[#31] placement: Implement container placement traverser

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/object_manager/placement/netmap.go

@@ -0,0 +1,32 @@
+package placement
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/pkg/netmap"
+	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	"github.com/pkg/errors"
+)
+
+type netMapBuilder struct {
+	nm *netmap.Netmap
+}
+
+func (b *netMapBuilder) BuildPlacement(a *object.Address, p *netmap.PlacementPolicy) ([]netmap.Nodes, error) {
+	aV2 := a.ToV2()
+
+	cn, err := b.nm.GetContainerNodes(p, aV2.GetContainerID().GetValue())
+	if err != nil {
+		return nil, errors.Wrap(err, "could not get container nodes")
+	}
+
+	oid := aV2.GetObjectID()
+	if oid == nil {
+		return cn.Replicas(), nil
+	}
+
+	on, err := b.nm.GetPlacementVectors(cn, oid.GetValue())
+	if err != nil {
+		return nil, errors.Wrap(err, "could not get placement vectors for object")
+	}
+
+	return on, nil
+}