outline general flow of naive attestation aggregation

beacon_chain/naive_attestation_aggregation.nim

@@ -1,8 +1,21 @@
+# beacon_chain
+# Copyright (c) 2018-2019 Status Research & Development GmbH
+# Licensed and distributed under either of
+#   * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
+#   * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
 import
-  sequtils,
+  options, sequtils,
   ./spec/[datatypes, crypto, digest, helpers, validator],
   ./ssz
 
+# TODO add tests
+# TODO gossipsub validation lives somewhere, maybe here
+# https://github.com/status-im/nim-beacon-chain/issues/122#issuecomment-562479965
+# it's conceptually separate, sort of, but depends on beaconstate, so isn't a
+# pure libp2p thing.
+
 # https://github.com/ethereum/eth2.0-specs/blob/v0.9.2/specs/validator/0_beacon-chain-validator.md#aggregation-selection
 func get_slot_signature(state: BeaconState, slot: Slot, privkey: ValidatorPrivKey):
     ValidatorSig =
@@ -23,6 +36,45 @@ func is_aggregator(state: BeaconState, slot: Slot, index: uint64,
   bytes_to_int(eth2hash(slot_signature.getBytes).data[0..7]) mod modulo == 0
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.9.2/specs/validator/0_beacon-chain-validator.md#aggregate-signature-1
-func get_aggregate_signature(attestations: seq[Attestation]): ValidatorSig =
+func get_aggregate_signature(attestations: openarray[Attestation]): ValidatorSig =
   let signatures = mapIt(attestations, it.signature)
   bls_aggregate_signatures(signatures)
+
+func should_aggregate(
+    state: BeaconState, index: uint64, privkey: ValidatorPrivKey): bool =
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.2/specs/validator/0_beacon-chain-validator.md#aggregation-selection
+  # A validator is selected to aggregate based upon the return value of
+  # is_aggregator().
+  let slot = state.slot - 1 # TODO this isn't correct
+
+  # the "Construct aggregate" section seems to suggest it's from a previous
+  # slot.
+  doAssert slot < state.slot
+  is_aggregator(state, slot, index, get_slot_signature(state, slot, privkey))
+
+func aggregate_attestations_naively*(
+    state: BeaconState, index: uint64, privkey: ValidatorPrivKey):
+    Option[Attestation] =
+  # Keep this code opt-in with clean entry point; if there's a part that
+  # mutates state, split that off and keep this separate.
+  # TODO alias CommitteeIndex to actual type then convert various uint64's here
+  # to it
+
+  # If the validator is selected to aggregate (`is_aggregator()`), they
+  # construct an aggregate attestation
+  if not should_aggregate(state, index, privkey):
+    return none(Attestation)
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.2/specs/validator/0_beacon-chain-validator.md#construct-aggregate
+  # Collect attestations seen via gossip during the slot that have an
+  # equivalent attestation_data to that constructed by the validator,
+  # and create an aggregate_attestation: Attestation with the following fields.
+  var aggregate_attestation = Attestation(
+  )
+
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.9.2/specs/validator/0_beacon-chain-validator.md#aggregate-signature-1
+  # Set aggregate_attestation.signature = aggregate_signature where
+  # aggregate_signature is obtained from get_aggregate_signature(...).
+  aggregate_attestation.signature = get_aggregate_signature([])
+
+  some(aggregate_attestation)