80 lines
3.0 KiB
Nim
80 lines
3.0 KiB
Nim
# beacon_chain
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# Copyright (c) 2018 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 http://opensource.org/licenses/MIT).
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# * Apache v2 license (license terms in the root directory or at http://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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# Helper functions
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import ../datatypes, sequtils, nimcrypto
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func getShuffling*(seed: Blake2_256_Digest, validatorCount: int): seq[int] {.noInit.}=
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## Pseudorandomly shuffles the validator set based on some seed
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assert validatorCount <= MaxValidatorCount
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let randMax = MaxValidatorCount - MaxValidatorCount mod validatorCount
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result = toSeq(0 ..< validatorCount)
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var source = seed
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var i = 0
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while i < validatorCount:
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source = blake2_256.digest source.data
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for pos in countup(0, 29, 3):
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let remaining = validatorCount - i
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if remaining == 0:
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break
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let m = source.data[pos].int shl 16 or source.data[pos+1].int shl 8 or source.data[pos+2].int
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if validatorCount < randMax:
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let replacementPos = m mod remaining + i
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swap result[i], result[replacementPos]
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inc i
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func getCutoffs*(validatorCount: int): tuple[height, shard: seq[int]] {.noInit.} =
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## Split up validators into groups at the start of every epoch,
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## determining at what height they can make attestations and what shard they are making crosslinks for
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## Implementation should do the following: http://vitalik.ca/files/ShuffleAndAssign.png
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## TODO: It doens't work.
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result.height = @[0]
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let cofactor = 39 # EpochLength / phi
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const StandardCommitteeSize = MaxValidatorCount div ShardCount
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var heightCount: int
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var heights: seq[int]
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if validatorCount < EpochLength * MinCommitteeSize:
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# If there are not enough validators to fill a minimally
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# sized committee at every height, skip some heights
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heightCount = validatorCount div MinCommitteeSize or 1 # TODO div/or precedence ?
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for i in 0 ..< heightCount:
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heights.add (i * cofactor) mod EpochLength
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else:
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# If there are enough validators, fill all the heights
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heightCount = EpochLength
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heights = toSeq(0 ..< EpochLength)
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var filled = 0
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for i in 0 ..< EpochLength - 1:
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if i notin heights: # TODO, this will be slow for seq, use intsets instead?
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result.height.add result.height[^1]
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else:
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inc filled
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result.height.add filled * validatorCount div heightCount
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result.height.add validatorCount
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# For the validators assigned to each height, split them up
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# into committees for different shards. Do not assign the
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# last END_EPOCH_GRACE_PERIOD heights in an epoch to any shards.
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result.shard = @[0]
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for i in 0 ..< EpochLength - EndEpochGracePeriod:
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let
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size = result.height[i+1] - result.height[i]
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shards = (size + StandardCommitteeSize - 1) div StandardCommitteeSize
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pre = result.shard[^1]
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for j in 1 .. shards:
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result.shard.add pre + size * j div shards
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