import java.util.*;
import javax.swing.*;
import java.awt.*;
/* A program to find all the knight's tours on a chessboard
 * A knight's tour is achieved when a knight visits each square
 * exactly once using legal knight moves
 * Rhys Price Jones
 * St Padraig 2009
 */
public class KnightsTourApplet extends JApplet implements Runnable {
    private static final int n = 8;          // size of the chessboard
    private static final int NUMKMOVES = 8;  // number of ways a knight can move
    // possible knight moves are:
    // 2 across and 1 up, 1 across and 2 up, ... etc, as in:
    private int[] across = {2, 1, -1, -2, -2, -1,  1,  2};
    private int[] up =     {1, 2,  2,  1, -1, -2, -2, -1};
    private int nsq = n*n;
    private int[][] board;
    private JButton[][] myButtons;
    private Panel myPanel;
    public void init() {
        setSize(400,400);
        setVisible(true);
        board = new int[n][n];
        myPanel = new Panel();
        myPanel.setLayout(new GridLayout(n,n));
        myButtons = new JButton[n][n];
        for (int i = 0; i < n; i++) 
            for (int j = 0; j < n; j++)  {
                board[i][j] = 0;
                myButtons[i][j] = new JButton(" ");
                myPanel.add(myButtons[i][j]);
            }
        add(myPanel, BorderLayout.CENTER);
        validate();
        board[0][0] = 1; 
        new Thread(this).start();
    }
    public void printSolution() { 
        // for the web version, let's just beep and pause
        Toolkit.getDefaultToolkit().beep();
        try {
            Thread.sleep(10000);
        }
        catch (InterruptedException ie) {}
        Toolkit.getDefaultToolkit().beep();
    }

    public void run() { 
        board[0][0] = 1; 
        myButtons[0][0].setText("1");
        findall(2, 0, 0);  // find all knight's tours that start at (0,0)
    }

    public void findall(int i, int x, int y) {
        // This is the workhorse
        // Proceeding from square (x,y) try to make the i'th move to (u,v)
        // If successful then make a recursive call to make the (i+1)th move
        // When you return from that, undo (u,v)
        // Effectively uses a stack because recursive calls use a stack
        // This is exhaustive search and should ultimately get all solutions
        // that start from the existing path
        int u, v;
        for (int k = 0; k < NUMKMOVES; k++) { // try each knight move
            u = x + across[k];
            v = y + up[k];
            if ((0 <= u) && (u < n) && (0 <= v) && (v < n)) // still on board?
                if (board[u][v] == 0) { // getting to an unvisited square?
                    board[u][v] = i;    // mark it
                    myButtons[u][v].setText(""+i);
                    if (i < nsq) findall(i+1, u, v);   // and proceed
                    else printSolution(); // unless solved, so print it
                    // backtrack
                    board[u][v] = 0;
                    myButtons[u][v].setText("");
                }
        }
    }
}