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Update validator guide with restricted sync committee computation

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Alex Stokes 2021-05-12 11:05:41 -07:00 committed by GitHub
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specs/altair/validator.md
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@ -177,7 +177,6 @@ At any given `epoch`, the `BeaconState` contains the current `SyncCommittee` and
Once every `EPOCHS_PER_SYNC_COMMITTEE_PERIOD` epochs, the next `SyncCommittee` becomes the current `SyncCommittee` and the next committee is computed and stored. Once every `EPOCHS_PER_SYNC_COMMITTEE_PERIOD` epochs, the next `SyncCommittee` becomes the current `SyncCommittee` and the next committee is computed and stored.
*Note*: The data required to compute a given committee is not cached in the `BeaconState` after committees are calculated at the period boundaries. *Note*: The data required to compute a given committee is not cached in the `BeaconState` after committees are calculated at the period boundaries.
This means that calling `get_sync_commitee()` in a given `epoch` can return a different result than what was computed during the relevant epoch transition.
For this reason, *always* get committee assignments via the fields of the `BeaconState` (`current_sync_committee` and `next_sync_committee`) or use the above reference code. For this reason, *always* get committee assignments via the fields of the `BeaconState` (`current_sync_committee` and `next_sync_committee`) or use the above reference code.
A validator should plan for future sync committee assignments by noting which sync committee periods they are selected for participation. A validator should plan for future sync committee assignments by noting which sync committee periods they are selected for participation.
@ -229,12 +228,12 @@ def process_sync_committee_contributions(block: BeaconBlock,
contributions: Set[SyncCommitteeContribution]) -> None: contributions: Set[SyncCommitteeContribution]) -> None:
sync_aggregate = SyncAggregate() sync_aggregate = SyncAggregate()
signatures = [] signatures = []
sync_subcommittee_size = SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT
for contribution in contributions: for contribution in contributions:
subcommittee_index = contribution.subcommittee_index subcommittee_index = contribution.subcommittee_index
for index, participated in enumerate(contribution.aggregation_bits): for index, participated in enumerate(contribution.aggregation_bits):
if participated: if participated:
sync_subcommittee_size = SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT
participant_index = sync_subcommittee_size * subcommittee_index + index participant_index = sync_subcommittee_size * subcommittee_index + index
sync_aggregate.sync_committee_bits[participant_index] = True sync_aggregate.sync_committee_bits[participant_index] = True
signatures.append(contribution.signature) signatures.append(contribution.signature)
@ -367,7 +366,7 @@ Set `contribution.subcommittee_index` to the index for the subcommittee index co
###### Aggregation bits ###### Aggregation bits
Let `contribution.aggregation_bits` be a `Bitvector[SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT]`, where the `index`th bit is set in the `Bitvector` for each corresponding validator included in this aggregate from the corresponding subcommittee. Let `contribution.aggregation_bits` be a `Bitvector[SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT]`, where the `index`th bit is set in the `Bitvector` for each corresponding validator included in this aggregate from the corresponding subcommittee.
An aggregator finds the index in the sync committee (as returned by `get_sync_committee_indices()`) for a given validator referenced by `sync_committee_signature.validator_index` and maps the sync committee index to an index in the subcommittee (along with the prior `subcommittee_index`). This index within the subcommittee is set in `contribution.aggegration_bits`. An aggregator finds the index in the sync committee (as determined by a reverse pubkey lookup on `state.current_sync_committee.pubkeys`) for a given validator referenced by `sync_committee_signature.validator_index` and maps the sync committee index to an index in the subcommittee (along with the prior `subcommittee_index`). This index within the subcommittee is set in `contribution.aggegration_bits`.
For example, if a validator with index `2044` is pseudo-randomly sampled to sync committee index `135`. This sync committee index maps to `subcommittee_index` `1` with position `7` in the `Bitvector` for the contribution. For example, if a validator with index `2044` is pseudo-randomly sampled to sync committee index `135`. This sync committee index maps to `subcommittee_index` `1` with position `7` in the `Bitvector` for the contribution.