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Fix aggregation bits example in Altair validator guide 

The sub-index calculation was incorrect when demonstrating how to map a validator in the sync committee to a particular bit in the `aggregation_bits` in the sync committee aggregation scheme. The correct index is given in this commit.
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Alex Stokes 2021-03-26 11:50:48 -07:00 committed by GitHub
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specs/altair/validator.md
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@ -354,7 +354,7 @@ Set `contribution.subcommittee_index` to the index for the subcommittee index co
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 needs to find 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 map the sync committee index to an index in the subcommittee (along with the prior `subcommittee_index`). This index within the subcommittee is the one set in the `Bitvector`. An aggregator needs to find 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 map the sync committee index to an index in the subcommittee (along with the prior `subcommittee_index`). This index within the subcommittee is the one set in the `Bitvector`.
For example, a validator with index `2044` could be at index `15` in the current sync committee. This sync committee index maps to `subcommittee_index` `1` with position `7` in the `Bitvector` for the contribution. For example, a validator with index `2044` could be at index `135` in the current sync committee. This sync committee index maps to `subcommittee_index` `1` with position `7` in the `Bitvector` for the contribution.
Also note that a validator **could be included multiple times** in a given subcommittee such that multiple bits are set for a single `SyncCommitteeSignature`. Also note that a validator **could be included multiple times** in a given subcommittee such that multiple bits are set for a single `SyncCommitteeSignature`.
###### Signature ###### Signature
@ -419,4 +419,4 @@ Some early sync committee rewards may be missed while the initial subnets form.
* To join a sync committee subnet, select a random number of epochs before the end of the current sync committee period between 1 and `SYNC_COMMITTEE_SUBNET_COUNT`, inclusive. * To join a sync committee subnet, select a random number of epochs before the end of the current sync committee period between 1 and `SYNC_COMMITTEE_SUBNET_COUNT`, inclusive.
Validators should join their member subnet at the beginning of the epoch they have randomly selected. Validators should join their member subnet at the beginning of the epoch they have randomly selected.
For example, if the next sync committee period starts at epoch `853,248` and the validator randomly selects an offset of `3`, they should join the subnet at the beginning of epoch `853,245`. For example, if the next sync committee period starts at epoch `853,248` and the validator randomly selects an offset of `3`, they should join the subnet at the beginning of epoch `853,245`.
Validators should leverage the lookahead period on sync committee assignments so that they can join the appropriate subnets ahead of their assigned sync committee period. Validators should leverage the lookahead period on sync committee assignments so that they can join the appropriate subnets ahead of their assigned sync committee period.