Add CalculateCommitteeSubscriptionResponse function to eth2exp

[xenowits]

Problem to be solved

Add a CalculateCommitteeSubscriptionResponse function to eth2exp/attagg.go that calculates is_aggregator from a BeaconCommitteeSubscription and returns a BeaconCommitteeSubscriptionResponse.

Proposed solution

• Make a HTTP request GET /eth/v1/beacon/states/{state_id}/committees?state_id=head&index=idx&slot=slot (https://ethereum.github.io/beacon-APIs/#/Beacon/getEpochCommittees)
• committeeLength = len(response.data.validators)

// CalculateCommitteeSubscriptionResponse returns a BeaconCommitteeSubscriptionResponse with IsAggregator field set to true if the validator is an aggregator.
func CalculateCommitteeSubscriptionResponse(subscription BeaconCommitteeSubscription) BeaconCommitteeSubscriptionResponse {
	// call IsAggregator() here
}

// https://github.com/prysmaticlabs/prysm/blob/b4d2395a38ea6cac15cb720672af12eeaf4d97a1/beacon-chain/core/helpers/attestation.go#L49
func isAggregator(committeeCount uint64, slotSig []byte) (bool, error) {
	var modulo = uint64(1)

	if committeeCount/targetAggregatorsPerCommittee > 1 {
		modulo = committeeCount / targetAggregatorsPerCommittee
	}

	b := eth2util.SHA256(slotSig)

	return binary.LittleEndian.Uint64(b[:8])%modulo == 0, nil
}

// https://github.com/ethereum/consensus-specs/blob/v0.9.3/specs/core/0_beacon-chain.md#hash
func SHA256(data []byte) [32]byte {
	h := sha256.New()
	h.Write(data)

	var b [32]byte
	h.Sum(b[:0])

	return b
}

Out of Scope

None