htaccess help please

[eluser]T I[/eluser]
I need a htaccess which is needed in my codeigniter application to run, which will do the following logic:

http://sitename.com/app will go to http://sitename.com/app/index.php

http://d.sitename.com/app will go to http://sitename.com/app/index.php/shop/index/d


http://d.sitename.com/app/a/b/c will go to http://sitename.com/app/index.php/a/b/c/d


Can anyone please write the rule? Is there any help possible?