use epochref for get_committee_assignments() to avoid repeated shuffling (#2463)

* use epochref for get_committee_assignments() to avoid repeated shuffling

* remove unnecessary imports and StateCache() construction

beacon_chain/consensus_object_pools/spec_cache.nim

@@ -8,10 +8,10 @@
 {.push raises: [Defect].}
 
 import
-  std/[algorithm, sequtils],
+  std/[algorithm, intsets, sequtils],
   chronicles,
   ../spec/[
-    crypto, datatypes, digest, helpers, presets, signatures,
+    crypto, datatypes, digest, helpers, network, presets, signatures,
     validator],
   ../extras,
   ./block_pools_types, ./blockchain_dag
@@ -193,3 +193,26 @@ func makeAttestationData*(
       root: epoch_boundary_block.blck.root
     )
   )
+
+# https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#validator-assignments
+iterator get_committee_assignments*(
+    epochRef: EpochRef, epoch: Epoch, validator_indices: IntSet):
+    tuple[validatorIndices: IntSet,
+      committeeIndex: CommitteeIndex,
+      subnetIndex: uint8, slot: Slot] =
+  let
+    committees_per_slot = get_committee_count_per_slot(epochRef)
+    start_slot = compute_start_slot_at_epoch(epoch)
+
+  for slot in start_slot ..< start_slot + SLOTS_PER_EPOCH:
+    for index in 0'u64 ..< committees_per_slot:
+      let
+        idx = index.CommitteeIndex
+        includedIndices =
+          toIntSet(get_beacon_committee(epochRef, slot, idx)) *
+            validator_indices
+      if includedIndices.len > 0:
+        yield (
+          includedIndices, idx,
+          compute_subnet_for_attestation(committees_per_slot, slot, idx).uint8,
+          slot)

beacon_chain/nimbus_beacon_node.nim

@@ -463,8 +463,6 @@ proc updateSubscriptionSchedule(node: BeaconNode, epoch: Epoch) {.async.} =
     attachedValidators = node.getAttachedValidators()
     validatorIndices = toIntSet(toSeq(attachedValidators.keys()))
 
-  var cache = StateCache()
-
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#lookahead
   # Only subscribe when this node should aggregate; libp2p broadcasting works
   # on subnet topics regardless.
@@ -496,8 +494,7 @@ proc updateSubscriptionSchedule(node: BeaconNode, epoch: Epoch) {.async.} =
   static: doAssert SLOTS_PER_EPOCH <= 32
 
   for (validatorIndices, committeeIndex, subnetIndex, slot) in
-      get_committee_assignments(
-        node.chainDag.headState.data.data, epoch, validatorIndices, cache):
+      get_committee_assignments(epochRef, epoch, validatorIndices):
 
     doAssert compute_epoch_at_slot(slot) == epoch
 
@@ -653,14 +650,13 @@ proc getInitialAttestationSubnets(node: BeaconNode): Table[uint8, Slot] =
     wallEpoch = node.beaconClock.now().slotOrZero().epoch
     validatorIndices = toIntSet(toSeq(node.getAttachedValidators().keys()))
 
-  var cache = StateCache()
-
   template mergeAttestationSubnets(epoch: Epoch) =
     # TODO when https://github.com/nim-lang/Nim/issues/15972 and
     # https://github.com/nim-lang/Nim/issues/16217 are fixed, in
     # Nimbus's Nim, use (_, _, subnetIndex, slot).
+    let epochRef = node.chainDag.getEpochRef(node.chainDag.head, epoch)
     for (_, ci, subnetIndex, slot) in get_committee_assignments(
-        node.chainDag.headState.data.data, epoch, validatorIndices, cache):
+        epochRef, epoch, validatorIndices):
       result.withValue(subnetIndex, v) do:
         v[] = max(v[], slot + 1)
       do:

beacon_chain/spec/network.nim

@@ -8,8 +8,8 @@
 {.push raises: [Defect].}
 
 import
-  std/[intsets, strformat],
-  ./datatypes, ./helpers, ./validator
+  std/strformat,
+  ./datatypes
 
 const
   # https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/p2p-interface.md#topics-and-messages
@@ -85,28 +85,3 @@ func getAttestationTopic*(forkDigest: ForkDigest, subnetIndex: uint64):
     &"/eth2/{$forkDigest}/beacon_attestation_{subnetIndex}/ssz"
   except ValueError as e:
     raiseAssert e.msg
-
-# https://github.com/ethereum/eth2.0-specs/blob/v1.0.1/specs/phase0/validator.md#validator-assignments
-iterator get_committee_assignments*(
-    state: BeaconState, epoch: Epoch,
-    validator_indices: IntSet,
-    cache: var StateCache):
-    tuple[validatorIndices: IntSet,
-      committeeIndex: CommitteeIndex,
-      subnetIndex: uint8, slot: Slot] =
-  let
-    committees_per_slot = get_committee_count_per_slot(state, epoch, cache)
-    start_slot = compute_start_slot_at_epoch(epoch)
-
-  for slot in start_slot ..< start_slot + SLOTS_PER_EPOCH:
-    for index in 0'u64 ..< committees_per_slot:
-      let
-        idx = index.CommitteeIndex
-        includedIndices =
-          toIntSet(get_beacon_committee(state, slot, idx, cache)) *
-            validator_indices
-      if includedIndices.len > 0:
-        yield (
-          includedIndices, idx,
-          compute_subnet_for_attestation(committees_per_slot, slot, idx).uint8,
-          slot)