nimbus-eth2/beacon_chain/spec/helpers.nim

# beacon_chain
# Copyright (c) 2018 Status Research & Development GmbH
# Licensed and distributed under either of
#   * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
#   * Apache v2 license (license terms in the root directory or at http://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

import ./datatypes, ./digest, sequtils, math

# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_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

# TODO reuse as necessary/useful for merkle proof building
func merkle_root*(values: openArray[Eth2Digest]): Eth2Digest =
  ## Merkleize ``values`` (where ``len(values)`` is a power of two) and return
  ## the Merkle root.
  ## https://crypto.stackexchange.com/questions/43430/what-is-the-reason-to-separate-domains-in-the-internal-hash-algorithm-of-a-merkl
  let num_values = len(values)

  # Simplifies boundary conditions
  doAssert is_power_of_two(num_values)
  doAssert num_values >= 2
  doAssert num_values mod 2 == 0

  # TODO reverse ``o`` order and use newSeqWith to avoid pointless zero-filling.
  var o = repeat(ZERO_HASH, len(values))
  var hash_buffer: array[2*32, byte]

  # These ``o`` indices get filled from ``values``.
  let highest_internally_filled_index = (num_values div 2) - 1
  doAssert (highest_internally_filled_index + 1) * 2 >= num_values

  for i in countdown(num_values-1, highest_internally_filled_index + 1):
    hash_buffer[0..31] = values[i*2 - num_values].data
    hash_buffer[32..63] = values[i*2+1 - num_values].data
    o[i] = eth2hash(hash_buffer)

  ## These ``o`` indices get filled from other ``o`` indices.
  doAssert highest_internally_filled_index * 2 + 1 < num_values

  for i in countdown(highest_internally_filled_index, 1):
    hash_buffer[0..31] = o[i*2].data
    hash_buffer[32..63] = o[i*2+1].data
    o[i] = eth2hash(hash_buffer)

  o[1]

# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compute_epoch_of_slot
func compute_epoch_of_slot*(slot: Slot|uint64): Epoch =
  # Return the epoch number of the given ``slot``.
  (slot div SLOTS_PER_EPOCH).Epoch

# https://github.com/ethereum/eth2.0-specs/blob/v0.7.1/specs/core/0_beacon-chain.md#compute_start_slot_of_epoch
func compute_start_slot_of_epoch*(epoch: Epoch): Slot =
  # Return the starting slot of the given ``epoch``.
  (epoch * SLOTS_PER_EPOCH).Slot

# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_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/v0.7.1/specs/core/0_beacon-chain.md#get_active_validator_indices
func get_active_validator_indices*(state: BeaconState, epoch: Epoch):
    seq[ValidatorIndex] =
  # Get active validator indices at ``epoch``.
  for idx, val in state.validators:
    if is_active_validator(val, epoch):
      result.add idx.ValidatorIndex

# https://github.com/ethereum/eth2.0-specs/blob/v0.7.1/specs/core/0_beacon-chain.md#get_epoch_committee_count
func get_epoch_committee_count*(state: BeaconState, epoch: Epoch): uint64 =
  # Return the number of committees at ``epoch``.
  let active_validator_indices = get_active_validator_indices(state, epoch)
  clamp(
    len(active_validator_indices) div SLOTS_PER_EPOCH div TARGET_COMMITTEE_SIZE,
    1, SHARD_COUNT div SLOTS_PER_EPOCH).uint64 * SLOTS_PER_EPOCH

# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_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_of_slot(state.slot)

# https://github.com/ethereum/eth2.0-specs/blob/v0.7.1/specs/core/0_beacon-chain.md#get_randao_mix
func get_randao_mix*(state: BeaconState,
                     epoch: Epoch): Eth2Digest =
    ## Returns the randao mix at a recent ``epoch``.
    ## ``epoch`` expected to be between (current_epoch -
    ## LATEST_RANDAO_MIXES_LENGTH, current_epoch].
    state.randao_mixes[epoch mod LATEST_RANDAO_MIXES_LENGTH]

# https://github.com/ethereum/eth2.0-specs/blob/v0.7.1/specs/core/0_beacon-chain.md#get_active_index_root
func get_active_index_root(state: BeaconState, epoch: Epoch): Eth2Digest =
  # Returns the index root at a recent ``epoch``.
  ## ``epoch`` expected to be between
  ##  (current_epoch - LATEST_ACTIVE_INDEX_ROOTS_LENGTH + ACTIVATION_EXIT_DELAY, current_epoch + ACTIVATION_EXIT_DELAY].
  ## TODO maybe assert this, but omission of such seems conspicuously
  ## intentional
  state.active_index_roots[epoch mod LATEST_ACTIVE_INDEX_ROOTS_LENGTH]

# https://github.com/ethereum/eth2.0-specs/blob/v0.7.1/specs/core/0_beacon-chain.md#bytes_to_int
func bytes_to_int*(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]

# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#int_to_bytes1-int_to_bytes2-
# Have 1, 4, 8, and 32-byte versions. 1+ more and maybe worth metaprogramming.
func int_to_bytes32*(x: uint64): array[32, byte] =
  ## Little-endian data representation
  ## TODO remove uint64 when those callers fade away
  for i in 0 ..< 8:
    result[24 + i] = byte((x shr i*8) and 0xff)

func int_to_bytes32*(x: Epoch): array[32, byte] {.borrow.}

func int_to_bytes8*(x: uint64): array[8, byte] =
  for i in 0 ..< 8:
    result[i] = byte((x shr i*8) and 0xff)

func int_to_bytes1*(x: int): array[1, byte] =
  doAssert x >= 0
  doAssert x < 256

  result[0] = x.byte

func int_to_bytes4*(x: uint64): array[4, byte] =
  doAssert x >= 0'u64
  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/v0.6.3/specs/core/0_beacon-chain.md#bls_domain
func compute_domain(domain_type: DomainType, fork_version: array[4, byte]):
    uint64 =
  var buf: array[8, byte]
  buf[0..3] = fork_version
  buf[4..7] = int_to_bytes4(domain_type.uint64)
  bytes_to_int(buf)

# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_domain
func get_domain*(
    state: BeaconState, domain_type: DomainType, message_epoch: Epoch): Domain =
  ## Return the signature domain (fork version concatenated with domain type)
  ## of a message.
  let
    epoch = message_epoch
    fork_version = if epoch < state.fork.epoch:
        state.fork.previous_version
      else:
        state.fork.current_version
  compute_domain(domain_type, fork_version)

func get_domain*(state: BeaconState, domain_type: DomainType): Domain =
  get_domain(state, domain_type, get_current_epoch(state))

# https://github.com/ethereum/eth2.0-specs/blob/v0.6.3/specs/core/0_beacon-chain.md#generate_seed
func generate_seed*(state: BeaconState, epoch: Epoch): Eth2Digest =
  # Generate a seed for the given ``epoch``.

  var seed_input : array[32*3, byte]

  # Detect potential underflow
  doAssert LATEST_RANDAO_MIXES_LENGTH >= MIN_SEED_LOOKAHEAD

  seed_input[0..31] =
    get_randao_mix(state,
      epoch + LATEST_RANDAO_MIXES_LENGTH - MIN_SEED_LOOKAHEAD).data
  seed_input[32..63] = get_active_index_root(state, epoch).data
  seed_input[64..95] = int_to_bytes32(epoch)
  eth2hash(seed_input)