86 lines
3.9 KiB
Nim
86 lines
3.9 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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# This file contains the implementation of the beacon chain fork choice rule.
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# The chosen rule is a hybrid that combines justification and finality
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# with Latest Message Driven (LMD) Greediest Heaviest Observed SubTree (GHOST)
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#
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# The latest version can be seen here:
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# https://github.com/ethereum/eth2.0-specs/blob/master/specs/beacon-chain.md
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#
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# How wrong the code is:
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# https://github.com/ethereum/eth2.0-specs/compare/126a7abfa86448091a0e037f52966b6a9531a857...master
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#
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# A standalone research implementation can be found here:
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# - https://github.com/ethereum/research/blob/94ac4e2100a808a7097715003d8ad1964df4dbd9/clock_disparity/lmd_node.py
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# A minispec in mathematical notation including proofs can be found here:
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# - https://ethresear.ch/t/beacon-chain-casper-ffg-rpj-mini-spec/2760
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# Note that it might be out-of-sync with the official spec.
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import
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tables, hashes,
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../../beacon_chain/spec/[datatypes, helpers, digest, crypto],
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../../beacon_chain/ssz
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type
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AttesterIdx = int
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BlockHash = Eth2Digest
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Store = object
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# This is private to each validator.
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# It holds the set of attestations and blocks that a validator
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# has observed and verified.
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#
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# We uniquely identify each block via it's block hash
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# and each attester via it's attester index (from AttestationRecord object)
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# TODO/Question: Should we use the public key? That would defeat the pubkey aggregation purpose
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verified_attestations: Table[AttesterIdx, ref seq[AttestationData]]
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# TODO: We assume that ref seq[AttestationData] is queue, ordered by
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# a pair (slot, observation time).
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verified_blocks: Table[BlockHash, BeaconBlock]
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finalized_head: BeaconBlock
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justified_head: BeaconBlock
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func hash(x: BlockHash): Hash =
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## Hash for Keccak digests for Nim hash tables
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# We just slice the first 4 or 8 bytes of the block hash
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# depending of if we are on a 32 or 64-bit platform
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const size = x.sizeof
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const num_hashes = size div sizeof(int)
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result = cast[array[num_hashes, Hash]](x)[0]
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func get_parent(store: Store, blck: BeaconBlock): BeaconBlock =
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store.verified_blocks[blck.parent_root]
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func get_ancestor(store: Store, blck: BeaconBlock, slot: uint64): BeaconBlock =
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## Find the ancestor with a specific slot number
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if blck.slot == slot:
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return blck
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else:
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return store.get_ancestor(store.get_parent(blck), slot)
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# TODO: what if the slot was never observed/verified?
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func get_latest_attestation(store: Store, validatorIdx: AttesterIdx): AttestationData =
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## Search for the attestation with the highest slot number
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## If multiple attestation have the same slot number, keep the first one.
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# We assume that in `verified_attestations: Table[AttesterIdx, seq[AttestationData]]`
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# `seq[AttestationSignedData]` is a queue ordered by (slot, observation time)
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let attestations = store.verified_attestations[validatorIdx]
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result = attestations[^1] # Pick the last attestation
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for idx in countdown(attestations[].len - 2, 0): # From the second to last attestation to 0, check if they have the same slot.
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if attestations[idx].slot == result.slot: # if yes it was observed earlier
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result = attestations[idx]
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else: # otherwise we are at the first observed attestation with the highest slot
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return
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func get_latest_attestation_target(store: Store, validatorIdx: AttesterIdx): BlockHash =
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store.get_latest_attestation(validatorIdx).beacon_block_root
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