Make Java recursive maze solver more efficient
Question
Update
I was able to get my algorithm working by increasing the thread size to a few gigabytes and was able to solve a 1803x1803 maze in a second or two.
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I started teaching myself recursion yesterday in Java. I created an algorithm that takes a photo of a maze and solves it. However, I get a stack overflow answer when doing mazes that are larger than about 200x200 px because I think the stacks of this algorithm get too long. How can I better this algorithm so that I can input images up to possibly 1000x1000?
Additionally, can you tell me what kind of algorithm I am currently using? I believe this is either DFS, but I am unsure.
Please explain why your solution is more efficient and the idea that it uses.
This is the main class for solving
public class BlackWhiteSolver {
static int[][] solutionSet = new int[203][203];
static int width, height;
static String originalImage;
static int correctX, correctY;
public static void convert() {
try {
BufferedImage original = ImageIO.read(new File(originalImage));
int red;
int threshold = 2;
width = original.getWidth();
height = original.getHeight();
for(int i=0; i<original.getWidth(); i++) {
for(int j=0; j<original.getHeight(); j++) {
red = new Color(original.getRGB(i, j)).getRed();
// 1 = white, 0 = black, 9 = tried, 5 = solved
if(red > threshold) { solutionSet[i][j] = 1; }
else { solutionSet[i][j] = 0; }
}
}
} catch (IOException e) {e.printStackTrace();}
}
public BlackWhiteSolver(int solvedX, int solvedY, String pic) {
correctX = solvedX;
correctY = solvedY;
originalImage = pic;
}
public boolean solve (int row, int column) {
boolean completed = false;
if (validPoint(row, column)) {
solutionSet[row][column] = 9;
if (row == correctX && column == correctY) {
completed = true;
} else {
completed = solve (row+1, column);
if (!completed) {
completed = solve (row, column+1);
}
if (!completed) {
completed = solve (row-1, column);
}
if (!completed) {
completed = solve (row, column-1);
}
}
if (completed) {
solutionSet[row][column] = 5;
}
}
return completed;
}
private boolean validPoint (int row, int column) {
boolean isValid = false;
if (row < height-1 && column < width-1 && row >= 1 && column >= 1 ) {
if (solutionSet[row][column] == 1) {
isValid = true;
}
}
return isValid;
}
public static void solvedFile() {
BufferedImage binarized = new BufferedImage(width, height,BufferedImage.TYPE_3BYTE_BGR);
int newPixel = 0;
int rgb = new Color(255, 0, 0).getRGB();
for(int i=0; i<width; i++){
for(int j=0; j<height; j++)
{
if (solutionSet[i][j] == 0) {
newPixel = 0;
newPixel = colorToRGB(1, newPixel, newPixel, newPixel);
} else if (solutionSet[i][j] == 1 || solutionSet[i][j] == 9) {
newPixel = 255;
newPixel = colorToRGB(1, newPixel, newPixel, newPixel);
} else if (solutionSet[i][j] == 5) {
newPixel = 16711680;
}
binarized.setRGB(i, j, newPixel);
}
}
try { ImageIO.write(binarized, "gif",new File("maze-complete") );} catch (IOException e) {e.printStackTrace();}
}
private static int colorToRGB(int alpha, int red, int green, int blue) {
int newPixel = 0;
newPixel += alpha;
newPixel = newPixel << 8;
newPixel += red; newPixel = newPixel << 8;
newPixel += green; newPixel = newPixel << 8;
newPixel += blue;
return newPixel;
}
}
This is the class that runs the maze
public class BlackWhiteInterface
{
public static void main (String[] args) {
BlackWhiteSolver puzzle = new BlackWhiteSolver(60, 202, "maze-4.gif");
System.out.println();
puzzle.convert();
if (puzzle.solve(0,34)) {
System.out.println("completed");
puzzle.solvedFile();
} else {
System.out.println("not possible");
}
}
}
Generates correct maze with start and end point
public class MazeBuilder {
static String start = "left";
static String end = "down";
public static void main(String[] args)
{
try
{
BufferedImage original = ImageIO.read(new File("mazeInput1.gif"));
BufferedImage binarized = new BufferedImage(original.getWidth(), original.getHeight(),BufferedImage.TYPE_BYTE_BINARY);
int red;
int redRightPixel;
int redUpPixel;
int newPixel;
int threshold = 2;
for(int i=0; i<original.getWidth(); i++)
{
for(int j=0; j<original.getHeight(); j++)
{
red = new Color(original.getRGB(i, j)).getRed();
int alpha = new Color(original.getRGB(i, j)).getAlpha();
if(red > threshold) { newPixel = 255; }
else { newPixel = 0; }
if (i == 0 || j == 0 || i == original.getWidth()-1 || j == original.getHeight() - 1){
newPixel = 0;
if (end == "left") {
} else if (end == "right") {
} else if (end == "up") {
} else if (end == "down") {
}
/*if (i == 1 || j == 1 || i == original.getWidth()-2 || j == original.getHeight() - 2 && red > 2) {
System.out.println("Start Point: (" + i + ", " + j + ")");
}
if (i == 0 && j > 0 && j < original.getHeight()-1) {
redRightPixel = new Color(original.getRGB(i+1, j)).getRed();
if (i == 0 && redRightPixel > 2) {
System.out.println("Start Point: (" + i + ", " + j + ")");
newPixel = 255;
}
}*/
/*if (j == original.getHeight()-1 && i > 0 && i < original.getWidth()-1) {
redUpPixel = new Color(original.getRGB(i, j-1)).getRed();
if (redUpPixel > 2) {
System.out.println("End Point: (" + i + ", " + j + ")");
newPixel = 255;
}
}*/
}
if (start == "left") {
if (i == 1 && j != 0 && j != original.getHeight()-1 && red > 2) {
System.out.println("Start Point: (" + i + ", " + j + ")");
}
} else if (start == "right") {
if (i == original.getHeight()-2 && j != 0 && j != original.getHeight()-1 && red > threshold) {
System.out.println("Start Point: (" + i + ", " + j + ")");
}
} else if (start == "up") {
if (j == 1 && i != 0 && i != original.getWidth()-1 && red > threshold) {
System.out.println("Start Point: (" + i + ", " + j + ")");
}
} else if (start == "down") {
if (j == original.getHeight()-2 && i != 0 && i != original.getWidth()-1 && red > threshold) {
System.out.println("Start Point: (" + i + ", " + j + ")");
}
}
if (end == "left") {
if (i == 1 && j != 0 && j != original.getHeight()-1 && red > 2) {
System.out.println("End Point: (" + i + ", " + j + ")");
}
} else if (end == "right") {
if (i == original.getHeight()-2 && j != 0 && j != original.getHeight()-1 && red > threshold) {
System.out.println("End Point: (" + i + ", " + j + ")");
}
} else if (end == "up") {
if (j == 1 && i != 0 && i != original.getWidth()-1 && red > threshold) {
System.out.println("End Point: (" + i + ", " + j + ")");
}
} else if (end == "down") {
if (j == original.getHeight()-2 && i != 0 && i != original.getWidth()-1 && red > threshold) {
System.out.println("End Point: (" + i + ", " + j + ")");
}
}
newPixel = colorToRGB(alpha, newPixel, newPixel, newPixel);
binarized.setRGB(i, j, newPixel);
}
}
ImageIO.write(binarized, "gif",new File("maze-4") );
}
catch (IOException e)
{
e.printStackTrace();
}
}
private static int colorToRGB(int alpha, int red, int green, int blue) {
int newPixel = 0;
newPixel += alpha;
newPixel = newPixel << 8;
newPixel += red; newPixel = newPixel << 8;
newPixel += green; newPixel = newPixel << 8;
newPixel += blue;
return newPixel;
}
}
Example output of a 203 x 203 maze
1 answers
solution1
0 2018-09-27 00:41:07
One simple way that's only slightly more efficient is to not store the path you've followed so far in the stack with recursion. Instead store the path you've followed so far in either a java.util.BitSet (where you store each path pixel in element y*width + x of the BitSet ) or you can simply use the red area of the picture that you've colored in to store the path.
This avoids stack overflows.
The basic algorithm is to start at the start point and go in one of the four cardinal directions unless you've already visited that direction (either trying it and finding it a dead end or having come from that direction to get here). When you go in a direction, you do the same thing there. It's a simple nonrecursive loop.
When you hit a dead end, you figure out how you got there originally by checking all four directions from where you are to see where the path came from. You remove the red from the spot where you're standing and go back in the direction you came from. If there is no red path in any direction, you're at the starting point again and you've tried everything, so there's no solution to the maze.
When you backtrack, you try the next direction you haven't tried yet at the older square on the path until all directions are dead ends.
If you ever reach the end point, you're done.
Here's some pseudocode that can't generally handle cycles (paths that go in a "circle"), that's grossly inefficient (for example, it should use a BitSet instead of a boolean[][] ), and that probably has some bugs, but it gives the general idea:
public class MazeSolver {
private static enum Direction { UP, RIGHT, DOWN, LEFT }
// Return array's element is true if that's part of the path
public static boolean[][] solve(final boolean[][] mazeWallHere,
int x, int y,
final int endX, final int endY) {
final int width = mazeWallHere.length;
final int height = mazeWallHere[0].length;
final boolean[][] path = new boolean[width][height];
Direction nextDirection = Direction.UP;
boolean backtrack = false;
while (true) {
// If this spot is a dead end in all new directions, head back
if (backtrack) {
backtrack = false;
// Unmark where we are
path[x][y] = false;
// Find where we came from and what direction we took to get here
// Then switch to the next direction
// If all directions have been tried, backtrack again
// If we can't backtrack, return null because there's no solution
// If we went up to get here, go back down and try going right.
if (y != 0 && path[x][y - 1]) {
y--;
nextDirection = Direction.RIGHT;
continue;
}
// If we went right to get here, go back left and try going down.
else if (x != 0 && path[x - 1][y]) {
x--;
nextDirection = Direction.DOWN;
continue;
}
// If we went down to get here, go back up and try going left.
else if (y < height && path[x][y + 1]) {
y++;
nextDirection = Direction.LEFT;
continue;
}
// If we went left to get here, go back right and backtrack again.
else if (x < width && path[x + 1][y]) {
x++;
backtrack = true;
continue;
}
// If we didn't come from anywhere, we're at the starting point
// All possible paths are dead ends
else return null;
}
// Mark where we are
path[x][y] = true;
// If we've solved it, return the solution
if (x == endX && y == endY) return path;
// Move unless we:
// * hit the edge of the maze
// * it's the direction we originally got here from
// * hit a wall
// If we can't go a certain direction, try the next direction
// If we're out of directions to try, backtrack
switch (nextDirection) {
case UP: if (y == height
|| path[x][y + 1]
|| mazeWallHere[x][y + 1]) {
nextDirection = Direction.RIGHT;
continue;
}
else y++;
break;
case RIGHT: if (x == width
|| path[x + 1][y]
|| mazeWallHere[x + 1][y]) {
nextDirection = Direction.DOWN;
continue;
}
else x++;
break;
case DOWN: if (y == 0
|| path[x][y - 1]
|| mazeWallHere[x][y - 1]) {
nextDirection = Direction.LEFT;
continue;
}
else y--;
break;
case LEFT: if (x == 0
|| path[x - 1][y]
|| mazeWallHere[x - 1][y]) {
backtrack = true;
continue;
}
else x--;
break;
}
}
}
}
If you want to handle cycles properly, make path an int[][] and store the move number instead of true so that you know which path is older.
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