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fix: correct native histogram bucket merge on scale-down #2274

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fix: correct native histogram bucket merge on scale-down #2274

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2 changes: 1 addition & 1 deletion prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java
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Original file line number Diff line number Diff line change
Expand Up @@ -614,7 +614,7 @@ private void doubleBucketWidth(Map<Integer, LongAdder> buckets) {
}
buckets.clear();
for (i = 0; i < keys.length; i++) {
int index = (keys[i] + 1) / 2;
int index = (keys[i] > 0 ? keys[i] + 1 : keys[i]) / 2;
LongAdder count = buckets.computeIfAbsent(index, k -> new LongAdder());
count.add(values[i]);
}
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33 changes: 33 additions & 0 deletions prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,9 @@
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.CompletionService;
import java.util.concurrent.CountDownLatch;
Expand All @@ -39,6 +41,7 @@
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.LongAdder;
import java.util.stream.Collectors;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
Expand Down Expand Up @@ -836,6 +839,36 @@ void testNativeBucketIndexToUpperBound()
}
}

@Test
void testDoubleBucketWidthMergesNegativeIndexesCorrectly()
throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
Histogram histogram = Histogram.builder().name("test").nativeOnly().build();
Histogram.DataPoint dataPoint = histogram.newDataPoint();

Method doubleBucketWidth =
Histogram.DataPoint.class.getDeclaredMethod("doubleBucketWidth", Map.class);
doubleBucketWidth.setAccessible(true);

int[] keys = {-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5};
int[] expectedMergedIndexes = {-2, -2, -1, -1, 0, 0, 1, 1, 2, 2, 3};

for (int i = 0; i < keys.length; i++) {
Map<Integer, LongAdder> buckets = new HashMap<>();
LongAdder adder = new LongAdder();
adder.add(7);
buckets.put(keys[i], adder);

doubleBucketWidth.invoke(dataPoint, buckets);

assertThat(buckets.keySet())
.as("merged index for original key " + keys[i])
.containsExactly(expectedMergedIndexes[i]);
assertThat(buckets.get(expectedMergedIndexes[i]).sum())
.as("count preserved for original key " + keys[i])
.isEqualTo(7);
}
}

/**
* Test if lowerBound < value <= upperBound is true for the bucket index returned by
* findBucketIndex()
Expand Down