nimbus-eth2/beacon_chain/spec/helpers.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.

# Uncategorized helper functions from the spec

{.push raises: [Defect].}

import
  # Standard lib
  std/[math, tables],
  # Third-party
  stew/endians2,
  # Internal
  ./datatypes, ./digest, ./crypto, ../ssz/merkleization

type
  # This solves an ambiguous identifier Error in some contexts
  # (other candidate is nativesockets.Domain)
  Domain = datatypes.Domain

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/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/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_epoch_at_slot
func compute_epoch_at_slot*(slot: Slot|uint64): Epoch =
  ## Return the epoch number at ``slot``.
  (slot div SLOTS_PER_EPOCH).Epoch

template epoch*(slot: Slot): Epoch =
  compute_epoch_at_slot(slot)

template isEpoch*(slot: Slot): bool =
  (slot mod SLOTS_PER_EPOCH) == 0

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_start_slot_at_epoch
func compute_start_slot_at_epoch*(epoch: Epoch): Slot =
  ## Return the start slot of ``epoch``.
  (epoch * SLOTS_PER_EPOCH).Slot

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/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

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_active_validator_indices
func get_active_validator_indices*(state: BeaconState, epoch: Epoch):
    seq[ValidatorIndex] =
  ## Return the sequence of active validator indices at ``epoch``.
  result = newSeqOfCap[ValidatorIndex](state.validators.len)
  for idx in 0..<state.validators.len:
    if is_active_validator(state.validators[idx], epoch):
      result.add idx.ValidatorIndex

func get_active_validator_indices_len*(state: BeaconState, epoch: Epoch): uint64 =
  for idx in 0..<state.validators.len:
    if is_active_validator(state.validators[idx], epoch):
      inc result

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_current_epoch
func get_current_epoch*(state: BeaconState): Epoch =
  ## Return the current epoch.
  doAssert state.slot >= GENESIS_SLOT, $state.slot
  compute_epoch_at_slot(state.slot)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_randao_mix
func get_randao_mix*(state: BeaconState,
                     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
  result = 0
  for i in countdown(7, 0):
    result = result * 256 + data[i]

# Have 1, 4, and 8-byte versions. Spec only defines 8-byte version, but useful
# to check invariants on rest.
func uint_to_bytes8*(x: uint64): array[8, byte] =
  x.toBytesLE()

func uint_to_bytes4*(x: uint64): array[4, byte] =
  doAssert x < 2'u64^32

  # Little-endian data representation
  result[0] = ((x shr  0) and 0xff).byte
  result[1] = ((x shr  8) and 0xff).byte
  result[2] = ((x shr 16) and 0xff).byte
  result[3] = ((x shr 24) and 0xff).byte

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_fork_data_root
func compute_fork_data_root(current_version: Version,
    genesis_validators_root: Eth2Digest): Eth2Digest =
  ## Return the 32-byte fork data root for the ``current_version`` and
  ## ``genesis_validators_root``.
  ## This is used primarily in signature domains to avoid collisions across
  ## forks/chains.
  hash_tree_root(ForkData(
    current_version: current_version,
    genesis_validators_root: genesis_validators_root
  ))

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_fork_digest
func compute_fork_digest*(current_version: Version,
                          genesis_validators_root: Eth2Digest): ForkDigest =
  ## Return the 4-byte fork digest for the ``current_version`` and
  ## ``genesis_validators_root``.
  ## This is a digest primarily used for domain separation on the p2p layer.
  ## 4-bytes suffices for practical separation of forks/chains.
  array[4, byte](result)[0..3] =
    compute_fork_data_root(
      current_version, genesis_validators_root).data.toOpenArray(0, 3)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_domain
func compute_domain*(
    domain_type: DomainType,
    fork_version: Version,
    genesis_validators_root: Eth2Digest = ZERO_HASH): Domain =
  ## 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] = uint_to_bytes4(domain_type.uint64)
  result[4..31] = fork_data_root.data.toOpenArray(0, 27)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_domain
func get_domain*(
    fork: Fork, domain_type: DomainType, epoch: Epoch, genesis_validators_root: Eth2Digest): Domain =
  ## 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: BeaconState, domain_type: DomainType, epoch: Epoch): Domain =
  ## 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/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_signing_root
func compute_signing_root*(ssz_object: auto, domain: Domain): 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/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_seed
func get_seed*(state: BeaconState, 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] = uint_to_bytes4(domain_type.uint64)
  seed_input[4..11] = uint_to_bytes8(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)