help with 15 piece puzzle please

[beginningjs]

http://pastebin.com/A2hRmSU2

hi so i am working on a group project for my CS class to build a 15 piece puzzle in javascript. The other group member wrote up some code which i dont understand he is unwilling to comment, so now i have to try and find moveable pieces but i have no idea how to go about it. His shuffling algorithm works perfectly well but seems uncomprehensible to me. Please help. I have posted the JS code on pastebin. Thanks

[Dormilich]

the code shown is basically a way (though not very efficient) to scramble the tiles without repetition. but that still doesn’t prevent half of the configurations to be unsolvable.

you might look at http://bytes.kulturbeutel-leipzig.net/MagicSquare.xhtml for inspiration, there’s some more explanation.