From 093798375e8bb418779952801a36a50f8f01c69f Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@victoriametrics.com>
Date: Wed, 7 Feb 2024 21:33:18 +0200
Subject: [PATCH] lib/protoparser/datadogsketches: add more permalinks to the
 original source code

These permalinks should help verifying the correctness of the code

This is a follow-up after 07213f4e0cc140a4f37f5b045afdfe7caf5728e0

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5775
---
 lib/protoparser/datadogsketches/parser.go     | 39 +++++++++++--------
 .../datadogsketches/parser_test.go            |  4 +-
 2 files changed, 24 insertions(+), 19 deletions(-)

diff --git a/lib/protoparser/datadogsketches/parser.go b/lib/protoparser/datadogsketches/parser.go
index c9242bc57..ec2b21268 100644
--- a/lib/protoparser/datadogsketches/parser.go
+++ b/lib/protoparser/datadogsketches/parser.go
@@ -9,11 +9,11 @@ import (
 )
 
 var (
-	// https://github.com/DataDog/opentelemetry-mapping-go/blob/48d52eeea60d28da2e14c154a24557c4d290c6e2/pkg/quantile/config.go
-	epsillon   = 1.0 / 128
-	gamma      = 1 + 2*epsillon
-	gammaLn    = math.Log(gamma)
-	defaultMin = 0.981e-9
+	// These constants were obtained from https://github.com/DataDog/opentelemetry-mapping-go/blob/48d52eeea60d28da2e14c154a24557c4d290c6e2/pkg/quantile/config.go
+	eps        = 1.0 / 128
+	gamma      = 1 + 2*eps
+	gammaLn    = math.Log1p(2 * eps)
+	defaultMin = 1e-9
 	bias       = 1 - int(math.Floor(math.Log(defaultMin)/gammaLn))
 	quantiles  = []float64{0.5, 0.75, 0.9, 0.95, 0.99}
 )
@@ -174,7 +174,7 @@ func (s *Sketch) ToSummary() []*Metric {
 			timestamp := d.Ts * 1000
 			points[j] = Point{
 				Timestamp: timestamp,
-				Value:     d.valueForQuantile(q),
+				Value:     d.quantile(q),
 			}
 			sumPoints[j] = Point{
 				Timestamp: timestamp,
@@ -285,7 +285,8 @@ func (d *Dogsketch) unmarshalProtobuf(src []byte) (err error) {
 	return nil
 }
 
-func (d *Dogsketch) valueForQuantile(q float64) float64 {
+// This function has been copied from https://github.com/DataDog/opentelemetry-mapping-go/blob/48d52eeea60d28da2e14c154a24557c4d290c6e2/pkg/quantile/sparse.go#L92
+func (d *Dogsketch) quantile(q float64) float64 {
 	switch {
 	case d.Cnt == 0:
 		return 0
@@ -312,10 +313,7 @@ func (d *Dogsketch) valueForQuantile(q float64) float64 {
 		weight := (cnt - rank) / float64(n)
 		vLow := f64(ks[i])
 		vHigh := vLow * gamma
-		switch i {
-		case len(ns):
-			vHigh = d.Max
-		case 0:
+		if i == 0 {
 			vLow = d.Min
 		}
 		return vLow*weight + vHigh*(1-weight)
@@ -323,15 +321,22 @@ func (d *Dogsketch) valueForQuantile(q float64) float64 {
 	return d.Max
 }
 
+// This function has been copied from https://github.com/DataDog/opentelemetry-mapping-go/blob/48d52eeea60d28da2e14c154a24557c4d290c6e2/pkg/quantile/config.go#L54
 func f64(k int32) float64 {
-	switch {
-	case k < 0:
-		return -f64(-k)
-	case k == math.MaxInt16 || k == math.MinInt16:
-		return math.Inf(int(k))
-	case k == 0:
+	// See https://github.com/DataDog/opentelemetry-mapping-go/blob/48d52eeea60d28da2e14c154a24557c4d290c6e2/pkg/quantile/key.go#L14
+	if k <= -((1 << 15) - 1) {
+		return math.Inf(-1)
+	}
+	if k >= ((1 << 15) - 1) {
+		return math.Inf(1)
+	}
+	if k == 0 {
 		return 0
 	}
+	if k < 0 {
+		return -f64(-k)
+	}
+
 	exp := float64(int(k) - bias)
 	return math.Pow(gamma, exp)
 }
diff --git a/lib/protoparser/datadogsketches/parser_test.go b/lib/protoparser/datadogsketches/parser_test.go
index 94c3bf0f1..161f073ef 100644
--- a/lib/protoparser/datadogsketches/parser_test.go
+++ b/lib/protoparser/datadogsketches/parser_test.go
@@ -5,10 +5,10 @@ import (
 	"testing"
 )
 
-func TestPointsForQuantile(t *testing.T) {
+func TestDogsketchQuantile(t *testing.T) {
 	f := func(d *Dogsketch, q float64, vExpected float64) {
 		t.Helper()
-		v := d.valueForQuantile(q)
+		v := d.quantile(q)
 		if math.Abs(v-vExpected) > 0.4 {
 			t.Fatalf("unexpected value; got %v; want %v", v, vExpected)
 		}