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nimbus-eth2/beacon_chain/spec/beaconstate.nim

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# beacon_chain
# Copyright (c) 2018-2019 Status Research & Development GmbH
# Licensed and distributed under either of
# * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
# * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.
import
tables, algorithm, math, options, sequtils,
json_serialization/std/sets, chronicles, stew/bitseqs,
../extras, ../ssz,
./crypto, ./datatypes, ./digest, ./helpers, ./validator
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#is_valid_merkle_branch
func is_valid_merkle_branch(leaf: Eth2Digest, branch: openarray[Eth2Digest], depth: uint64, index: uint64, root: Eth2Digest): bool =
## Check if ``leaf`` at ``index`` verifies against the Merkle ``root`` and
## ``branch``.
var
value = leaf
buf: array[64, byte]
for i in 0 ..< depth.int:
if (index div (1'u64 shl i)) mod 2 != 0:
buf[0..31] = branch[i.int].data
buf[32..63] = value.data
else:
buf[0..31] = value.data
buf[32..63] = branch[i.int].data
value = eth2hash(buf)
value == root
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#increase_balance
func increase_balance*(
state: var BeaconState, index: ValidatorIndex, delta: Gwei) =
# Increase the validator balance at index ``index`` by ``delta``.
state.balances[index] += delta
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#decrease_balance
func decrease_balance*(
state: var BeaconState, index: ValidatorIndex, delta: Gwei) =
## Decrease the validator balance at index ``index`` by ``delta``, with
## underflow protection.
state.balances[index] =
if delta > state.balances[index]:
0'u64
else:
state.balances[index] - delta
# https://github.com/ethereum/eth2.0-specs/blob/v0.7.1/specs/core/0_beacon-chain.md#deposits
func process_deposit*(
state: var BeaconState, deposit: Deposit, flags: UpdateFlags = {}): bool =
# Process an Eth1 deposit, registering a validator or increasing its balance.
# Verify the Merkle branch
# TODO enable this check, but don't use doAssert
if not is_valid_merkle_branch(
hash_tree_root(deposit.data),
deposit.proof,
DEPOSIT_CONTRACT_TREE_DEPTH,
state.eth1_deposit_index,
state.eth1_data.deposit_root,
):
## TODO: a notice-like mechanism which works in a func
## here and elsewhere, one minimal approach is a check-if-true
## and return false iff so.
## obviously, better/more principled ones exist, but
## generally require broader rearchitecting, and this is what
## mostly happens now, just haphazardly.
discard
# Deposits must be processed in order
state.eth1_deposit_index += 1
let
pubkey = deposit.data.pubkey
amount = deposit.data.amount
validator_pubkeys = mapIt(state.validators, it.pubkey)
index = validator_pubkeys.find(pubkey)
if index == -1:
# Verify the deposit signature (proof of possession)
if skipValidation notin flags and not bls_verify(
pubkey, signing_root(deposit.data).data, deposit.data.signature,
compute_domain(DOMAIN_DEPOSIT)):
return false
# Add validator and balance entries
state.validators.add(Validator(
pubkey: pubkey,
withdrawal_credentials: deposit.data.withdrawal_credentials,
activation_eligibility_epoch: FAR_FUTURE_EPOCH,
activation_epoch: FAR_FUTURE_EPOCH,
exit_epoch: FAR_FUTURE_EPOCH,
withdrawable_epoch: FAR_FUTURE_EPOCH,
effective_balance: min(amount - amount mod EFFECTIVE_BALANCE_INCREMENT,
MAX_EFFECTIVE_BALANCE)
))
state.balances.add(amount)
else:
# Increase balance by deposit amount
increase_balance(state, index.ValidatorIndex, amount)
true
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#compute_activation_exit_epoch
func compute_activation_exit_epoch*(epoch: Epoch): Epoch =
## Return the epoch during which validator activations and exits initiated in
## ``epoch`` take effect.
epoch + 1 + MAX_SEED_LOOKAHEAD
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_validator_churn_limit
func get_validator_churn_limit(state: BeaconState): uint64 =
# Return the validator churn limit for the current epoch.
let active_validator_indices =
get_active_validator_indices(state, get_current_epoch(state))
max(MIN_PER_EPOCH_CHURN_LIMIT,
len(active_validator_indices) div CHURN_LIMIT_QUOTIENT).uint64
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#initiate_validator_exit
func initiate_validator_exit*(state: var BeaconState,
index: ValidatorIndex) =
# Initiate the exit of the validator with index ``index``.
# Return if validator already initiated exit
let validator = addr state.validators[index]
if validator.exit_epoch != FAR_FUTURE_EPOCH:
return
# Compute exit queue epoch
# TODO try zero-functional here
var exit_epochs = mapIt(
filterIt(state.validators, it.exit_epoch != FAR_FUTURE_EPOCH),
it.exit_epoch)
exit_epochs.add compute_activation_exit_epoch(get_current_epoch(state))
var exit_queue_epoch = max(exit_epochs)
let exit_queue_churn = foldl(
state.validators,
a + (if b.exit_epoch == exit_queue_epoch: 1'u64 else: 0'u64),
0'u64)
if exit_queue_churn >= get_validator_churn_limit(state):
exit_queue_epoch += 1
# Set validator exit epoch and withdrawable epoch
validator.exit_epoch = exit_queue_epoch
validator.withdrawable_epoch =
validator.exit_epoch + MIN_VALIDATOR_WITHDRAWABILITY_DELAY
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#slash_validator
func slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex,
stateCache: var StateCache) =
# Slash the validator with index ``index``.
let epoch = get_current_epoch(state)
initiate_validator_exit(state, slashed_index)
let validator = addr state.validators[slashed_index]
validator.slashed = true
validator.withdrawable_epoch =
max(validator.withdrawable_epoch, epoch + EPOCHS_PER_SLASHINGS_VECTOR)
state.slashings[epoch mod EPOCHS_PER_SLASHINGS_VECTOR] +=
validator.effective_balance
decrease_balance(state, slashed_index,
validator.effective_balance div MIN_SLASHING_PENALTY_QUOTIENT)
let
proposer_index = get_beacon_proposer_index(state, stateCache)
# Spec has whistleblower_index as optional param, but it's never used.
whistleblower_index = proposer_index
whistleblowing_reward =
(validator.effective_balance div WHISTLEBLOWER_REWARD_QUOTIENT).Gwei
proposer_reward = whistleblowing_reward div PROPOSER_REWARD_QUOTIENT
increase_balance(state, proposer_index, proposer_reward)
# TODO: evaluate if spec bug / underflow can be triggered
doAssert(whistleblowing_reward >= proposer_reward, "Spec bug: underflow in slash_validator")
increase_balance(
state, whistleblower_index, whistleblowing_reward - proposer_reward)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.4/specs/core/0_beacon-chain.md#genesis
func initialize_beacon_state_from_eth1*(
eth1_block_hash: Eth2Digest,
eth1_timestamp: uint64,
deposits: openArray[Deposit],
flags: UpdateFlags = {}): BeaconState =
## Get the genesis ``BeaconState``.
##
## Before the beacon chain starts, validators will register in the Eth1 chain
## and deposit ETH. When enough many validators have registered, a
## `ChainStart` log will be emitted and the beacon chain can start beaconing.
##
## Because the state root hash is part of the genesis block, the beacon state
## must be calculated before creating the genesis block.
# Induct validators
# Not in spec: the system doesn't work unless there are at least SLOTS_PER_EPOCH
# validators - there needs to be at least one member in each committee -
# good to know for testing, though arguably the system is not that useful at
# at that point :)
doAssert deposits.len >= SLOTS_PER_EPOCH
const SECONDS_PER_DAY = uint64(60*60*24)
var state = BeaconState(
genesis_time:
eth1_timestamp + 2'u64 * SECONDS_PER_DAY -
(eth1_timestamp mod SECONDS_PER_DAY),
eth1_data:
Eth1Data(block_hash: eth1_block_hash, deposit_count: uint64(len(deposits))),
latest_block_header:
BeaconBlockHeader(
body_root: hash_tree_root(BeaconBlockBody(
randao_reveal: BlsValue[Signature](kind: OpaqueBlob)
)),
# TODO - Pure BLSSig cannot be zero: https://github.com/status-im/nim-beacon-chain/issues/374
signature: BlsValue[Signature](kind: OpaqueBlob)
)
)
# Process deposits
let leaves = deposits.mapIt(it.data)
for i, deposit in deposits:
let deposit_data_list = leaves[0..i]
state.eth1_data.deposit_root = hash_tree_root(
sszList(deposit_data_list, 2'i64^DEPOSIT_CONTRACT_TREE_DEPTH))
discard process_deposit(state, deposit, flags)
# Process activations
for validator_index in 0 ..< state.validators.len:
let
balance = state.balances[validator_index]
validator = addr state.validators[validator_index]
validator.effective_balance = min(
balance - balance mod EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)
if validator.effective_balance == MAX_EFFECTIVE_BALANCE:
validator.activation_eligibility_epoch = GENESIS_EPOCH
validator.activation_epoch = GENESIS_EPOCH
state
proc is_valid_genesis_state*(state: BeaconState): bool =
if state.genesis_time < MIN_GENESIS_TIME:
return false
if len(get_active_validator_indices(state, GENESIS_EPOCH)) < MIN_GENESIS_ACTIVE_VALIDATOR_COUNT:
return false
return true
# TODO candidate for spec?
# https://github.com/ethereum/eth2.0-specs/blob/0.5.1/specs/core/0_beacon-chain.md#on-genesis
func get_initial_beacon_block*(state: BeaconState): BeaconBlock =
BeaconBlock(
slot: GENESIS_SLOT,
state_root: hash_tree_root(state),
body: BeaconBlockBody(
# TODO: This shouldn't be necessary if OpaqueBlob is the default
randao_reveal: BlsValue[Signature](kind: OpaqueBlob)),
# TODO: This shouldn't be necessary if OpaqueBlob is the default
signature: BlsValue[Signature](kind: OpaqueBlob))
# parent_root, randao_reveal, eth1_data, signature, and body automatically
# initialized to default values.
# TODO remove when shim layer isn't needed
func get_epoch_and_shard(
state: BeaconState, slot: Slot, committee_index: uint64): auto =
# linearize, get compute_committee(...) index, then back-calculate
# what shard to use for the given epoch:
# (shard + SHARD_COUNT - get_start_shard(state, epoch)) mod SHARD_COUNT
let
committees_per_slot = get_committee_count_at_slot(state, slot)
gbc_index =
(slot mod SLOTS_PER_EPOCH) * committees_per_slot + committee_index
epoch = slot.compute_epoch_at_slot
# `index` is constructed as
# mod get_committee_count_at_slot(state, compute_start_slot_at_epoch(epoch))
# which is <= MAX_COMMITTEES_PER_SLOT (4 in minimal, 64 in mainnet).
# get_committee_count_at_slot(...) is constant across an epoch, so:
let
shard_offset_constant =
SHARD_COUNT.uint64 - get_start_shard(state, epoch).uint64
shard =
(gbc_index + SHARD_COUNT - shard_offset_constant) mod SHARD_COUNT
let (roundtrip_slot, roundtrip_index) = get_slot_and_index(
state, epoch, shard)
doAssert roundtrip_slot == slot
doAssert committee_index == roundtrip_index
(epoch, shard)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.4/specs/core/0_beacon-chain.md#get_attestation_data_slot
func get_attestation_data_slot*(state: BeaconState,
data: AttestationData, committee_count: uint64): Slot =
# Return the slot corresponding to the attestation ``data``.
let
(epoch, shard) = get_epoch_and_shard(state, data.slot, data.index)
offset = (shard + SHARD_COUNT -
get_start_shard(state, epoch)) mod SHARD_COUNT
compute_start_slot_at_epoch(epoch) + offset div
(committee_count div SLOTS_PER_EPOCH)
# This is the slower (O(n)), spec-compatible signature.
func get_attestation_data_slot*(state: BeaconState,
data: AttestationData): Slot =
get_attestation_data_slot(
state, data, get_committee_count(state, data.target.epoch))
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_block_root_at_slot
func get_block_root_at_slot*(state: BeaconState,
slot: Slot): Eth2Digest =
# Return the block root at a recent ``slot``.
doAssert state.slot <= slot + SLOTS_PER_HISTORICAL_ROOT
doAssert slot < state.slot
state.block_roots[slot mod SLOTS_PER_HISTORICAL_ROOT]
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_block_root
func get_block_root*(state: BeaconState, epoch: Epoch): Eth2Digest =
# Return the block root at the start of a recent ``epoch``.
get_block_root_at_slot(state, compute_start_slot_at_epoch(epoch))
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_total_balance
func get_total_balance*(state: BeaconState, validators: auto): Gwei =
## Return the combined effective balance of the ``indices``. (1 Gwei minimum
## to avoid divisions by zero.)
max(1'u64,
foldl(validators, a + state.validators[b].effective_balance, 0'u64)
)
# XXX: Move to state_transition_epoch.nim?
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#registry-updates
func process_registry_updates*(state: var BeaconState) =
## Process activation eligibility and ejections
## Try to avoid caching here, since this could easily become undefined
for index, validator in state.validators:
if validator.activation_eligibility_epoch == FAR_FUTURE_EPOCH and
validator.effective_balance == MAX_EFFECTIVE_BALANCE:
state.validators[index].activation_eligibility_epoch =
get_current_epoch(state)
if is_active_validator(validator, get_current_epoch(state)) and
validator.effective_balance <= EJECTION_BALANCE:
initiate_validator_exit(state, index.ValidatorIndex)
## Queue validators eligible for activation and not dequeued for activation
## prior to finalized epoch
var activation_queue : seq[tuple[a: Epoch, b: int]] = @[]
for index, validator in state.validators:
if validator.activation_eligibility_epoch != FAR_FUTURE_EPOCH and
validator.activation_epoch >=
compute_activation_exit_epoch(state.finalized_checkpoint.epoch):
activation_queue.add (
state.validators[index].activation_eligibility_epoch, index)
activation_queue.sort(system.cmp)
## Dequeued validators for activation up to churn limit (without resetting
## activation epoch)
let churn_limit = get_validator_churn_limit(state)
for i, epoch_and_index in activation_queue:
if i.uint64 >= churn_limit:
break
let
(epoch, index) = epoch_and_index
validator = addr state.validators[index]
if validator.activation_epoch == FAR_FUTURE_EPOCH:
validator.activation_epoch =
compute_activation_exit_epoch(get_current_epoch(state))
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#is_valid_indexed_attestation
proc is_valid_indexed_attestation*(
state: BeaconState, indexed_attestation: IndexedAttestation): bool =
## Check if ``indexed_attestation`` has valid indices and signature.
# TODO: this is noSideEffect besides logging
# https://github.com/status-im/nim-chronicles/issues/62
let
bit_0_indices = indexed_attestation.custody_bit_0_indices.asSeq
bit_1_indices = indexed_attestation.custody_bit_1_indices.asSeq
# Verify no index has custody bit equal to 1 [to be removed in phase 1]
if len(bit_1_indices) != 0: # [to be removed in phase 1]
notice "indexed attestation: custody_bit equal to 1"
return false # [to be removed in phase 1]
# Verify max number of indices
let combined_len = len(bit_0_indices) + len(bit_1_indices)
if not (combined_len <= MAX_VALIDATORS_PER_COMMITTEE):
notice "indexed attestation: validator index beyond max validators per committee"
return false
# Verify index sets are disjoint
if len(intersection(bit_0_indices.toSet, bit_1_indices.toSet)) != 0:
notice "indexed attestation: indices set not disjoint"
return false
# Verify indices are sorted
if bit_0_indices != sorted(bit_0_indices, system.cmp):
notice "indexed attestation: indices 0 not sorted"
return false
if bit_1_indices != sorted(bit_1_indices, system.cmp):
notice "indexed attestation: indices 0 not sorted"
return false
# Verify aggregate signature
let
pubkeys = @[ # TODO, bls_verify_multiple should accept openarray
bls_aggregate_pubkeys(mapIt(bit_0_indices, state.validators[it.int].pubkey)),
bls_aggregate_pubkeys(mapIt(bit_1_indices, state.validators[it.int].pubkey)),
]
msg1 = AttestationDataAndCustodyBit(
data: indexed_attestation.data, custody_bit: false)
msg2 = AttestationDataAndCustodyBit(
data: indexed_attestation.data, custody_bit: true)
message_hashes = [
hash_tree_root(msg1),
hash_tree_root(msg2),
]
domain = get_domain(state, DOMAIN_BEACON_ATTESTER, indexed_attestation.data.target.epoch)
result = bls_verify_multiple(
pubkeys,
message_hashes,
indexed_attestation.signature,
domain,
)
if not result:
notice "indexed attestation: signature verification failure"
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_attesting_indices
func get_attesting_indices*(state: BeaconState,
data: AttestationData,
bits: CommitteeValidatorsBits,
stateCache: var StateCache):
HashSet[ValidatorIndex] =
# Return the set of attesting indices corresponding to ``data`` and ``bits``.
result = initSet[ValidatorIndex]()
let committee = get_beacon_committee(state, data.slot, data.index, stateCache)
for i, index in committee:
if bits[i]:
result.incl index
# TODO remove after removing attestation pool legacy usage
func get_attesting_indices_seq*(state: BeaconState,
attestation_data: AttestationData,
bits: CommitteeValidatorsBits): seq[ValidatorIndex] =
var cache = get_empty_per_epoch_cache()
toSeq(items(get_attesting_indices(
state, attestation_data, bits, cache)))
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#get_indexed_attestation
func get_indexed_attestation*(state: BeaconState, attestation: Attestation,
stateCache: var StateCache): IndexedAttestation =
# Return the indexed attestation corresponding to ``attestation``.
let
attesting_indices =
get_attesting_indices(
state, attestation.data, attestation.aggregation_bits, stateCache)
custody_bit_1_indices =
get_attesting_indices(
state, attestation.data, attestation.custody_bits, stateCache)
doAssert custody_bit_1_indices <= attesting_indices
let
# custody_bit_0_indices = attesting_indices.difference(custody_bit_1_indices)
custody_bit_0_indices =
filterIt(toSeq(items(attesting_indices)), it notin custody_bit_1_indices)
## TODO No fundamental reason to do so many type conversions
## verify_indexed_attestation checks for sortedness but it's
## entirely a local artifact, seemingly; networking uses the
## Attestation data structure, which can't be unsorted. That
## the conversion here otherwise needs sorting is due to the
## usage of HashSet -- order only matters in one place (that
## 0.6.3 highlights and explicates) except in that the spec,
## for no obvious reason, verifies it.
IndexedAttestation(
custody_bit_0_indices: CustodyBitIndices sorted(
mapIt(custody_bit_0_indices, it.uint64), system.cmp),
# toSeq pointlessly constructs int-indexable copy so mapIt can infer type;
# see above
custody_bit_1_indices: CustodyBitIndices sorted(
mapIt(toSeq(items(custody_bit_1_indices)), it.uint64),
system.cmp),
data: attestation.data,
signature: attestation.signature,
)
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.0/specs/core/0_beacon-chain.md#attestations
proc check_attestation*(
state: BeaconState, attestation: Attestation, flags: UpdateFlags,
stateCache: var StateCache): bool =
## Check that an attestation follows the rules of being included in the state
## at the current slot. When acting as a proposer, the same rules need to
## be followed!
let stateSlot =
if nextSlot in flags: state.slot + 1
else: state.slot
let data = attestation.data
trace "process_attestation: beginning",
attestation=attestation
if not (data.target.epoch == get_previous_epoch(state) or
data.target.epoch == get_current_epoch(state)):
warn("Target epoch not current or previous epoch")
return
if not (data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= stateSlot):
warn("Attestation too new",
attestation_slot = shortLog(data.slot),
state_slot = shortLog(stateSlot))
return
if not (stateSlot <= data.slot + SLOTS_PER_EPOCH):
warn("Attestation too old",
attestation_slot = shortLog(data.slot),
state_slot = shortLog(stateSlot))
return
let committee = get_beacon_committee(state, data.slot, data.index, stateCache)
if attestation.aggregation_bits.len != attestation.custody_bits.len:
warn("Inconsistent aggregation and custody bits",
aggregation_bits_len = attestation.aggregation_bits.len,
custody_bits_len = attestation.custody_bits.len
)
return
if attestation.aggregation_bits.len != committee.len:
warn("Inconsistent aggregation and committee length",
aggregation_bits_len = attestation.aggregation_bits.len,
committee_len = committee.len
)
return
let ffg_check_data = (data.source.epoch, data.source.root, data.target.epoch)
if data.target.epoch == get_current_epoch(state):
if not (ffg_check_data == (state.current_justified_checkpoint.epoch,
state.current_justified_checkpoint.root, get_current_epoch(state))):
warn("FFG data not matching current justified epoch")
return
else:
if not (ffg_check_data == (state.previous_justified_checkpoint.epoch,
state.previous_justified_checkpoint.root, get_previous_epoch(state))):
warn("FFG data not matching current justified epoch")
return
if not is_valid_indexed_attestation(
state, get_indexed_attestation(state, attestation, stateCache)):
warn("process_attestation: signature or bitfields incorrect")
return
true
proc process_attestation*(
state: var BeaconState, attestation: Attestation, flags: UpdateFlags,
stateCache: var StateCache): bool =
# In the spec, attestation validation is mixed with state mutation, so here
# we've split it into two functions so that the validation logic can be
# reused when looking for suitable blocks to include in attestations.
# TODO don't log warnings when looking for attestations (return
# Result[void, cstring] instead of logging in check_attestation?)
if check_attestation(state, attestation, flags, stateCache):
let
attestation_slot = get_attestation_data_slot(state, attestation.data)
pending_attestation = PendingAttestation(
data: attestation.data,
aggregation_bits: attestation.aggregation_bits,
inclusion_delay: state.slot - attestation_slot,
proposer_index: get_beacon_proposer_index(state, stateCache).uint64,
)
if attestation.data.target.epoch == get_current_epoch(state):
trace "process_attestation: current_epoch_attestations.add",
pending_attestation = pending_attestation,
indices = get_attesting_indices(
state, attestation.data, attestation.aggregation_bits, stateCache).len
state.current_epoch_attestations.add(pending_attestation)
else:
trace "process_attestation: previous_epoch_attestations.add",
pending_attestation = pending_attestation,
indices = get_attesting_indices(
state, attestation.data, attestation.aggregation_bits, stateCache).len
state.previous_epoch_attestations.add(pending_attestation)
true
else:
false
proc makeAttestationData*(
state: BeaconState, shard: uint64,
beacon_block_root: Eth2Digest): AttestationData =
## Create an attestation / vote for the block `beacon_block_root` using the
## data in `state` to fill in the rest of the fields.
## `state` is the state corresponding to the `beacon_block_root` advanced to
## the slot we're attesting to.
## https://github.com/ethereum/eth2.0-specs/blob/v0.8.4/specs/validator/0_beacon-chain-validator.md#construct-attestation
let
current_epoch = get_current_epoch(state)
start_slot = compute_start_slot_at_epoch(current_epoch)
epoch_boundary_block_root =
if start_slot == state.slot: beacon_block_root
else: get_block_root_at_slot(state, start_slot)
(a_slot, a_index) = get_slot_and_index(state, current_epoch, shard)
AttestationData(
slot: a_slot,
index: a_index,
beacon_block_root: beacon_block_root,
source: state.current_justified_checkpoint,
target: Checkpoint(
epoch: current_epoch,
root: epoch_boundary_block_root
)
)
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