core: introduce ValidatorsCount, make a proper count

We were completely lacking ValidatorsCount that is supposed to track the
number of votes with particular count of consensus nodes which in theory can
change the number of active consensus nodes (if it ever to exceed the number
of standby validators), so we were not producing the right count and based on
that not giving the right set of validators.

Fixes #512.

pkg/core/blockchain.go

@@ -648,6 +648,15 @@ func processTXWithValidatorsAdd(output *transaction.Output, account *state.Accou
 				return err
 			}
 		}
+		vc, err := dao.GetValidatorsCount()
+		if err != nil {
+			return err
+		}
+		vc[len(account.Votes)-1] += output.Amount
+		err = dao.PutValidatorsCount(vc)
+		if err != nil {
+			return err
+		}
 	}
 	return nil
 }
@@ -669,6 +678,17 @@ func processTXWithValidatorsSubtract(account *state.Account, dao *cachedDao, toS
 			}
 		}
 	}
+	if len(account.Votes) > 0 {
+		vc, err := dao.GetValidatorsCount()
+		if err != nil {
+			return err
+		}
+		vc[len(account.Votes)-1] -= toSubtract
+		err = dao.PutValidatorsCount(vc)
+		if err != nil {
+			return err
+		}
+	}
 	return nil
 }
 
@@ -713,6 +733,10 @@ func processAccountStateDescriptor(descriptor *transaction.StateDescriptor, dao
 		if err != nil {
 			return err
 		}
+		if len(votes) > state.MaxValidatorsVoted {
+			return errors.New("voting candidate limit exceeded")
+		}
+		if len(votes) > 0 {
 			account.Votes = votes
 			for _, vote := range account.Votes {
 				validatorState, err := dao.GetValidatorStateOrNew(vote)
@@ -724,6 +748,18 @@ func processAccountStateDescriptor(descriptor *transaction.StateDescriptor, dao
 					return err
 				}
 			}
+			vc, err := dao.GetValidatorsCount()
+			if err != nil {
+				return err
+			}
+			vc[len(account.Votes)-1] += balance
+			err = dao.PutValidatorsCount(vc)
+			if err != nil {
+				return err
+			}
+		} else {
+			account.Votes = nil
+		}
 		return dao.PutAccountState(account)
 	}
 	return nil
@@ -1357,7 +1393,11 @@ func (bc *Blockchain) GetValidators(txes ...*transaction.Transaction) ([]*keys.P
 		return validators[i].PublicKey.Cmp(validators[j].PublicKey) == -1
 	})
 
-	count := state.GetValidatorsWeightedAverage(validators)
+	validatorsCount, err := cache.GetValidatorsCount()
+	if err != nil {
+		return nil, err
+	}
+	count := validatorsCount.GetWeightedAverage()
 	standByValidators, err := bc.GetStandByValidators()
 	if err != nil {
 		return nil, err

pkg/core/dao.go

@@ -266,6 +266,24 @@ func (dao *dao) DeleteValidatorState(vs *state.Validator) error {
 	return dao.store.Delete(key)
 }
 
+// GetValidatorsCount returns current ValidatorsCount or new one if there is none
+// in the DB.
+func (dao *dao) GetValidatorsCount() (*state.ValidatorsCount, error) {
+	vc := &state.ValidatorsCount{}
+	key := []byte{byte(storage.IXValidatorsCount)}
+	err := dao.GetAndDecode(vc, key)
+	if err != nil && err != storage.ErrKeyNotFound {
+		return nil, err
+	}
+	return vc, nil
+}
+
+// PutValidatorsCount put given ValidatorsCount in the store.
+func (dao *dao) PutValidatorsCount(vc *state.ValidatorsCount) error {
+	key := []byte{byte(storage.IXValidatorsCount)}
+	return dao.Put(vc, key)
+}
+
 // -- end validator.
 
 // -- start notification event.

pkg/core/state/validator.go

@@ -6,6 +6,9 @@ import (
 	"github.com/CityOfZion/neo-go/pkg/util"
 )
 
+// MaxValidatorsVoted limits the number of validators that one can vote for.
+const MaxValidatorsVoted = 1024
+
 // Validator holds the state of a validator.
 type Validator struct {
 	PublicKey  *keys.PublicKey
@@ -13,6 +16,10 @@ type Validator struct {
 	Votes      util.Fixed8
 }
 
+// ValidatorsCount represents votes with particular number of consensus nodes
+// for this number to be changeable by the voting system.
+type ValidatorsCount [MaxValidatorsVoted]util.Fixed8
+
 // RegisteredAndHasVotes returns true or false whether Validator is registered and has votes.
 func (vs *Validator) RegisteredAndHasVotes() bool {
 	return vs.Registered && vs.Votes > util.Fixed8(0)
@@ -38,67 +45,64 @@ func (vs *Validator) DecodeBinary(reader *io.BinReader) {
 	vs.Votes.DecodeBinary(reader)
 }
 
-// GetValidatorsWeightedAverage applies weighted filter based on votes for validator and returns number of validators.
-// Get back to it with further investigation in https://github.com/nspcc-dev/neo-go/issues/512.
-func GetValidatorsWeightedAverage(validators []*Validator) int {
-	return int(weightedAverage(applyWeightedFilter(validators)))
-}
-
-// applyWeightedFilter is an implementation of the filter for validators votes.
-// C# reference https://github.com/neo-project/neo/blob/41caff115c28d6c7665b2a7ac72967e7ce82e921/neo/Helper.cs#L273
-func applyWeightedFilter(validators []*Validator) map[*Validator]float64 {
-	var validatorsWithVotes []*Validator
-	var amount float64
-
-	weightedVotes := make(map[*Validator]float64)
-	start := 0.25
-	end := 0.75
-	sum := float64(0)
-	current := float64(0)
-
-	for _, validator := range validators {
-		if validator.Votes > util.Fixed8(0) {
-			validatorsWithVotes = append(validatorsWithVotes, validator)
-			amount += validator.Votes.FloatValue()
+// EncodeBinary encodes ValidatorCount to the given BinWriter.
+func (vc *ValidatorsCount) EncodeBinary(w *io.BinWriter) {
+	for i := range vc {
+		vc[i].EncodeBinary(w)
 	}
 }
 
-	for _, validator := range validatorsWithVotes {
-		if current >= end {
+// DecodeBinary decodes ValidatorCount from the given BinReader.
+func (vc *ValidatorsCount) DecodeBinary(r *io.BinReader) {
+	for i := range vc {
+		vc[i].DecodeBinary(r)
+	}
+}
+
+// GetWeightedAverage returns an average count of validators that's been voted
+// for not counting 1/4 of minimum and maximum numbers.
+func (vc *ValidatorsCount) GetWeightedAverage() int {
+	const (
+		lowerThreshold = 0.25
+		upperThreshold = 0.75
+	)
+	var (
+		sumWeight, sumValue, overallSum, slidingSum util.Fixed8
+		slidingRatio                                float64
+	)
+
+	for i := range vc {
+		overallSum += vc[i]
+	}
+
+	for i := range vc {
+		if slidingRatio >= upperThreshold {
 			break
 		}
-		weight := validator.Votes.FloatValue()
-		sum += weight
-		old := current
-		current = sum / amount
+		weight := vc[i]
+		slidingSum += weight
+		previousRatio := slidingRatio
+		slidingRatio = slidingSum.FloatValue() / overallSum.FloatValue()
 
-		if current <= start {
+		if slidingRatio <= lowerThreshold {
 			continue
 		}
 
-		if old < start {
-			if current > end {
-				weight = (end - start) * amount
+		if previousRatio < lowerThreshold {
+			if slidingRatio > upperThreshold {
+				weight = util.Fixed8FromFloat((upperThreshold - lowerThreshold) * overallSum.FloatValue())
 			} else {
-				weight = (current - start) * amount
+				weight = util.Fixed8FromFloat((slidingRatio - lowerThreshold) * overallSum.FloatValue())
 			}
-		} else if current > end {
-			weight = (end - old) * amount
+		} else if slidingRatio > upperThreshold {
+			weight = util.Fixed8FromFloat((upperThreshold - previousRatio) * overallSum.FloatValue())
 		}
-		weightedVotes[validator] = weight
-	}
-	return weightedVotes
-}
-
-func weightedAverage(weightedVotes map[*Validator]float64) float64 {
-	sumWeight := float64(0)
-	sumValue := float64(0)
-	for vState, weight := range weightedVotes {
 		sumWeight += weight
-		sumValue += vState.Votes.FloatValue() * weight
+		// Votes with N values get stored with N-1 index, thus +1 here.
+		sumValue += util.Fixed8(i+1) * weight
 	}
 	if sumValue == 0 || sumWeight == 0 {
 		return 0
 	}
-	return sumValue / sumWeight
+	return int(sumValue / sumWeight)
 }