Oh and about designing for difficulty level — I think it's sort of reverse engineered from the fully solved state by continually removing numbers until it's mor

Oh and about designing for difficulty level — I think it's sort of reverse engineered from the fully solved state by continually removing numbers until it's more sparse / harder. I believe some of the apps do it by incubating puzzles then gathering aggregate statistics about how long it took people to solve then bucket < 5 min --> easy, 5-10 min --> medium, 10-15 --> hard, 15+ min --> evil, etc.