prysm-pulse/beacon-chain/core/helpers/attestation.go

package helpers

import (
	"github.com/gogo/protobuf/proto"
	"github.com/pkg/errors"
	pb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
	ethpb "github.com/prysmaticlabs/prysm/proto/eth/v1alpha1"
	"github.com/prysmaticlabs/prysm/shared/bls"

	"github.com/prysmaticlabs/prysm/shared/params"
)

var (
	// ErrAttestationDataSlotNilState is returned when a nil state argument
	// is provided to AttestationDataSlot.
	ErrAttestationDataSlotNilState = errors.New("nil state provided for AttestationDataSlot")
	// ErrAttestationDataSlotNilData is returned when a nil attestation data
	// argument is provided to AttestationDataSlot.
	ErrAttestationDataSlotNilData = errors.New("nil data provided for AttestationDataSlot")
)

// AttestationDataSlot returns current slot of AttestationData for given state
//
// Spec pseudocode definition:
//   def get_attestation_data_slot(state: BeaconState, data: AttestationData) -> Slot:
//    """
//    Return the slot corresponding to the attestation ``data``.
//    """
//    committee_count = get_committee_count(state, data.target.epoch)
//    offset = (data.crosslink.shard + SHARD_COUNT - get_start_shard(state, data.target.epoch)) % SHARD_COUNT
//    return Slot(compute_start_slot_of_epoch(data.target.epoch) + offset // (committee_count // SLOTS_PER_EPOCH))
func AttestationDataSlot(state *pb.BeaconState, data *ethpb.AttestationData) (uint64, error) {
	if state == nil {
		return 0, ErrAttestationDataSlotNilState
	}
	if data == nil {
		return 0, ErrAttestationDataSlotNilData
	}

	committeeCount, err := CommitteeCount(state, data.Target.Epoch)
	if err != nil {
		return 0, err
	}

	epochStartShardNumber, err := StartShard(state, data.Target.Epoch)
	if err != nil { // This should never happen if CommitteeCount was successful
		return 0, errors.Wrap(err, "could not determine epoch start shard")
	}
	offset := (data.Crosslink.Shard + params.BeaconConfig().ShardCount -
		epochStartShardNumber) % params.BeaconConfig().ShardCount

	return StartSlot(data.Target.Epoch) + (offset / (committeeCount / params.BeaconConfig().SlotsPerEpoch)), nil
}

// AggregateAttestations such that the minimal number of attestations are returned.
// Note: this is currently a naive implementation to the order of O(n^2).
func AggregateAttestations(atts []*ethpb.Attestation) ([]*ethpb.Attestation, error) {
	if len(atts) <= 1 {
		return atts, nil
	}

	// Naive aggregation. O(n^2) time.
	for i, a := range atts {
		if i >= len(atts) {
			break
		}
		for j := i + 1; j < len(atts); j++ {
			b := atts[j]
			if !a.AggregationBits.Overlaps(b.AggregationBits) {
				var err error
				a, err = AggregateAttestation(a, b)
				if err != nil {
					return nil, err
				}
				// Delete b
				atts = append(atts[:j], atts[j+1:]...)
				j--
				atts[i] = a
			}
		}
	}

	// Naive deduplication of identical aggregations. O(n^2) time.
	for i, a := range atts {
		for j := i + 1; j < len(atts); j++ {
			b := atts[j]
			if a.AggregationBits.Contains(b.AggregationBits) {
				// If b is fully contained in a, then b can be removed.
				atts = append(atts[:j], atts[j+1:]...)
				j--
			} else if b.AggregationBits.Contains(a.AggregationBits) {
				// if a is fully contained in b, then a can be removed.
				atts = append(atts[:i], atts[i+1:]...)
				i--
				break // Stop the inner loop, advance a.
			}
		}
	}

	return atts, nil
}

// BLS aggregate signature aliases for testing / benchmark substitution. These methods are
// significantly more expensive than the inner logic of AggregateAttestations so they must be
// substituted for benchmarks which analyze AggregateAttestations.
var aggregateSignatures = bls.AggregateSignatures
var signatureFromBytes = bls.SignatureFromBytes

// AggregateAttestation aggregates attestations a1 and a2 together.
func AggregateAttestation(a1 *ethpb.Attestation, a2 *ethpb.Attestation) (*ethpb.Attestation, error) {
	baseAtt := proto.Clone(a1).(*ethpb.Attestation)
	newAtt := proto.Clone(a2).(*ethpb.Attestation)
	if newAtt.AggregationBits.Count() > baseAtt.AggregationBits.Count() {
		baseAtt, newAtt = newAtt, baseAtt
	}

	if baseAtt.AggregationBits.Contains(newAtt.AggregationBits) {
		return baseAtt, nil
	}

	newBits := baseAtt.AggregationBits.Or(newAtt.AggregationBits)
	newSig, err := signatureFromBytes(newAtt.Signature)
	if err != nil {
		return nil, err
	}
	baseSig, err := signatureFromBytes(baseAtt.Signature)
	if err != nil {
		return nil, err
	}

	aggregatedSig := aggregateSignatures([]*bls.Signature{baseSig, newSig})
	baseAtt.Signature = aggregatedSig.Marshal()
	baseAtt.AggregationBits = newBits

	return baseAtt, nil
}