[#477] metabase: Move MergeSplitInfo to storage/util pkg

This function already reused in different storage engine parts
so it makes sense to keep it in separate package.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/local_object_storage/util/splitinfo_test.go

@@ -0,0 +1,66 @@
+package util_test
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/util"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMergeSplitInfo(t *testing.T) {
+	uid, err := uuid.NewUUID()
+	require.NoError(t, err)
+
+	splitID := object.NewSplitID()
+	splitID.SetUUID(uid)
+
+	var rawLinkID, rawLastID [32]byte
+	linkID := object.NewID()
+	lastID := object.NewID()
+
+	_, err = rand.Read(rawLinkID[:])
+	require.NoError(t, err)
+	linkID.SetSHA256(rawLinkID)
+
+	_, err = rand.Read(rawLastID[:])
+	require.NoError(t, err)
+	lastID.SetSHA256(rawLastID)
+
+	target := object.NewSplitInfo() // target is SplitInfo struct with all fields set
+	target.SetSplitID(splitID)
+	target.SetLastPart(lastID)
+	target.SetLink(linkID)
+
+	t.Run("merge empty", func(t *testing.T) {
+		to := object.NewSplitInfo()
+
+		result := util.MergeSplitInfo(target, to)
+		require.Equal(t, result, target)
+	})
+
+	t.Run("merge link", func(t *testing.T) {
+		from := object.NewSplitInfo()
+		from.SetSplitID(splitID)
+		from.SetLastPart(lastID)
+
+		to := object.NewSplitInfo()
+		to.SetLink(linkID)
+
+		result := util.MergeSplitInfo(from, to)
+		require.Equal(t, result, target)
+	})
+	t.Run("merge last", func(t *testing.T) {
+		from := object.NewSplitInfo()
+		from.SetSplitID(splitID)
+		from.SetLink(linkID)
+
+		to := object.NewSplitInfo()
+		to.SetLastPart(lastID)
+
+		result := util.MergeSplitInfo(from, to)
+		require.Equal(t, result, target)
+	})
+}