# 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

import
  # Standard lib
  math,
  # Third-party
  stew/endians2,
  # Internal
  ./datatypes, ./digest, ../ssz

# https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/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/v0.11.0/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)

# https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/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/v0.11.0/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/v0.11.0/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``.
  for idx, val in state.validators:
    if is_active_validator(val, epoch):
      result.add idx.ValidatorIndex

# https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/specs/phase0/beacon-chain.md#get_committee_count_at_slot
func get_committee_count_at_slot*(state: BeaconState, slot: Slot): uint64 =
  # Return the number of committees at ``slot``.
  let epoch = compute_epoch_at_slot(slot)
  let active_validator_indices = get_active_validator_indices(state, epoch)
  let committees_per_slot = clamp(
    len(active_validator_indices) div SLOTS_PER_EPOCH div TARGET_COMMITTEE_SIZE,
    1, MAX_COMMITTEES_PER_SLOT).uint64
  result = committees_per_slot

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

# https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/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/v0.11.0/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_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]

# 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
  result[0..<7] = x.toBytesLE()

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

func int_to_bytes8*(x: uint64): array[8, byte] =
  x.toBytesLE()

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/v0.11.0/specs/phase0/beacon-chain.md#compute_fork_data_root
func compute_fork_data_root(current_version: array[4, byte],
    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/v0.11.0/specs/phase0/beacon-chain.md#compute_fork_digest
func compute_fork_digest(current_version: array[4, byte],
    genesis_validators_root: Eth2Digest): array[4, byte] =
  # 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.
  result[0..3] =
    compute_fork_data_root(current_version, genesis_validators_root).data[0..3]

# https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/specs/phase0/beacon-chain.md#compute_domain
func compute_domain*(
    domain_type: DomainType,
    fork_version: array[4, byte] = [0'u8, 0, 0, 0],
    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] = int_to_bytes4(domain_type.uint64)
  result[4..31] = fork_data_root.data[0..27]

# https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/beacon-chain.md#get_domain
func get_domain*(
    fork: Fork, domain_type: DomainType, epoch: Epoch): 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)

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.
  get_domain(state.fork, domain_type, message_epoch)

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/v0.11.0/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 = SigningRoot(
    object_root: hash_tree_root(ssz_object),
    domain: domain
  )
  hash_tree_root(domain_wrapped_object)

# https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/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] = int_to_bytes4(domain_type.uint64)
  seed_input[4..11] = int_to_bytes8(epoch.uint64)
  seed_input[12..43] =
    get_randao_mix(state, # Avoid underflow
      epoch + EPOCHS_PER_HISTORICAL_VECTOR - MIN_SEED_LOOKAHEAD - 1).data
  eth2hash(seed_input)