386 lines
16 KiB
Nim
386 lines
16 KiB
Nim
# Copyright (c) 2018-2020 Status Research & Development GmbH
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# Licensed and distributed under either of
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# * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
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# * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
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# at your option. This file may not be copied, modified, or distributed except according to those terms.
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# Helpers and functions pertaining to managing the validator set
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{.push raises: [Defect].}
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import
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options, math, tables,
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./datatypes, ./digest, ./helpers
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const
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SEED_SIZE = sizeof(Eth2Digest)
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ROUND_SIZE = 1
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POSITION_WINDOW_SIZE = 4
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PIVOT_VIEW_SIZE = SEED_SIZE + ROUND_SIZE
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TOTAL_SIZE = PIVOT_VIEW_SIZE + POSITION_WINDOW_SIZE
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#compute_shuffled_index
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#compute_committee
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# Port of https://github.com/protolambda/zrnt/blob/master/eth2/beacon/shuffle.go
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# Shuffles or unshuffles, depending on the `dir` (true for shuffling, false for unshuffling
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func shuffle_list*(input: var seq[ValidatorIndex], seed: Eth2Digest) =
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let list_size = input.lenu64
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if list_size <= 1: return
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var buf {.noinit.}: array[TOTAL_SIZE, byte]
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# Seed is always the first 32 bytes of the hash input, we never have to change
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# this part of the buffer.
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buf[0..<32] = seed.data
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# The original code includes a direction flag, but only the reverse direction
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# is used in eth2, so we simplify it here
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for r in 0'u8..<SHUFFLE_ROUND_COUNT.uint8:
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# spec: pivot = bytes_to_int(hash(seed + int_to_bytes1(round))[0:8]) % list_size
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# This is the "int_to_bytes1(round)", appended to the seed.
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buf[SEED_SIZE] = (SHUFFLE_ROUND_COUNT.uint8 - r - 1)
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# Seed is already in place, now just hash the correct part of the buffer,
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# and take a uint64 from it, and modulo it to get a pivot within range.
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let
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pivotDigest = eth2digest(buf.toOpenArray(0, PIVOT_VIEW_SIZE - 1))
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pivot = bytes_to_uint64(pivotDigest.data.toOpenArray(0, 7)) mod listSize
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# Split up the for-loop in two:
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# 1. Handle the part from 0 (incl) to pivot (incl). This is mirrored around
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# (pivot / 2)
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# 2. Handle the part from pivot (excl) to N (excl). This is mirrored around
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# ((pivot / 2) + (size/2))
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# The pivot defines a split in the array, with each of the splits mirroring
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# their data within the split.
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# Print out some example even/odd sized index lists, with some even/odd pivots,
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# and you can deduce how the mirroring works exactly.
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# Note that the mirror is strict enough to not consider swapping the index
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# @mirror with itself.
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# Since we are iterating through the "positions" in order, we can just
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# repeat the hash every 256th position.
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# No need to pre-compute every possible hash for efficiency like in the
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# example code.
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# We only need it consecutively (we are going through each in reverse order
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# however, but same thing)
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# spec: source = hash(seed + int_to_bytes1(round) + int_to_bytes4(position // 256))
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# - seed is still in 0:32 (excl., 32 bytes)
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# - round number is still in 32
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# - mix in the position for randomness, except the last byte of it,
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# which will be used later to select a bit from the resulting hash.
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# We start from the pivot position, and work back to the mirror position
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# (of the part left to the pivot).
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# This makes us process each pear exactly once (instead of unnecessarily
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# twice, like in the spec)
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buf[33..<37] = uint_to_bytes4(pivot shr 8)
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var
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mirror = (pivot + 1) shr 1
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source = eth2digest(buf)
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byteV = source.data[(pivot and 0xff) shr 3]
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i = 0'u64
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j = pivot
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template shuffle =
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while i < mirror:
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# The pair is i,j. With j being the bigger of the two, hence the "position" identifier of the pair.
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# Every 256th bit (aligned to j).
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if (j and 0xff) == 0xff:
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# just overwrite the last part of the buffer, reuse the start (seed, round)
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buf[33..<37] = uint_to_bytes4(j shr 8)
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source = eth2digest(buf)
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# Same trick with byte retrieval. Only every 8th.
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if (j and 0x07) == 0x7:
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byteV = source.data[(j and 0xff'u64) shr 3]
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let
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bitV = (byteV shr (j and 0x7)) and 0x1
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if bitV == 1:
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swap(input[i], input[j])
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i.inc
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j.dec
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shuffle
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# Now repeat, but for the part after the pivot.
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mirror = (pivot + list_size + 1) shr 1
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let lend = list_size - 1
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# Again, seed and round input is in place, just update the position.
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# We start at the end, and work back to the mirror point.
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# This makes us process each pear exactly once (instead of unnecessarily twice, like in the spec)
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buf[33..<37] = uint_to_bytes4(lend shr 8)
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source = eth2digest(buf)
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byteV = source.data[(lend and 0xff) shr 3]
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i = pivot + 1'u64
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j = lend
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shuffle
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func get_shuffled_active_validator_indices*(state: BeaconState, epoch: Epoch):
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seq[ValidatorIndex] =
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# Non-spec function, to cache a data structure from which one can cheaply
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# compute both get_active_validator_indexes() and get_beacon_committee().
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var active_validator_indices = get_active_validator_indices(state, epoch)
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shuffle_list(
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active_validator_indices, get_seed(state, epoch, DOMAIN_BEACON_ATTESTER))
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active_validator_indices
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func get_shuffled_active_validator_indices*(
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cache: var StateCache, state: BeaconState, epoch: Epoch):
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var seq[ValidatorIndex] =
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# `cache` comes first because of nim's borrowing rules for the `var` return -
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# the `var` returns avoids copying the validator set.
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cache.shuffled_active_validator_indices.withValue(epoch, validator_indices) do:
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return validator_indices[]
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do:
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let indices = get_shuffled_active_validator_indices(state, epoch)
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return cache.shuffled_active_validator_indices.mgetOrPut(epoch, indices)
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#get_active_validator_indices
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func count_active_validators*(state: BeaconState,
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epoch: Epoch,
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cache: var StateCache): uint64 =
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cache.get_shuffled_active_validator_indices(state, epoch).lenu64
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#get_committee_count_per_slot
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func get_committee_count_per_slot*(num_active_validators: uint64): uint64 =
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clamp(
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num_active_validators div SLOTS_PER_EPOCH div TARGET_COMMITTEE_SIZE,
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1'u64, MAX_COMMITTEES_PER_SLOT)
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func get_committee_count_per_slot*(state: BeaconState,
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epoch: Epoch,
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cache: var StateCache): uint64 =
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# Return the number of committees at ``slot``.
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# TODO this is mostly used in for loops which have indexes which then need to
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# be converted to CommitteeIndex types for get_beacon_committee(...); replace
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# with better and more type-safe use pattern, probably beginning with using a
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# CommitteeIndex return type here.
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let
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active_validator_count = count_active_validators(state, epoch, cache)
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result = get_committee_count_per_slot(active_validator_count)
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# Otherwise, get_beacon_committee(...) cannot access some committees.
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doAssert (SLOTS_PER_EPOCH * MAX_COMMITTEES_PER_SLOT) >= uint64(result)
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func get_committee_count_per_slot*(state: BeaconState,
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slot: Slot,
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cache: var StateCache): uint64 =
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get_committee_count_per_slot(state, slot.compute_epoch_at_slot, cache)
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#get_previous_epoch
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func get_previous_epoch*(current_epoch: Epoch): Epoch =
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# Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
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if current_epoch == GENESIS_EPOCH:
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current_epoch
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else:
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current_epoch - 1
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func get_previous_epoch*(state: BeaconState): Epoch =
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# Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
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get_previous_epoch(get_current_epoch(state))
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#compute_committee
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func compute_committee_slice*(
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active_validators, index, count: uint64): Slice[int] =
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doAssert active_validators <= ValidatorIndex.high.uint64
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let
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start = (active_validators * index) div count
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endIdx = (active_validators * (index + 1)) div count
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start.int..(endIdx.int - 1)
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func compute_committee*(shuffled_indices: seq[ValidatorIndex],
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index: uint64, count: uint64): seq[ValidatorIndex] =
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## Return the committee corresponding to ``indices``, ``seed``, ``index``,
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## and committee ``count``.
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## In this version, we pass in the shuffled indices meaning we no longer need
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## the seed.
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let
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slice = compute_committee_slice(shuffled_indices.lenu64, index, count)
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# In spec, this calls get_shuffled_index() every time, but that's wasteful
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# Here, get_beacon_committee() gets the shuffled version.
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shuffled_indices[slice]
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func compute_committee_len*(
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active_validators, index, count: uint64): uint64 =
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## Return the committee corresponding to ``indices``, ``seed``, ``index``,
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## and committee ``count``.
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let
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slice = compute_committee_slice(active_validators, index, count)
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(slice.b - slice.a + 1).uint64
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#get_beacon_committee
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func get_beacon_committee*(
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state: BeaconState, slot: Slot, index: CommitteeIndex,
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cache: var StateCache): seq[ValidatorIndex] =
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# Return the beacon committee at ``slot`` for ``index``.
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let
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epoch = compute_epoch_at_slot(slot)
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committees_per_slot = get_committee_count_per_slot(state, epoch, cache)
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compute_committee(
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cache.get_shuffled_active_validator_indices(state, epoch),
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(slot mod SLOTS_PER_EPOCH) * committees_per_slot +
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index.uint64,
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committees_per_slot * SLOTS_PER_EPOCH
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)
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#get_beacon_committee
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func get_beacon_committee_len*(
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state: BeaconState, slot: Slot, index: CommitteeIndex,
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cache: var StateCache): uint64 =
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# Return the number of members in the beacon committee at ``slot`` for ``index``.
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let
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epoch = compute_epoch_at_slot(slot)
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committees_per_slot = get_committee_count_per_slot(state, epoch, cache)
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compute_committee_len(
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count_active_validators(state, epoch, cache),
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(slot mod SLOTS_PER_EPOCH) * committees_per_slot +
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index.uint64,
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committees_per_slot * SLOTS_PER_EPOCH
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)
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#compute_shuffled_index
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func compute_shuffled_index(
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index: uint64, index_count: uint64, seed: Eth2Digest): uint64 =
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# Return the shuffled index corresponding to ``seed`` (and ``index_count``).
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doAssert index < index_count
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var
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pivot_buffer: array[(32+1), byte]
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source_buffer: array[(32+1+4), byte]
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cur_idx_permuted = index
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pivot_buffer[0..31] = seed.data
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source_buffer[0..31] = seed.data
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# Swap or not (https://link.springer.com/content/pdf/10.1007%2F978-3-642-32009-5_1.pdf)
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# See the 'generalized domain' algorithm on page 3
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for current_round in 0'u8 ..< SHUFFLE_ROUND_COUNT.uint8:
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pivot_buffer[32] = current_round
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source_buffer[32] = current_round
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let
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# If using multiple indices, can amortize this
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pivot =
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bytes_to_uint64(eth2digest(pivot_buffer).data.toOpenArray(0, 7)) mod
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index_count
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flip = ((index_count + pivot) - cur_idx_permuted) mod index_count
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position = max(cur_idx_permuted.int, flip.int)
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source_buffer[33..36] = uint_to_bytes4((position div 256).uint64)
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let
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source = eth2digest(source_buffer).data
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byte_value = source[(position mod 256) div 8]
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bit = (byte_value shr (position mod 8)) mod 2
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cur_idx_permuted = if bit != 0: flip else: cur_idx_permuted
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cur_idx_permuted
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#compute_proposer_index
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func compute_proposer_index(state: BeaconState, indices: seq[ValidatorIndex],
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seed: Eth2Digest): Option[ValidatorIndex] =
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# Return from ``indices`` a random index sampled by effective balance.
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const MAX_RANDOM_BYTE = 255
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if len(indices) == 0:
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return none(ValidatorIndex)
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let seq_len = indices.lenu64
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var
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i = 0'u64
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buffer: array[32+8, byte]
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buffer[0..31] = seed.data
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while true:
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buffer[32..39] = uint_to_bytes8(i div 32)
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let
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candidate_index =
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indices[compute_shuffled_index(i mod seq_len, seq_len, seed)]
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random_byte = (eth2digest(buffer).data)[i mod 32]
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effective_balance = state.validators[candidate_index].effective_balance
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if effective_balance * MAX_RANDOM_BYTE >=
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MAX_EFFECTIVE_BALANCE * random_byte:
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return some(candidate_index)
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i += 1
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#get_beacon_proposer_index
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func get_beacon_proposer_index*(state: BeaconState, cache: var StateCache, slot: Slot):
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Option[ValidatorIndex] =
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cache.beacon_proposer_indices.withValue(slot, proposer) do:
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return proposer[]
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do:
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# Return the beacon proposer index at the current slot.
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let epoch = get_current_epoch(state)
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var buffer: array[32 + 8, byte]
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buffer[0..31] = get_seed(state, epoch, DOMAIN_BEACON_PROPOSER).data
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# There's exactly one beacon proposer per slot.
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let
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# active validator indices are kept in cache but sorting them takes
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# quite a while
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indices = get_active_validator_indices(state, epoch)
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start = epoch.compute_start_slot_at_epoch()
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var res: Option[ValidatorIndex]
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for i in 0..<SLOTS_PER_EPOCH:
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buffer[32..39] = uint_to_bytes8((start + i).uint64)
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let seed = eth2digest(buffer)
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let pi = compute_proposer_index(state, indices, seed)
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if start + i == slot:
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res = pi
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cache.beacon_proposer_indices[start + i] = pi
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return res
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#get_beacon_proposer_index
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func get_beacon_proposer_index*(state: BeaconState, cache: var StateCache):
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Option[ValidatorIndex] =
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get_beacon_proposer_index(state, cache, state.slot)
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# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/validator.md#validator-assignments
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func get_committee_assignment*(
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state: BeaconState, epoch: Epoch,
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validator_index: ValidatorIndex):
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Option[tuple[a: seq[ValidatorIndex], b: CommitteeIndex, c: Slot]] =
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# Return the committee assignment in the ``epoch`` for ``validator_index``.
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# ``assignment`` returned is a tuple of the following form:
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# * ``assignment[0]`` is the list of validators in the committee
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# * ``assignment[1]`` is the index to which the committee is assigned
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# * ``assignment[2]`` is the slot at which the committee is assigned
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# Return None if no assignment.
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let next_epoch = get_current_epoch(state) + 1
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doAssert epoch <= next_epoch
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var cache = StateCache()
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let
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start_slot = compute_start_slot_at_epoch(epoch)
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committee_count_per_slot =
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get_committee_count_per_slot(state, epoch, cache)
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for slot in start_slot ..< start_slot + SLOTS_PER_EPOCH:
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for index in 0'u64 ..< committee_count_per_slot:
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let idx = index.CommitteeIndex
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let committee = get_beacon_committee(state, slot, idx, cache)
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if validator_index in committee:
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return some((committee, idx, slot))
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none(tuple[a: seq[ValidatorIndex], b: CommitteeIndex, c: Slot])
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