histogram: Fix bounds of buckets returned by floatBucketIterator

The bounds weren't really used so far, so no actual bug in the code so
far. But it's obviously confusing if the bounds returned by a
floatBucketIterator with a target schema different from the original
schema are wrong.

Signed-off-by: beorn7 <beorn@grafana.com>

model/histogram/float_histogram.go

@@ -818,6 +818,11 @@ type floatBucketIterator struct {
 	absoluteStartValue float64 // Never return buckets with an upper bound ≤ this value.
 }
 
+func (i *floatBucketIterator) At() Bucket[float64] {
+	// Need to use i.targetSchema rather than i.baseBucketIterator.schema.
+	return i.baseBucketIterator.at(i.targetSchema)
+}
+
 func (i *floatBucketIterator) Next() bool {
 	if i.spansIdx >= len(i.spans) {
 		return false

model/histogram/float_histogram_test.go

@@ -2205,3 +2205,50 @@ func TestAllReverseFloatBucketIterator(t *testing.T) {
 		})
 	}
 }
+
+func TestFloatBucketIteratorTargetSchema(t *testing.T) {
+	h := FloatHistogram{
+		Count:  405,
+		Sum:    1008.4,
+		Schema: 1,
+		PositiveSpans: []Span{
+			{Offset: 0, Length: 4},
+			{Offset: 1, Length: 3},
+			{Offset: 2, Length: 3},
+		},
+		PositiveBuckets: []float64{100, 344, 123, 55, 3, 63, 2, 54, 235, 33},
+		NegativeSpans: []Span{
+			{Offset: 0, Length: 3},
+			{Offset: 7, Length: 4},
+			{Offset: 1, Length: 3},
+		},
+		NegativeBuckets: []float64{10, 34, 1230, 54, 67, 63, 2, 554, 235, 33},
+	}
+	expPositiveBuckets := []Bucket[float64]{
+		{Lower: 0.25, Upper: 1, LowerInclusive: false, UpperInclusive: true, Count: 100, Index: 0},
+		{Lower: 1, Upper: 4, LowerInclusive: false, UpperInclusive: true, Count: 522, Index: 1},
+		{Lower: 4, Upper: 16, LowerInclusive: false, UpperInclusive: true, Count: 68, Index: 2},
+		{Lower: 16, Upper: 64, LowerInclusive: false, UpperInclusive: true, Count: 322, Index: 3},
+	}
+	expNegativeBuckets := []Bucket[float64]{
+		{Lower: -1, Upper: -0.25, LowerInclusive: true, UpperInclusive: false, Count: 10, Index: 0},
+		{Lower: -4, Upper: -1, LowerInclusive: true, UpperInclusive: false, Count: 1264, Index: 1},
+		{Lower: -64, Upper: -16, LowerInclusive: true, UpperInclusive: false, Count: 184, Index: 3},
+		{Lower: -256, Upper: -64, LowerInclusive: true, UpperInclusive: false, Count: 791, Index: 4},
+		{Lower: -1024, Upper: -256, LowerInclusive: true, UpperInclusive: false, Count: 33, Index: 5},
+	}
+
+	it := h.floatBucketIterator(true, 0, -1)
+	for i, b := range expPositiveBuckets {
+		require.True(t, it.Next(), "positive iterator exhausted too early")
+		require.Equal(t, b, it.At(), "bucket %d", i)
+	}
+	require.False(t, it.Next(), "positive iterator not exhausted")
+
+	it = h.floatBucketIterator(false, 0, -1)
+	for i, b := range expNegativeBuckets {
+		require.True(t, it.Next(), "negative iterator exhausted too early")
+		require.Equal(t, b, it.At(), "bucket %d", i)
+	}
+	require.False(t, it.Next(), "negative iterator not exhausted")
+}

model/histogram/generic.go

@@ -102,16 +102,22 @@ type baseBucketIterator[BC BucketCount, IBC InternalBucketCount] struct {
 }
 
 func (b baseBucketIterator[BC, IBC]) At() Bucket[BC] {
+	return b.at(b.schema)
+}
+
+// at is an internal version of the exported At to enable using a different
+// schema.
+func (b baseBucketIterator[BC, IBC]) at(schema int32) Bucket[BC] {
 	bucket := Bucket[BC]{
 		Count: BC(b.currCount),
 		Index: b.currIdx,
 	}
 	if b.positive {
-		bucket.Upper = getBound(b.currIdx, b.schema)
-		bucket.Lower = getBound(b.currIdx-1, b.schema)
+		bucket.Upper = getBound(b.currIdx, schema)
+		bucket.Lower = getBound(b.currIdx-1, schema)
 	} else {
-		bucket.Lower = -getBound(b.currIdx, b.schema)
-		bucket.Upper = -getBound(b.currIdx-1, b.schema)
+		bucket.Lower = -getBound(b.currIdx, schema)
+		bucket.Upper = -getBound(b.currIdx-1, schema)
 	}
 	bucket.LowerInclusive = bucket.Lower < 0
 	bucket.UpperInclusive = bucket.Upper > 0