MATH-1683: Make precondition explicit.

Author: Gilles Sadowski

commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/ml/clustering/ElkanKMeansPlusPlusClusterer.java

@@ -234,6 +234,10 @@ private double[][] seed(final List<T> points) {
             sumSqDist += minDistances[i];
         }
 
+        if (sumSqDist == 0d) { // MATH-1683.
+            throw new IllegalArgumentException("All points are identical");
+        }
+
         while (++idx < k) {
             final double p = sumSqDist * random.nextDouble();
             int next = 0;
@@ -253,7 +257,6 @@ private double[][] seed(final List<T> points) {
         return result;
     }
 
-
     /**
      * Once initial centers are chosen, we can actually go through data points and assign points to the
      * cluster based on the distance between initial centers and points.

commons-math-legacy/src/test/java/org/apache/commons/math4/legacy/ml/clustering/ElkanKMeansPlusPlusClustererTest.java

@@ -112,6 +112,19 @@ public void oneClusterCenterShouldBeTheMean() {
         Assert.assertArrayEquals(mean.getResult(), clusters.get(0).getCenter().getPoint(), 1e-6);
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testAllPointsIdentical() {
+        final DoublePoint p = new DoublePoint(new double[] {0.0, 0.0});
+        // Create 4 identical points.
+        final List<DoublePoint> points = Arrays.asList(p, p, p, p);
+
+        // Attempt to cluster into 2 clusters.
+        ElkanKMeansPlusPlusClusterer<DoublePoint> clusterer =
+            new ElkanKMeansPlusPlusClusterer<>(2);
+
+        clusterer.cluster(points);
+    }
+
     /**
      * Generates a list of random uncorrelated points to cluster.
      *

src/changes/changes.xml

@@ -96,6 +96,9 @@ Caveat:
  to support the whole codebase (it was one of the main reasons for
  creating more focused components).
 ">
+      <action dev="erans" type="fix" issue="MATH-1683" due-to="Ruiqi Dong">
+        "ElkanKMeansPlusPlusClusterer": More explicit error message.
+      </action>
       <action dev="erans" type="fix" issue="MATH-1682" due-to="Ruiqi Dong">
         "SparseEntryIterator": Prevent exception upon iterating an empty vector.
       </action>