Closest pair of points using sweep line algorithm in Java

Question

First off; I'm doing this as an assignment for school, and that's why I'm using the sweep line algorithm. I'm basing it off of the pseudocode given by my teacher.

I've done an implementation of my own using TreeMap instead of a balanced binary search tree, which I was told would provide the same functionality. (Don't know if this is true, though?)

However I don't get the proper end result, and I really have no idea why. I've been staring myself blind.

Below is the part of my code that performs the actual computation. I've omitted the creation of the points-list, and other unimportant stuff.

            count = 0;

            TreeMap<Double, Point> tree = new TreeMap<Double, Point>();

            double dist = Double.POSITIVE_INFINITY;

            // Sorts points on x-axis
            Collections.sort(points); 

            // Gets left-most point
            Point q = points.get(count++);

            for (Point p : points) {

                while (q.getX() < p.getX() - dist) {
                    tree.remove(q.getY());
                    q = points.get(count++);
                }

                NavigableSet<Double> keys = tree.navigableKeySet();

                // Look at the 4 points above 'p'
                int i = 1;
                Iterator<Double> iterHi = keys.tailSet(p.getY()).iterator();

                while (i <= 4 && iterHi.hasNext()) {
                    double tmp = p.distanceTo(tree.get(iterHi.next()));
                    if (tmp < dist) {
                        dist = tmp;
                        pClosest = p;
                        qClosest = q;
                    }
                    i++;
                }

                // Look at the 4 points below 'p'
                i = 1;
                Iterator<Double> iterLo = keys.headSet(p.getY()).iterator();

                while (i <= 4 && iterLo.hasNext()) {
                    double tmp = q.distanceTo(tree.get(iterLo.next()));
                    if (tmp < dist) {
                        dist = tmp;
                        pClosest = p;
                        qClosest = q;
                    }
                    i++;
                }

                tree.put(p.getY(), p);
            }

            double finalDist = pClosest.distanceTo(qClosest);

Edit: The pseudocode can be found here: http://pastebin.com/i0XbPp1a . It's based on notes taken from what my teacher wrote on the whiteboard.

Regarding results: Using the following points (X, Y): (0, 2) - (6, 67) - (43, 71) - (39, 107) - (189, 140)

I should get ~36, but I'm getting ~65.

What is the end result you expect ? What are you getting instead ? — Florent Bayle, Jan 09 '15 at 11:20
Surely the left most point is at index 0, not 1? Point q = points.get(count++); — ConMan, Jan 09 '15 at 11:24
Could you provide the pseudo-code given by your teacher? What is your expected output and how is it different to the actual output? — Tim Hallyburton, Jan 09 '15 at 12:03
ConMan: Good point. I've changed my code to increment after "get". — Cethy, Jan 09 '15 at 12:07
Tim Hallyburton and Florent Bayle: I've edited the original post to answer your questions. — Cethy, Jan 09 '15 at 12:13

score 2 · Accepted Answer · answered Jan 09 '15 at 11:51

I have already found several bugs in your code(I'm not sure that there are no others):

What if several points have the same y coordinate? A TreeMap can hold only one point for each y value. Is that what you want?
When you look at points below and above the current one, you compute a distance to the iterHi.next(): double tmp = p.distanceTo(tree.get(iterHi.next()));, but then assign qClosest to q. It is not correct(obviously, iterHi.next() and q is not the same point).
In the second inner loop, you compute the distance from q to the element of the set: double tmp = q.distanceTo(tree.get(iterLo.next()));. It should be p instead.

I would also recommend maintaining a TreeSet of Point instead of using a TreeMap(they should compared by their y coordinate, of course).

1. In the set of points I'm currently using all the y-values are exclusive, so that shouldn't be a problem, atleast for the moment. 2. I'm a bit uncertain about that, since the pseudocode says to assign it to 'q', which was given a value in the while-loop at the top. 3. Again; This is what was written in the pseudocode. But what you're saying makes sense, and I'll try and change it. — Cethy, Jan 09 '15 at 12:20
Okay, applying the changes in 2 and 3 gave me the correct results. Thanks a lot, ILoveCoding! — Cethy, Jan 09 '15 at 12:27

Closest pair of points using sweep line algorithm in Java

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