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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