public SudokuPuzzle GenerateSudoku(SudokuDifficuty difficulty, int length = 9)
{
// get randomly filled out sudoku
SudokuPuzzle solution = SolveSudoku(new SudokuPuzzle());
SudokuPuzzle sudoku = (SudokuPuzzle)solution.Clone();
// check how many fields need to be removed (depending on desired difficulty)
int fieldsToRemove = ((length * length) - (int)difficulty);
while (fieldsToRemove > 0)
{
int freeFieldsCount = sudoku.GetFreeFields().Count;
foreach (var field in sudoku.GetSetFields().Shuffle())
{
if (fieldsToRemove > 0 && isFieldDetermined(sudoku, solution, field.RowIndex, field.ColumnIndex))
{
field.SetValue(0);
fieldsToRemove--;
}
}
// check if progress was made (if not => try with another sudoku puzzle)
if ((freeFieldsCount - sudoku.GetFreeFields().Count) == 0)
{
sudoku = GenerateSudoku(difficulty, length);
break;
}
}
// check if the generated sudoku is unique. if not create a new one.
return((HasSudokuUniqueSolution(sudoku)) ? sudoku : GenerateSudoku(difficulty, length));
}
public bool HasSudokuUniqueSolution(SudokuPuzzle sudoku)
{
// sudokus with 16 or less determinded fields cannot have a unique solution
// source: https://www.technologyreview.com/s/426554/mathematicians-solve-minimum-sudoku-problem/
bool ret = (sudoku.GetSetFields()?.Count > 16);
if (ret)
{
// solve sudoku (gets one solution out of possibly many solutions)
var solution = SolveSudoku(sudoku);
ret = (solution != null);
if (ret)
{
var temp = (SudokuPuzzle)sudoku.Clone();
// go through all empty fields (filled out fields are ignored)
while (ret && !temp.IsSolved())
{
// choose a free field with a minimum amount of remaining possibilities
int minPossibleValuesCount = temp.GetFreeFields().Select(x => x.GetPossibleValuesCount()).Min();
var field = temp.GetFreeFields().Where(x => x.GetPossibleValuesCount() == minPossibleValuesCount).ChooseRandom();
// check if the field is determined; if not, there is a second solution => return false
ret = isFieldDetermined(temp, solution, field);
// apply the determined value to the sudoku => less possibilities
field.SetValue(solution.Fields[field.RowIndex, field.ColumnIndex].Value);
}
}
}
return(ret);
}
private bool removeField(SudokuPuzzle sudoku, SudokuPuzzle solution)
{
foreach (SudokuField field in sudoku.GetSetFields().Shuffle())
{
// clone sudoku and set the field value to 0
SudokuPuzzle temp = (SudokuPuzzle)sudoku.Clone();
SudokuField tempField = temp.Fields[field.RowIndex, field.ColumnIndex];
tempField.SetValue(0);
if (HasSudokuUniqueSolution(temp))
{
field.SetValue(0);
return(true);
}
}
return(false);
}
