Cool! May I ask, is each level pre-made or are they all partly randomly generated?
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You may! So, they are randomly generated. It throws down noise, carves out a slot for the target piece (making sure the walls and "floor" are solid), and then makes sure no row already has lines. Not very elegant but it works!
It does have some other rules like making the first few levels increase linearly in height before going random, and from level 5+ it will start rotating the piece randomly before carving it upwards. :)