satellite/audit: merge support for reservoirs

Change-Id: Ibbedd2a0043412210159fa2523f9e63d987276c3

satellite/audit/reservoir.go

@@ -8,6 +8,8 @@ import (
 	"math/rand"
 	"time"
 
+	"github.com/zeebo/errs"
+
 	"storj.io/common/uuid"
 	"storj.io/storj/satellite/metabase"
 	"storj.io/storj/satellite/metabase/segmentloop"
@@ -55,6 +57,10 @@ func (reservoir *Reservoir) Keys() []float64 {
 // article: https://en.wikipedia.org/wiki/Reservoir_sampling#Algorithm_A-Res
 func (reservoir *Reservoir) Sample(r *rand.Rand, segment *segmentloop.Segment) {
 	k := -math.Log(r.Float64()) / float64(segment.EncryptedSize)
+	reservoir.sample(k, segment)
+}
+
+func (reservoir *Reservoir) sample(k float64, segment *segmentloop.Segment) {
 	if reservoir.index < reservoir.size {
 		reservoir.segments[reservoir.index] = *segment
 		reservoir.keys[reservoir.index] = k
@@ -73,6 +79,17 @@ func (reservoir *Reservoir) Sample(r *rand.Rand, segment *segmentloop.Segment) {
 	}
 }
 
+// Merge merges the given reservoir into the first. Both reservoirs must have the same size.
+func (reservoir *Reservoir) Merge(operand *Reservoir) error {
+	if reservoir.size != operand.size {
+		return errs.New("cannot merge: mismatched size: expected %d but got %d", reservoir.size, operand.size)
+	}
+	for i := int8(0); i < operand.index; i++ {
+		reservoir.sample(operand.keys[i], &operand.segments[i])
+	}
+	return nil
+}
+
 // Segment is a segment to audit.
 type Segment struct {
 	StreamID      uuid.UUID

satellite/audit/reservoir_test.go

@@ -23,13 +23,11 @@ import (
 func TestReservoir(t *testing.T) {
 	rng := rand.New(rand.NewSource(time.Now().Unix()))
 
-	seg := func(n int) segmentloop.Segment { return segmentloop.Segment{StreamID: uuid.UUID{0: byte(n)}} }
-
 	for size := 0; size < maxReservoirSize; size++ {
 		t.Run(fmt.Sprintf("size %d", size), func(t *testing.T) {
 			samples := []segmentloop.Segment{}
 			for i := 0; i < size; i++ {
-				samples = append(samples, seg(i))
+				samples = append(samples, makeSegment(i))
 			}
 
 			// If we sample N segments, less than the max, we should record all N
@@ -43,6 +41,46 @@ func TestReservoir(t *testing.T) {
 	}
 }
 
+func TestReservoirMerge(t *testing.T) {
+	t.Run("merge successful", func(t *testing.T) {
+		// Use a fixed rng so we get deterministic sampling results.
+		segments := []segmentloop.Segment{
+			makeSegment(0), makeSegment(1), makeSegment(2),
+			makeSegment(3), makeSegment(4), makeSegment(5),
+		}
+		rng := rand.New(rand.NewSource(999))
+		r1 := NewReservoir(3)
+		r1.Sample(rng, &segments[0])
+		r1.Sample(rng, &segments[1])
+		r1.Sample(rng, &segments[2])
+
+		r2 := NewReservoir(3)
+		r2.Sample(rng, &segments[3])
+		r2.Sample(rng, &segments[4])
+		r2.Sample(rng, &segments[5])
+
+		err := r1.Merge(r2)
+		require.NoError(t, err)
+
+		// Segments should contain a cross section from r1 and r2. If the rng
+		// changes, this result will likely change too since that will affect
+		// the keys. and therefore how they are merged.
+		require.Equal(t, []segmentloop.Segment{
+			segments[5],
+			segments[1],
+			segments[2],
+		}, r1.Segments())
+	})
+
+	t.Run("mismatched size", func(t *testing.T) {
+		r1 := NewReservoir(2)
+		r2 := NewReservoir(1)
+		err := r1.Merge(r2)
+		require.EqualError(t, err, "cannot merge: mismatched size: expected 2 but got 1")
+	})
+
+}
+
 func TestReservoirWeights(t *testing.T) {
 	var weight10StreamID = testrand.UUID()
 	var weight5StreamID = testrand.UUID()
@@ -159,3 +197,10 @@ type uint64Slice []uint64
 func (us uint64Slice) Len() int           { return len(us) }
 func (us uint64Slice) Swap(i, j int)      { us[i], us[j] = us[j], us[i] }
 func (us uint64Slice) Less(i, j int) bool { return us[i] < us[j] }
+
+func makeSegment(n int) segmentloop.Segment {
+	return segmentloop.Segment{
+		StreamID:      uuid.UUID{0: byte(n)},
+		EncryptedSize: int32(n * 1000),
+	}
+}