SudokuGenerator.java

import java.util.Random;

public class SudokuGenerator {
    private static final int SIZE = 9;
    private static final int SUBGRID = 3;
    private int[][] board = new int[SIZE][SIZE];
    private Random rand = new Random();

    public int[][] generateFullBoard() {
        fillBoard(0, 0);
        return board;
    }

    private boolean fillBoard(int row, int col) {
        if (row == SIZE)
            return true;
        if (col == SIZE)
            return fillBoard(row + 1, 0);

        int[] nums = shuffleArray();
        for (int num : nums) {
            if (isSafe(row, col, num)) {
                board[row][col] = num;
                if (fillBoard(row, col + 1))
                    return true;
                board[row][col] = 0;
            }
        }
        return false;
    }

    private int[] shuffleArray() {
        int[] arr = new int[SIZE];
        for (int i = 0; i < SIZE; i++)
            arr[i] = i + 1;
        for (int i = SIZE - 1; i > 0; i--) {
            int j = rand.nextInt(i + 1);
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        }
        return arr;
    }

    private boolean isSafe(int row, int col, int num) {
        for (int i = 0; i < SIZE; i++) {
            if (board[row][i] == num || board[i][col] == num)
                return false;
        }
        int startRow = row - row % SUBGRID;
        int startCol = col - col % SUBGRID;
        for (int i = 0; i < SUBGRID; i++) {
            for (int j = 0; j < SUBGRID; j++) {
                if (board[startRow + i][startCol + j] == num)
                    return false;
            }
        }
        return true;
    }

    public int[][] generatePuzzleBoard(Difficulty level) {
        int[][] puzzleBoard = deepCopy(board);
        removeCells(puzzleBoard, level.getCellsToRemove());
        return puzzleBoard;
    }

    private void removeCells(int[][] puzzleBoard, int cellsToRemove) {
        int count = 0;
        while (count < cellsToRemove) {
            int row = rand.nextInt(9);
            int col = rand.nextInt(9);

            if (puzzleBoard[row][col] != 0) {
                puzzleBoard[row][col] = 0;
                count++;
            }
        }
    }

    private int[][] deepCopy(int[][] original) {
        int[][] copy = new int[original.length][original[0].length];
        for (int i = 0; i < original.length; i++) {
            System.arraycopy(original[i], 0, copy[i], 0, original[0].length);
        }
        return copy;
    }

}