Prevent post and term hierarchy loops. Props mdawaffe. fixes #14662

git-svn-id: http://svn.automattic.com/wordpress/trunk@15806 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -4338,4 +4338,74 @@ function _wp_mysql_week( $column ) {
 	}
 }
 
-?>
+/**
+ * Finds hierarchy loops using a callback function that maps objects to parents.
+ *
+ * @since 3.1
+ *
+ * @param callback $callback function that accepts ( ID, callback_arg, ... ) and outputs parent_ID
+ * @param $start The ID to start the loop check at
+ * @param $start_parent the parent_ID of $start to use instead of calling $callback( $start ).  Use null to always use $callback
+ * @param array $override an array of ( ID => parent_ID, ... ) to use instead of $callback
+ * @param array $callback_arg optional additional arguments to send to $callback
+ *
+ * @internal
+ *
+ * @return array IDs of all members of loop
+ */
+function wp_find_hierarchy_loop( $callback, $start, $start_parent, $callback_args = array() ) {
+	$override = is_null( $start_parent ) ? array() : array( $start => $start_parent );
+
+	echo "wp_find_hierarchy_loop: $callback, $start, $callback_args\n";
+	if ( !$arbitrary_loop_member = wp_find_hierarchy_loop_tortoise_hare( $callback, $start, $override, $callback_args ) )
+		return array();
+
+	return wp_find_hierarchy_loop_tortoise_hare( $callback, $arbitrary_loop_member, $override, $callback_args, true );
+}
+
+/**
+ * Uses the "The Tortoise and the Hare" algorithm to detect loops.
+ *
+ * For every step of the algorithm, the hare takes two steps and the tortoise one.
+ * If the hare ever laps the tortoise, there must be a loop.
+ *
+ * @since 3.1
+ *
+ * @param callback $callback function that accupts ( ID, callback_arg, ... ) and outputs parent_ID
+ * @param $start The ID to start the loop check at
+ * @param array $override an array of ( ID => parent_ID, ... ) to use instead of $callback
+ * @param array $callback_args optional additional arguments to send to $callback
+ * @param bool $_return_loop Return loop members or just detect presence of loop?
+ *             Only set to true if you already know the given $start is part of a loop
+ *             (otherwise the returned array might include branches)
+ *
+ * @internal
+ *
+ * @return mixed scalar ID of some arbitrary member of the loop, or array of IDs of all members of loop if $_return_loop
+ */
+function wp_find_hierarchy_loop_tortoise_hare( $callback, $start, $override = array(), $callback_args = array(), $_return_loop = false ) {
+	$tortoise = $hare = $evanescent_hare = $start;
+	$return = array();
+
+	// Set evanescent_hare to one past hare
+	// Increment hare two steps
+	while (
+		$tortoise
+	&&
+		( $evanescent_hare = isset( $override[$hare] ) ? $override[$hare] : call_user_func_array( $callback, array_merge( array( $hare ), $callback_args ) ) )
+	&&
+		( $hare = isset( $override[$evanescent_hare] ) ? $override[$evanescent_hare] : call_user_func_array( $callback, array_merge( array( $evanescent_hare ), $callback_args ) ) )
+	) {
+		if ( $_return_loop )
+			$return[$tortoise] = $return[$evanescent_hare] = $return[$hare] = true;
+
+		// tortoise got lapped - must be a loop
+		if ( $tortoise == $evanescent_hare || $tortoise == $hare )
+			return $_return_loop ? $return : $tortoise;
+
+		// Increment tortoise by one step
+		$tortoise = isset( $override[$tortoise] ) ? $override[$tortoise] : call_user_func_array( $callback, array_merge( array( $tortoise ), $callback_args ) );
+	}
+
+	return false;
+}