nimbus-eth2/beacon_chain/spec/validator.nim

# 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.

# Helpers and functions pertaining to managing the validator set

{.push raises: [Defect].}

import
  std/[options, math, tables],
  ./datatypes, ./digest, ./helpers

const
  SEED_SIZE = sizeof(Eth2Digest)
  ROUND_SIZE = 1
  POSITION_WINDOW_SIZE = 4
  PIVOT_VIEW_SIZE = SEED_SIZE + ROUND_SIZE
  TOTAL_SIZE = PIVOT_VIEW_SIZE + POSITION_WINDOW_SIZE

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_shuffled_index
# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_committee
# Port of https://github.com/protolambda/zrnt/blob/master/eth2/beacon/shuffle.go
# Shuffles or unshuffles, depending on the `dir` (true for shuffling, false for unshuffling
func shuffle_list*(input: var seq[ValidatorIndex], seed: Eth2Digest) =
  let list_size = input.lenu64

  if list_size <= 1: return

  var buf {.noinit.}: array[TOTAL_SIZE, byte]

  # Seed is always the first 32 bytes of the hash input, we never have to change
  # this part of the buffer.
  buf[0..<32] = seed.data

  # The original code includes a direction flag, but only the reverse direction
  # is used in eth2, so we simplify it here
  for r in 0'u8..<SHUFFLE_ROUND_COUNT.uint8:
    # spec: pivot = bytes_to_int(hash(seed + int_to_bytes1(round))[0:8]) % list_size
    # This is the "int_to_bytes1(round)", appended to the seed.
    buf[SEED_SIZE] = (SHUFFLE_ROUND_COUNT.uint8 - r - 1)

    # Seed is already in place, now just hash the correct part of the buffer,
    # and take a uint64 from it, and modulo it to get a pivot within range.
    let
      pivotDigest = eth2digest(buf.toOpenArray(0, PIVOT_VIEW_SIZE - 1))
      pivot = bytes_to_uint64(pivotDigest.data.toOpenArray(0, 7)) mod listSize

    # Split up the for-loop in two:
    #  1. Handle the part from 0 (incl) to pivot (incl). This is mirrored around
    #     (pivot / 2)
    #  2. Handle the part from pivot (excl) to N (excl). This is mirrored around
    #     ((pivot / 2) + (size/2))
    # The pivot defines a split in the array, with each of the splits mirroring
    # their data within the split.
    # Print out some example even/odd sized index lists, with some even/odd pivots,
    # and you can deduce how the mirroring works exactly.
    # Note that the mirror is strict enough to not consider swapping the index
    # @mirror with itself.
    # Since we are iterating through the "positions" in order, we can just
    # repeat the hash every 256th position.
    # No need to pre-compute every possible hash for efficiency like in the
    # example code.
    # We only need it consecutively (we are going through each in reverse order
    # however, but same thing)

    # spec: source = hash(seed + int_to_bytes1(round) + int_to_bytes4(position // 256))
    # - seed is still in 0:32 (excl., 32 bytes)
    # - round number is still in 32
    # - mix in the position for randomness, except the last byte of it,
    #     which will be used later to select a bit from the resulting hash.
    # We start from the pivot position, and work back to the mirror position
    # (of the part left to the pivot).
    # This makes us process each pear exactly once (instead of unnecessarily
    # twice, like in the spec)
    buf[33..<37] = uint_to_bytes4(pivot shr 8)

    var
      mirror = (pivot + 1) shr 1
      source = eth2digest(buf)
      byteV = source.data[(pivot and 0xff) shr 3]
      i = 0'u64
      j = pivot

    template shuffle =
      while i < mirror:
        # The pair is i,j. With j being the bigger of the two, hence the "position" identifier of the pair.
        # Every 256th bit (aligned to j).
        if (j and 0xff) == 0xff:
          # just overwrite the last part of the buffer, reuse the start (seed, round)
          buf[33..<37] = uint_to_bytes4(j shr 8)
          source = eth2digest(buf)

        # Same trick with byte retrieval. Only every 8th.
        if (j and 0x07) == 0x7:
          byteV = source.data[(j and 0xff'u64) shr 3]

        let
          bitV = (byteV shr (j and 0x7)) and 0x1

        if bitV == 1:
          swap(input[i], input[j])

        i.inc
        j.dec

    shuffle

    # Now repeat, but for the part after the pivot.
    mirror = (pivot + list_size + 1) shr 1
    let lend = list_size - 1
    # Again, seed and round input is in place, just update the position.
    # We start at the end, and work back to the mirror point.
    # This makes us process each pear exactly once (instead of unnecessarily twice, like in the spec)
    buf[33..<37] = uint_to_bytes4(lend shr 8)

    source = eth2digest(buf)
    byteV = source.data[(lend and 0xff) shr 3]
    i = pivot + 1'u64
    j = lend

    shuffle

func get_shuffled_active_validator_indices*(state: BeaconState, epoch: Epoch):
    seq[ValidatorIndex] =
  # Non-spec function, to cache a data structure from which one can cheaply
  # compute both get_active_validator_indexes() and get_beacon_committee().
  var active_validator_indices = get_active_validator_indices(state, epoch)

  shuffle_list(
    active_validator_indices, get_seed(state, epoch, DOMAIN_BEACON_ATTESTER))

  active_validator_indices

func get_shuffled_active_validator_indices*(
    cache: var StateCache, state: BeaconState, epoch: Epoch):
    var seq[ValidatorIndex] =
  # `cache` comes first because of nim's borrowing rules for the `var` return -
  # the `var` returns avoids copying the validator set.
  cache.shuffled_active_validator_indices.withValue(epoch, validator_indices) do:
    return validator_indices[]
  do:
    let indices = get_shuffled_active_validator_indices(state, epoch)
    return cache.shuffled_active_validator_indices.mgetOrPut(epoch, indices)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_active_validator_indices
func count_active_validators*(state: BeaconState,
                              epoch: Epoch,
                              cache: var StateCache): uint64 =
  cache.get_shuffled_active_validator_indices(state, epoch).lenu64

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_committee_count_per_slot
func get_committee_count_per_slot*(num_active_validators: uint64): uint64 =
  clamp(
    num_active_validators div SLOTS_PER_EPOCH div TARGET_COMMITTEE_SIZE,
    1'u64, MAX_COMMITTEES_PER_SLOT)

func get_committee_count_per_slot*(state: BeaconState,
                                   epoch: Epoch,
                                   cache: var StateCache): uint64 =
  # Return the number of committees at ``slot``.

  let
    active_validator_count = count_active_validators(state, epoch, cache)
  result = get_committee_count_per_slot(active_validator_count)

  # Otherwise, get_beacon_committee(...) cannot access some committees.
  doAssert (SLOTS_PER_EPOCH * MAX_COMMITTEES_PER_SLOT) >= uint64(result)

func get_committee_count_per_slot*(state: BeaconState,
                                   slot: Slot,
                                   cache: var StateCache): uint64 =
  get_committee_count_per_slot(state, slot.compute_epoch_at_slot, cache)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_previous_epoch
func get_previous_epoch*(current_epoch: Epoch): Epoch =
  # Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
  if current_epoch == GENESIS_EPOCH:
    current_epoch
  else:
    current_epoch - 1

func get_previous_epoch*(state: BeaconState): Epoch =
  # Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
  get_previous_epoch(get_current_epoch(state))

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_committee
func compute_committee_slice*(
    active_validators, index, count: uint64): Slice[int] =
  doAssert active_validators <= ValidatorIndex.high.uint64

  let
    start = (active_validators * index) div count
    endIdx = (active_validators * (index + 1)) div count

  start.int..(endIdx.int - 1)

iterator compute_committee*(shuffled_indices: seq[ValidatorIndex],
    index: uint64, count: uint64): ValidatorIndex =
  let
    slice = compute_committee_slice(shuffled_indices.lenu64, index, count)
  for i in slice:
    yield shuffled_indices[i]

func compute_committee*(shuffled_indices: seq[ValidatorIndex],
    index: uint64, count: uint64): seq[ValidatorIndex] =
  ## Return the committee corresponding to ``indices``, ``seed``, ``index``,
  ## and committee ``count``.
  ## In this version, we pass in the shuffled indices meaning we no longer need
  ## the seed.
  let
    slice = compute_committee_slice(shuffled_indices.lenu64, index, count)

  # In spec, this calls get_shuffled_index() every time, but that's wasteful
  # Here, get_beacon_committee() gets the shuffled version.
  shuffled_indices[slice]

func compute_committee_len*(
    active_validators, index, count: uint64): uint64 =
  ## Return the committee corresponding to ``indices``, ``seed``, ``index``,
  ## and committee ``count``.

  let
    slice = compute_committee_slice(active_validators, index, count)

  (slice.b - slice.a + 1).uint64

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_beacon_committee
iterator get_beacon_committee*(
    state: BeaconState, slot: Slot, index: CommitteeIndex,
    cache: var StateCache): ValidatorIndex =
  ## Return the beacon committee at ``slot`` for ``index``.
  let
    epoch = compute_epoch_at_slot(slot)
    committees_per_slot = get_committee_count_per_slot(state, epoch, cache)
  for idx in compute_committee(
    cache.get_shuffled_active_validator_indices(state, epoch),
    (slot mod SLOTS_PER_EPOCH) * committees_per_slot +
      index.uint64,
    committees_per_slot * SLOTS_PER_EPOCH
  ): yield idx

func get_beacon_committee*(
    state: BeaconState, slot: Slot, index: CommitteeIndex,
    cache: var StateCache): seq[ValidatorIndex] =
  ## Return the beacon committee at ``slot`` for ``index``.
  let
    epoch = compute_epoch_at_slot(slot)
    committees_per_slot = get_committee_count_per_slot(state, epoch, cache)
  compute_committee(
    cache.get_shuffled_active_validator_indices(state, epoch),
    (slot mod SLOTS_PER_EPOCH) * committees_per_slot +
      index.uint64,
    committees_per_slot * SLOTS_PER_EPOCH
  )

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_beacon_committee
func get_beacon_committee_len*(
    state: BeaconState, slot: Slot, index: CommitteeIndex,
    cache: var StateCache): uint64 =
  # Return the number of members in the beacon committee at ``slot`` for ``index``.
  let
    epoch = compute_epoch_at_slot(slot)
    committees_per_slot = get_committee_count_per_slot(state, epoch, cache)

  compute_committee_len(
    count_active_validators(state, epoch, cache),
    (slot mod SLOTS_PER_EPOCH) * committees_per_slot +
      index.uint64,
    committees_per_slot * SLOTS_PER_EPOCH
  )

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_shuffled_index
func compute_shuffled_index(
    index: uint64, index_count: uint64, seed: Eth2Digest): uint64 =
  ## Return the shuffled index corresponding to ``seed`` (and ``index_count``).
  doAssert index < index_count

  var
    source_buffer {.noinit.}: array[(32+1+4), byte]
    cur_idx_permuted = index

  source_buffer[0..31] = seed.data

  # Swap or not (https://link.springer.com/content/pdf/10.1007%2F978-3-642-32009-5_1.pdf)
  # See the 'generalized domain' algorithm on page 3
  for current_round in 0'u8 ..< SHUFFLE_ROUND_COUNT.uint8:
    source_buffer[32] = current_round

    let
      # If using multiple indices, can amortize this
      pivot =
        bytes_to_uint64(eth2digest(source_buffer.toOpenArray(0, 32)).data.toOpenArray(0, 7)) mod
          index_count

      flip = ((index_count + pivot) - cur_idx_permuted) mod index_count
      position = max(cur_idx_permuted, flip)
    source_buffer[33..36] = uint_to_bytes4((position shr 8))
    let
      source = eth2digest(source_buffer).data
      byte_value = source[(position mod 256) shr 3]
      bit = (byte_value shr (position mod 8)) mod 2

    cur_idx_permuted = if bit != 0: flip else: cur_idx_permuted

  cur_idx_permuted

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#compute_proposer_index
func compute_proposer_index(state: BeaconState, indices: seq[ValidatorIndex],
    seed: Eth2Digest): Option[ValidatorIndex] =
  ## Return from ``indices`` a random index sampled by effective balance.
  const MAX_RANDOM_BYTE = 255

  if len(indices) == 0:
    return none(ValidatorIndex)

  let seq_len = indices.lenu64

  var
    i = 0'u64
    buffer: array[32+8, byte]
  buffer[0..31] = seed.data
  while true:
    buffer[32..39] = uint_to_bytes8(i div 32)
    let
      candidate_index =
        indices[compute_shuffled_index(i mod seq_len, seq_len, seed)]
      random_byte = (eth2digest(buffer).data)[i mod 32]
      effective_balance = state.validators[candidate_index].effective_balance
    if effective_balance * MAX_RANDOM_BYTE >=
        MAX_EFFECTIVE_BALANCE * random_byte:
      return some(candidate_index)
    i += 1

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_beacon_proposer_index
func get_beacon_proposer_index*(state: BeaconState, cache: var StateCache, slot: Slot):
    Option[ValidatorIndex] =
  cache.beacon_proposer_indices.withValue(slot, proposer) do:
    return proposer[]
  do:

    # Return the beacon proposer index at the current slot.
    let epoch = get_current_epoch(state)

    var buffer: array[32 + 8, byte]
    buffer[0..31] = get_seed(state, epoch, DOMAIN_BEACON_PROPOSER).data

    # There's exactly one beacon proposer per slot.

    let
      # active validator indices are kept in cache but sorting them takes
      # quite a while
      indices = get_active_validator_indices(state, epoch)
      start = epoch.compute_start_slot_at_epoch()

    var res: Option[ValidatorIndex]
    for i in 0..<SLOTS_PER_EPOCH:
      buffer[32..39] = uint_to_bytes8((start + i).uint64)
      let seed = eth2digest(buffer)
      let pi = compute_proposer_index(state, indices, seed)
      if start + i == slot:
        res = pi
      cache.beacon_proposer_indices[start + i] = pi

    return res

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/beacon-chain.md#get_beacon_proposer_index
func get_beacon_proposer_index*(state: BeaconState, cache: var StateCache):
    Option[ValidatorIndex] =
  get_beacon_proposer_index(state, cache, state.slot)

# https://github.com/ethereum/eth2.0-specs/blob/v1.0.0/specs/phase0/validator.md#validator-assignments
func get_committee_assignment*(
    state: BeaconState, epoch: Epoch,
    validator_index: ValidatorIndex):
    Option[tuple[a: seq[ValidatorIndex], b: CommitteeIndex, c: Slot]] =
  ## Return the committee assignment in the ``epoch`` for ``validator_index``.
  ## ``assignment`` returned is a tuple of the following form:
  ##     * ``assignment[0]`` is the list of validators in the committee
  ##     * ``assignment[1]`` is the index to which the committee is assigned
  ##     * ``assignment[2]`` is the slot at which the committee is assigned
  ## Return None if no assignment.
  let next_epoch = get_current_epoch(state) + 1
  doAssert epoch <= next_epoch

  var cache = StateCache()

  let
    start_slot = compute_start_slot_at_epoch(epoch)
    committee_count_per_slot =
      get_committee_count_per_slot(state, epoch, cache)
  for slot in start_slot ..< start_slot + SLOTS_PER_EPOCH:
    for index in 0'u64 ..< committee_count_per_slot:
      let idx = index.CommitteeIndex
      let committee = get_beacon_committee(state, slot, idx, cache)
      if validator_index in committee:
        return some((committee, idx, slot))
  none(tuple[a: seq[ValidatorIndex], b: CommitteeIndex, c: Slot])