001package algs13.xbacktrack.xsudoku;
002
003/**
004 * A mutable cell in a Sudoku grid. A mutable cell's digit can be
005 * changed as the solver works to find the solution to the puzzle.
006 */
007final class XMutableCell extends XSudokuCell {
008
009    // The next cell for which a choice will be made after
010    // a choice is made for this cell. If the next cell is null,
011    // this cell is the last cell for which we need to choose a digit.
012    final XMutableCell nextCell;
013
014    // The current digit assigned to this mutable cell.
015    // A digit value of 0 indicates that the digit for the cell
016    // has not been assigned.
017    private int digit;
018
019    // Constructor for a Mutable cell.
020    XMutableCell(final int x, final int y, final XMutableCell next) {
021        super(x, y);
022        this.nextCell = next;
023        digit = 0;
024    }
025
026    // Get the current digit assigned to this cell.
027    int getDigit() {
028        return digit;
029    }
030
031    // Get the next digit to try for this cell.
032    // If the digit value wraps around to 0, we know we've tried all valid
033    // digits for this cell that could be tried given all the previous choices
034    // we have made. In this case we cannot try anything else, so the digit we
035    // assigned to the previously chosen cell cannot yield a valid result for
036    // the Sudoku puzzle.
037    int nextDigit() {
038        digit = (digit + 1) % (MySudoku.GRID_SIZE + 1);
039        return digit;
040    }
041
042    @Override
043    public String toString() {
044        return String.format("(%d, %d): %d", x, y, digit);
045    }
046}