~20bn sounds about right. 9! * 4^9 / 4 ~ 23.8bn. Divide by 4 due to rotational symmetry.
Absolutely mind-boggling to me that such a small, simple puzzle has such an extreme search space! Great example of P vs NP, time to build a Where’s Waldo matching game cryptosystem.
Absolutely mind-boggling to me that such a small, simple puzzle has such an extreme search space! Great example of P vs NP, time to build a Where’s Waldo matching game cryptosystem.