Java: Iteration to find a coordinate point that hasn't been used?

Question

I am using the following method to try to find a point (coordinate) that hasn't been previously used, and isn't within the bounds of items that have previously used and coordinates.

The way it works is I am rendering "bases" (RTS top-down game), and I am creating two random variable locations for x and y. I pass these, along with the bases texture, into the following method. The method loops through a list of rectangles that are the rectangles of each previously rendered base. If the point is within any of the rectangles, the method is called again using a different set of coordinates. It does this until it finds a set that isn't within a rectangle. It then adds a new rectangle to the list at these coordinates, and returns them so the game can render a new base.

However, the bases still overlap.

Here is the method:

private Point getCoords(int x, int y, Texture t){
    for (int i=bases.size()-1; i> -1; i--) {
        if (bases.get(i).contains(new Point(x,y))){
            x = new Random().nextInt(map.getWidth() * map.getTileWidth());
            y = new Random().nextInt(map.getHeight() * map.getTileHeight());
            getCoords(x, y, t);
        }
    }
    bases.add(new Rectangle(x,y,t.getImage().getWidth(), t.getImage().getHeight()));
    return new Point(x, y);
}

And here is where it is being called:

switch(ran){
            default:
                int x = new Random().nextInt(map.getWidth() * map.getTileWidth());
                int y = new Random().nextInt(map.getHeight() * map.getTileHeight());
                Point p = getCoords(x, y, temp);
                map.generateBase("air", p.x, p.y);
                break;
        }

Any ideas what is wrong here?

Thanks

score 1 · Answer 1 · answered Feb 19 '16 at 14:13

1

            int x = new Random().nextInt(map.getWidth() * map.getTileHeight());

Maybe a bad copy paste. It may be :

            int x = new Random().nextInt(map.getWidth() * map.getTileWidth());

In both codes :-D

answered Feb 19 '16 at 14:13

Benjamin

3,350
4
24
49

I'm afraid it made no difference, they still overlap. But thanks for pointing it out :) – Kris Rice Feb 19 '16 at 14:19

score 1 · Accepted Answer · answered Feb 19 '16 at 14:39

There are several problems:

Your algorithm might be overwritting good coordinates (free ones) with wrong coordinates, you dont have any condition to exit the loop/recursion if you find a good place
You are checking for if rectangle contains the point, but later you are adding a rectanble, so it may not contain the point, but the rectangle created later may collide

try this

private Point getCoords(int x, int y, Texture t){
    boolean found = false;
    final int width = map.getTileWidth();
    final int height = map.getTileHeight();
    while(!found) {
            x = new Random().nextInt(map.getWidth() * width);
            y = new Random().nextInt(map.getHeight() * height);
            for (int i=bases.size()-1; i> -1; i--) {
                if (!bases.get(i).intersects(new Rectanble(x,y, width, height))){
                        found = true;
                } else found = false;
            }
    }

        bases.add(new Rectangle(x,y,t.getImage().getWidth(), t.getImage().getHeight()));
        return new Point(x, y);
}

*** EDIT: Im not sure if I had to use TileWidth and TileHeight or image width and image height for width and height :D

Thank you for your response. There is a issue with this though. I am using threads to update and render the game. The while loop causes the thread to stop updating other things, and at the moment multi threading is overcomplicated for a task so trivial. I was trying to avoid using while loops for this reason. Is there any other way to go about this? — Kris Rice, Feb 19 '16 at 14:48
There is no real difference in the execution flow between the for + recursion and the while + for. The only difference could be that the code you posted perform lighter operations and return wrong positions, so it has to perform much less iterations/calls to itself to find a point. If you are using threads already, just spawn a new one to call this method: When you have the results, the main thread will update the game with the solution, but wont block looking for a point. — Nadir, Feb 19 '16 at 14:55

score 0 · Answer 3 · answered Feb 19 '16 at 15:15

Okay so after some playing around, I found the issue is the rectangles that are saved are saved with a fixed location which means as the map moves, the rectangles don't. The fix is to loop through each bases and get the base's map position, rather than screen position, and check against this. Also, I found i was checking for a point in a rectangle, which may be outside the rectangle but leaves my bases overlapping still. So i now check for rectangle-rectangle collision instead

Java: Iteration to find a coordinate point that hasn't been used?

3 Answers3