# beacon_chain
# Copyright (c) 2018-2022 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.

# Uncategorized helper functions from the spec

{.push raises: [Defect].}

# References to `vFuture` refer to the pre-release proposal of the libp2p based
# light client sync protocol. Conflicting release versions are not in use.
# https://github.com/ethereum/consensus-specs/pull/2802

import
  # Standard lib
  std/[algorithm, math, sequtils, sets, tables],
  # Status libraries
  stew/[bitops2, byteutils, endians2, objects],
  chronicles,
  # Internal
  ./datatypes/[phase0, altair, bellatrix],
  "."/[eth2_merkleization, forks, ssz_codec]

# TODO although eth2_merkleization already exports ssz_codec, *sometimes* code
# fails to compile if the export is not done here also
export
  forks, eth2_merkleization, ssz_codec

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#integer_squareroot
func integer_squareroot*(n: SomeInteger): SomeInteger =
  ## Return the largest integer ``x`` such that ``x**2 <= n``.
  doAssert n >= 0'u64

  var
    x = n
    y = (x + 1) div 2
  while y < x:
    x = y
    y = (x + n div x) div 2
  x

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/ssz/merkle-proofs.md#generalized_index_sibling
template generalized_index_sibling*(
    index: GeneralizedIndex): GeneralizedIndex =
  index xor 1.GeneralizedIndex

template generalized_index_sibling_left(
    index: GeneralizedIndex): GeneralizedIndex =
  index and not 1.GeneralizedIndex

template generalized_index_sibling_right(
    index: GeneralizedIndex): GeneralizedIndex =
  index or 1.GeneralizedIndex

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/ssz/merkle-proofs.md#generalized_index_parent
template generalized_index_parent*(
    index: GeneralizedIndex): GeneralizedIndex =
  index shr 1

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/ssz/merkle-proofs.md#merkle-multiproofs
iterator get_branch_indices*(
    tree_index: GeneralizedIndex): GeneralizedIndex =
  ## Get the generalized indices of the sister chunks along the path
  ## from the chunk with the given tree index to the root.
  var index = tree_index
  while index > 1.GeneralizedIndex:
    yield generalized_index_sibling(index)
    index = generalized_index_parent(index)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/ssz/merkle-proofs.md#merkle-multiproofs
iterator get_path_indices*(
    tree_index: GeneralizedIndex): GeneralizedIndex =
  ## Get the generalized indices of the chunks along the path
  ## from the chunk with the given tree index to the root.
  var index = tree_index
  while index > 1.GeneralizedIndex:
    yield index
    index = generalized_index_parent(index)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/ssz/merkle-proofs.md#merkle-multiproofs
func get_helper_indices*(
    indices: openArray[GeneralizedIndex]): seq[GeneralizedIndex] =
  ## Get the generalized indices of all "extra" chunks in the tree needed
  ## to prove the chunks with the given generalized indices. Note that the
  ## decreasing order is chosen deliberately to ensure equivalence to the order
  ## of hashes in a regular single-item Merkle proof in the single-item case.
  var all_helper_indices = initHashSet[GeneralizedIndex]()
  for index in indices:
    for idx in get_branch_indices(index):
      all_helper_indices.incl idx
  for index in indices:
    for idx in get_path_indices(index):
      all_helper_indices.excl idx

  var res = newSeqOfCap[GeneralizedIndex](all_helper_indices.len)
  for idx in all_helper_indices:
    res.add idx
  res.sort(SortOrder.Descending)
  res

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/ssz/merkle-proofs.md#merkle-multiproofs
func check_multiproof_acceptable*(
    indices: openArray[GeneralizedIndex]): Result[void, string] =
  # Check that proof verification won't allocate excessive amounts of memory.
  const max_multiproof_complexity = nextPowerOfTwo(256)
  if indices.len > max_multiproof_complexity:
    trace "Max multiproof complexity exceeded",
      num_indices=indices.len, max_multiproof_complexity
    return err("Unsupported multiproof complexity (" & $indices.len & ")")

  if indices.len == 0:
    return err("No indices specified")
  if indices.anyIt(it == 0.GeneralizedIndex):
    return err("Invalid index specified")
  ok()

func calculate_multi_merkle_root_impl(
    leaves: openArray[Eth2Digest],
    proof: openArray[Eth2Digest],
    indices: openArray[GeneralizedIndex],
    helper_indices: openArray[GeneralizedIndex]): Result[Eth2Digest, string] =
  # All callers have already verified the checks in check_multiproof_acceptable,
  # as well as whether lengths of leaves/indices and proof/helper_indices match.

  # Helper to retrieve a value from a table that is statically known to exist.
  template getExisting[A, B](t: var Table[A, B], key: A): var B =
    try: t[key]
    except KeyError: raiseAssert "Unreachable"

  # Populate data structure with all leaves.
  # This data structure only scales with the number of `leaves`,
  # in contrast to the spec one that also scales with the number of `proof`
  # items and the number of all intermediate roots, potentially the entire tree.
  let capacity = nextPowerOfTwo(leaves.len)
  var objects = initTable[GeneralizedIndex, Eth2Digest](capacity)
  for i, index in indices:
    if objects.mgetOrPut(index, leaves[i]) != leaves[i]:
      return err("Conflicting roots for same index")

  # Create list with keys of all active nodes that need to be visited.
  # This list is sorted in descending order, same as `helper_indices`.
  # Pulling from `objects` instead of from `indices` deduplicates the list.
  var keys = newSeqOfCap[GeneralizedIndex](objects.len)
  for index in objects.keys:
    if index > 1.GeneralizedIndex: # For the root, no work needs to be done.
      keys.add index
  keys.sort(SortOrder.Descending)

  # The merkle tree is processed from bottom to top, pulling in helper
  # indices from `proof` as needed. During processing, the `keys` list
  # may temporarily end up being split into two parts, sorted individually.
  # An additional index tracks the current maximum element of the list.
  var
    completed = 0         # All key indices before this are fully processed.
    maxIndex = completed  # Index of the list's largest key.
    helper = 0            # Helper index from `proof` to be pulled next.

  # Processing is done when there are no more keys to process.
  while completed < keys.len:
    let
      k = keys[maxIndex]
      sibling = generalized_index_sibling(k)
      left = generalized_index_sibling_left(k)
      right = generalized_index_sibling_right(k)
      parent = generalized_index_parent(k)
      parentRight = generalized_index_sibling_right(parent)

    # Keys need to be processed in descending order to ensure that intermediate
    # roots remain available until they are no longer needed. This ensures that
    # conflicting roots are detected in all cases.
    keys[maxIndex] =
      if not objects.hasKey(k):
        # A previous computation did already merge this key with its sibling.
        0.GeneralizedIndex
      else:
        # Compute expected root for parent. This deletes child roots.
        # Because the list is sorted in descending order, they are not needed.
        let root = withEth2Hash:
          if helper < helper_indices.len and helper_indices[helper] == sibling:
            # The next proof item is required to form the parent hash.
            if sibling == left:
              h.update proof[helper].data
              h.update objects.getExisting(right).data; objects.del right
            else:
              h.update objects.getExisting(left).data;  objects.del left
              h.update proof[helper].data
            inc helper
          else:
            # Both siblings are already known.
            h.update objects.getExisting(left).data;  objects.del left
            h.update objects.getExisting(right).data; objects.del right

        # Store parent root, and replace the current list entry with its parent.
        if objects.hasKeyOrPut(parent, root):
          if objects.getExisting(parent) != root:
            return err("Conflicting roots for same index")
          0.GeneralizedIndex
        elif parent > 1.GeneralizedIndex:
          # Note that the list may contain further nodes that are on a layer
          # beneath the parent, so this may break the strictly descending order
          # of the list. For example, given [12, 9], this will lead to [6, 9].
          # This will resolve itself after the additional nodes are processed,
          # i.e., [6, 9] -> [6, 4] -> [3, 4] -> [3, 2] -> [1].
          parent
        else:
          0.GeneralizedIndex
    if keys[maxIndex] != 0.GeneralizedIndex:
      # The list may have been temporarily split up into two parts that are
      # individually sorted in descending order. Have to first process further
      # nodes until the list is sorted once more.
      inc maxIndex

    # Determine whether descending sort order has been restored.
    let isSorted =
      if maxIndex == completed: true
      else:
        while maxIndex < keys.len and keys[maxIndex] == 0.GeneralizedIndex:
          inc maxIndex
        maxIndex >= keys.len or keys[maxIndex] <= parentRight
    if isSorted:
      # List is sorted once more. Reset `maxIndex` to its start.
      while completed < keys.len and keys[completed] == 0.GeneralizedIndex:
        inc completed
      maxIndex = completed

  # Proof is guaranteed to provide all info needed to reach the root.
  doAssert helper == helper_indices.len
  doAssert objects.len == 1
  ok(objects.getExisting(1.GeneralizedIndex))

func calculate_multi_merkle_root*(
    leaves: openArray[Eth2Digest],
    proof: openArray[Eth2Digest],
    indices: openArray[GeneralizedIndex],
    helper_indices: openArray[GeneralizedIndex]): Result[Eth2Digest, string] =
  doAssert proof.len == helper_indices.len
  if leaves.len != indices.len:
    return err("Length mismatch for leaves and indices")
  ? check_multiproof_acceptable(indices)
  calculate_multi_merkle_root_impl(
    leaves, proof, indices, helper_indices)

func calculate_multi_merkle_root*(
    leaves: openArray[Eth2Digest],
    proof: openArray[Eth2Digest],
    indices: openArray[GeneralizedIndex]): Result[Eth2Digest, string] =
  if leaves.len != indices.len:
    return err("Length mismatch for leaves and indices")
  ? check_multiproof_acceptable(indices)
  calculate_multi_merkle_root_impl(
    leaves, proof, indices, get_helper_indices(indices))

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/ssz/merkle-proofs.md#merkle-multiproofs
func verify_merkle_multiproof*(
    leaves: openArray[Eth2Digest],
    proof: openArray[Eth2Digest],
    indices: openArray[GeneralizedIndex],
    helper_indices: openArray[GeneralizedIndex],
    root: Eth2Digest): bool =
  let calc = calculate_multi_merkle_root(leaves, proof, indices, helper_indices)
  if calc.isErr: return false
  calc.get == root

func verify_merkle_multiproof*(
    leaves: openArray[Eth2Digest],
    proof: openArray[Eth2Digest],
    indices: openArray[GeneralizedIndex],
    root: Eth2Digest): bool =
  let calc = calculate_multi_merkle_root(leaves, proof, indices)
  if calc.isErr: return false
  calc.get == root

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#is_valid_merkle_branch
func is_valid_merkle_branch*(leaf: Eth2Digest, branch: openArray[Eth2Digest],
                             depth: int, 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:
    if (index div (1'u64 shl i)) mod 2 != 0:
      buf[0..31] = branch[i].data
      buf[32..63] = value.data
    else:
      buf[0..31] = value.data
      buf[32..63] = branch[i].data
    value = eth2digest(buf)
  value == root

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/tests/core/pyspec/eth2spec/test/helpers/merkle.py#L4-L21
func build_proof_impl(anchor: object, leaf_index: uint64,
                      proof: var openArray[Eth2Digest]) =
  let
    bottom_length = nextPow2(typeof(anchor).totalSerializedFields.uint64)
    tree_depth = log2trunc(bottom_length)
    parent_index =
      if leaf_index < bottom_length shl 1:
        0'u64
      else:
        var i = leaf_index
        while i >= bottom_length shl 1:
          i = i shr 1
        i

  var
    prefix_len = 0
    proof_len = log2trunc(leaf_index)
    cache = newSeq[Eth2Digest](bottom_length shl 1)
  block:
    var i = bottom_length
    anchor.enumInstanceSerializedFields(fieldNameVar, fieldVar):
      if i == parent_index:
        when fieldVar is object:
          prefix_len = log2trunc(leaf_index) - tree_depth
          proof_len -= prefix_len
          let
            bottom_bits = leaf_index and not (uint64.high shl prefix_len)
            prefix_leaf_index = (1'u64 shl prefix_len) + bottom_bits
          build_proof_impl(fieldVar, prefix_leaf_index, proof)
        else: raiseAssert "Invalid leaf_index"
      cache[i] = hash_tree_root(fieldVar)
      i += 1
    for i in countdown(bottom_length - 1, 1):
      cache[i] = withEth2Hash:
        h.update cache[i shl 1].data
        h.update cache[i shl 1 + 1].data

  var i = if parent_index != 0: parent_index
          else: leaf_index
  doAssert i > 0 and i < bottom_length shl 1
  for proof_index in prefix_len ..< prefix_len + proof_len:
    let b = (i and 1) != 0
    i = i shr 1
    proof[proof_index] = if b: cache[i shl 1]
                         else: cache[i shl 1 + 1]

func build_proof*(anchor: object, leaf_index: uint64,
                  proof: var openArray[Eth2Digest]) =
  doAssert leaf_index > 0
  doAssert proof.len == log2trunc(leaf_index)
  build_proof_impl(anchor, leaf_index, proof)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#is_active_validator
func is_active_validator*(validator: Validator, epoch: Epoch): bool =
  ## Check if ``validator`` is active
  validator.activation_epoch <= epoch and epoch < validator.exit_epoch

func is_exited_validator*(validator: Validator, epoch: Epoch): bool =
  ## Check if ``validator`` is exited
  validator.exit_epoch <= epoch

func is_withdrawable_validator*(validator: Validator, epoch: Epoch): bool =
  epoch >= validator.withdrawable_epoch

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#get_active_validator_indices
iterator get_active_validator_indices*(state: ForkyBeaconState, epoch: Epoch):
    ValidatorIndex =
  for vidx in state.validators.vindices:
    if is_active_validator(state.validators[vidx], epoch):
      yield vidx

func get_active_validator_indices*(state: ForkyBeaconState, epoch: Epoch):
    seq[ValidatorIndex] =
  ## Return the sequence of active validator indices at ``epoch``.
  var res = newSeqOfCap[ValidatorIndex](state.validators.len)
  for vidx in get_active_validator_indices(state, epoch):
    res.add vidx
  res

func get_active_validator_indices_len*(state: ForkyBeaconState, epoch: Epoch):
    uint64 =
  for vidx in state.validators.vindices:
    if is_active_validator(state.validators[vidx], epoch):
      inc result

func get_active_validator_indices_len*(
    state: ForkedHashedBeaconState; epoch: Epoch): uint64 =
  withState(state):
    get_active_validator_indices_len(state.data, epoch)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#get_current_epoch
func get_current_epoch*(state: ForkyBeaconState): Epoch =
  ## Return the current epoch.
  state.slot.epoch

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#get_current_epoch
func get_current_epoch*(state: ForkedHashedBeaconState): Epoch =
  ## Return the current epoch.
  withState(state): get_current_epoch(state.data)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#get_randao_mix
func get_randao_mix*(state: ForkyBeaconState, epoch: Epoch): Eth2Digest =
  ## Returns the randao mix at a recent ``epoch``.
  state.randao_mixes[epoch mod EPOCHS_PER_HISTORICAL_VECTOR]

func bytes_to_uint64*(data: openArray[byte]): uint64 =
  doAssert data.len == 8

  # Little-endian data representation
  uint64.fromBytesLE(data)

func uint_to_bytes*(x: uint64): array[8, byte] = toBytesLE(x)
func uint_to_bytes*(x: uint32): array[4, byte] = toBytesLE(x)
func uint_to_bytes*(x: uint16): array[2, byte] = toBytesLE(x)
func uint_to_bytes*(x: uint8): array[1, byte] = toBytesLE(x)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#compute_domain
func compute_domain*(
    domain_type: DomainType,
    fork_version: Version,
    genesis_validators_root: Eth2Digest = ZERO_HASH): Eth2Domain =
  ## Return the domain for the ``domain_type`` and ``fork_version``.
  let fork_data_root =
    compute_fork_data_root(fork_version, genesis_validators_root)
  result[0..3] = domain_type.data
  result[4..31] = fork_data_root.data.toOpenArray(0, 27)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#get_domain
func get_domain*(
    fork: Fork,
    domain_type: DomainType,
    epoch: Epoch,
    genesis_validators_root: Eth2Digest): Eth2Domain =
  ## Return the signature domain (fork version concatenated with domain type)
  ## of a message.
  let fork_version =
    if epoch < fork.epoch:
      fork.previous_version
    else:
      fork.current_version
  compute_domain(domain_type, fork_version, genesis_validators_root)

func get_domain*(
    state: ForkyBeaconState, domain_type: DomainType, epoch: Epoch): Eth2Domain =
  ## Return the signature domain (fork version concatenated with domain type)
  ## of a message.
  get_domain(state.fork, domain_type, epoch, state.genesis_validators_root)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#compute_signing_root
func compute_signing_root*(ssz_object: auto, domain: Eth2Domain): Eth2Digest =
  ## Return the signing root of an object by calculating the root of the
  ## object-domain tree.
  let domain_wrapped_object = SigningData(
    object_root: hash_tree_root(ssz_object),
    domain: domain
  )
  hash_tree_root(domain_wrapped_object)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/beacon-chain.md#get_seed
func get_seed*(state: ForkyBeaconState, epoch: Epoch, domain_type: DomainType):
    Eth2Digest =
  ## Return the seed at ``epoch``.

  var seed_input : array[4+8+32, byte]

  # Detect potential underflow
  static:
    doAssert EPOCHS_PER_HISTORICAL_VECTOR > MIN_SEED_LOOKAHEAD

  seed_input[0..3] = domain_type.data
  seed_input[4..11] = uint_to_bytes(epoch.uint64)
  seed_input[12..43] =
    get_randao_mix(state, # Avoid underflow
      epoch + EPOCHS_PER_HISTORICAL_VECTOR - MIN_SEED_LOOKAHEAD - 1).data
  eth2digest(seed_input)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/altair/beacon-chain.md#add_flag
func add_flag*(flags: ParticipationFlags, flag_index: int): ParticipationFlags =
  let flag = ParticipationFlags(1'u8 shl flag_index)
  flags or flag

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/altair/beacon-chain.md#has_flag
func has_flag*(flags: ParticipationFlags, flag_index: int): bool =
  let flag = ParticipationFlags(1'u8 shl flag_index)
  (flags and flag) == flag

# https://github.com/ethereum/consensus-specs/blob/vFuture/specs/altair/sync-protocol.md#is_sync_committee_update
template is_sync_committee_update*(update: SomeLightClientUpdate): bool =
  when update is SomeLightClientUpdateWithSyncCommittee:
    not isZeroMemory(update.next_sync_committee_branch)
  else:
    false

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/altair/sync-protocol.md#get_active_header
template is_finality_update*(update: SomeLightClientUpdate): bool =
  when update is SomeLightClientUpdateWithFinality:
    not isZeroMemory(update.finality_branch)
  else:
    false

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/altair/sync-protocol.md#get_subtree_index
func get_subtree_index*(idx: GeneralizedIndex): uint64 =
  doAssert idx > 0
  uint64(idx mod (type(idx)(1) shl log2trunc(idx)))

# https://github.com/ethereum/consensus-specs/blob/vFuture/specs/altair/sync-protocol.md#is_next_sync_committee_known
template is_next_sync_committee_known*(store: LightClientStore): bool =
  not isZeroMemory(store.next_sync_committee)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/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/vFuture/specs/altair/sync-protocol.md#is_better_update
type LightClientUpdateMetadata* = object
  attested_slot*, finalized_slot*, signature_slot*: Slot
  has_sync_committee*, has_finality*: bool
  num_active_participants*: uint64

func toMeta*(update: SomeLightClientUpdate): LightClientUpdateMetadata =
  var meta {.noinit.}: LightClientUpdateMetadata
  meta.attested_slot =
    update.attested_header.slot
  meta.finalized_slot =
    when update is SomeLightClientUpdateWithFinality:
      update.finalized_header.slot
    else:
      GENESIS_SLOT
  meta.signature_slot =
    update.signature_slot
  meta.has_sync_committee =
    when update is SomeLightClientUpdateWithSyncCommittee:
      not update.next_sync_committee_branch.isZeroMemory
    else:
      false
  meta.has_finality =
    when update is SomeLightClientUpdateWithFinality:
      not update.finality_branch.isZeroMemory
    else:
      false
  meta.num_active_participants =
    countOnes(update.sync_aggregate.sync_committee_bits).uint64
  meta

func is_better_data*(new_meta, old_meta: LightClientUpdateMetadata): bool =
  # Compare supermajority (> 2/3) sync committee participation
  const max_active_participants = SYNC_COMMITTEE_SIZE.uint64
  let
    new_has_supermajority =
      new_meta.num_active_participants * 3 >= max_active_participants * 2
    old_has_supermajority =
      old_meta.num_active_participants * 3 >= max_active_participants * 2
  if new_has_supermajority != old_has_supermajority:
    return new_has_supermajority > old_has_supermajority
  if not new_has_supermajority:
    if new_meta.num_active_participants != old_meta.num_active_participants:
      return new_meta.num_active_participants > old_meta.num_active_participants

  # Compare presence of relevant sync committee
  let
    new_has_relevant_sync_committee = new_meta.has_sync_committee and
      new_meta.attested_slot.sync_committee_period ==
      new_meta.signature_slot.sync_committee_period
    old_has_relevant_sync_committee = old_meta.has_sync_committee and
      old_meta.attested_slot.sync_committee_period ==
      old_meta.signature_slot.sync_committee_period
  if new_has_relevant_sync_committee != old_has_relevant_sync_committee:
    return new_has_relevant_sync_committee > old_has_relevant_sync_committee

  # Compare indication of any finality
  if new_meta.has_finality != old_meta.has_finality:
    return new_meta.has_finality > old_meta.has_finality

  # Compare sync committee finality
  if new_meta.has_finality:
    let
      new_has_sync_committee_finality =
        new_meta.finalized_slot.sync_committee_period ==
        new_meta.attested_slot.sync_committee_period
      old_has_sync_committee_finality =
        old_meta.finalized_slot.sync_committee_period ==
        old_meta.attested_slot.sync_committee_period
    if new_has_sync_committee_finality != old_has_sync_committee_finality:
      return new_has_sync_committee_finality > old_has_sync_committee_finality

  # Tiebreaker 1: Sync committee participation beyond supermajority
  if new_meta.num_active_participants != old_meta.num_active_participants:
    return new_meta.num_active_participants > old_meta.num_active_participants

  # Tiebreaker 2: Prefer older data (fewer changes to best data)
  new_meta.attested_slot < old_meta.attested_slot

template is_better_update*[A, B: SomeLightClientUpdate](
    new_update: A, old_update: B): bool =
  is_better_data(toMeta(new_update), toMeta(old_update))

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/bellatrix/beacon-chain.md#is_merge_transition_complete
func is_merge_transition_complete*(state: bellatrix.BeaconState): bool =
  const defaultExecutionPayloadHeader = default(ExecutionPayloadHeader)
  state.latest_execution_payload_header != defaultExecutionPayloadHeader

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/bellatrix/beacon-chain.md#is_merge_transition_block
func is_merge_transition_block(
    state: bellatrix.BeaconState,
    body: bellatrix.BeaconBlockBody | bellatrix.TrustedBeaconBlockBody |
          bellatrix.SigVerifiedBeaconBlockBody): bool =
  const defaultBellatrixExecutionPayload = default(bellatrix.ExecutionPayload)
  not is_merge_transition_complete(state) and
    body.execution_payload != defaultBellatrixExecutionPayload

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/bellatrix/beacon-chain.md#is_execution_enabled
func is_execution_enabled*(
    state: bellatrix.BeaconState,
    body: bellatrix.BeaconBlockBody | bellatrix.TrustedBeaconBlockBody |
          bellatrix.SigVerifiedBeaconBlockBody): bool =
  is_merge_transition_block(state, body) or is_merge_transition_complete(state)

# https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/bellatrix/beacon-chain.md#compute_timestamp_at_slot
func compute_timestamp_at_slot*(state: ForkyBeaconState, slot: Slot): uint64 =
  # Note: This function is unsafe with respect to overflows and underflows.
  let slots_since_genesis = slot - GENESIS_SLOT
  state.genesis_time + slots_since_genesis * SECONDS_PER_SLOT

# https://github.com/ethereum/consensus-specs/blob/v1.1.10/tests/core/pyspec/eth2spec/test/helpers/execution_payload.py#L1-L31
func build_empty_execution_payload*(state: bellatrix.BeaconState): ExecutionPayload =
  ## Assuming a pre-state of the same slot, build a valid ExecutionPayload
  ## without any transactions.
  let
    latest = state.latest_execution_payload_header
    timestamp = compute_timestamp_at_slot(state, state.slot)
    randao_mix = get_randao_mix(state, get_current_epoch(state))

  var payload = ExecutionPayload(
    parent_hash: latest.block_hash,
    state_root: latest.state_root, # no changes to the state
    receipts_root: static(Eth2Digest.fromHex(
      "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421")),
    block_number: latest.block_number + 1,
    prev_randao: randao_mix,
    gas_limit: latest.gas_limit, # retain same limit
    gas_used: 0, # empty block, 0 gas
    timestamp: timestamp,
    base_fee_per_gas: latest.base_fee_per_gas) # retain same base_fee

  payload.block_hash = withEth2Hash:
    h.update payload.hash_tree_root().data
    h.update cast[array[13, uint8]]("FAKE RLP HASH")

  payload