I covered how to create Sudoku problems in the “Creating a Sudoku program” section. Here, I will explain about the program maximum level.
The problems on this website are created automatically using a program. When you reach around 26 clue boxes, the chances of encountering a problem you cannot solve increases.
An unsolvable problem is not a valid problem, so in the program I use on this website, I create around 36 clue boxes automatically first, then delete 1 clue boxes at a time, and every time the clue box gets deleted, the Sudoku solver program is used to solve the problem. If it cannot solve it, the deleted clue box will be recovered and a different clue box will be deleted, which is then put into the solver program once again.
This process is repeated to decrease the number of clue boxes.
Normally, when the clue box reaches a certain amount (for instance, 26 in level 5), it gets saved as a problem, but the program maximum level removes this limit and keeps decreasing the clue boxes until it cannot be solved anymore.
Although I used the word edge, the minimum amount of clue boxes was 21. I was hoping for something smaller like 16, but it wasn’t so easy. In order to decrease the number of clue boxes even further, I need to either make the solver program smarter or find an alternative way to create problems. In the future I want to try and make problems with fewer clue boxes, but for now, this will do.