rpc/nns: add InferHash convenience function

Refs. #348

Signed-off-by: Roman Khimov <roman@nspcc.ru>

rpc/nns/hashes.go

@@ -0,0 +1,29 @@
+package nns
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/core/state"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+// ID is the default NNS contract ID in all NeoFS networks. NeoFS networks
+// always deploy NNS first and can't work without it, therefore it always gets
+// and ID of 1.
+const ID = 1
+
+// ContractStateGetter is the interface required for contract state resolution
+// using a known contract ID.
+type ContractStateGetter interface {
+	GetContractStateByID(int32) (*state.Contract, error)
+}
+
+// InferHash simplifies resolving NNS contract hash in existing NeoFS networks.
+// It assumes that NNS follows [ID] assignment assumptions which likely won't
+// be the case for any non-NeoFS network.
+func InferHash(sg ContractStateGetter) (util.Uint160, error) {
+	c, err := sg.GetContractStateByID(ID)
+	if err != nil {
+		return util.Uint160{}, err
+	}
+
+	return c.Hash, nil
+}

rpc/nns/hashes_test.go

@@ -0,0 +1,37 @@
+package nns
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/core/state"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/stretchr/testify/require"
+)
+
+type stateGetter struct {
+	f func(int32) (*state.Contract, error)
+}
+
+func (s stateGetter) GetContractStateByID(id int32) (*state.Contract, error) {
+	return s.f(id)
+}
+
+func TestInferHash(t *testing.T) {
+	var sg stateGetter
+	sg.f = func(int32) (*state.Contract, error) {
+		return nil, errors.New("bad")
+	}
+	_, err := InferHash(sg)
+	require.Error(t, err)
+	sg.f = func(int32) (*state.Contract, error) {
+		return &state.Contract{
+			ContractBase: state.ContractBase{
+				Hash: util.Uint160{0x01, 0x02, 0x03},
+			},
+		}, nil
+	}
+	h, err := InferHash(sg)
+	require.NoError(t, err)
+	require.Equal(t, util.Uint160{0x01, 0x02, 0x03}, h)
+}