    // use the default position (the last of the target node's children)
                    $position = count($target_children);

            }

            // we are about to do an insert and some nodes need to be updated first

            // if target has no children nodes OR the copy is to be inserted as the target node's first child node
            if (empty($target_children) || $position == 0)

                // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
                // the insert, and will need to be updated
                // if parent is not found (meaning that we're inserting a topmost node) set the boundary to 0
                $target_boundary = isset($this->lookup[$target]) ? $this->lookup[$target][$this->properties['left_column']] : 0;

            // if target has children nodes and/or the copy needs to be inserted at a specific position
            else {

                // find the target's child node that currently exists at the position where the new node needs to be inserted to
                $slice = array_slice($target_children, $position - 1, 1);

                $target_children = array_shift($slice);

                // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
                // the insert, and will need to be updated
                $target_boundary = $target_children[$this->properties['right_column']];

            }

            // iterate through the nodes in the lookup array
            foreach ($this->lookup as $id => $properties) {

                // if the "left" value of node is outside the boundary
                if ($properties[$this->properties['left_column']] > $target_boundary)

                    // increment it
                    $this->lookup[$id][$this->properties['left_column']] += $source_rl_difference;

                // if the "right" value of node is outside the boundary
                if ($properties[$this->properties['right_column']] > $target_boundary)

                    // increment it
                    $this->lookup[$id][$this->properties['right_column']] += $source_rl_difference;

            }

            // lock table to prevent other sessions from modifying the data and thus preserving data integrity
            mysqli_query($this->link, 'LOCK TABLE `' . $this->properties['table_name'] . '` WRITE') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // update the nodes in the database having their "left"/"right" values outside the boundary
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['left_column'] . '` = `' . $this->properties['left_column'] . '` + ' . $source_rl_difference . '
                WHERE
                    `' . $this->properties['left_column'] . '` > ' . $target_boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['right_column'] . '` = `' . $this->properties['right_column'] . '` + ' . $source_rl_difference . '
                WHERE
                    `' . $this->properties['right_column'] . '` > ' . $target_boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // finally, the nodes that are to be inserted need to have their "left" and "right" values updated
            $shift = $target_boundary - $source_boundary + 1;

            // iterate through the nodes that are to be inserted
            foreach ($sources as $id => &$properties) {

                // update "left" value
                $properties[$this->properties['left_column']] += $shift;

                // update "right" value
                $properties[$this->properties['right_column']] += $shift;

                // insert into the database
                mysqli_query($this->link, '
                    INSERT INTO
                        `' . $this->properties['table_name'] . '`
                        (
                            `' . $this->properties['title_column'] . '`,
                            `' . $this->properties['left_column'] . '`,
                            `' . $this->properties['right_column'] . '`,
                            `' . $this->properties['parent_column'] . '`
                        )
                    VALUES
                        (
                            "' . mysqli_real_escape_string($this->link, $properties[$this->properties['title_column']]) . '",
                            ' . $properties[$this->properties['left_column']] . ',
                            ' . $properties[$this->properties['right_column']] . ',
                            ' . $properties[$this->properties['parent_column']] . '
                        )
                ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

                // get the ID of the newly inserted node
                $node_id = mysqli_insert_id($this->link);

                // because the node may have children nodes and its ID just changed
                // we need to find its children and update the reference to the parent ID
                foreach ($sources as $key => $value)

                    // if a child node was found
                    if ($value[$this->properties['parent_column']] == $properties[$this->properties['id_column']])

                        // update the reference to the parent ID
                        $sources[$key][$this->properties['parent_column']] = $node_id;

                // update the node's properties with the ID
                $properties[$this->properties['id_column']] = $node_id;

                // update the array of inserted items
                $sources[$id] = $properties;

            }

            // a reference of a $properties and the last array element remain even after the foreach loop
            // we have to destroy it
            unset($properties);

            // release table lock
            mysqli_query($this->link, 'UNLOCK TABLES') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // at this point, we have the nodes in the database but we need to also update the lookup array

            $parents = array();

            // iterate through the inserted nodes
            foreach ($sources as $id => $properties) {

                // if the node has any parents
                if (count($parents) > 0)

                    // iterate through the array of parent nodes
                    while ($parents[count($parents) - 1]['right'] < $properties[$this->properties['right_column']])

                        // and remove those which are not parents of the current node
                        array_pop($parents);

                // if there are any parents left
                if (count($parents) > 0)

                    // the last node in the $parents array is the current node's parent
                    $properties[$this->properties['parent_column']] = $parents[count($parents) - 1]['id'];

                // update the lookup array
                $this->lookup[$properties[$this->properties['id_column']]] = $properties;

                // add current node to the stack
                $parents[] = array(

                    'id'    =>  $properties[$this->properties['id_column']],
                    'right' =>  $properties[$this->properties['right_column']]

                );

            }

            // reorder the lookup array
            $this->_reorder_lookup_array();

            // return the ID of the copy
            return $sources[0][$this->properties['id_column']];

        }

        // if scripts gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Deletes a node, including the node's descendant nodes.
     *
     *  <code>
     *  // add a topmost node
     *  $node = $mptt->add(0, 'Main');
     *
     *  // add child node
     *  $child1 = $mptt->add($node, 'Child 1');
     *
     *  // add another child node
     *  $child2 = $mptt->add($node, 'Child 2');
     *
     *  // delete the "Child 2" node
     *  $mptt->delete($child2);
     *  </code>
     *
     *  @param  integer     $node       The ID of the node to delete.
     *
     *  @return boolean                 TRUE on success or FALSE on error.
     */
    public function delete($node) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // continue only if target node exists in the lookup array
        if (isset($this->lookup[$node])) {

            // get target node's descendant nodes (if any)
            $descendants = $this->get_descendants($node, false);

            // iterate through target node's descendant nodes
            foreach ($descendants as $descendant)

                // remove node from the lookup array
                unset($this->lookup[$descendant[$this->properties['id_column']]]);

            // lock table to prevent other sessions from modifying the data and thus preserving data integrity
            mysqli_query($this->link, 'LOCK TABLE `' . $this->properties['table_name'] . '` WRITE') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // also remove nodes from the database
            mysqli_query($this->link, '

                DELETE
                FROM
                    `' . $this->properties['table_name'] . '`
                WHERE
                    `' . $this->properties['left_column'] . '` >= ' . $this->lookup[$node][$this->properties['left_column']] . ' AND
                    `' . $this->properties['right_column'] . '` <= ' . $this->lookup[$node][$this->properties['right_column']] . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // the value with which items outside the boundary set below, are to be updated with
            $target_rl_difference =

                $this->lookup[$node][$this->properties['right_column']] -

                $this->lookup[$node][$this->properties['left_column']]

                + 1;

            // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
            // the insert, and will need to be updated
            $boundary = $this->lookup[$node][$this->properties['left_column']];

            // remove the target node from the lookup array
            unset($this->lookup[$node]);

            // iterate through nodes in the lookup array
            foreach ($this->lookup as $id => $properties) {

                // if the "left" value of node is outside the boundary
                if ($this->lookup[$id][$this->properties['left_column']] > $boundary)

                    // decrement it
                    $this->lookup[$id][$this->properties['left_column']] -= $target_rl_difference;

                // if the "right" value of node is outside the boundary
                if ($this->lookup[$id][$this->properties['right_column']] > $boundary)

                    // decrement it
                    $this->lookup[$id][$this->properties['right_column']] -= $target_rl_difference;

            }

            // update the nodes in the database having their "left"/"right" values outside the boundary
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['left_column'] . '` = `' . $this->properties['left_column'] . '` - ' . $target_rl_difference . '
                WHERE
                    `' . $this->properties['left_column'] . '` > ' . $boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['right_column'] . '` = `' . $this->properties['right_column'] . '` - ' . $target_rl_difference . '
                WHERE
                    `' . $this->properties['right_column'] . '` > ' . $boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // release table lock
            mysqli_query($this->link, 'UNLOCK TABLES') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // return true as everything went well
            return true;

        }

        // if script gets this far, something must've went wrong so we return false
        return false;

    }

    /**
     *  Returns an unidimensional (flat) array with the descendant nodes of a given parent node.
     *
     *  <i>For a multidimensional array use the {@link get_tree()} method.</i>
     *
     *  @param  integer     $node                       (Optional) The ID of a node for which to return the descendant nodes.
     *
     *                                                  When not specified or given as "0", the "root" node is implied.
     *
     *  @param  boolean     $direct_descendants_only    (Optional) Set this to FALSE if you want <b>all the descendants</b>
     *                                                  (including descendants of descendants), and not just the <b>direct
     *                                                  descendants</b> (children) of the node.
     *
     *                                                  Default is TRUE
     *
     *  @return array                                   Returns an unidimensional array with the descendant nodes of a
     *                                                  given parent node.
     */
    public function get_descendants($node = 0, $direct_descendants_only = true) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // if parent node exists in the lookup array OR we're looking for the topmost nodes
        if (isset($this->lookup[$node]) || $node === 0) {

            $descendants = array();

            // get the keys in the lookup array
            $keys = array_keys($this->lookup);

            // iterate through the available keys
            foreach ($keys as $item)

                // if
                if (

                    // node's "left" is higher than parent node's "left" (or, if parent is 0, if it is higher than 0)
                    $this->lookup[$item][$this->properties['left_column']] > ($node !== 0 ? $this->lookup[$node][$this->properties['left_column']] : 0) &&

                    // node's "left" is smaller than parent node's "right" (or, if parent is 0, if it is smaller than PHP's maximum integer value)
                    $this->lookup[$item][$this->properties['left_column']] < ($node !== 0 ? $this->lookup[$node][$this->properties['right_column']] : PHP_INT_MAX) &&

                    // if we only need the first level children, check if children node's parent node is the parent given as argument
                    (!$direct_descendants_only || $this->lookup[$item][$this->properties['parent_column']] == $node)

                // save to array
                ) $descendants[$this->lookup[$item][$this->properties['id_column']]] = $this->lookup[$item];

            // return children nodes
            return $descendants;

        }

        // if script gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Returns the number of descendant nodes of a given node.
     *
     *  @param  integer     $node                       The ID of the node for which to return the number of direct
     *                                                  descendant nodes.
     *
     *  @param  boolean     $direct_descendants_only    (Optional) Specifies whether to count <b>direct descendants only</b>,
     *                                                  or to count <b>all the descendants</b> (including descendants of
     *                                                  descendants)
     *
     *                                                  Default is TRUE
     *
     *  @return integer                                 Returns the number of direct descendant nodes of a parent node,
     *                                                  or FALSE on error.
     *
     *                                                  <i>Since this method may return both "0" and FALSE, make sure you
     *                                                  use === to verify the returned result!</i>
     */
    public function get_descendant_count($node, $direct_descendants_only = true) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // if parent node exists in the lookup array
        if (isset($this->lookup[$node]))

            // if we require all the descendants (not direct only)
            if (!$direct_descendants_only)

                // return the total number of descendant nodes
                return ($this->lookup[$node][$this->properties['right_column']] - $this->lookup[$node][$this->properties['left_column']] - 1) / 2;

            // if we require direct descendants only
            else {

                $result = 0;

                // iterate through all the records in the lookup array
                foreach ($this->lookup as $id => $properties)

                    // if node is a direct descendant of the parent node
                    if ($this->lookup[$id][$this->properties['parent_column']] == $node)

                        // increment the number of direct descendant nodes
                        $result++;

                // return the number of direct descendant nodes
                return $result;

            }

        // if script gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Returns the next sibling of a node.
     *
     *  @param  integer     $node           The ID of a node for which to return the next sibling node.
     *
     *  @return mixed                       Returns a node's next sibling node, "0" if a next sibling doesn't exist, or
     *                                      FALSE on error (if the node doesn't exist).
     *
     *                                      <i>Since this method may return both "0" and FALSE, make sure you use === to
     *                                      verify the returned result!</i>
     *
     *  @since  2.2.6
     */
    public function get_next_sibling($node) {

        // if node exists, get its siblings
        // (if $node exists this will never be an empty array as it will contain at least $node)
        if ($siblings = $this->get_siblings($node, true)) {

            // get the node's position among the siblings
            $node_position = array_search($node, array_keys($siblings));

            // get next node
            $sibling = array_slice($siblings, $node_position + 1, 1);

            // return result
            return !empty($sibling) ? array_pop($sibling) : 0;

        }

        // if script gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Returns an array containing a node's direct parent node if the node has a parent node, or "0" if the node is a
     *  topmost node.
     *
     *  @param  integer     $node               The ID of a node for which to return the parent node.
     *
     *  @return mixed                           Returns an array containing a node's direct parent node if the node has a
     *                                          parent node, or "0" if the node is a topmost node.
     *
     *                                          <i>Since this method may return both "0" and FALSE, make sure you use ===
     *                                          to verify the returned result!</>
     */
    public function get_parent($node) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // if node exists in the lookup array
        if (isset($this->lookup[$node]))

            // if node has a parent node, return the parent node's properties
            // also, return 0 if the node is a topmost node
            return isset($this->lookup[$this->lookup[$node][$this->properties['parent_column']]]) ? $this->lookup[$this->lookup[$node][$this->properties['parent_column']]] : 0;

        // if script gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Returns an unidimensional (flat) array with the path to the given node (including the node itself).
     *
     *  @param  integer     $node               The ID of a node for which to return the path.
     *
     *  @return array                           Returns an unidimensional array with the path to the given node.
     */
    public function get_path($node) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        $parents = array();

        // if node exists in the lookup array
        if (isset($this->lookup[$node]))

            // iterate through all the nodes in the lookup array
            foreach ($this->lookup as $id => $properties)

                // if
                if (

                    // node is a parent node
                    $properties[$this->properties['left_column']] < $this->lookup[$node][$this->properties['left_column']] &&

                    $properties[$this->properties['right_column']] > $this->lookup[$node][$this->properties['right_column']]

                // save the parent node's information
                ) $parents[$properties[$this->properties['id_column']]] = $properties;

        // add also the node given as argument
        $parents[$node] = $this->lookup[$node];

        // return the path to the node
        return $parents;

    }

    /**
     *  Returns the previous sibling of a node.
     *
     *  @param  integer     $node           The ID of a node for which to return the previous sibling node.
     *
     *  @return mixed                       Returns a node's previous sibling node, "0" if a previous sibling doesn't
     *                                      exist, or FALSE on error (if the node doesn't exist).
     *
     *                                      <i>Since this method may return both "0" and FALSE, make sure you use === to
     *                                      verify the returned result!</i>
     *
     *  @since  2.2.6
     */
    public function get_previous_sibling($node) {

        // if node exists, get its siblings
        // (if $node exists this will never be an empty array as it will contain at least $node)
        if ($siblings = $this->get_siblings($node, true)) {

            // get the node's position among the siblings
            $node_position = array_search($node, array_keys($siblings));

            // get previous node
            $sibling = $node_position > 0 ? array_slice($siblings, $node_position - 1, 1) : array();

            // return result
            return !empty($sibling) ? array_pop($sibling) : 0;

        }

        // if script gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Returns an array with a node's sibling nodes.
     *
     *  @param  integer     $node           The ID of a node for which to return the node's sibling nodes.
     *
     *  @param  boolean     $include_self   Whether the node given as argument should also be present in the returned
     *                                      array.
     *
     *  @return mixed                       Returns an array with a node's sibling nodes, an empty array if the node has
     *                                      no siblings, or FALSE on error (if the node doesn't exist)
     *
     *  @since  2.2.6
     */
    public function get_siblings($node, $include_self = false) {

        // if parent node exists in the lookup array OR we're looking for the topmost nodes
        if (isset($this->lookup[$node])) {

            // properties of the node
            $properties = $this->lookup[$node];

            // get node's siblings
            $siblings = $this->get_descendants($properties['parent']);

            // remove self, if required so
            if (!$include_self) unset($siblings[$node]);

            // return siblings
            return $siblings;

        }

        // if script gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Returns a multidimensional array with all the descendant nodes (including children nodes of children nodes of
     *  children nodes and so on) of a given node.
     *
     *  @param  integer     $node               (Optional) The ID of a node for which to return all descendant nodes, as
     *                                          a multidimensional array.
     *
     *                                          Not given or given as "0", will return all the nodes.
     *
     *  @return array                           Returns a multi dimensional array with all the descendant nodes (including
     *                                          children nodes of children nodes of children nodes and so on) of a given
     *                                          node.
     */
    public function get_tree($node = 0) {

        // get direct children nodes
        $descendants = $this->get_descendants($node);

        // iterate through the direct children nodes
        foreach ($descendants as $id => $properties)

            // for each child node create a "children" property
            // and get the node's children nodes, recursively
            $descendants[$id]['children'] = $this->get_tree($id);

        // return the array
        return $descendants;

    }

    /**
     *  Moves a node, including the node's descendants nodes, into another node (becoming that node's child), or
     *  after/before a node (becoming that node's sibling)
     *
     *  <code>
     *  // insert a topmost node
     *  $node = $mptt->add(0, 'Main');
     *
     *  // add a child node
     *  $child1 = $mptt->add($node, 'Child 1');
     *
     *  // add another child node
     *  $child2 = $mptt->add($node, 'Child 2');
     *
     *  // add another child node
     *  $child3 = $mptt->add($node, 'Child 3');
     *
     *  // move "Child 2" node to be the first of "Main"'s children nodes
     *  $mptt->move($child2, $node, 0);
     *
     *  // move "Child 2" node into "Child 1"
     *  $mptt->move($child2, $child1);
     *
     *  // move "Child 1" after "Child 3"
     *  $mptt->move($child1, $child3, 'after');
     *  </code>
     *
     *  @param  integer     $source     The ID of a node to move
     *
     *  @param  integer     $target     The ID of the node relative to which the source node needs to be moved. Use
     *                                  "0" if the node does not need a parent node (making it a topmost node).
     *
     *  @param  integer     $position   (Optional) The position where to move the node, relative to the target node.
     *
     *                                  Can be a numerical value, indicating that the source node needs to be moved to
     *                                  become a <b>child of the target node</b>, inserted at the indicated position (
     *                                  the values are 0-based, meaning that if you want the node to be inserted as
     *                                  the first child of the target node, you have to use "0", if you want it to
     *                                  be second, use "1", and so on)
     *
     *                                  Can also be the literal "after" or "before" string, indicating the the source
     *                                  node needs to be moved <b>after/before the target node</b>.
     *
     *                                  If not given (or given as boolean FALSE), the node will be inserted as the last
     *                                  of the target node's children nodes.
     *
     *  @return boolean                 TRUE on success or FALSE on error
     */
    public function move($source, $target, $position = false) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // continue only if
        if (

            // source node exists in the lookup array AND
            isset($this->lookup[$source]) &&

            // target node exists in the lookup array OR is 0 (indicating a topmost node)
            (isset($this->lookup[$target]) || $target == 0) &&

            // target node is not a child node of the source node (that would cause infinite loop)
            !in_array($target, array_keys($this->get_descendants($source, false)))

        ) {

            // if we have to move the node after/before another node
            if ($position === 'after' || $position === 'before') {

                // get the target's parent node
                $target_parent = $target == 0 ? 0 : $this->lookup[$target]['parent'];

                // get the target's parent's descendant nodes
                $descendants = $this->get_descendants($target_parent);

                // get the target's position among the descendants
                $keys = array_keys($descendants);
                $target_position = array_search($target, $keys);

                // move the source node to the desired position
                if ($position == 'after') return $this->move($source, $target_parent, $target_position + 1);
                else return $this->move($source, $target_parent, $target_position == 0 ? 0 : $target_position - 1);

            }

            // the source's parent node's ID becomes the target node's ID
            $this->lookup[$source][$this->properties['parent_column']] = $target;

            // get source node's descendant nodes (if any)
            $source_descendants = $this->get_descendants($source, false);

            // this array will hold the nodes we need to move
            // by default we add the source node to it
            $sources = array($this->lookup[$source]);

            // iterate through source node's descendants
            foreach ($source_descendants as $descendant) {

                // save them for later use
                $sources[] = $this->lookup[$descendant[$this->properties['id_column']]];

                // for now, remove them from the lookup array
                unset($this->lookup[$descendant[$this->properties['id_column']]]);

            }

            // the value with which nodes outside the boundary set below, are to be updated with
            $source_rl_difference =

                $this->lookup[$source][$this->properties['right_column']] -

                $this->lookup[$source][$this->properties['left_column']]

                + 1;

            // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
            // the insert, and will need to be updated
            $source_boundary = $this->lookup[$source][$this->properties['left_column']];

            // lock table to prevent other sessions from modifying the data and thus preserving data integrity
            mysqli_query($this->link, 'LOCK TABLE `' . $this->properties['table_name'] . '` WRITE') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // we'll multiply the "left" and "right" values of the nodes we're about to move with "-1", in order to
            // prevent the values being changed further in the script
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['left_column'] . '` = `' . $this->properties['left_column'] . '` * -1,
                    `' . $this->properties['right_column'] . '` = `' . $this->properties['right_column'] . '` * -1
                WHERE
                    `' . $this->properties['left_column'] . '` >= ' . $this->lookup[$source][$this->properties['left_column']] . ' AND
                    `' . $this->properties['right_column'] . '` <= ' . $this->lookup[$source][$this->properties['right_column']] . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // remove the source node from the list
            unset($this->lookup[$source]);

            // iterate through the remaining nodes in the lookup array
            foreach ($this->lookup as $id=>$properties) {

                // if the "left" value of node is outside the boundary
                if ($this->lookup[$id][$this->properties['left_column']] > $source_boundary)

                    // decrement it
                    $this->lookup[$id][$this->properties['left_column']] -= $source_rl_difference;

                // if the "right" value of item is outside the boundary
                if ($this->lookup[$id][$this->properties['right_column']] > $source_boundary)

                    // decrement it
                    $this->lookup[$id][$this->properties['right_column']] -= $source_rl_difference;

            }

            // update the nodes in the database having their "left"/"right" values outside the boundary
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['left_column'] . '` = `' . $this->properties['left_column'] . '` - ' . $source_rl_difference . '
                WHERE
                    `' . $this->properties['left_column'] . '` > ' . $source_boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['right_column'] . '` = `' . $this->properties['right_column'] . '` - ' . $source_rl_difference . '
                WHERE
                    `' . $this->properties['right_column'] . '` > ' . $source_boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // get descendant nodes of target node (first level only)
            $target_descendants = $this->get_descendants((int)$target);

            // if node is to be inserted in the default position (as the last of target node's children nodes)
            // give a numerical value to the position
            if ($position === false) $position = count($target_descendants);

            // if a custom position was specified
            else {

                // make sure given position is an integer value
                $position = (int)$position;

                // if position is a bogus number
                if ($position > count($target_descendants) || $position < 0)

                    // use the default position (as the last of the target node's children)
                    $position = count($target_descendants);

            }

            // because of the insert, some nodes need to have their "left" and/or "right" values adjusted

            // if target node has no descendant nodes OR the node is to be inserted as the target node's first child node
            if (empty($target_descendants) || $position == 0)

                // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
                // the insert, and will need to be updated
                // if parent is not found (meaning that we're inserting a topmost node) set the boundary to 0
                $target_boundary = isset($this->lookup[$target]) ? $this->lookup[$target][$this->properties['left_column']] : 0;

            // if target has any descendant nodes and/or the node needs to be inserted at a specific position
            else {

                // find the target's child node that currently exists at the position where the new node needs to be inserted to
                $slice = array_slice($target_descendants, $position - 1, 1);

                $target_descendants = array_shift($slice);

                // set the boundary - nodes having their "left"/"right" values outside this boundary will be affected by
                // the insert, and will need to be updated
                $target_boundary = $target_descendants[$this->properties['right_column']];

            }

            // iterate through the records in the lookup array
            foreach ($this->lookup as $id => $properties) {

                // if the "left" value of node is outside the boundary
                if ($properties[$this->properties['left_column']] > $target_boundary)

                    // increment it
                    $this->lookup[$id][$this->properties['left_column']] += $source_rl_difference;

                // if the "left" value of node is outside the boundary
                if ($properties[$this->properties['right_column']] > $target_boundary)

                    // increment it
                    $this->lookup[$id][$this->properties['right_column']] += $source_rl_difference;

            }

            // update the nodes in the database having their "left"/"right" values outside the boundary
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['left_column'] . '` = `' . $this->properties['left_column'] . '` + ' . $source_rl_difference . '
                WHERE
                    `' . $this->properties['left_column'] . '` > ' . $target_boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['right_column'] . '` = `' . $this->properties['right_column'] . '` + ' . $source_rl_difference . '
                WHERE
                    `' . $this->properties['right_column'] . '` > ' . $target_boundary . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // finally, the nodes that are to be inserted need to have their "left" and "right" values updated
            $shift = $target_boundary - $source_boundary + 1;

            // iterate through the nodes to be inserted
            foreach ($sources as $properties) {

                // update "left" value
                $properties[$this->properties['left_column']] += $shift;

                // update "right" value
                $properties[$this->properties['right_column']] += $shift;

                // add the item to our lookup array
                $this->lookup[$properties[$this->properties['id_column']]] = $properties;

            }

            // also update the entries in the database
            // (notice that we're subtracting rather than adding and that finally we multiply by -1 so that the values
            // turn positive again)
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['left_column'] . '` = (`' . $this->properties['left_column'] . '` - ' . $shift . ') * -1,
                    `' . $this->properties['right_column'] . '` = (`' . $this->properties['right_column'] . '` - ' . $shift . ') * -1
                WHERE
                    `' . $this->properties['left_column'] . '` < 0

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // finally, update the parent of the source node
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['parent_column'] . '` = ' . $target . '
                WHERE
                    `' . $this->properties['id_column'] . '` = ' . $source . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // release table lock
            mysqli_query($this->link, 'UNLOCK TABLES') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // reorder the lookup array
            $this->_reorder_lookup_array();

            // return true as everything went well
            return true;

        }

        // if scripts gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Updates a node's title.
     *
     *  <code>
     *  // add a topmost node
     *  $node = $mptt->add(0, 'Main');
     *
     *  // change the node's title
     *  $mptt->update($node, 'Primary');
     *  </code>
     *
     *  @param  integer     $node       The ID of a node to update the title for.
     *
     *  @param  string      $title      The new title to be set for the node.
     *
     *  @return boolean                 TRUE on success or FALSE on error.
     *
     *  @since  2.2.5
     */
    public function update($node, $title) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // continue only if target node exists in the lookup array
        if (isset($this->lookup[$node])) {

            // lock table to prevent other sessions from modifying the data and thus preserving data integrity
            mysqli_query($this->link, 'LOCK TABLE `' . $this->properties['table_name'] . '` WRITE') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // update node's title
            mysqli_query($this->link, '

                UPDATE
                    `' . $this->properties['table_name'] . '`
                SET
                    `' . $this->properties['title_column'] . '` = "' . $title . '"
                WHERE
                    `' . $this->properties['id_column'] . '` = ' . $node . '

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // release table lock
            mysqli_query($this->link, 'UNLOCK TABLES') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            // update lookup array
            $this->lookup[$node][$this->properties['title_column']] = $title;

            // return true as everything went well
            return true;

        }

        // if scripts gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Returns an unidimensional (flat) array with the descendant nodes of the node given as argument, indented using
     *  whatever given in the <i>$separator</i> argument, and ready to be used in a <select> element.
     *
     *  <code>
     *  $selectables = $mptt->to_select($node_id);
     *
     *  echo '<select name="myselect">';
     *
     *  foreach ($selectables as $value => $caption)
     *
     *      echo '<option value="' . $value . '">' . $caption . '</option>';
     *
     *  echo '</select>';
     *  </code>
     *
     *  @param  integer     $node               (Optional) The ID of a node for which to get the descendant nodes and
     *                                          return everything as a unidimensional (flat) array, indented using whatever
     *                                          given in the <i>$separator</i> argument, and ready to be used in a <select>
     *                                          control.
     *
     *                                          When not given, or given as "0", will return an array with *all* the
     *                                          available nodes.
     *
     *  @param  string      $separator          (Optional) A string used to separate nodes.
     *
     *                                          Default is " &rarr; "
     *
     *  @param  boolean     $show_full_path     (Optional) By default, parent nodes are not shown.
     *
     *                                          Set this to TRUE to show full path to each node.
     *
     *                                          Default is FALSE.
     *
     *                                          This option was added in <b>2.3.6</b>
     *
     *  @return array                           Returns an array of children nodes of a node given as argument, indented
     *                                          and ready to be used in a <select> control.
     */
    public function to_select($node = 0, $separator = ' &rarr; ', $show_full_path = false) {

        // lazy connection: touch the database only when the data is required for the first time and not at object instantiation
        $this->_init();

        // continue only if parent node exists in the lookup array OR is 0 (indicating topmost node)
        if (isset($this->lookup[$node]) || $node == 0) {

            // the resulting array and a temporary array
            $result = $parents = array();

            // get node's descendant nodes
            $descendants = $this->get_descendants($node, false);

            // if node is not 0, prepend the item itself to the list
            if ($node != 0) array_unshift($descendants, $this->lookup[$node]);

            // iterate through the nodes
            foreach ($descendants as $properties) {

                // if we find a topmost node
                if ($properties[$this->properties['parent_column']] == 0) {

                    // if the $nodes variable is set, save the what we have so far
                    if (isset($nodes)) $result += $nodes;

                    // reset the categories and parents arrays
                    $nodes = $parents = array();

                }

                // if the node has any parents
                if (count($parents) > 0) {

                    $keys = array_keys($parents);

                    // iterate through the array of parent nodes
                    while (array_pop($keys) < $properties[$this->properties['right_column']])

                        // and remove parents that are not parents of current node
                        array_pop($parents);

                }

                // add node to the stack of nodes
                $nodes[$properties[$this->properties['id_column']]] = (!empty($parents) ? ($show_full_path ? implode($separator, $parents) . $separator : str_repeat($separator, count($parents))) : '') . $properties[$this->properties['title_column']];

                // add node to the stack of parents
                $parents[$properties[$this->properties['right_column']]] = $properties[$this->properties['title_column']];

            }

            // may not be set when there are no nodes at all
            // finalize the result
            if (isset($nodes)) $result += $nodes;

            // return the resulting array
            return $result;

        }

        // if the script gets this far, return false as something must've went wrong
        return false;

    }

    /**
     *  Transforms a node and it's subnodes to an ordered/unordered list.
     *
     *  The list items will have the class attribute set to "zebra_mptt_item zebra_mptt_item_xx" where "xx" is the ID
     *  of the respective node.
     *
     *  <i>You can further customize the output with regular expressions to suit your needs</i>
     *
     *  <code>
     *  // instantiate the class
     *  $mptt = new Zebra_Mptt();
     *
     *  // make a list out of all nodes as an ordered list and with the
     *  // main list having the class "mylist"
     *  echo $mptt->to_list(0, 'ol', 'class="mylist"');
     *  </code>
     *
     *  @param  integer     $node           The ID of a node.
     *
     *                                      When given as "0", the "root" node is implied.
     *
     *  @param  string      $list_type      (Optional) Can be either "ul" (for an unordered list) or "ol" (for an ordered
     *                                      list).
     *
     *                                      Default is "ul".
     *
     *  @param  string      $attributes     Additional HTML attributes to set for the main list, like "class" or "style".
     *
     *  @return string
     *
     *  @since  2.2.3
     */
    public function to_list($node, $list_type = 'ul', $attributes = '') {

        // if node is an ID, get the descendant nodes
        //  (when called recursively this is an array)
        if (!is_array($node)) $node = $this->get_tree($node);

        // if there are any elements
        if (!empty($node)) {

            // start generating the output
            $out = '<' . $list_type . ($attributes != '' ? ' ' . $attributes : '') . '>';

            // iterate through each node
            foreach ($node as $elem)

                // generate output and if the node has children nodes, call this method recursively
                $out .= '<li class="zebra_mptt_item zebra_mptt_item_' . $elem[$this->properties['id_column']] . '">' .
                    $elem[$this->properties['title_column']] . (is_array($elem['children']) ? $this->to_list($elem['children'], $list_type) : '') .
                '</li>';

            // return generated output
            return $out . '</' . $list_type . '>';

        }

    }

    /**
     *  Reads the data from the MySQL table and creates a lookup array. Searches will be done in the lookup array
     *  rather than always querying the database.
     *
     *  @return void
     *
     *  @access private
     */
    private function _init() {

        // if the results are not already cached
        if (!isset($this->lookup)) {

            // fetch data from the database
            $result = mysqli_query($this->link, '

                SELECT
                    *
                FROM
                    `' . $this->properties['table_name'] . '`
                ORDER BY
                    `' . $this->properties['left_column'] . '`

            ') or trigger_error(mysqli_error($this->link), E_USER_ERROR);

            $this->lookup = array();

            // iterate through the found records
            while ($row = mysqli_fetch_assoc($result))

                // put all records in an array; use the ID column as index
                $this->lookup[$row[$this->properties['id_column']]] = $row;

        }

    }

    /**
     *  Updates the lookup array after inserts and deletes.
     *
     *  @return void
     *
     *  @access private
     */
    private function _reorder_lookup_array() {

        // reorder the lookup array

        // iterate through the nodes in the lookup array
        foreach ($this->lookup as $properties)

            // create a new array with the name of "left" column, having the values from the "left" column
            ${$this->properties['left_column']}[] = $properties[$this->properties['left_column']];

        // order the array by the left column
        // in the ordering process, the keys are lost
        array_multisort(${$this->properties['left_column']}, SORT_ASC, $this->lookup);

        $tmp = array();

        // iterate through the existing nodes
        foreach ($this->lookup as $properties)

            // and save them to a different array, this time with the correct ID
            $tmp[$properties[$this->properties['id_column']]] = $properties;

        // the updated lookup array
        $this->lookup = $tmp;

        // free memory
        unset($tmp);

    }

}