histogram: Fix bug in bucket key calculation

The current code doesn't work fork negative schemas if the observed value should go into a bucket with a non-positive key. Signed-off-by: beorn7 <beorn@grafana.com>
prometheus · May 25, 2023 · 77e97da · 77e97da
1 parent 3094812
commit 77e97da
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Showing 2 changed files with 5 additions and 5 deletions.
diff --git a/prometheus/histogram.go b/prometheus/histogram.go
@@ -639,8 +639,8 @@ func (hc *histogramCounts) observe(v float64, bucket int, doSparse bool) {
 			if frac == 0.5 {
 				key--
 			}
-			div := 1 << -schema
-			key = (key + div - 1) / div
+			offset := (1 << -schema) - 1
+			key = (key + offset) >> -schema
 		}
 		if isInf {
 			key++

diff --git a/prometheus/histogram_test.go b/prometheus/histogram_test.go
@@ -513,13 +513,13 @@ func TestNativeHistogram(t *testing.T) {
 		{
 			name: "factor 17 results in schema -2",
 			observations: []float64{
-				0.0156251, 0.0625, 0.1, 0.25, // Bucket -1: (0.015625, 0.25]
-				0.5, 1, // Bucket 0: (0.0625, 1]
+				0.0156251, 0.0625, // Bucket -1: (0.015625, 0.0625]
+				0.1, 0.25, 0.5, 1, // Bucket 0: (0.0625, 1]
 				1.5, 2, 3, 3.5, 5, 6, 7, // Bucket 1: (1, 16]
 				33.33, // Bucket 2: (16, 256]
 			},
 			factor: 17,
-			want:   `sample_count:14 sample_sum:63.2581251 schema:-2 zero_threshold:2.938735877055719e-39 zero_count:0 positive_span:<offset:-1 length:4 > positive_delta:4 positive_delta:-2 positive_delta:5 positive_delta:-6 `,
+			want:   `sample_count:14 sample_sum:63.2581251 schema:-2 zero_threshold:2.938735877055719e-39 zero_count:0 positive_span:<offset:-1 length:4 > positive_delta:2 positive_delta:2 positive_delta:3 positive_delta:-6 `,
 		},
 		{
 			name:         "negative buckets",