[#170] Implement validator voting function in inner ring

This function calculates validator key based on inner ring index
and invokes `Vote` method of corresponding alphabet contract.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/innerring/state.go

@@ -1,5 +1,10 @@
 package innerring
 
+import (
+	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	"github.com/nspcc-dev/neofs-node/pkg/innerring/invoke"
+)
+
 // EpochCounter is a getter for a global epoch counter.
 func (s *Server) EpochCounter() uint64 {
 	return s.epochCounter.Load()
@@ -21,3 +26,28 @@ func (s *Server) IsActive() bool {
 func (s *Server) Index() int32 {
 	return s.innerRingIndex.Load()
 }
+
+func (s *Server) voteForSidechainValidator(validators []keys.PublicKey) error {
+	index := s.Index()
+	if index < 0 || index >= alphabetContractsN {
+		s.log.Info("ignore validator vote: node not in alphabet range")
+
+		return nil
+	}
+
+	if len(validators) == 0 {
+		s.log.Info("ignore validator vote: empty validators list")
+
+		return nil
+	}
+
+	// len(validators) can be less than 7, in this case we split votes by
+	// using `mod` operations, e.g: with two validators:
+	// - node[0] vote for validators[0],
+	// - node[1] vote for validators[1],
+	// - node[2] vote for validators[0], etc.
+	// If there are more than 7 validators, this function will ignore the rest.
+	candidate := validators[int(index)%len(validators)]
+
+	return invoke.AlphabetVote(s.morphClient, s.contracts.alphabet[index], candidate)
+}