[#303] morph/netmap: Implement getting network map snapshot by epoch

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

pkg/morph/client/netmap/netmap.go

@@ -23,6 +23,18 @@ type GetNetMapValues struct {
 	peers [][]byte
 }
 
+// EpochSnapshotArgs groups the arguments
+// of snapshot by epoch test invoke call.
+type EpochSnapshotArgs struct {
+	epoch uint64
+}
+
+// EpochSnapshotValues groups the stack parameters
+// returned by snapshot by epoch test invoke.
+type EpochSnapshotValues struct {
+	*GetNetMapValues
+}
+
 const nodeInfoFixedPrmNumber = 1
 
 // SetDiff sets argument for snapshot method of
@@ -31,6 +43,11 @@ func (g *GetSnapshotArgs) SetDiff(d uint64) {
 	g.diff = d
 }
 
+// SetEpoch sets epoch number to get snapshot.
+func (a *EpochSnapshotArgs) SetEpoch(d uint64) {
+	a.epoch = d
+}
+
 // Peers return the list of peers from
 // network map in a binary format.
 func (g GetNetMapValues) Peers() [][]byte {
@@ -69,6 +86,29 @@ func (c *Client) Snapshot(a GetSnapshotArgs) (*GetNetMapValues, error) {
 	return peersFromStackItems(prms, c.snapshotMethod)
 }
 
+// EpochSnapshot performs the test invoke of get snapshot of network map by epoch
+// from NeoFS Netmap contract.
+func (c *Client) EpochSnapshot(args EpochSnapshotArgs) (*EpochSnapshotValues, error) {
+	prms, err := c.client.TestInvoke(
+		c.epochSnapshotMethod,
+		int64(args.epoch),
+	)
+	if err != nil {
+		return nil, errors.Wrapf(err,
+			"could not perform test invocation (%s)",
+			c.epochSnapshotMethod)
+	}
+
+	nmVals, err := peersFromStackItems(prms, c.epochSnapshotMethod)
+	if err != nil {
+		return nil, err
+	}
+
+	return &EpochSnapshotValues{
+		GetNetMapValues: nmVals,
+	}, nil
+}
+
 func peersFromStackItems(stack []stackitem.Item, method string) (*GetNetMapValues, error) {
 	if ln := len(stack); ln != 1 {
 		return nil, errors.Errorf("unexpected stack item count (%s): %d",