From 775ef5493af82b23a8aa636919f809d2d08ee555 Mon Sep 17 00:00:00 2001
From: Arnab Nandy <arnab_nandy7@yahoo.com>
Date: Fri, 3 Jul 2026 18:14:43 +0530
Subject: [PATCH] Fix native histogram bucket index merge on scale-down

Signed-off-by: Arnab Nandy <arnab_nandy7@yahoo.com>
---
 .../metrics/core/metrics/Histogram.java       |  2 +-
 .../metrics/core/metrics/HistogramTest.java   | 33 +++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java b/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java
index 9a7f9b7c9..c4bb1f5fe 100644
--- a/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java
+++ b/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java
@@ -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]);
       }
diff --git a/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java b/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java
index 69518205d..cbfd5fade 100644
--- a/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java
+++ b/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java
@@ -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;
@@ -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;
@@ -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()