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nimbus-eth2/beacon_chain/spec/helpers.nim

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# 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
# TODO spec candidate? there's bits in nim-ranges but that one has some API
# issues regarding bit endianess that need resolving..
func bitSet*(bitfield: var openArray[byte], index: int) =
bitfield[index div 8] = bitfield[index div 8] or 1'u8 shl (7 - (index mod 8))
func get_bitfield_bit*(bitfield: openarray[byte], i: int): byte =
# https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#get_bitfield_bit
# Extract the bit in ``bitfield`` at position ``i``.
(bitfield[i div 8] shr (7 - (i mod 8))) mod 2
func verify_bitfield*(bitfield: openarray[byte], committee_size: int): bool =
# https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#verify_bitfield
# Verify ``bitfield`` against the ``committee_size``.
if len(bitfield) != (committee_size + 7) div 8:
return false
for i in committee_size + 1 ..< committee_size - (committee_size mod 8) + 8:
if get_bitfield_bit(bitfield, i) == 0b1:
return false
true
func mod_get[T](arr: openarray[T], pos: Natural): T =
arr[pos mod arr.len]
func shuffle*[T](values: seq[T], seed: Eth2Digest): seq[T] =
## Returns the shuffled ``values`` with seed as entropy.
## TODO: this calls out for tests, but I odn't particularly trust spec
## right now.
let values_count = values.len
const
# Entropy is consumed from the seed in 3-byte (24 bit) chunks.
rand_bytes = 3
# The highest possible result of the RNG.
rand_max = 2^(rand_bytes * 8) - 1
# The range of the RNG places an upper-bound on the size of the list that
# may be shuffled. It is a logic error to supply an oversized list.
assert values_count < rand_max
result = values
var
source = seed
index = 0
while index < values_count - 1:
# Re-hash the `source` to obtain a new pattern of bytes.
source = eth2hash source.data
# Iterate through the `source` bytes in 3-byte chunks.
for pos in countup(0, 29, 3):
let remaining = values_count - index
if remaining == 1:
break
# Read 3-bytes of `source` as a 24-bit big-endian integer.
let sample_from_source =
source.data[pos].ValidatorIndex shl 16 or
source.data[pos+1].ValidatorIndex shl 8 or
source.data[pos+2].ValidatorIndex
# Sample values greater than or equal to `sample_max` will cause
# modulo bias when mapped into the `remaining` range.
let sample_max = rand_max - rand_max mod remaining
# Perform a swap if the consumed entropy will not cause modulo bias.
if sample_from_source < sample_max:
# Select a replacement index for the current index.
let replacement_position = sample_from_source mod remaining + index
swap result[index], result[replacement_position]
inc index
func split*[T](lst: openArray[T], N: Positive): seq[seq[T]] =
## split lst in N pieces, with each piece having `len(lst) div N` or
## `len(lst) div N + 1` pieces
# TODO: implement as an iterator
result = newSeq[seq[T]](N)
for i in 0 ..< N:
result[i] = lst[lst.len * i div N ..< lst.len * (i+1) div N] # TODO: avoid alloc via toOpenArray
func get_new_recent_block_roots*(old_block_roots: seq[Eth2Digest],
parent_slot, current_slot: int64,
parent_hash: Eth2Digest
): seq[Eth2Digest] =
# Should throw for `current_slot - CYCLE_LENGTH * 2 - 1` according to spec comment
let d = current_slot - parent_slot
result = old_block_roots[d .. ^1]
for _ in 0 ..< min(d, old_block_roots.len):
result.add parent_hash
func ceil_div8*(v: int): int = (v + 7) div 8 # TODO use a proper bitarray!
func repeat_hash*(v: Eth2Digest, n: SomeInteger): Eth2Digest =
# Spec version:
# if n == 0: v
# else: repeat_hash(eth2hash(v.data), n - 1)
# Nim is pretty bad at recursion though (max 2k levels / no tco), so:
result = v
var n = n
while n != 0:
result = eth2hash(result.data)
dec n
func get_shard_committees_index*(state: BeaconState, slot: uint64): uint64 =
# TODO temporary adapter; remove when all users gone
## Warning: as it stands, this helper only works during state updates _after_
## state.slot has been incremented but before shard_committees_at_slots has
## been updated!
# TODO spec unsigned-unsafe here
doAssert slot + (state.slot mod EPOCH_LENGTH) + EPOCH_LENGTH > state.slot
slot + (state.slot mod EPOCH_LENGTH) + EPOCH_LENGTH - state.slot
func integer_squareroot*(n: SomeInteger): SomeInteger =
## The largest integer ``x`` such that ``x**2`` is less than ``n``.
var
x = n
y = (x + 1) div 2
while y < x:
x = y
y = (x + n div x) div 2
x
func get_fork_version*(fork: Fork, epoch: EpochNumber): uint64 =
if epoch < fork.epoch:
fork.previous_version
else:
fork.current_version
func get_domain*(
fork: Fork, slot: uint64, domain_type: SignatureDomain): uint64 =
# TODO Slot overflow? Or is slot 32 bits for all intents and purposes?
(get_fork_version(fork, slot) shl 32) + domain_type.uint32
func is_power_of_2*(v: uint64): bool = (v and (v-1)) == 0
func merkle_root*(values: openArray[Eth2Digest]): Eth2Digest =
# o = [0] * len(values) + values
# for i in range(len(values)-1, 0, -1):
# o[i] = hash(o[i*2] + o[i*2+1])
# return o[1]
# TODO
discard
proc is_double_vote*(attestation_data_1: AttestationData,
attestation_data_2: AttestationData): bool =
## Assumes ``attestation_data_1`` is distinct from ``attestation_data_2``.
## Returns True if the provided ``AttestationData`` are slashable
## due to a 'double vote'.
## A double vote is when a validator votes for two attestations within the
## same slot - doing so means risking getting slashed.
attestation_data_1.slot == attestation_data_2.slot
func slot_to_epoch*(slot: SlotNumber): EpochNumber =
slot div EPOCH_LENGTH
proc is_surround_vote*(attestation_data_1: AttestationData,
attestation_data_2: AttestationData): bool =
## Assumes ``attestation_data_1`` is distinct from ``attestation_data_2``.
## Returns True if the provided ``AttestationData`` are slashable
## due to a 'surround vote'.
## Note: parameter order matters as this function only checks
## that ``attestation_data_1`` surrounds ``attestation_data_2``.
let
source_epoch_1 = attestation_data_1.justified_epoch
source_epoch_2 = attestation_data_2.justified_epoch
target_epoch_1 = slot_to_epoch(attestation_data_1.slot)
target_epoch_2 = slot_to_epoch(attestation_data_2.slot)
(
(source_epoch_1 < source_epoch_2) and
(source_epoch_2 + 1 == target_epoch_2) and
(target_epoch_2 < target_epoch_1)
)
func is_active_validator*(validator: Validator, epoch: EpochNumber): bool =
### Checks if validator is active
validator.activation_epoch <= epoch and epoch < validator.exit_epoch
func get_active_validator_indices*(validators: openArray[Validator], epoch: EpochNumber): seq[ValidatorIndex] =
## Gets indices of active validators from validators
for idx, val in validators:
if is_active_validator(val, epoch):
result.add idx.ValidatorIndex
func get_epoch_committee_count*(active_validator_count: int): uint64 =
clamp(
active_validator_count div EPOCH_LENGTH div TARGET_COMMITTEE_SIZE,
1, SHARD_COUNT div EPOCH_LENGTH).uint64 * EPOCH_LENGTH
func get_current_epoch_committee_count*(state: BeaconState): uint64 =
let current_active_validators = get_active_validator_indices(
state.validator_registry,
state.current_calculation_epoch,
)
return get_epoch_committee_count(len(current_active_validators))
func get_current_epoch*(state: BeaconState): EpochNumber =
slot_to_epoch(state.slot)
## TODO I've been moving things into helpers because of layering issues
## but create DAG of which helper functions ref others and topo sort to
## refactor all of this mess.
func get_randao_mix*(state: BeaconState,
epoch: EpochNumber): Eth2Digest =
## Returns the randao mix at a recent ``epoch``.
assert get_current_epoch(state) - LATEST_RANDAO_MIXES_LENGTH < epoch
assert epoch <= get_current_epoch(state)
state.latest_randao_mixes[epoch mod LATEST_RANDAO_MIXES_LENGTH]
func get_active_index_root(state: BeaconState, epoch: EpochNumber): Eth2Digest =
# Returns the index root at a recent ``epoch``.
assert get_current_epoch(state) - LATEST_INDEX_ROOTS_LENGTH < epoch
assert epoch <= get_current_epoch(state) + 1
state.latest_index_roots[epoch mod LATEST_INDEX_ROOTS_LENGTH]
func generate_seed*(state: BeaconState, epoch: EpochNumber): Eth2Digest =
# Generate a seed for the given ``epoch``.
var seed_input : array[32*2, byte]
seed_input[0..31] = get_randao_mix(state, epoch - SEED_LOOKAHEAD).data
seed_input[32..63] = get_active_index_root(state, epoch).data
eth2hash(seed_input)
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