/// <summary>
/// Removes numbers from an existing <see cref="SudokuPuzzle"/> according to its difficulty.
/// </summary>
/// <param name="sudoku">The <see cref="SudokuPuzzle"/> to remove numbers from.</param>
/// <exception cref="ArgumentNullException"><c><paramref name="sudoku"/></c> is <c>null</c>.</exception>
public static void RemoveNumbers(SudokuPuzzle sudoku)
{
if (sudoku is null)
{
throw new ArgumentNullException(nameof(sudoku));
}
sudoku.ClearReadOnlyProperties();
int attempts = 45;
switch (sudoku.Difficulty)
{
case SudokuDifficulty.Medium:
attempts = 60;
break;
case SudokuDifficulty.Hard:
attempts = 75;
break;
}
do
{
int row, column;
do
{
row = SudokuGenerator.Random.Next(sudoku.Size);
column = SudokuGenerator.Random.Next(sudoku.Size);
} while (sudoku[row, column] == 0);
int value = sudoku[row, column];
sudoku[row, column] = 0;
bool solvable = SudokuSolver.CheckSolvable(sudoku, out bool multipleSolutions);
if (!solvable || multipleSolutions)
{
sudoku[row, column] = value;
}
} while (attempts-- > 0);
sudoku.SetReadOnlyProperties();
}