Some other rhythm game simulators are open source.
Here's where to find code.
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Delight Delight Reduplication [Boost Software License]
http://sourceforge.jp/projects/delight/svn/view/trunk/src/?root=delight

Direct Move [unknown license]
http://zsoo.net/38

pydance [see http://kai.vm.bytemark.co.uk/svn/pydance/trunk/pydance/LICENSE]
http://kai.vm.bytemark.co.uk/svn/pydance/trunk/pydance/

be-pachi music [MIT license]
http://x-raise.cjb.net/bpm.html

marfitude [GPL license]
http://www.erestar.net/games/marfitude/