BFS: PriorityQueue not getting empty

Question

So I want to solve the problem described in my last post: Terrain/Mountain algorithm not working as intended. A copy of the problem is the following:

I want to create a terrain with a mountain on it, using a very basic principle, shown by this height mapping:

0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 1 0 0 0 0 0
0 0 0 0 1 2 1 0 0 0 0
0 0 0 1 2 3 2 1 0 0 0
0 0 1 2 3 4 3 2 1 0 0
0 0 0 1 2 3 2 1 0 0 0
0 0 0 0 1 2 1 0 0 0 0
0 0 0 0 0 1 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0

It starts at a random point with height = 4, and then gradually decreases it amonst the neighbours.

The recursive idea is simple, I start a point, recurse to the top/down/left/right with height - 1 (in this example), and only if not encountered yet, I set their values.

So I went ahead and implemented it with a BFS:

private void createMountain(final float[][] heightMapping, final float startHeight) {
    boolean[][] traversed = new boolean[width][depth];
    boolean positive = (startHeight >= 0f);
    int x = random.nextInt(width);
    int z = random.nextInt(depth);
    PriorityQueue<QueueElement> priorityQueue = new PriorityQueue<>((o1, o2) -> (int)Math.signum(o1.height - o2.height));
    priorityQueue.offer(new QueueElement(x, z, startHeight));
    while (!priorityQueue.isEmpty()) {
        QueueElement current = priorityQueue.poll();
        if (current.x < 0 || current.x >= width || current.z < 0 || current.z >= depth) {
            continue;
        }
        if (traversed[current.x][current.z]) {
            continue;
        }
        if ((positive && current.height <= 0f) || (!positive && current.height >= 0f)) {
            continue;
        }
        heightMapping[x][z] = current.height;
        priorityQueue.offer(new QueueElement(x, z - 1, calculateNewHeight(current.height, positive)));
        priorityQueue.offer(new QueueElement(x, z + 1, calculateNewHeight(current.height, positive)));
        priorityQueue.offer(new QueueElement(x - 1, z, calculateNewHeight(current.height, positive)));
        priorityQueue.offer(new QueueElement(x + 1, z, calculateNewHeight(current.height, positive)));
    }
}

private class QueueElement {
    public int x, z;
    public float height;

    public QueueElement(final int x, final int z, final float height) {
        this.x = x;
        this.z = z;
        this.height = height;
    }
}

private float calculateNewHeight(final float startHeight, final boolean positive) {
    float delta = startHeight / maxDecayFactor;
    return (positive) ? startHeight - delta : startHeight + delta;
}

Now the code never stops, I tried debugging it a few times, but either got no useful result, or a terrain that was completely flat.

The only real clue I have currently is that the priorityQueue.size() keeps increasing by 3 the whole time.
Does anyone have an idea of what is going on?

Update: Even after the fixes, it is still not working as intended.

0.6  0.5  0.4  0.3  0.3  0.2  0.2  0.1  0.1  0.1  0.1  0.1  0.0  0.0  0.0  0.0  
0.8  1.0  1.2  1.6  1.9  2.4  3.0  3.8  4.7  5.9  7.4  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  
0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0

What could possibly be wrong? Is the PriorityQueue now messing things up?

score 4 · Accepted Answer · answered Jan 31 '14 at 18:54

4

You forgot to set traversed[x][z] = true; when you poll a new element.
Make sure you do it AFTER you query if (traversed[current.x][current.z]) ...

Also, as a side note, for your implementation - a Queue will be a better idea I think. (when I wrote a PriorityQueue will be better I had multiple peaks in mind - which seems not to be the case for you, I think a simple Queue will be best).

answered Jan 31 '14 at 18:54

amit

175,853
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I was relocating the `traversed[x][z] = true` and forgot to put it back. But actually more things were wrong, I now added an epsilon value and also I needed to use `current.x` and `current.z` in most occasions, and not the initial `x` and `z`! The only problem I have now, is that the terrain still looks wrong, I'll add extra output to the OP. – skiwi Jan 31 '14 at 19:02
With using a `LinkedBlockingQueue` it now suddenly works! I yet have no clue though why it didn't work with a `PriorityQueue`. – skiwi Jan 31 '14 at 19:48
@skiwi what did you put as a comparator function to the priority queue? – amit Jan 31 '14 at 19:49
`PriorityQueue priorityQueue = new PriorityQueue<>((o1, o2) -> (int)Math.signum(o1.height - o2.height));`, which is using lambdas shorthand for maximum first. – skiwi Jan 31 '14 at 19:51
@skiwi I think it happened because you chagned `o2.height` for objects that are already in the priority queue (deeper in it) - which resulted in this behavior, but I could be wrong. – amit Jan 31 '14 at 19:56
A priority queue actually just violates the whole purpose of using a queue, nwo I think about it. – skiwi Jan 31 '14 at 21:01

BFS: PriorityQueue not getting empty

1 Answers1