raise ValueError('puzzle is unsolvable')

Very interest Kata :) Thank you
luckily i found the right method. my algorithm solved everything in 6 seconds, only on one Sudoku from your set it was required to take 1 step back.
also my algorithm solved in 2 seconds "the most difficult sudoku in the world" by taking 14 steps back. I'm glad that my program coped with it!

my algorithm restores the level so that you don't have to remove extra walls. but I knew that someone will face this problem.
you can make an additional algorithm for filling empty spaces to remove extra walls. or rebuild the logic of your algorithm.

when I created the kata, I suggested rank 4. but those who decided it first changed to level 5. Apparently for some this is a very simple task.

i wrote a simple soko-solver. it works with brute-force.
unfortunately it is still very slow. if I manage to optimize it, it will probably be a great kata :)

Thank you!
I thought about it. but I didn't know how to do it right...
I also need adjust the coefficients for levels 50 * 50 so that there is less empty space

ok. thank you!
this is my first kata :)

i fix problem with empy output.
also random generate now twice as fast

I agree with you. i saw this error so i run the generate loop multiple times. this is a strange behavior of the random number generator.
because of this, the function is slow. I will try to fix it soon...

after I added random level generation, three solutions became "invalidated". apparently they run again after adding new tests.

I was also interested in a new problem for solving small levels. maybe I'll do it...

done

done

i fixe my error. thank you!
also i added many big random levels :)

soko-solver this is very-very hard task... possible for small levels

hmm. strange. i will check it.
ok i try added more big random levels

you need to find the initial state of the level.