[#60] morph/netmap: Implement Epoch method on Client

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

pkg/morph/client/netmap/epoch.go

@@ -0,0 +1,49 @@
+package netmap
+
+import (
+	"github.com/nspcc-dev/neofs-node/pkg/morph/client"
+	"github.com/pkg/errors"
+)
+
+// EpochArgs groups the arguments
+// of get epoch number test invoke call.
+type EpochArgs struct {
+}
+
+// EpochValues groups the stack parameters
+// returned by get epoch number test invoke.
+type EpochValues struct {
+	num int64
+}
+
+// Number return the number of NeoFS epoch.
+func (e EpochValues) Number() int64 {
+	return e.num
+}
+
+// Epoch performs the test invoke of get epoch number
+// method of NeoFS Netmap contract.
+func (c *Client) Epoch(_ EpochArgs) (*EpochValues, error) {
+	items, err := c.client.TestInvoke(
+		c.epochMethod,
+	)
+	if err != nil {
+		return nil, errors.Wrapf(err,
+			"could not perform test invocation (%s)",
+			c.epochMethod)
+	}
+
+	if ln := len(items); ln != 1 {
+		return nil, errors.Errorf("unexpected stack item count (%s): %d",
+			c.epochMethod, ln)
+	}
+
+	num, err := client.IntFromStackItem(items[0])
+	if err != nil {
+		return nil, errors.Wrapf(err, "could not get number from stack item (%s)", c.epochMethod)
+	}
+
+	return &EpochValues{
+		num: num,
+	}, nil
+}