[#82] netmap: Add functionality for working with subnets

* Implement `NodeInfo.EnterSubnet` method which adds node's subnet.
  * Implement `NodeInfo.IterateSubnets` which behaves similar to
  eponymous function from neofs-api-go/v2.
  * Implement `BelongsToSubnet` which checks if `NodeInfo` describes the
  node belonging to a specific subnet.

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

netmap/subnet_test.go

@@ -0,0 +1,102 @@
+package netmap_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-sdk-go/netmap"
+	subnetid "github.com/nspcc-dev/neofs-sdk-go/subnet/id"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNodeInfoSubnets(t *testing.T) {
+	t.Run("enter subnet", func(t *testing.T) {
+		var id subnetid.ID
+
+		id.SetNumber(13)
+
+		var node netmap.NodeInfo
+
+		node.EnterSubnet(id)
+
+		mIDs := make(map[string]struct{})
+
+		err := node.IterateSubnets(func(id subnetid.ID) error {
+			mIDs[id.String()] = struct{}{}
+			return nil
+		})
+
+		require.NoError(t, err)
+
+		_, ok := mIDs[id.String()]
+		require.True(t, ok)
+	})
+
+	t.Run("iterate with removal", func(t *testing.T) {
+		t.Run("not last", func(t *testing.T) {
+			var id, idrm subnetid.ID
+
+			id.SetNumber(13)
+			idrm.SetNumber(23)
+
+			var node netmap.NodeInfo
+
+			node.EnterSubnet(id)
+			node.EnterSubnet(idrm)
+
+			err := node.IterateSubnets(func(id subnetid.ID) error {
+				if subnetid.IsZero(id) || id.Equals(&idrm) {
+					return netmap.ErrRemoveSubnet
+				}
+
+				return nil
+			})
+
+			require.NoError(t, err)
+
+			mIDs := make(map[string]struct{})
+
+			err = node.IterateSubnets(func(id subnetid.ID) error {
+				mIDs[id.String()] = struct{}{}
+				return nil
+			})
+
+			require.NoError(t, err)
+
+			var zeroID subnetid.ID
+
+			_, ok := mIDs[zeroID.String()]
+			require.False(t, ok)
+
+			_, ok = mIDs[idrm.String()]
+			require.False(t, ok)
+
+			_, ok = mIDs[id.String()]
+			require.True(t, ok)
+		})
+
+		t.Run("last", func(t *testing.T) {
+			var node netmap.NodeInfo
+
+			err := node.IterateSubnets(func(id subnetid.ID) error {
+				return netmap.ErrRemoveSubnet
+			})
+
+			require.Error(t, err)
+		})
+	})
+}
+
+func TestBelongsToSubnet(t *testing.T) {
+	var id, idMiss, idZero subnetid.ID
+
+	id.SetNumber(13)
+	idMiss.SetNumber(23)
+
+	var node netmap.NodeInfo
+
+	node.EnterSubnet(id)
+
+	require.True(t, netmap.BelongsToSubnet(&node, idZero))
+	require.True(t, netmap.BelongsToSubnet(&node, id))
+	require.False(t, netmap.BelongsToSubnet(&node, idMiss))
+}