Adding test case.

Fixing a couple of bugs.

src/main/java/org/forkalsrud/util/VpTree.java

@@ -29,7 +29,7 @@ public class VpTree<T> {
     }
 
     
-    private class WithDistance implements Comparable<WithDistance> {
+    public class WithDistance implements Comparable<WithDistance> {
 
         private T value;
         private double distance;
@@ -52,13 +52,18 @@ public class VpTree<T> {
             double delta = other.distance - this.distance;
             return (int) Math.signum(delta);
         }
+        
+        @Override
+        public String toString() {
+            return String.valueOf(value) + " (" + distance + ")";
+        }
     }
 
     public List<WithDistance> search(final T target, int numResults) {
 
-        PriorityQueue<WithDistance> heap = new PriorityQueue<WithDistance>(numResults);
-        root.search(new Nearest(target, numResults));
-
+        Nearest acc = new Nearest(target, numResults);
+        root.search(acc);
+        PriorityQueue<WithDistance> heap = acc.matches;
         ArrayList<WithDistance> results = new ArrayList<WithDistance>(heap.size());
         while (!heap.isEmpty()) {
             results.add(heap.remove());
@@ -76,23 +81,27 @@ public class VpTree<T> {
     
     Node buildFromPoints(ArrayList<WithDistance> points, int left, int right) {
 
-        if (right <= left) {
-            return null;
+        WithDistance vp = points.get(left);
+        Node leftChild = null;
+        Node rightChild = null;
+        double threshold = 0d;
+        
+        if (right - left > 1) {
+            // choose an arbitrary point and move it to the start
+            int len = right - left;
+            int pos = left + random.nextInt(len);
+            Collections.swap(points, left, pos);
+            vp = points.get(left);
+            for (int i = left + 1; i < right; i++) {
+                WithDistance wd = points.get(i);
+                wd.distance = metric.distance(vp.value, wd.value);
+            }
+            Collections.sort(points.subList(left + 1, right));
+            int midpoint = (left + 1 + right) / 2;
+            threshold = points.get(midpoint).distance;
+            leftChild = buildFromPoints(points, left + 1, midpoint);
+            rightChild = buildFromPoints(points, midpoint, right);
         }
-        // choose an arbitrary point and move it to the start
-        int len = right - left;
-        int pos = left + random.nextInt(len);
-        Collections.swap(points, left, pos);
-        final WithDistance vp = points.get(left);
-        for (int i = left + 1; i < right; i++) {
-            WithDistance wd = points.get(i); 
-            wd.distance = metric.distance(vp.value, wd.value);
-        }
-        Collections.sort(points.subList(left + 1, right));
-        int midpoint = (left + 1 + right) / 2;
-        double threshold = points.get(midpoint).distance;
-        Node leftChild = buildFromPoints(points, left + 1, midpoint);
-        Node rightChild = buildFromPoints(points, midpoint, right);
         return new Node(vp.value, threshold, leftChild, rightChild);
     }
 
@@ -105,7 +114,7 @@ public class VpTree<T> {
         public Nearest(T query, int numMatches) {
             this.query = query;
             this.numMatches = numMatches;
-            this.matches = new PriorityQueue<WithDistance>(numMatches, Collections.reverseOrder());
+            this.matches = new PriorityQueue<WithDistance>(numMatches);
         }
 
         public double distanceFrom(T value) {

src/main/java/org/forkalsrud/util/VpTreeTest.java

@@ -0,0 +1,34 @@
+package org.forkalsrud.util;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.forkalsrud.util.VpTree.Metric;
+import static org.junit.Assert.*;
+import org.junit.Test;
+
+public class VpTreeTest {
+
+    @Test
+    public void testSearch() {
+        
+        double epsilon = 0.0003d;
+        
+        VpTree<Integer> vp = new VpTree<Integer>(Arrays.asList(4, 1, 7, 2, 9, 3, 5), new Metric<Integer>() {
+
+            @Override
+            public double distance(Integer a, Integer b) {
+                return Math.abs(b - a);
+            }
+            
+        });
+        
+        
+        List<VpTree<Integer>.WithDistance> result = vp.search(6, 3);
+        assertEquals(3, result.size());
+        assertEquals(1d, result.get(0).getDistance(), epsilon);
+        assertEquals(1d, result.get(1).getDistance(), epsilon);
+        assertEquals(2d, result.get(2).getDistance(), epsilon);
+    }
+
+}