nimbus-eth2/beacon_chain/spec/light_client_sync.nim

import
  stew/[bitops2, objects],
  datatypes/altair,
  helpers

# https://github.com/ethereum/consensus-specs/blob/v1.1.9/specs/altair/sync-protocol.md#get_active_header
func get_active_header(update: LightClientUpdate): BeaconBlockHeader =
  # The "active header" is the header that the update is trying to convince
  # us to accept. If a finalized header is present, it's the finalized
  # header, otherwise it's the attested header
  if not update.finalized_header.isZeroMemory:
    update.finalized_header
  else:
    update.attested_header

# https://github.com/ethereum/consensus-specs/blob/v1.1.9/specs/altair/sync-protocol.md#validate_light_client_update
proc validate_light_client_update*(store: LightClientStore,
                                   update: LightClientUpdate,
                                   current_slot: Slot,
                                   genesis_validators_root: Eth2Digest): bool =
  # Verify update slot is larger than slot of current best finalized header
  let active_header = get_active_header(update)
  if not (current_slot >= active_header.slot and
          active_header.slot > store.finalized_header.slot):
    return false

  # Verify update does not skip a sync committee period
  let
    finalized_period = sync_committee_period(store.finalized_header.slot)
    update_period = sync_committee_period(active_header.slot)

  if update_period notin [finalized_period, finalized_period + 1]:
    return false

  # Verify that the `finalized_header`, if present, actually is the finalized
  # header saved in the state of the `attested header`
  if update.finalized_header.isZeroMemory:
    if not update.finality_branch.isZeroMemory:
      return false
  else:
    if not is_valid_merkle_branch(hash_tree_root(update.finalized_header),
                                  update.finality_branch,
                                  log2trunc(FINALIZED_ROOT_INDEX),
                                  get_subtree_index(FINALIZED_ROOT_INDEX),
                                  update.attested_header.state_root):
      return false

  # Verify update next sync committee if the update period incremented
  # TODO: Use a view type instead of `unsafeAddr`
  let sync_committee = if update_period == finalized_period:
    if not update.next_sync_committee_branch.isZeroMemory:
      return false
    unsafeAddr store.current_sync_committee
  else:
    if not is_valid_merkle_branch(hash_tree_root(update.next_sync_committee),
                                  update.next_sync_committee_branch,
                                  log2trunc(NEXT_SYNC_COMMITTEE_INDEX),
                                  get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
                                  active_header.state_root):
      return false
    unsafeAddr store.next_sync_committee

  template sync_aggregate(): auto = update.sync_aggregate
  let sync_committee_participants_count = countOnes(sync_aggregate.sync_committee_bits)

  # Verify sync committee has sufficient participants
  if sync_committee_participants_count < MIN_SYNC_COMMITTEE_PARTICIPANTS:
    return false

  # Verify sync committee aggregate signature
  # participant_pubkeys = [pubkey for (bit, pubkey) in zip(sync_aggregate.sync_committee_bits, sync_committee.pubkeys) if bit]
  var participant_pubkeys = newSeqOfCap[ValidatorPubKey](sync_committee_participants_count)
  for idx, bit in sync_aggregate.sync_committee_bits:
    if bit:
      participant_pubkeys.add(sync_committee.pubkeys[idx])

  let domain = compute_domain(
    DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
  let signing_root = compute_signing_root(update.attested_header, domain)

  blsFastAggregateVerify(
    participant_pubkeys, signing_root.data, sync_aggregate.sync_committee_signature)

# https://github.com/ethereum/consensus-specs/blob/v1.1.9/specs/altair/sync-protocol.md#apply_light_client_update
func apply_light_client_update(
    store: var LightClientStore, update: LightClientUpdate) =
  let
    active_header = get_active_header(update)
    finalized_period = sync_committee_period(store.finalized_header.slot)
    update_period = sync_committee_period(active_header.slot)
  if update_period == finalized_period + 1:
    store.current_sync_committee = store.next_sync_committee
    store.next_sync_committee = update.next_sync_committee
  store.finalized_header = active_header

# https://github.com/ethereum/consensus-specs/blob/v1.1.9/specs/altair/sync-protocol.md#get_safety_threshold
func get_safety_threshold(store: LightClientStore): uint64 =
  max(
    store.previous_max_active_participants,
    store.current_max_active_participants
  ) div 2

# https://github.com/ethereum/consensus-specs/blob/v1.1.9/specs/altair/sync-protocol.md#process_light_client_update
proc process_light_client_update*(store: var LightClientStore,
                                  update: LightClientUpdate,
                                  current_slot: Slot,
                                  genesis_validators_root: Eth2Digest): bool =
  if not validate_light_client_update(
      store, update, current_slot, genesis_validators_root):
    return false

  let
    sync_committee_bits = update.sync_aggregate.sync_committee_bits
    sum_sync_committee_bits = countOnes(sync_committee_bits)

  # Update the best update in case we have to force-update to it if the
  # timeout elapses
  if  store.best_valid_update.isNone or
      sum_sync_committee_bits > countOnes(
        store.best_valid_update.get.sync_aggregate.sync_committee_bits):
    store.best_valid_update = some(update)

  # Track the maximum number of active participants in the committee signatures
  store.current_max_active_participants = max(
    store.current_max_active_participants,
    sum_sync_committee_bits.uint64,
  )

  # Update the optimistic header
  if  sum_sync_committee_bits.uint64 > get_safety_threshold(store) and
      update.attested_header.slot > store.optimistic_header.slot:
    store.optimistic_header = update.attested_header

  # Update finalized header
  if  sum_sync_committee_bits * 3 >= len(sync_committee_bits) * 2 and
      not update.finalized_header.isZeroMemory:
    # Normal update through 2/3 threshold
    apply_light_client_update(store, update)
    store.best_valid_update = none(LightClientUpdate)

  true