nimbus-eth2/beacon_chain/spec/state_transition_epoch.nim

# beacon_chain
# Copyright (c) 2018-2020 Status Research & Development GmbH
# Licensed and distributed under either of
#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.

# State transition - epoch processing, as described in
# https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
#
# The entry point is `process_epoch`, which is at the bottom of this file.
#
# General notes about the code:
# * Weird styling - the sections taken from the spec use python styling while
#   the others use NEP-1 - helps grepping identifiers in spec
# * When updating the code, add TODO sections to mark where there are clear
#   improvements to be made - other than that, keep things similar to spec unless
#   motivated by security or performance considerations

{.push raises: [Defect].}

import
  std/[math, sequtils, tables, algorithm],
  stew/[bitops2], chronicles,
  ../extras,
  ../ssz/merkleization,
  ./beaconstate, ./crypto, ./datatypes, ./digest, ./helpers, ./validator,
  ../../nbench/bench_lab

# Logging utilities
# --------------------------------------------------------

logScope: topics = "consens"

type
  # Caches for computing justificiation, rewards and penalties - based on
  # implementation in Lighthouse:
  # https://github.com/sigp/lighthouse/blob/master/consensus/state_processing/src/per_epoch_processing/validator_statuses.rs
  Delta* = object
    rewards*: Gwei
    penalties*: Gwei

  InclusionInfo = object
    # The distance between the attestation slot and the slot that attestation
    # was included in block.
    delay: uint64
    # The index of the proposer at the slot where the attestation was included.
    proposer_index: uint64

  ValidatorStatus = object
    # True if the validator has been slashed, ever.
    is_slashed: bool
    # True if the validator can withdraw in the current epoch.
    is_withdrawable_in_current_epoch: bool
    # True if the validator was active in the state's _current_ epoch.
    is_active_in_current_epoch: bool
    # True if the validator was active in the state's _previous_ epoch.
    is_active_in_previous_epoch: bool
    # The validator's effective balance in the _current_ epoch.
    current_epoch_effective_balance: uint64

    # True if the validator had an attestation included in the _current_ epoch.
    is_current_epoch_attester: bool
    # True if the validator's beacon block root attestation for the first slot
    # of the _current_ epoch matches the block root known to the state.
    is_current_epoch_target_attester: bool
    # True if the validator had an attestation included in the _previous_ epoch.
    is_previous_epoch_attester: Option[InclusionInfo]
    # Set if the validator's beacon block root attestation for the first slot of
    # the _previous_ epoch matches the block root known to the state.
    # Information used to reward the block producer of this validators
    # earliest-included attestation.
    is_previous_epoch_target_attester: bool
    # True if the validator's beacon block root attestation in the _previous_
    # epoch at the attestation's slot (`attestation_data.slot`) matches the
    # block root known to the state.
    is_previous_epoch_head_attester: bool

    inclusion_info: Option[InclusionInfo]

    # Total rewards and penalties for this validator
    delta: Delta

  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_total_balance
  TotalBalances = object
    # The total effective balance of all active validators during the _current_
    # epoch.
    current_epoch_raw: Gwei
    # The total effective balance of all active validators during the _previous_
    # epoch.
    previous_epoch_raw: Gwei
    # The total effective balance of all validators who attested during the
    # _current_ epoch.
    current_epoch_attesters_raw: Gwei
    # The total effective balance of all validators who attested during the
    # _current_ epoch and agreed with the state about the beacon block at the
    # first slot of the _current_ epoch.
    current_epoch_target_attesters_raw: Gwei
    # The total effective balance of all validators who attested during the
    # _previous_ epoch.
    previous_epoch_attesters_raw: Gwei
    # The total effective balance of all validators who attested during the
    # _previous_ epoch and agreed with the state about the beacon block at the
    # first slot of the _previous_ epoch.
    previous_epoch_target_attesters_raw: Gwei
    # The total effective balance of all validators who attested during the
    # _previous_ epoch and agreed with the state about the beacon block at the
    # time of attestation.
    previous_epoch_head_attesters_raw: Gwei

  ValidatorStatuses* = object
    statuses*: seq[ValidatorStatus]
    total_balances*: TotalBalances

# Accessors that implement the max condition in `get_total_balance`:
# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_total_balance
template current_epoch*(v: TotalBalances): Gwei =
  max(EFFECTIVE_BALANCE_INCREMENT, v.current_epoch_raw)
template previous_epoch*(v: TotalBalances): Gwei =
  max(EFFECTIVE_BALANCE_INCREMENT, v.previous_epoch_raw)
template current_epoch_attesters*(v: TotalBalances): Gwei =
  max(EFFECTIVE_BALANCE_INCREMENT, v.current_epoch_attesters_raw)
template current_epoch_target_attesters*(v: TotalBalances): Gwei =
  max(EFFECTIVE_BALANCE_INCREMENT, v.current_epoch_target_attesters_raw)
template previous_epoch_attesters*(v: TotalBalances): Gwei =
  max(EFFECTIVE_BALANCE_INCREMENT, v.previous_epoch_attesters_raw)
template previous_epoch_target_attesters*(v: TotalBalances): Gwei =
  max(EFFECTIVE_BALANCE_INCREMENT, v.previous_epoch_target_attesters_raw)
template previous_epoch_head_attesters*(v: TotalBalances): Gwei =
  max(EFFECTIVE_BALANCE_INCREMENT, v.previous_epoch_head_attesters_raw)

func init*(T: type ValidatorStatuses, state: BeaconState): T =
  result.statuses = newSeq[ValidatorStatus](state.validators.len)
  for i, v in state.validators:
    result.statuses[i].is_slashed = v.slashed
    result.statuses[i].is_withdrawable_in_current_epoch =
      state.get_current_epoch() >= v.withdrawable_epoch
    result.statuses[i].current_epoch_effective_balance = v.effective_balance

    if v.is_active_validator(state.get_current_epoch()):
      result.statuses[i].is_active_in_current_epoch = true
      result.total_balances.current_epoch_raw += v.effective_balance

    if v.is_active_validator(state.get_previous_epoch()):
      result.statuses[i].is_active_in_previous_epoch = true
      result.total_balances.previous_epoch_raw += v.effective_balance

func add(a: var Delta, b: Delta) =
  a.rewards += b.rewards
  a.penalties += b.penalties

func process_attestation(
    self: var ValidatorStatuses, state: BeaconState, a: PendingAttestation,
    cache: var StateCache) =
  # Collect information about the attestation
  var
    is_current_epoch_attester, is_current_epoch_target_attester: bool
    is_previous_epoch_target_attester: bool
    is_previous_epoch_head_attester: bool
    is_previous_epoch_attester: Option[InclusionInfo]

  if a.data.target.epoch == state.get_current_epoch():
    is_current_epoch_attester = true

    if a.data.target.root == get_block_root(state, state.get_current_epoch()):
      is_current_epoch_target_attester = true;

  elif a.data.target.epoch == state.get_previous_epoch():
    is_previous_epoch_attester = some(InclusionInfo(
      delay: a.inclusion_delay,
      proposer_index: a.proposer_index,
    ))

    if a.data.target.root == get_block_root(state, state.get_previous_epoch()):
      is_previous_epoch_target_attester = true;

      if a.data.beacon_block_root == get_block_root_at_slot(state, a.data.slot):
        is_previous_epoch_head_attester = true

  # Update the cache for all participants
  for validator_index in get_attesting_indices(
      state, a.data, a.aggregation_bits, cache):
    template v(): untyped = self.statuses[validator_index]
    if is_current_epoch_attester:
      v.is_current_epoch_attester = true

    if is_current_epoch_target_attester:
      v.is_current_epoch_target_attester = true

    if is_previous_epoch_attester.isSome:
      if v.is_previous_epoch_attester.isSome:
        if is_previous_epoch_attester.get().delay <
            v.is_previous_epoch_attester.get().delay:
          v.is_previous_epoch_attester = is_previous_epoch_attester
      else:
        v.is_previous_epoch_attester = is_previous_epoch_attester

    if is_previous_epoch_target_attester:
      v.is_previous_epoch_target_attester = true

    if is_previous_epoch_head_attester:
      v.is_previous_epoch_head_attester = true

func process_attestations*(
    self: var ValidatorStatuses, state: BeaconState, cache: var StateCache) =
  # Walk state attestations and update the status information
  for a in state.previous_epoch_attestations:
    process_attestation(self, state, a, cache)
  for a in state.current_epoch_attestations:
    process_attestation(self, state, a, cache)

  for idx, v in self.statuses:
    if v.is_slashed:
      continue

    let validator_balance = state.validators[idx].effective_balance

    if v.is_current_epoch_attester:
      self.total_balances.current_epoch_attesters_raw += validator_balance

    if v.is_current_epoch_target_attester:
      self.total_balances.current_epoch_target_attesters_raw += validator_balance

    if v.is_previous_epoch_attester.isSome():
      self.total_balances.previous_epoch_attesters_raw += validator_balance

    if v.is_previous_epoch_target_attester:
      self.total_balances.previous_epoch_target_attesters_raw += validator_balance

    if v.is_previous_epoch_head_attester:
      self.total_balances.previous_epoch_head_attesters_raw += validator_balance

func is_eligible_validator*(validator: ValidatorStatus): bool =
  validator.is_active_in_previous_epoch or
    (validator.is_slashed and (not validator.is_withdrawable_in_current_epoch))

# Spec
# --------------------------------------------------------

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_total_active_balance
func get_total_active_balance*(state: BeaconState, cache: var StateCache): Gwei =
  ## Return the combined effective balance of the active validators.
  # Note: ``get_total_balance`` returns ``EFFECTIVE_BALANCE_INCREMENT`` Gwei
  # minimum to avoid divisions by zero.

  let epoch = state.get_current_epoch()

  get_total_balance(
    state, cache.get_shuffled_active_validator_indices(state, epoch))

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#justification-and-finalization
proc process_justification_and_finalization*(state: var BeaconState,
    total_balances: TotalBalances, updateFlags: UpdateFlags = {}) {.nbench.} =
  # Initial FFG checkpoint values have a `0x00` stub for `root`.
  # Skip FFG updates in the first two epochs to avoid corner cases that might
  # result in modifying this stub.
  if get_current_epoch(state) <= GENESIS_EPOCH + 1:
    return

  let
    previous_epoch = get_previous_epoch(state)
    current_epoch = get_current_epoch(state)
    old_previous_justified_checkpoint = state.previous_justified_checkpoint
    old_current_justified_checkpoint = state.current_justified_checkpoint

  # Process justifications
  state.previous_justified_checkpoint = state.current_justified_checkpoint

  ## Spec:
  ## state.justification_bits[1:] = state.justification_bits[:-1]
  ## state.justification_bits[0] = 0b0

  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#constants
  const JUSTIFICATION_BITS_LENGTH = 4

  state.justification_bits = (state.justification_bits shl 1) and
    cast[uint8]((2^JUSTIFICATION_BITS_LENGTH) - 1)

  let total_active_balance = total_balances.current_epoch
  if total_balances.previous_epoch_target_attesters * 3 >=
      total_active_balance * 2:
    state.current_justified_checkpoint =
      Checkpoint(epoch: previous_epoch,
                 root: get_block_root(state, previous_epoch))
    state.justification_bits.setBit 1

    trace "Justified with previous epoch",
      current_epoch = current_epoch,
      checkpoint = shortLog(state.current_justified_checkpoint)
  elif verifyFinalization in updateFlags:
    warn "Low attestation participation in previous epoch",
      total_balances, epoch = get_current_epoch(state)

  if total_balances.current_epoch_target_attesters * 3 >=
      total_active_balance * 2:
    state.current_justified_checkpoint =
      Checkpoint(epoch: current_epoch,
                 root: get_block_root(state, current_epoch))
    state.justification_bits.setBit 0

    trace "Justified with current epoch",
      current_epoch = current_epoch,
      checkpoint = shortLog(state.current_justified_checkpoint)

  # Process finalizations
  let bitfield = state.justification_bits

  ## The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th
  ## as source
  if (bitfield and 0b1110) == 0b1110 and
     old_previous_justified_checkpoint.epoch + 3 == current_epoch:
    state.finalized_checkpoint = old_previous_justified_checkpoint

    trace "Finalized with rule 234",
      current_epoch = current_epoch,
      checkpoint = shortLog(state.finalized_checkpoint)

  ## The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as
  ## source
  if (bitfield and 0b110) == 0b110 and
     old_previous_justified_checkpoint.epoch + 2 == current_epoch:
    state.finalized_checkpoint = old_previous_justified_checkpoint

    trace "Finalized with rule 23",
      current_epoch = current_epoch,
      checkpoint = shortLog(state.finalized_checkpoint)

  ## The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as
  ## source
  if (bitfield and 0b111) == 0b111 and
     old_current_justified_checkpoint.epoch + 2 == current_epoch:
    state.finalized_checkpoint = old_current_justified_checkpoint

    trace "Finalized with rule 123",
      current_epoch = current_epoch,
      checkpoint = shortLog(state.finalized_checkpoint)

  ## The 1st/2nd most recent epochs are justified, the 1st using the 2nd as
  ## source
  if (bitfield and 0b11) == 0b11 and
     old_current_justified_checkpoint.epoch + 1 == current_epoch:
    state.finalized_checkpoint = old_current_justified_checkpoint

    trace "Finalized with rule 12",
      current_epoch = current_epoch,
      checkpoint = shortLog(state.finalized_checkpoint)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#helpers
func get_base_reward_sqrt*(state: BeaconState, index: ValidatorIndex,
    total_balance_sqrt: auto): Gwei =
  # Spec function recalculates total_balance every time, which creates an
  # O(n^2) situation.
  let effective_balance = state.validators[index].effective_balance
  effective_balance * BASE_REWARD_FACTOR div
    total_balance_sqrt div BASE_REWARDS_PER_EPOCH

func get_proposer_reward(base_reward: Gwei): Gwei =
  # Spec version recalculates get_total_active_balance(state) quadratically
  base_reward div PROPOSER_REWARD_QUOTIENT

func is_in_inactivity_leak(finality_delay: uint64): bool =
  finality_delay > MIN_EPOCHS_TO_INACTIVITY_PENALTY

func get_finality_delay(state: BeaconState): uint64 =
  get_previous_epoch(state) - state.finalized_checkpoint.epoch

func get_attestation_component_delta(is_unslashed_attester: bool,
                                     attesting_balance: Gwei,
                                     total_balance: Gwei,
                                     base_reward: uint64,
                                     finality_delay: uint64): Delta =
  # Helper with shared logic for use by get source, target, and head deltas
  # functions
  if is_unslashed_attester:
    if is_in_inactivity_leak(finality_delay):
      # Since full base reward will be canceled out by inactivity penalty deltas,
      # optimal participation receives full base reward compensation here.
      Delta(rewards: base_reward)
    else:
      let reward_numerator =
        base_reward * (attesting_balance div EFFECTIVE_BALANCE_INCREMENT)
      Delta(rewards:
        reward_numerator div (total_balance div EFFECTIVE_BALANCE_INCREMENT))
  else:
    Delta(penalties: base_reward)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#components-of-attestation-deltas
func get_source_delta*(validator: ValidatorStatus,
                       base_reward: uint64,
                       total_balances: TotalBalances,
                       finality_delay: uint64): Delta =
  ## Return attester micro-rewards/penalties for source-vote for each validator.
  get_attestation_component_delta(
    validator.is_previous_epoch_attester.isSome() and (not validator.is_slashed),
    total_balances.previous_epoch_attesters,
    total_balances.current_epoch,
    base_reward,
    finality_delay)

func get_target_delta*(validator: ValidatorStatus,
                       base_reward: uint64,
                       total_balances: TotalBalances,
                       finality_delay: uint64): Delta =
  ## Return attester micro-rewards/penalties for target-vote for each validator.
  get_attestation_component_delta(
    validator.is_previous_epoch_target_attester and (not validator.is_slashed),
    total_balances.previous_epoch_target_attesters,
    total_balances.current_epoch,
    base_reward,
    finality_delay)

func get_head_delta*(validator: ValidatorStatus,
                     base_reward: uint64,
                     total_balances: TotalBalances,
                     finality_delay: uint64): Delta =
  ## Return attester micro-rewards/penalties for head-vote for each validator.
  get_attestation_component_delta(
    validator.is_previous_epoch_head_attester and (not validator.is_slashed),
    total_balances.previous_epoch_head_attesters,
    total_balances.current_epoch,
    base_reward,
    finality_delay)

func get_inclusion_delay_delta*(validator: ValidatorStatus,
                                base_reward: uint64):
                                  (Delta, Option[(uint64, Delta)]) =
  ## Return proposer and inclusion delay micro-rewards/penalties for each validator.
  if validator.is_previous_epoch_attester.isSome() and (not validator.is_slashed):
    let
      inclusion_info = validator.is_previous_epoch_attester.get()
      proposer_reward = get_proposer_reward(base_reward)
      proposer_delta = Delta(rewards: proposer_reward)

    let
      max_attester_reward = base_reward - proposer_reward
      delta = Delta(rewards: max_attester_reward div inclusion_info.delay)
      proposer_index = inclusion_info.proposer_index;
    return (delta, some((proposer_index, proposer_delta)))

func get_inactivity_penalty_delta*(validator: ValidatorStatus,
                                   base_reward: Gwei,
                                   finality_delay: uint64): Delta =
  ## Return inactivity reward/penalty deltas for each validator.
  var delta: Delta

  if is_in_inactivity_leak(finality_delay):
    # If validator is performing optimally this cancels all rewards for a neutral balance
    delta.penalties +=
      BASE_REWARDS_PER_EPOCH * base_reward - get_proposer_reward(base_reward)

    # Additionally, all validators whose FFG target didn't match are penalized extra
    # This condition is equivalent to this condition from the spec:
    # `index not in get_unslashed_attesting_indices(state, matching_target_attestations)`
    if validator.is_slashed or (not validator.is_previous_epoch_target_attester):
        delta.penalties +=
          validator.current_epoch_effective_balance * finality_delay div
            INACTIVITY_PENALTY_QUOTIENT

  delta

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_attestation_deltas
func get_attestation_deltas(
    state: BeaconState, validator_statuses: var ValidatorStatuses) =
  ## Update validator_statuses with attestation reward/penalty deltas for each validator.

  let
    finality_delay = get_finality_delay(state)
    total_balance = validator_statuses.total_balances.current_epoch
    total_balance_sqrt = integer_squareroot(total_balance)
  # Filter out ineligible validators. All sub-functions of the spec do this
  # except for `get_inclusion_delay_deltas`. It's safe to do so here because
  # any validator that is in the unslashed indices of the matching source
  # attestations is active, and therefore eligible.
  for index, validator in validator_statuses.statuses.mpairs():
    if not is_eligible_validator(validator):
      continue

    let
      base_reward = get_base_reward_sqrt(
        state, index.ValidatorIndex, total_balance_sqrt)

    let
      source_delta = get_source_delta(
        validator, base_reward, validator_statuses.total_balances, finality_delay)
      target_delta = get_target_delta(
        validator, base_reward, validator_statuses.total_balances, finality_delay)
      head_delta = get_head_delta(
        validator, base_reward, validator_statuses.total_balances, finality_delay)
      (inclusion_delay_delta, proposer_delta) =
        get_inclusion_delay_delta(validator, base_reward)
      inactivity_delta = get_inactivity_penalty_delta(
        validator, base_reward, finality_delay)

    validator.delta.add source_delta
    validator.delta.add target_delta
    validator.delta.add head_delta
    validator.delta.add inclusion_delay_delta
    validator.delta.add inactivity_delta

    if proposer_delta.isSome:
      let proposer_index = proposer_delta.get()[0]
      if proposer_index < validator_statuses.statuses.lenu64:
        validator_statuses.statuses[proposer_index].delta.add(
          proposer_delta.get()[1])

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#process_rewards_and_penalties
func process_rewards_and_penalties(
    state: var BeaconState, validator_statuses: var ValidatorStatuses) {.nbench.} =
  # No rewards are applied at the end of `GENESIS_EPOCH` because rewards are
  # for work done in the previous epoch
  doAssert validator_statuses.statuses.len == state.validators.len

  if get_current_epoch(state) == GENESIS_EPOCH:
    return

  get_attestation_deltas(state, validator_statuses)

  for idx, v in validator_statuses.statuses:
    increase_balance(state, idx.ValidatorIndex, v.delta.rewards)
    decrease_balance(state, idx.ValidatorIndex, v.delta.penalties)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#slashings
func process_slashings*(state: var BeaconState, total_balance: Gwei) {.nbench.}=
  let
    epoch = get_current_epoch(state)
    adjusted_total_slashing_balance =
      min(sum(state.slashings) * PROPORTIONAL_SLASHING_MULTIPLIER, total_balance)

  for index, validator in state.validators:
    if validator.slashed and epoch + EPOCHS_PER_SLASHINGS_VECTOR div 2 ==
        validator.withdrawable_epoch:
      let increment = EFFECTIVE_BALANCE_INCREMENT # Factored out from penalty
                                                  # numerator to avoid uint64 overflow
      let penalty_numerator =
        validator.effective_balance div increment *
        adjusted_total_slashing_balance
      let penalty = penalty_numerator div total_balance * increment
      decrease_balance(state, index.ValidatorIndex, penalty)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#final-updates
func process_final_updates*(state: var BeaconState) {.nbench.}=
  let
    current_epoch = get_current_epoch(state)
    next_epoch = current_epoch + 1

  # Reset eth1 data votes
  if next_epoch mod EPOCHS_PER_ETH1_VOTING_PERIOD == 0:
    state.eth1_data_votes = default(type state.eth1_data_votes)

  # Update effective balances with hysteresis
  for index, validator in state.validators:
    let balance = state.balances[index]
    const
      HYSTERESIS_INCREMENT =
        EFFECTIVE_BALANCE_INCREMENT div HYSTERESIS_QUOTIENT
      DOWNWARD_THRESHOLD =
        HYSTERESIS_INCREMENT * HYSTERESIS_DOWNWARD_MULTIPLIER
      UPWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_UPWARD_MULTIPLIER
    if balance + DOWNWARD_THRESHOLD < validator.effective_balance or
        validator.effective_balance + UPWARD_THRESHOLD < balance:
      state.validators[index].effective_balance =
        min(
          balance - balance mod EFFECTIVE_BALANCE_INCREMENT,
          MAX_EFFECTIVE_BALANCE)

  # Reset slashings
  state.slashings[int(next_epoch mod EPOCHS_PER_SLASHINGS_VECTOR)] = 0.Gwei

  # Set randao mix
  state.randao_mixes[next_epoch mod EPOCHS_PER_HISTORICAL_VECTOR] =
    get_randao_mix(state, current_epoch)

  # Set historical root accumulator
  if next_epoch mod (SLOTS_PER_HISTORICAL_ROOT div SLOTS_PER_EPOCH) == 0:
    # Equivalent to hash_tree_root(foo: HistoricalBatch), but without using
    # significant additional stack or heap.
    # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#historicalbatch
    # In response to https://github.com/status-im/nimbus-eth2/issues/921
    state.historical_roots.add hash_tree_root(
      [hash_tree_root(state.block_roots), hash_tree_root(state.state_roots)])

  # Rotate current/previous epoch attestations
  state.previous_epoch_attestations = state.current_epoch_attestations
  state.current_epoch_attestations = default(type state.current_epoch_attestations)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#epoch-processing
proc process_epoch*(state: var BeaconState, updateFlags: UpdateFlags,
    cache: var StateCache) {.nbench.} =
  let currentEpoch = get_current_epoch(state)
  trace "process_epoch",
    current_epoch = currentEpoch
  var validator_statuses = ValidatorStatuses.init(state)
  validator_statuses.process_attestations(state, cache)

  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#justification-and-finalization
  process_justification_and_finalization(
    state, validator_statuses.total_balances, updateFlags)

  # state.slot hasn't been incremented yet.
  if verifyFinalization in updateFlags and currentEpoch >= 2:
    doAssert state.current_justified_checkpoint.epoch + 2 >= currentEpoch

  if verifyFinalization in updateFlags and currentEpoch >= 3:
    # Rule 2/3/4 finalization results in the most pessimal case. The other
    # three finalization rules finalize more quickly as long as the any of
    # the finalization rules triggered.
    doAssert state.finalized_checkpoint.epoch + 3 >= currentEpoch

  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#rewards-and-penalties-1
  process_rewards_and_penalties(state, validator_statuses)

  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#registry-updates
  process_registry_updates(state, cache)

  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#slashings
  process_slashings(state, validator_statuses.total_balances.current_epoch)

  # https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#final-updates
  process_final_updates(state)