storage: add deduplication function

This adds a function to deduplicate two series sets given that duplicate
series have equivalent data points.

storage/interface.go

@@ -90,3 +90,66 @@ type SeriesIterator interface {
 	// Err returns the current error.
 	Err() error
 }
+
+// dedupedSeriesSet takes two series sets and returns them deduplicated.
+// The input sets must be sorted and identical if two series exist in both, i.e.
+// if their label sets are equal, the datapoints must be equal as well.
+type dedupedSeriesSet struct {
+	a, b SeriesSet
+
+	cur          Series
+	adone, bdone bool
+}
+
+// DeduplicateSeriesSet merges two SeriesSet and removes duplicates.
+// If two series exist in both sets, their datapoints must be equal.
+func DeduplicateSeriesSet(a, b SeriesSet) SeriesSet {
+	s := &dedupedSeriesSet{a: a, b: b}
+	s.adone = !s.a.Next()
+	s.bdone = !s.b.Next()
+
+	return s
+}
+
+func (s *dedupedSeriesSet) At() Series {
+	return s.cur
+}
+
+func (s *dedupedSeriesSet) Err() error {
+	if s.a.Err() != nil {
+		return s.a.Err()
+	}
+	return s.b.Err()
+}
+
+func (s *dedupedSeriesSet) compare() int {
+	if s.adone {
+		return 1
+	}
+	if s.bdone {
+		return -1
+	}
+	return labels.Compare(s.a.At().Labels(), s.b.At().Labels())
+}
+
+func (s *dedupedSeriesSet) Next() bool {
+	if s.adone && s.bdone || s.Err() != nil {
+		return false
+	}
+
+	d := s.compare()
+
+	// Both sets contain the current series. Chain them into a single one.
+	if d > 0 {
+		s.cur = s.b.At()
+		s.bdone = !s.b.Next()
+	} else if d < 0 {
+		s.cur = s.a.At()
+		s.adone = !s.a.Next()
+	} else {
+		s.cur = s.a.At()
+		s.adone = !s.a.Next()
+		s.bdone = !s.b.Next()
+	}
+	return true
+}