Set index limits in the grow funcs

output/cloud/expv2/hdr.go

@@ -116,22 +116,28 @@ func (h *histogram) addToBucket(v float64) {
 
 	index := resolveBucketIndex(v)
 
-	blen := len(h.Buckets)
-	if blen == 0 {
+	if len(h.Buckets) == 0 {
 		h.FirstNotZeroBucket = index
 		h.LastNotZeroBucket = index
 		h.Buckets = append(h.Buckets, 1)
 		return
 	}
 
-	if index < h.FirstNotZeroBucket {
+	// they grow the current Buckets slice if there isn't enough capacity.
+	//
+	// An example with growRight:
+	// With Buckets [4, 1] and index equals to 5
+	// then we expect a slice like [4,1,0,0,0,0]
+	// then the counter at 5th position will be incremented
+	// generating the final slice [4,1,0,0,0,1]
+	switch {
+	case index < h.FirstNotZeroBucket:
 		h.growLeft(index)
-		h.FirstNotZeroBucket = index
-	} else if index > h.LastNotZeroBucket {
+	case index > h.LastNotZeroBucket:
 		h.growRight(index)
-		h.LastNotZeroBucket = index
+	default:
+		h.Buckets[index-h.FirstNotZeroBucket]++
 	}
-	h.Buckets[index-h.FirstNotZeroBucket]++
 }
 
 func (h *histogram) growLeft(index uint32) {
@@ -143,12 +149,15 @@ func (h *histogram) growLeft(index uint32) {
 
 	// TODO: we may consider to swap by sub-groups
 	// e.g  [4, 1] => [4, 1, 0, 0] => [0, 0, 4, 1]
-	// It requires benchmark if it is better than
-	// just copy
+	// It requires a benchmark if it is better than just copy it.
 
 	newBuckets := make([]uint32, newLen)
 	copy(newBuckets[h.FirstNotZeroBucket-index:], h.Buckets)
 	h.Buckets = newBuckets
+
+	// Update the stats
+	h.Buckets[0] = 1
+	h.FirstNotZeroBucket = index
 }
 
 // growRight expands the buckets slice
@@ -171,6 +180,10 @@ func (h *histogram) growRight(index uint32) {
 		copy(newBuckets, h.Buckets)
 		h.Buckets = newBuckets
 	}
+
+	// Update the stats
+	h.Buckets[len(h.Buckets)-1] = 1
+	h.LastNotZeroBucket = index
 }
 
 // histogramAsProto converts the histogram into the equivalent Protobuf version.

output/cloud/expv2/hdr_test.go

@@ -41,12 +41,28 @@ func TestValueBacket(t *testing.T) {
 func TestNewHistogramWithSimpleValue(t *testing.T) {
 	t.Parallel()
 
-	// Add a lower bucket index within slice capacity
+	// Zero as value
 	res := newHistogram()
+	res.addToBucket(0)
+	exp := histogram{
+		Buckets:            []uint32{1},
+		FirstNotZeroBucket: 0,
+		LastNotZeroBucket:  0,
+		ExtraLowBucket:     0,
+		ExtraHighBucket:    0,
+		Max:                0,
+		Min:                0,
+		Sum:                0,
+		Count:              1,
+	}
+	require.Equal(t, exp, res)
+
+	// Add a lower bucket index within slice capacity
+	res = newHistogram()
 	res.addToBucket(8)
 	res.addToBucket(5)
 
-	exp := histogram{
+	exp = histogram{
 		Buckets:            []uint32{1, 0, 0, 1},
 		FirstNotZeroBucket: 5,
 		LastNotZeroBucket:  8,
@@ -233,7 +249,7 @@ func TestHistogramGrowRight(t *testing.T) {
 	h.growRight(5)
 	assert.Len(t, h.Buckets, 6)
 	assert.Equal(t, uint32(101), h.Buckets[2])
-	assert.Equal(t, uint32(0), h.Buckets[5])
+	assert.Equal(t, uint32(1), h.Buckets[5])
 
 	// it is not possible to request an index smaller than
 	// the last already available index