Fix spike detection for short spikes at the tail of the data. (#119637) (#119668)

* Fix spike detection for short spikes at the tail of the data.

* Update docs/changelog/119637.yaml

Author: jan-elastic

docs/changelog/119637.yaml

@@ -0,0 +1,5 @@
+pr: 119637
+summary: Fix spike detection for short spikes at the tail of the data
+area: Machine Learning
+type: bug
+issues: []

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/aggs/changepoint/SpikeAndDipDetector.java

@@ -58,7 +58,7 @@ private SpikeOrDip findSpikeOrDip(double[] values, int extent, boolean negate) {
         int maxEnd = Math.min(maxStart + extent, values.length);
         double maxSum = sum(values, maxStart, maxEnd, negate);
         for (int start = maxStart + 1; start <= argmax; start++) {
-            if (start + extent >= values.length) {
+            if (start + extent > values.length) {
                 break;
             }
             double average = sum(values, start, start + extent, negate);

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/aggs/changepoint/SpikeAndDipDetectorTests.java

@@ -184,4 +184,14 @@ public void testMissingBuckets() {
         assertThat(change, instanceOf(ChangeType.Spike.class));
         assertThat(change.changePoint(), equalTo(10));
     }
+
+    public void testSpikeAtTail() {
+        MlAggsHelper.DoubleBucketValues bucketValues = new MlAggsHelper.DoubleBucketValues(
+            null,
+            new double[] { 2, 2, 2, 2, 3, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 2, 2, 9, 8 }
+        );
+        ChangeType change = new SpikeAndDipDetector(bucketValues).detect(0.01);
+        assertThat(change, instanceOf(ChangeType.Spike.class));
+        assertThat(change.changePoint(), equalTo(27));
+    }
 }