Generate a sudoku puzzle

[jbranchaud]

The general idea is to:

1. Generate a unique, valid, filled-in sudoku grid. Start with a solution and work backward to a puzzle.
2. Dig holes: remove values from the grid until you have stepped far enough away from the solution to make an interesting puzzle but still have a puzzle that has a unique solution. As you're stepping backward, you'll reach a point where the grid no longer has a unique solution. You can either undo that hole and continue somewhere else on the board. Or you can undo that hole and consider that to be the puzzle.
3. Rank the difficulty of the generated puzzles either manually or with an automated set of heuristics.
4. Rotate the puzzle and rearrange its value to produce tons of variations on a single puzzle. With enough starting puzzles and randomization, there will be more than enough variety.

References

• https://gamedev.stackexchange.com/a/76170/188085
• Sudoku Puzzle Generating: From Easy to Evil (pdf)
• https://stackoverflow.com/questions/6924216/how-to-generate-sudoku-boards-with-unique-solutions