RosettaCodeData/Task/Maze-solving/Java/maze-solving-1.java

import java.io.*;
import java.util.*;

public class MazeSolver
{
    /**
     * Reads a file into an array of strings, one per line.
     */
    private static String[] readLines (InputStream f) throws IOException
    {
        BufferedReader r =
            new BufferedReader (new InputStreamReader (f, "US-ASCII"));
        ArrayList<String> lines = new ArrayList<String>();
        String line;
        while ((line = r.readLine()) != null)
            lines.add (line);
        return lines.toArray(new String[0]);
    }

    /**
     * Makes the maze half as wide (i. e. "+---+" becomes "+-+"), so that
     * each cell in the maze is the same size horizontally as vertically.
     * (Versus the expanded version, which looks better visually.)
     * Also, converts each line of the maze from a String to a
     * char[], because we'll want mutability when drawing the solution later.
     */
    private static char[][] decimateHorizontally (String[] lines)
    {
        final int width = (lines[0].length() + 1) / 2;
        char[][] c = new char[lines.length][width];
        for (int i = 0  ;  i < lines.length  ;  i++)
            for (int j = 0  ;  j < width  ;  j++)
                c[i][j] = lines[i].charAt (j * 2);
        return c;
    }

    /**
     * Given the maze, the x and y coordinates (which must be odd),
     * and the direction we came from, return true if the maze is
     * solvable, and draw the solution if so.
     */
    private static boolean solveMazeRecursively (char[][] maze,
                                                 int x, int y, int d)
    {
        boolean ok = false;
        for (int i = 0  ;  i < 4  &&  !ok  ;  i++)
            if (i != d)
                switch (i)
                    {
                        // 0 = up, 1 = right, 2 = down, 3 = left
                    case 0:
                        if (maze[y-1][x] == ' ')
                            ok = solveMazeRecursively (maze, x, y - 2, 2);
                        break;
                    case 1:
                        if (maze[y][x+1] == ' ')
                            ok = solveMazeRecursively (maze, x + 2, y, 3);
                        break;
                    case 2:
                        if (maze[y+1][x] == ' ')
                            ok = solveMazeRecursively (maze, x, y + 2, 0);
                        break;
                    case 3:
                        if (maze[y][x-1] == ' ')
                            ok = solveMazeRecursively (maze, x - 2, y, 1);
                        break;
                    }
        // check for end condition
        if (x == 1  &&  y == 1)
            ok = true;
        // once we have found a solution, draw it as we unwind the recursion
        if (ok)
            {
                maze[y][x] = '*';
                switch (d)
                    {
                    case 0:
                        maze[y-1][x] = '*';
                        break;
                    case 1:
                        maze[y][x+1] = '*';
                        break;
                    case 2:
                        maze[y+1][x] = '*';
                        break;
                    case 3:
                        maze[y][x-1] = '*';
                        break;
                    }
            }
        return ok;
    }

    /**
     * Solve the maze and draw the solution.  For simplicity,
     * assumes the starting point is the lower right, and the
     * ending point is the upper left.
     */
    private static void solveMaze (char[][] maze)
    {
        solveMazeRecursively (maze, maze[0].length - 2, maze.length - 2, -1);
    }

    /**
     * Opposite of decimateHorizontally().  Adds extra characters to make
     * the maze "look right", and converts each line from char[] to
     * String at the same time.
     */
    private static String[] expandHorizontally (char[][] maze)
    {
        char[] tmp = new char[3];
        String[] lines = new String[maze.length];
        for (int i = 0  ;  i < maze.length  ;  i++)
            {
                StringBuilder sb = new StringBuilder(maze[i].length * 2);
                for (int j = 0  ;  j < maze[i].length  ;  j++)
                    if (j % 2 == 0)
                        sb.append (maze[i][j]);
                    else
                        {
                            tmp[0] = tmp[1] = tmp[2] = maze[i][j];
                            if (tmp[1] == '*')
                                tmp[0] = tmp[2] = ' ';
                            sb.append (tmp);
                        }
                lines[i] = sb.toString();
            }
        return lines;
    }

    /**
     * Accepts a maze as generated by:
     * http://rosettacode.org/wiki/Maze_generation#Java
     * in a file whose name is specified as a command-line argument,
     * or on standard input if no argument is specified.
     */
    public static void main (String[] args) throws IOException
    {
        InputStream f = (args.length > 0
                         ?  new FileInputStream (args[0])
                         :  System.in);
        String[] lines = readLines (f);
        char[][] maze = decimateHorizontally (lines);
        solveMaze (maze);
        String[] solvedLines = expandHorizontally (maze);
        for (int i = 0  ;  i < solvedLines.length  ;  i++)
            System.out.println (solvedLines[i]);
    }
}