quick search at our udf-library gives at least these 2 perfectly fitting examples:
http://www.kixtart.org/board/ultimatebb.php?ubb=get_topic;f=12;t=000256
http://www.kixtart.org/board/ultimatebb.php?ubb=get_topic;f=12;t=000143

the first is kinda based on:
http://www.kixtart.org/board/ultimatebb.php?ubb=get_topic;f=12;t=000254

and the latter has quite many dependencies...
if you want to search yourself, browse to:
http://www.gwspikval.com/jooel/UDF
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