deltatocumulative: Fix selection of the target scale for exponential histograms (#37432)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
While addressing comments a bug was added to the logic of calculating of
the desired scale and it slipped through tests.
Fix the bug (use `min` instead of `max`) and update tests to avoid
regressions in the future.

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes #37416 

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Update tests to separately cover positive and negative buckets.

<!--Describe the documentation added.-->
#### Documentation
n/a

<!--Please delete paragraphs that you did not use before submitting.-->

Author: euroelessar

.chloggen/expo-histogram-fix-downscaling.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: deltatocumulativeprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: In order to cap number of histogram buckets take the min of desired scale across negative and positive buckets instead of the max
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37416]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

processor/deltatocumulativeprocessor/internal/data/add.go

@@ -80,7 +80,7 @@ func (dp ExpHistogram) Add(in ExpHistogram) ExpHistogram {
 
 	// Downscale if an expected number of buckets after the merge is too large.
 	from := expo.Scale(dp.Scale())
-	to := max(
+	to := min(
 		expo.Limit(maxBuckets, from, dp.Positive(), in.Positive()),
 		expo.Limit(maxBuckets, from, dp.Negative(), in.Negative()),
 	)

processor/deltatocumulativeprocessor/internal/data/expo_test.go

@@ -27,10 +27,11 @@ func TestExpoAdd(t *testing.T) {
 	defer func() { maxBuckets = prevMaxBuckets }()
 
 	cases := []struct {
-		name   string
-		dp, in expdp
-		want   expdp
-		flip   bool
+		name            string
+		dp, in          expdp
+		want            expdp
+		flip            bool
+		alsoTryEachSign bool
 	}{{
 		name: "noop",
 		dp:   expdp{PosNeg: bins{0, 0, 0, 0, 0, 0, 0, 0}.Into(), Count: 0},
@@ -108,6 +109,7 @@ func TestExpoAdd(t *testing.T) {
 			PosNeg: bins{3, 3, 3, 3, 3, 3, 3, 3}.Into(),
 			Count:  24,
 		},
+		alsoTryEachSign: true,
 	}, {
 		name: "scale/downscale_once_exceeds_limit",
 		dp: expdp{
@@ -125,6 +127,7 @@ func TestExpoAdd(t *testing.T) {
 			PosNeg: rawbs([]uint64{2, 2, 2, 6, 4, 4, 4}, 0),
 			Count:  24,
 		},
+		alsoTryEachSign: true,
 	}, {
 		name: "scale/downscale_multiple_times_until_within_limit",
 		dp: expdp{
@@ -142,6 +145,7 @@ func TestExpoAdd(t *testing.T) {
 			PosNeg: rawbs([]uint64{2, 4, 2, 4, 8, 4}, -2),
 			Count:  24,
 		},
+		alsoTryEachSign: true,
 	}, {
 		name: "scale/ignore_leading_trailing_zeros_in_bucket_count",
 		dp: expdp{
@@ -159,6 +163,7 @@ func TestExpoAdd(t *testing.T) {
 			PosNeg: rawbs([]uint64{1, 7, 7, 4, 3, 2, 2}, 0),
 			Count:  26,
 		},
+		alsoTryEachSign: true,
 	}, {
 		name: "scale/downscale_with_leading_trailing_zeros",
 		dp: expdp{
@@ -176,17 +181,18 @@ func TestExpoAdd(t *testing.T) {
 			PosNeg: rawbs([]uint64{11, 11, 0, 0, 12, 12}, -1),
 			Count:  46,
 		},
+		alsoTryEachSign: true,
 	}}
 
 	for _, cs := range cases {
-		run := func(dp, in expdp) func(t *testing.T) {
+		run := func(dp, in, want expdp) func(t *testing.T) {
 			return func(t *testing.T) {
 				is := datatest.New(t)
 
 				var (
 					dp   = ExpHistogram{dp.Into()}
 					in   = ExpHistogram{in.Into()}
-					want = ExpHistogram{cs.want.Into()}
+					want = ExpHistogram{want.Into()}
 				)
 
 				dp.SetTimestamp(0)
@@ -199,14 +205,32 @@ func TestExpoAdd(t *testing.T) {
 		}
 
 		if cs.flip {
-			t.Run(cs.name+"-dp", run(cs.dp, cs.in))
-			t.Run(cs.name+"-in", run(cs.in, cs.dp))
+			t.Run(cs.name+"-dp", run(cs.dp, cs.in, cs.want))
+			t.Run(cs.name+"-in", run(cs.in, cs.dp, cs.want))
 			continue
 		}
-		t.Run(cs.name, run(cs.dp, cs.in))
+		if cs.alsoTryEachSign {
+			t.Run(cs.name+"-pos", run(clonePosExpdp(cs.dp), clonePosExpdp(cs.in), clonePosExpdp(cs.want)))
+			t.Run(cs.name+"-neg", run(cloneNegExpdp(cs.dp), cloneNegExpdp(cs.in), cloneNegExpdp(cs.want)))
+		}
+		t.Run(cs.name, run(cs.dp, cs.in, cs.want))
 	}
 }
 
+func cloneNegExpdp(dp expotest.Histogram) expotest.Histogram {
+	dp.Neg = pmetric.NewExponentialHistogramDataPointBuckets()
+	dp.PosNeg.CopyTo(dp.Neg)
+	dp.PosNeg = expo.Buckets{}
+	return dp
+}
+
+func clonePosExpdp(dp expotest.Histogram) expotest.Histogram {
+	dp.Pos = pmetric.NewExponentialHistogramDataPointBuckets()
+	dp.PosNeg.CopyTo(dp.Pos)
+	dp.PosNeg = expo.Buckets{}
+	return dp
+}
+
 func rawbs(data []uint64, offset int32) expo.Buckets {
 	bs := pmetric.NewExponentialHistogramDataPointBuckets()
 	bs.BucketCounts().FromRaw(data)