[#308] audit: Add CollectMembers function

Add new function to build storage group using OIDs of virtual objects.

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

audit/collect.go

@@ -0,0 +1,76 @@
+package audit
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/nspcc-dev/neofs-sdk-go/checksum"
+	cid "github.com/nspcc-dev/neofs-sdk-go/container/id"
+	"github.com/nspcc-dev/neofs-sdk-go/object"
+	oid "github.com/nspcc-dev/neofs-sdk-go/object/id"
+	"github.com/nspcc-dev/neofs-sdk-go/object/relations"
+	"github.com/nspcc-dev/neofs-sdk-go/storagegroup"
+	"github.com/nspcc-dev/tzhash/tz"
+)
+
+type Collector interface {
+	Head(ctx context.Context, addr oid.Address) (*object.Object, error)
+	relations.Relations
+}
+
+// CollectMembers creates new storage group structure and fills it
+// with information about members collected via HeadReceiver.
+//
+// Resulting storage group consists of physically stored objects only.
+func CollectMembers(ctx context.Context, collector Collector, cnr cid.ID, members []oid.ID, tokens relations.Tokens, calcHomoHash bool) (*storagegroup.StorageGroup, error) {
+	var (
+		err        error
+		sumPhySize uint64
+		phyMembers []oid.ID
+		phyHashes  [][]byte
+		addr       oid.Address
+		sg         storagegroup.StorageGroup
+	)
+
+	addr.SetContainer(cnr)
+
+	for i := range members {
+		if phyMembers, err = relations.ListRelations(ctx, collector, cnr, members[i], tokens, false); err != nil {
+			return nil, err
+		}
+
+		for _, phyMember := range phyMembers {
+			addr.SetObject(phyMember)
+			leaf, err := collector.Head(ctx, addr)
+			if err != nil {
+				return nil, fmt.Errorf("head phy member '%s': %w", phyMember.EncodeToString(), err)
+			}
+
+			sumPhySize += leaf.PayloadSize()
+			cs, _ := leaf.PayloadHomomorphicHash()
+
+			if calcHomoHash {
+				phyHashes = append(phyHashes, cs.Value())
+			}
+		}
+	}
+
+	sg.SetMembers(phyMembers)
+	sg.SetValidationDataSize(sumPhySize)
+
+	if calcHomoHash {
+		sumHash, err := tz.Concat(phyHashes)
+		if err != nil {
+			return nil, err
+		}
+
+		var cs checksum.Checksum
+		tzHash := [64]byte{}
+		copy(tzHash[:], sumHash)
+		cs.SetTillichZemor(tzHash)
+
+		sg.SetValidationDataHash(cs)
+	}
+
+	return &sg, nil
+}