<?php
/**
 * Elgg relationships.
 * Stub containing relationship functions, making import and export easier.
 *
 * @package Elgg
 * @subpackage Core
 * @author Curverider Ltd <info@elgg.com>
 * @link http://elgg.org/
 */

/**
 * Relationship class.
 *
 * @author Curverider Ltd
 * @package Elgg
 * @subpackage Core
 */
class ElggRelationship implements
	Importable,
	Exportable,
	Loggable,	// Can events related to this object class be logged
	Iterator,	// Override foreach behaviour
	ArrayAccess // Override for array access
	{
	/**
	 * This contains the site's main properties (id, etc)
	 * @var array
	 */
	protected $attributes;

	/**
	 * Construct a new site object, optionally from a given id value or row.
	 *
	 * @param mixed $id
	 */
	function __construct($id = null) {
		$this->attributes = array();

		if (!empty($id)) {
			if ($id instanceof stdClass) {
				$relationship = $id; // Create from db row
			} else {
				$relationship = get_relationship($id);
			}

			if ($relationship) {
				$objarray = (array) $relationship;
				foreach($objarray as $key => $value) {
					$this->attributes[$key] = $value;
				}
			}
		}
	}

	/**
	 * Class member get overloading
	 *
	 * @param string $name
	 * @return mixed
	 */
	function __get($name) {
		if (isset($this->attributes[$name])) {
			return $this->attributes[$name];
		}

		return null;
	}

	/**
	 * Class member set overloading
	 *
	 * @param string $name
	 * @param mixed $value
	 * @return mixed
	 */
	function __set($name, $value) {
		$this->attributes[$name] = $value;
		return true;
	}

	/**
	 * Save the relationship
	 *
	 * @return int the relationship id
	 */
	public function save() {
		if ($this->id > 0) {
			delete_relationship($this->id);
		}

		$this->id = add_entity_relationship($this->guid_one, $this->relationship, $this->guid_two);
		if (!$this->id) {
			throw new IOException(sprintf(elgg_new('IOException:UnableToSaveNew'), get_class()));
		}

		return $this->id;
	}

	/**
	 * Delete a given relationship.
	 */
	public function delete() {
		return delete_relationship($this->id);
	}

	/**
	 * Get a URL for this relationship.
	 *
	 * @return string
	 */
	public function getURL() {
		return get_relationship_url($this->id);
	}

	// EXPORTABLE INTERFACE ////////////////////////////////////////////////////////////

	/**
	 * Return an array of fields which can be exported.
	 */
	public function getExportableValues() {
		return array(
			'id',
			'guid_one',
			'relationship',
			'guid_two'
		);
	}

	/**
	 * Export this relationship
	 *
	 * @return array
	 */
	public function export() {
		$uuid = get_uuid_from_object($this);
		$relationship = new ODDRelationship(
			guid_to_uuid($this->guid_one),
			$this->relationship,
			guid_to_uuid($this->guid_two)
		);

		$relationship->setAttribute('uuid', $uuid);

		return $relationship;
	}

	// IMPORTABLE INTERFACE ////////////////////////////////////////////////////////////

	/**
	 * Import a relationship
	 *
	 * @param array $data
	 * @param int $version
	 * @return ElggRelationship
	 * @throws ImportException
	 */
	public function import(ODD $data) {
		if (!($element instanceof ODDRelationship)) {
			throw new InvalidParameterException(elgg_echo('InvalidParameterException:UnexpectedODDClass'));
		}

		$uuid_one = $data->getAttribute('uuid1');
		$uuid_two = $data->getAttribute('uuid2');

		// See if this entity has already been imported, if so then we need to link to it
		$entity1 = get_entity_from_uuid($uuid_one);
		$entity2 = get_entity_from_uuid($uuid_two);
		if (($entity1) && ($entity2)) {
			// Set the item ID
			$this->attributes['guid_one'] = $entity1->getGUID();
			$this->attributes['guid_two'] = $entity2->getGUID();

			// Map verb to relationship
			//$verb = $data->getAttribute('verb');
			//$relationship = get_relationship_from_verb($verb);
			$relationship = $data->getAttribute('type');

			if ($relationship) {
				$this->attributes['relationship'] = $relationship;
				// save
				$result = $this->save();
				if (!$result) {
					throw new ImportException(sprintf(elgg_echo('ImportException:ProblemSaving'), get_class()));
				}

				return $this;
			}
		}
	}

	// SYSTEM LOG INTERFACE ////////////////////////////////////////////////////////////

	/**
	 * Return an identification for the object for storage in the system log.
	 * This id must be an integer.
	 *
	 * @return int
	 */
	public function getSystemLogID() {
		return $this->id;
	}

	/**
	 * Return the class name of the object.
	 */
	public function getClassName() {
		return get_class($this);
	}

	/**
	 * For a given ID, return the object associated with it.
	 * This is used by the river functionality primarily.
	 * This is useful for checking access permissions etc on objects.
	 */
	public function getObjectFromID($id) {
		return get_relationship($id);
	}

	/**
	 * Return the GUID of the owner of this object.
	 */
	public function getObjectOwnerGUID() {
		return $this->owner_guid;
	}

	/**
	 * Return a type of the object - eg. object, group, user, relationship, metadata, annotation etc
	 */
	public function getType() {
		return 'relationship';
	}

	/**
	 * Return a subtype. For metadata & annotations this is the 'name' and for relationship this is the relationship type.
	 */
	public function getSubtype() {
		return $this->relationship;
	}

	// ITERATOR INTERFACE //////////////////////////////////////////////////////////////
	/*
	 * This lets an entity's attributes be displayed using foreach as a normal array.
	 * Example: http://www.sitepoint.com/print/php5-standard-library
	 */

	private $valid = FALSE;

	function rewind() {
		$this->valid = (FALSE !== reset($this->attributes));
	}

	function current() {
		return current($this->attributes);
	}

	function key() {
		return key($this->attributes);
	}

	function next() {
		$this->valid = (FALSE !== next($this->attributes));
	}

	function valid() {
		return $this->valid;
	}

	// ARRAY ACCESS INTERFACE //////////////////////////////////////////////////////////
	/*
	 * This lets an entity's attributes be accessed like an associative array.
	 * Example: http://www.sitepoint.com/print/php5-standard-library
	 */

	function offsetSet($key, $value) {
		if ( array_key_exists($key, $this->attributes) ) {
			$this->attributes[$key] = $value;
		}
	}

	function offsetGet($key) {
		if ( array_key_exists($key, $this->attributes) ) {
			return $this->attributes[$key];
		}
	}

	function offsetUnset($key) {
		if ( array_key_exists($key, $this->attributes) ) {
			$this->attributes[$key] = ""; // Full unsetting is dangerious for our objects
		}
	}

	function offsetExists($offset) {
		return array_key_exists($offset, $this->attributes);
	}
}


/**
 * Convert a database row to a new ElggRelationship
 *
 * @param stdClass $row
 * @return stdClass or ElggMetadata
 */
function row_to_elggrelationship($row) {
	if (!($row instanceof stdClass)) {
		return $row;
	}

	return new ElggRelationship($row);
}

/**
 * Return a relationship.
 *
 * @param int $id
 */
function get_relationship($id) {
	global $CONFIG;

	$id = (int)$id;

	return row_to_elggrelationship(get_data_row("SELECT * from {$CONFIG->dbprefix}entity_relationships where id=$id"));
}

/**
 * Delete a specific relationship.
 *
 * @param int $id
 */
function delete_relationship($id) {
	global $CONFIG;

	$id = (int)$id;

	$relationship = get_relationship($id);

	if (trigger_elgg_event('delete', 'relationship', $relationship)) {
		return delete_data("delete from {$CONFIG->dbprefix}entity_relationships where id=$id");
	}

	return FALSE;
}

/**
 * Define an arbitrary relationship between two entities.
 * This relationship could be a friendship, a group membership or a site membership.
 *
 * This function lets you make the statement "$guid_one is a $relationship of $guid_two".
 *
 * @param int $guid_one
 * @param string $relationship
 * @param int $guid_two
 */
function add_entity_relationship($guid_one, $relationship, $guid_two) {
	global $CONFIG;

	$guid_one = (int)$guid_one;
	$relationship = sanitise_string($relationship);
	$guid_two = (int)$guid_two;

	// Check for duplicates
	if (check_entity_relationship($guid_one, $relationship, $guid_two)) {
		return false;
	}

	$result = insert_data("INSERT into {$CONFIG->dbprefix}entity_relationships (guid_one, relationship, guid_two) values ($guid_one, '$relationship', $guid_two)");

	if ($result!==false) {
		$obj = get_relationship($result);
		if (trigger_elgg_event('create', $relationship, $obj)) {
			return true;
		} else {
			delete_relationship($result);
		}
	}

	return false;
}

/**
 * Determine whether or not a relationship between two entities exists and returns the relationship object if it does
 *
 * @param int $guid_one The GUID of the entity "owning" the relationship
 * @param string $relationship The type of relationship
 * @param int $guid_two The GUID of the entity the relationship is with
 * @return object|false Depending on success
 */
function check_entity_relationship($guid_one, $relationship, $guid_two) {
	global $CONFIG;

	$guid_one = (int)$guid_one;
	$relationship = sanitise_string($relationship);
	$guid_two = (int)$guid_two;

	if ($row = get_data_row("SELECT * FROM {$CONFIG->dbprefix}entity_relationships WHERE guid_one=$guid_one AND relationship='$relationship' AND guid_two=$guid_two limit 1")) {
		return $row;
	}

	return false;
}

/**
 * Remove an arbitrary relationship between two entities.
 *
 * @param int $guid_one
 * @param string $relationship
 * @param int $guid_two
 */
function remove_entity_relationship($guid_one, $relationship, $guid_two) {
	global $CONFIG;

	$guid_one = (int)$guid_one;
	$relationship = sanitise_string($relationship);
	$guid_two = (int)$guid_two;

	$obj = check_entity_relationship($guid_one, $relationship, $guid_two);
	if ($obj == false) {
		return false;
	}

	if (trigger_elgg_event('delete', $relationship, $obj)) {
		return delete_data("DELETE from {$CONFIG->dbprefix}entity_relationships where guid_one=$guid_one and relationship='$relationship' and guid_two=$guid_two");
	} else {
		return false;
	}
}

/**
 * Removes all arbitrary relationships originating from a particular entity
 *
 * @param int $guid_one The GUID of the entity
 * @param string $relationship The name of the relationship (optionally)
 * @param true|false $inverse Whether we're deleting inverse relationships (default false)
 * @param string $type The type of entity to limit this relationship delete to (defaults to all)
 * @return true|false Depending on success
 */
function remove_entity_relationships($guid_one, $relationship = "", $inverse = false, $type = '') {
	global $CONFIG;

	$guid_one = (int) $guid_one;

	if (!empty($relationship)) {
		$relationship = sanitise_string($relationship);
		$where = "and er.relationship='$relationship'";
	} else {
		$where = "";
	}

	if (!empty($type)) {
		$type = sanitise_string($type);
		if (!$inverse) {
			$join = " join {$CONFIG->dbprefix}entities e on e.guid = er.guid_two ";
		} else {
			$join = " join {$CONFIG->dbprefix}entities e on e.guid = er.guid_one ";
			$where .= " and ";
		}
		$where .= " and e.type = '{$type}' ";
	} else {
		$join = "";
	}

	if (!$inverse) {
		$sql = "DELETE er from {$CONFIG->dbprefix}entity_relationships as er {$join} where guid_one={$guid_one} {$where}";
		return delete_data($sql);
	} else {
		$sql = "DELETE er from {$CONFIG->dbprefix}entity_relationships as er {$join} where guid_two={$guid_one} {$where}";
		return delete_data($sql);
	}
}

/**
 * Get all the relationships for a given guid.
 *
 * @param int $guid
 */
function get_entity_relationships($guid, $inverse_relationship = FALSE) {
	global $CONFIG;

	$guid = (int)$guid;

	$where = ($inverse_relationship ? "guid_two='$guid'" : "guid_one='$guid'");

	$query = "SELECT * from {$CONFIG->dbprefix}entity_relationships where {$where}";

	return get_data($query, "row_to_elggrelationship");
}


/**
 * Return entities matching a given query joining against a relationship.
 *
 * @param array $options Array in format:
 *
 * 	relationship => NULL|STR relationship
 *
 * 	relationship_guid => NULL|INT Guid of relationship to test
 *
 * 	inverse_relationship => BOOL Inverse the relationship
 *
 * @return array
 */
function elgg_get_entities_from_relationship($options) {
	$defaults = array(
		'relationship' => NULL,
		'relationship_guid' => NULL,
		'inverse_relationship' => FALSE
	);

	$options = array_merge($defaults, $options);

	$clauses = elgg_get_entity_relationship_where_sql('e', $options['relationship'],
		$options['relationship_guid'], $options['inverse_relationship']);

	if ($clauses) {
		// merge wheres to pass to get_entities()
		if (isset($options['wheres']) && !is_array($options['wheres'])) {
			$options['wheres'] = array($options['wheres']);
		} elseif (!isset($options['wheres'])) {
			$options['wheres'] = array();
		}

		$options['wheres'] = array_merge($options['wheres'], $clauses['wheres']);

		// merge joins to pass to get_entities()
		if (isset($options['joins']) && !is_array($options['joins'])) {
			$options['joins'] = array($options['joins']);
		} elseif (!isset($options['joins'])) {
			$options['joins'] = array();
		}

		$options['joins'] = array_merge($options['joins'], $clauses['joins']);
	}

	return elgg_get_entities_from_metadata($options);
}

/**
 * Returns sql appropriate for relationship joins and wheres
 *
 * @todo add support for multiple relationships and guids.
 *
 * @param $table Entities table name
 * @param $relationship relationship string
 * @param $entity_guid entity guid to check
 * @return mixed
 */
function elgg_get_entity_relationship_where_sql($table, $relationship = NULL, $relationship_guid = NULL, $inverse_relationship = FALSE) {
	if ($relationship == NULL && $entity_guid == NULL) {
		return '';
	}

	global $CONFIG;

	$wheres = array();
	$joins = array();

	if ($inverse_relationship) {
		$joins[] = "JOIN {$CONFIG->dbprefix}entity_relationships r on r.guid_one = e.guid";
		//$wheres[] = "{$table}.guid = {$CONFIG->dbprefix}entity_relationships.guid_two";
	} else {
		$joins[] = "JOIN {$CONFIG->dbprefix}entity_relationships r on r.guid_two = e.guid";
		//$wheres[] = "{$table}.guid = {$CONFIG->dbprefix}entity_relationships.guid_one";
	}

	if ($relationship) {
		$wheres[] = "r.relationship = '" . sanitise_string($relationship) . "'";
	}

	if ($relationship_guid) {
		if ($inverse_relationship) {
			$wheres[] = "r.guid_two = '$relationship_guid'";
		} else {
			$wheres[] = "r.guid_one = '$relationship_guid'";
		}
	}

	if ($where_str = implode(' AND ' , $wheres)) {

		return array('wheres' => array("($where_str)"), 'joins' => $joins);
	}

	return '';
}

/**
 * @deprecated 1.7.  Use elgg_get_entities_from_relationship().
 * @param $relationship
 * @param $relationship_guid
 * @param $inverse_relationship
 * @param $type
 * @param $subtype
 * @param $owner_guid
 * @param $order_by
 * @param $limit
 * @param $offset
 * @param $count
 * @param $site_guid
 * @return unknown_type
 */
function get_entities_from_relationship($relationship, $relationship_guid, $inverse_relationship = false,
$type = "", $subtype = "", $owner_guid = 0, $order_by = "", $limit = 10, $offset = 0,
$count = false, $site_guid = 0) {

	elgg_deprecated_notice('get_entities_from_relationship() was deprecated by elgg_get_entities_from_relationship()!', 1.7);

	$options = array();

	$options['relationship'] = $relationship;
	$options['relationship_guid'] = $relationship_guid;
	$options['inverse_relationship'] = $inverse_relationship;

	if ($type) {
		$options['types'] = $type;
	}

	if ($subtype) {
		$options['subtypes'] = $subtype;
	}

	if ($owner_guid) {
		$options['owner_guid'] = $owner_guid;
	}

	if ($limit) {
		$options['limit'] = $limit;
	}

	if ($offset) {
		$options['offset'] = $offset;
	}

	if ($order_by) {
		$options['order_by'];
	}

	if ($site_guid) {
		$options['site_guid'];
	}

	if ($count) {
		$options['count'] = $count;
	}

	return elgg_get_entities_from_relationship($options);
}

/**
 * Returns a viewable list of entities by relationship
 *
 * @see elgg_view_entity_list
 *
 * @param string $relationship The relationship eg "friends_of"
 * @param int $relationship_guid The guid of the entity to use query
 * @param bool $inverse_relationship Reverse the normal function of the query to instead say "give me all entities for whome $relationship_guid is a $relationship of"
 * @param string $type The type of entity (eg 'object')
 * @param string $subtype The entity subtype
 * @param int $owner_guid The owner (default: all)
 * @param int $limit The number of entities to display on a page
 * @param true|false $fullview Whether or not to display the full view (default: true)
 * @param true|false $viewtypetoggle Whether or not to allow gallery view
 * @param true|false $pagination Whether to display pagination (default: true)
 * @return string The viewable list of entities
 */
function list_entities_from_relationship($relationship, $relationship_guid, $inverse_relationship = false, $type = ELGG_ENTITIES_ANY_VALUE, $subtype = ELGG_ENTITIES_ANY_VALUE, $owner_guid = 0, $limit = 10, $fullview = true, $viewtypetoggle = false, $pagination = true) {
	$limit = (int) $limit;
	$offset = (int) get_input('offset');
	$options = array(
		'relationship' => $relationship, 
		'relationship_guid' => $relationship_guid, 
		'inverse_relationship' => $inverse_relationship, 
		'types' => $type, 
		'subtypes' => $subtype, 
		'owner_guid' => $owner_guid, 
		'order_by' => '', 
		'limit' => $limit, 
		'offset' => $offset, 
		'count' => TRUE
	);
	$count = elgg_get_entities_from_relationship($options);
	$options['count'] = FALSE;
	$entities = elgg_get_entities_from_relationship($options);

	return elgg_view_entity_list($entities, $count, $offset, $limit, $fullview, $viewtypetoggle, $pagination);
}

/**
 * Gets the number of entities by a the number of entities related to them in a particular way.
 * This is a good way to get out the users with the most friends, or the groups with the most members.
 *
 * @param string $relationship The relationship eg "friends_of"
 * @param bool $inverse_relationship Reverse the normal function of the query to instead say "give me all entities for whome $relationship_guid is a $relationship of" (default: true)
 * @param string $type The type of entity (default: all)
 * @param string $subtype The entity subtype (default: all)
 * @param int $owner_guid The owner of the entities (default: none)
 * @param int $limit
 * @param int $offset
 * @param boolean $count Set to true if you want to count the number of entities instead (default false)
 * @param int $site_guid The site to get entities for. Leave as 0 (default) for the current site; -1 for all sites.
 * @return array|int|false An array of entities, or the number of entities, or false on failure
 */

function get_entities_by_relationship_count($relationship, $inverse_relationship = true, $type = "", $subtype = "", $owner_guid = 0, $limit = 10, $offset = 0, $count = false, $site_guid = 0) {
	global $CONFIG;

	$relationship = sanitise_string($relationship);
	$inverse_relationship = (bool)$inverse_relationship;
	$type = sanitise_string($type);
	if ($subtype AND !$subtype = get_subtype_id($type, $subtype)) {
		return false;
	}
	$owner_guid = (int)$owner_guid;
	$order_by = sanitise_string($order_by);
	$limit = (int)$limit;
	$offset = (int)$offset;
	$site_guid = (int) $site_guid;
	if ($site_guid == 0) {
		$site_guid = $CONFIG->site_guid;
	}

	//$access = get_access_list();

	$where = array();

	if ($relationship!="") {
		$where[] = "r.relationship='$relationship'";
	}

	if ($inverse_relationship) {
		$on = 'e.guid = r.guid_two';
	} else {
		$on = 'e.guid = r.guid_one';
	}
	if ($type != "") {
		$where[] = "e.type='$type'";
	}

	if ($subtype) {
		$where[] = "e.subtype=$subtype";
	}

	if ($owner_guid != "") {
		$where[] = "e.container_guid='$owner_guid'";
	}

	if ($site_guid > 0) {
		$where[] = "e.site_guid = {$site_guid}";
	}

	if ($count) {
		$query = "SELECT count(distinct e.guid) as total ";
	} else {
		$query = "SELECT e.*, count(e.guid) as total ";
	}

	$query .= " from {$CONFIG->dbprefix}entity_relationships r JOIN {$CONFIG->dbprefix}entities e on {$on} where ";

	if (!empty($where)) {
		foreach ($where as $w) {
			$query .= " $w and ";
		}
	}
	$query .= get_access_sql_suffix("e"); // Add access controls

	if (!$count) {
		$query .= " group by e.guid ";
		$query .= " order by total desc limit {$offset}, {$limit}"; // Add order and limit
		return get_data($query, "entity_row_to_elggstar");
	} else {
		if ($count = get_data_row($query)) {
			return $count->total;
		}
	}

	return false;
}

/**
 * Displays a human-readable list of entities
 *
 * @param string $relationship The relationship eg "friends_of"
 * @param bool $inverse_relationship Reverse the normal function of the query to instead say "give me all entities for whome $relationship_guid is a $relationship of" (default: true)
 * @param string $type The type of entity (eg 'object')
 * @param string $subtype The entity subtype
 * @param int $owner_guid The owner (default: all)
 * @param int $limit The number of entities to display on a page
 * @param true|false $fullview Whether or not to display the full view (default: true)
 * @param true|false $viewtypetoggle Whether or not to allow gallery view
 * @param true|false $pagination Whether to display pagination (default: true)
 * @return string The viewable list of entities
 */

function list_entities_by_relationship_count($relationship, $inverse_relationship = true, $type = "", $subtype = "", $owner_guid = 0, $limit = 10, $fullview = true, $viewtypetoggle = false, $pagination = true) {
	$limit = (int) $limit;
	$offset = (int) get_input('offset');
	$count = get_entities_by_relationship_count($relationship,$inverse_relationship,$type,$subtype,$owner_guid,0,0,true);
	$entities = get_entities_by_relationship_count($relationship,$inverse_relationship,$type,$subtype,$owner_guid,$limit,$offset);

	return elgg_view_entity_list($entities, $count, $offset, $limit, $fullview, $viewtypetoggle, $pagination);
}

/**
 * Gets the number of entities by a the number of entities related to them in a particular way also constrained by
 * metadata
 *
 * @param string $relationship The relationship eg "friends_of"
 * @param int $relationship_guid The guid of the entity to use query
 * @param bool $inverse_relationship Reverse the normal function of the query to instead say "give me all entities for whome $relationship_guid is a $relationship of" (default: true)
 * @param String $meta_name The metadata name
 * @param String $meta_value The metadata value
 * @param string $type The type of entity (default: all)
 * @param string $subtype The entity subtype (default: all)
 * @param int $owner_guid The owner of the entities (default: none)
 * @param int $limit
 * @param int $offset
 * @param boolean $count Set to true if you want to count the number of entities instead (default false)
 * @param int $site_guid The site to get entities for. Leave as 0 (default) for the current site; -1 for all sites.
 * @return array|int|false An array of entities, or the number of entities, or false on failure
 */
function get_entities_from_relationships_and_meta($relationship, $relationship_guid, $inverse_relationship = false, $meta_name = "", $meta_value = "", $type = "", $subtype = "", $owner_guid = 0, $limit = 10, $offset = 0, $count = false, $site_guid = 0) {
	elgg_deprecated_notice('get_entities_from_relationship_and_meta() was deprecated by elgg_get_entities_from_relationship()!', 1.7);

	$options = array();

	$options['relationship'] = $relationship;
	$options['relationship_guid'] = $relationship_guid;
	$options['inverse_relationship'] = $inverse_relationship;

	if ($meta_value) {
		$options['values'] = $meta_value;
	}

	if ($entity_type) {
		$options['types'] = $entity_type;
	}

	if ($type) {
		$options['types'] = $type;
	}

	if (subtype) {
		$options['subtypes'] = $subtype;
	}

	if ($owner_guid) {
		$options['owner_guid'] = $owner_guid;
	}

	if ($limit) {
		$options['limit'] = $limit;
	}

	if ($offset) {
		$options['offset'] = $offset;
	}

	if ($order_by) {
		$options['order_by'];
	}

	if ($site_guid) {
		$options['site_guid'];
	}

	if ($count) {
		$options['count'] = $count;
	}

	return elgg_get_entities_from_relationship($options);
}

/**
 * Sets the URL handler for a particular relationship type
 *
 * @param string $function_name The function to register
 * @param string $relationship_type The relationship type.
 * @return true|false Depending on success
 */
function register_relationship_url_handler($function_name, $relationship_type = "all") {
	global $CONFIG;

	if (!is_callable($function_name)) {
		return false;
	}

	if (!isset($CONFIG->relationship_url_handler)) {
		$CONFIG->relationship_url_handler = array();
	}

	$CONFIG->relationship_url_handler[$relationship_type] = $function_name;

	return true;
}

/**
 * Get the url for a given relationship.
 *
 * @param unknown_type $id
 * @return unknown
 */
function get_relationship_url($id) {
	global $CONFIG;

	$id = (int)$id;

	if ($relationship = get_relationship($id)) {
		$view = elgg_get_viewtype();

		$guid = $relationship->guid_one;
		$type = $relationship->relationship;

		$url = "";

		$function = "";
		if (isset($CONFIG->relationship_url_handler[$type])) {
			$function = $CONFIG->relationship_url_handler[$type];
		}
		if (isset($CONFIG->relationship_url_handler['all'])) {
			$function = $CONFIG->relationship_url_handler['all'];
		}

		if (is_callable($function)) {
			$url = $function($relationship);
		}

		if ($url == "") {
			$nameid = $relationship->id;

			$url = $CONFIG->wwwroot  . "export/$view/$guid/relationship/$nameid/";
		}

		return $url;
	}

	return false;
}

/**** HELPER FUNCTIONS FOR RELATIONSHIPS OF TYPE 'ATTACHED' ****/

/**
 * Function to determine if the object trying to attach to other, has already done so
 * @param int $guid_one This is the target object
 * @param int $guid_two This is the object trying to attach to $guid_one
 * @return true | false
 **/

function already_attached($guid_one, $guid_two){
	if ($attached = check_entity_relationship($guid_one, "attached", $guid_two)){
		return true;
	} else {
		return false;
	}
}

/**
 * Function to get all objects attached to a particular object
 * @param int $guid
 * @param string $type - the type of object to return e.g. 'file', 'friend_of' etc
 * @return an array of objects
**/

function get_attachments($guid, $type=""){
	$attached = elgg_get_entities_from_relationship(array('relationship' => 'attached', 'relationship_guid' => $guid, 'inverse_relationship' => $inverse_relationship = false, 'types' => $type, 'subtypes' => '', 'owner_guid' => 0, 'order_by' => 'time_created desc', 'limit' => 10, 'offset' => 0, 'count' => false, 'site_guid' => 0));
	return $attached;
}

/**
 * Function to remove a particular attachment between two objects
 * @param int $guid_one This is the target object
 * @param int $guid_two This is the object to remove from $guid_one
 * @return a view
**/

function remove_attachment($guid_one, $guid_two){
	if(already_attached($guid_one, $guid_two)) {
		remove_entity_relationship($guid_one, "attached", $guid_two);
	}
}

/**
 * Function to start the process of attaching one object to another
 * @param int $guid_one This is the target object
 * @param int $guid_two This is the object trying to attach to $guid_one
 * @return a view
**/

function make_attachment($guid_one, $guid_two){
	if (!(already_attached($guid_one, $guid_two))) {
		if (add_entity_relationship($guid_one, "attached", $guid_two)) {
			return true;
		}
	}
}

/**
 *  Handler called by trigger_plugin_hook on the "import" event.
 */
function import_relationship_plugin_hook($hook, $entity_type, $returnvalue, $params) {
	$element = $params['element'];

	$tmp = NULL;

	if ($element instanceof ODDRelationship) {
		$tmp = new ElggRelationship();
		$tmp->import($element);

		return $tmp;
	}
}

/**
 *  Handler called by trigger_plugin_hook on the "export" event.
 */
function export_relationship_plugin_hook($hook, $entity_type, $returnvalue, $params) {
	global $CONFIG;

	// Sanity check values
	if ((!is_array($params)) && (!isset($params['guid']))) {
		throw new InvalidParameterException(elgg_echo('InvalidParameterException:GUIDNotForExport'));
	}

	if (!is_array($returnvalue)) {
		throw new InvalidParameterException(elgg_echo('InvalidParameterException:NonArrayReturnValue'));
	}

	$guid = (int)$params['guid'];

	$result = get_entity_relationships($guid);

	if ($result) {
		foreach ($result as $r) {
			$returnvalue[] = $r->export();
		}
	}

	return $returnvalue;
}

/**
 * An event listener which will notify users based on certain events.
 *
 * @param unknown_type $event
 * @param unknown_type $object_type
 * @param unknown_type $object
 */
function relationship_notification_hook($event, $object_type, $object) {
	global $CONFIG;

	if (
		($object instanceof ElggRelationship) &&
		($event == 'create') &&
		($object_type == 'friend')
	)
	{
		$user_one = get_entity($object->guid_one);
		$user_two = get_entity($object->guid_two);

		// Notify target user
		return notify_user($object->guid_two, $object->guid_one, sprintf(elgg_echo('friend:newfriend:subject'), $user_one->name),
			sprintf(elgg_echo("friend:newfriend:body"), $user_one->name, $CONFIG->site->url . "pg/profile/" . $user_one->username)
		);
	}
}

/** Register the import hook */
register_plugin_hook("import", "all", "import_relationship_plugin_hook", 3);

/** Register the hook, ensuring entities are serialised first */
register_plugin_hook("export", "all", "export_relationship_plugin_hook", 3);

/** Register event to listen to some events **/
register_elgg_event_handler('create','friend','relationship_notification_hook');