ShufflingRef approach to next-epoch validator duty calculation/prediction (#5414)

* ShufflingRef approach to next-epoch validator duty calculation/prediction

* refactor action_tracker.updateActions to take ShufflingRef + beacon_proposers; refactor maybeUpdateActionTrackerNextEpoch to be separate and reused function; add actual fallback logic

* document one possible set of conditions

* check epoch participation flags and inactivity scores to ensure no penalties and MAX_EFFECTIVE_BALANCE to ensure rewards don't matter

* correctly (un)shuffle each proposer index

* remove debugging assertion

AllTests-mainnet.md

@@ -261,13 +261,14 @@ OK: 2/2 Fail: 0/2 Skip: 0/2
 ## Honest validator
 ```diff
 + General pubsub topics                                                                      OK
++ Index shuffling and unshuffling invert                                                     OK
 + Liveness failsafe conditions                                                               OK
 + Mainnet attestation topics                                                                 OK
 + Stability subnets                                                                          OK
 + isNearSyncCommitteePeriod                                                                  OK
 + is_aggregator                                                                              OK
 ```
-OK: 6/6 Fail: 0/6 Skip: 0/6
+OK: 7/7 Fail: 0/7 Skip: 0/7
 ## ImportKeystores requests [Beacon Node] [Preset: mainnet]
 ```diff
 + ImportKeystores/ListKeystores/DeleteKeystores [Beacon Node] [Preset: mainnet]              OK
@@ -704,4 +705,4 @@ OK: 2/2 Fail: 0/2 Skip: 0/2
 OK: 9/9 Fail: 0/9 Skip: 0/9
 
 ---TOTAL---
-OK: 397/402 Fail: 0/402 Skip: 5/402
+OK: 398/403 Fail: 0/403 Skip: 5/403

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -544,11 +544,8 @@ func putEpochRef(dag: ChainDAGRef, epochRef: EpochRef) =
 func init*(
     T: type ShufflingRef, state: ForkedHashedBeaconState,
     cache: var StateCache, epoch: Epoch): T =
-  let
+  let attester_dependent_root =
-    dependent_epoch =
+    withState(state): forkyState.dependent_root(epoch.get_previous_epoch)
-      if epoch < 1: Epoch(0) else: epoch - 1
-    attester_dependent_root =
-      withState(state): forkyState.dependent_root(dependent_epoch)
 
   ShufflingRef(
     epoch: epoch,

beacon_chain/nimbus_beacon_node.nim

@@ -1061,6 +1061,83 @@ proc doppelgangerChecked(node: BeaconNode, epoch: Epoch) =
     for validator in node.attachedValidators[]:
       validator.doppelgangerChecked(epoch - 1)
 
+from ./spec/state_transition_epoch import effective_balance_might_update
+
+proc maybeUpdateActionTrackerNextEpoch(
+    node: BeaconNode, forkyState: ForkyHashedBeaconState, nextEpoch: Epoch) =
+  if node.consensusManager[].actionTracker.needsUpdate(
+      forkyState, nextEpoch):
+    template epochRefFallback() =
+      let epochRef =
+        node.dag.getEpochRef(node.dag.head, nextEpoch, false).expect(
+          "Getting head EpochRef should never fail")
+      node.consensusManager[].actionTracker.updateActions(
+        epochRef.shufflingRef, epochRef.beacon_proposers)
+
+    when forkyState is phase0.HashedBeaconState:
+      # The previous_epoch_participation-based logic requires Altair or newer
+      epochRefFallback()
+    else:
+      let
+        shufflingRef = node.dag.getShufflingRef(node.dag.head, nextEpoch, false).valueOr:
+          # epochRefFallback() won't work in this case either
+          return
+        nextEpochProposers = get_beacon_proposer_indices(
+          forkyState.data, shufflingRef.shuffled_active_validator_indices,
+          nextEpoch)
+        nextEpochFirstProposer = nextEpochProposers[0].valueOr:
+          # All proposers except the first can be more straightforwardly and
+          # efficiently (re)computed correctly once in that epoch.
+          epochRefFallback()
+          return
+
+      # Has to account for potential epoch transition TIMELY_SOURCE_FLAG_INDEX,
+      # TIMELY_TARGET_FLAG_INDEX, and inactivity penalties, resulting from spec
+      # functions get_flag_index_deltas() and get_inactivity_penalty_deltas().
+      #
+      # There are no penalties associated with TIMELY_HEAD_FLAG_INDEX, but a
+      # reward exists. effective_balance == MAX_EFFECTIVE_BALANCE ensures if
+      # even so, then the effective balanace cannot change as a result.
+      #
+      # It's not truly necessary to avoid all rewards and penalties, but only
+      # to bound them to ensure they won't unexpected alter effective balance
+      # during the upcoming epoch transition.
+      #
+      # During genesis epoch, the check for epoch participation is against current,
+      # not previous, epoch, and therefore there's a possibility of checking for if
+      # a validator has participated in an epoch before it will happen.
+      #
+      # Because process_rewards_and_penalties() in epoch processing happens
+      # before the current/previous participation swap, previous is correct
+      # even here, and consistent with what the epoch transition uses.
+      #
+      # Whilst slashing, proposal, and sync committee rewards and penalties do
+      # update the balances as they occur, they don't update effective_balance
+      # until the end of epoch, so detect via effective_balance_might_update.
+      #
+      # On EF mainnet epoch 233906, this matches 99.5% of active validators;
+      # with Holesky epoch 2041, 83% of active validators.
+      let
+        participation_flags =
+          forkyState.data.previous_epoch_participation.item(
+            nextEpochFirstProposer)
+        effective_balance = forkyState.data.validators.item(
+          nextEpochFirstProposer).effective_balance
+
+      if  participation_flags.has_flag(TIMELY_SOURCE_FLAG_INDEX) and
+          participation_flags.has_flag(TIMELY_TARGET_FLAG_INDEX) and
+          effective_balance == MAX_EFFECTIVE_BALANCE and
+          forkyState.data.slot.epoch != GENESIS_EPOCH and
+          forkyState.data.inactivity_scores.item(
+            nextEpochFirstProposer) == 0 and
+          not effective_balance_might_update(
+            forkyState.data.balances.item(nextEpochFirstProposer),
+            effective_balance):
+        node.consensusManager[].actionTracker.updateActions(
+          shufflingRef, nextEpochProposers)
+      else:
+        epochRefFallback()
+
 proc updateGossipStatus(node: BeaconNode, slot: Slot) {.async.} =
   ## Subscribe to subnets that we are providing stability for or aggregating
   ## and unsubscribe from the ones that are no longer relevant.
@@ -1133,13 +1210,10 @@ proc updateGossipStatus(node: BeaconNode, slot: Slot) {.async.} =
           forkyState, slot.epoch):
         let epochRef = node.dag.getEpochRef(head, slot.epoch, false).expect(
           "Getting head EpochRef should never fail")
-        node.consensusManager[].actionTracker.updateActions(epochRef)
+        node.consensusManager[].actionTracker.updateActions(
+          epochRef.shufflingRef, epochRef.beacon_proposers)
 
-      if node.consensusManager[].actionTracker.needsUpdate(
+      node.maybeUpdateActionTrackerNextEpoch(forkyState, slot.epoch + 1)
-          forkyState, slot.epoch + 1):
-        let epochRef = node.dag.getEpochRef(head, slot.epoch + 1, false).expect(
-          "Getting head EpochRef should never fail")
-        node.consensusManager[].actionTracker.updateActions(epochRef)
 
   if node.gossipState.card > 0 and targetGossipState.card == 0:
     debug "Disabling topic subscriptions",
@@ -1260,11 +1334,19 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
   let head = node.dag.head
   if node.isSynced(head) and head.executionValid:
     withState(node.dag.headState):
-      if node.consensusManager[].actionTracker.needsUpdate(
+      # maybeUpdateActionTrackerNextEpoch might not account for balance changes
-          forkyState, slot.epoch + 1):
+      # from the process_rewards_and_penalties() epoch transition but only from
-        let epochRef = node.dag.getEpochRef(head, slot.epoch + 1, false).expect(
+      # process_block() and other per-slot sources. This mainly matters insofar
-          "Getting head EpochRef should never fail")
+      # as it might trigger process_effective_balance_updates() changes in that
-        node.consensusManager[].actionTracker.updateActions(epochRef)
+      # same epoch transition, which function is therefore potentially blind to
+      # but which might then affect beacon proposers.
+      #
+      # Because this runs every slot, it can account naturally for slashings,
+      # which affect balances via slash_validator() when they happen, and any
+      # missed sync committee participation via process_sync_aggregate(), but
+      # attestation penalties for example, need, specific handling.
+      # checked by maybeUpdateActionTrackerNextEpoch.
+      node.maybeUpdateActionTrackerNextEpoch(forkyState, slot.epoch + 1)
 
   let
     nextAttestationSlot =

beacon_chain/spec/state_transition.nim

@@ -357,7 +357,7 @@ func partialBeaconBlock*(
     parent_root: state.latest_block_root,
     body: consensusFork.BeaconBlockBody(
       randao_reveal: randao_reveal,
-      eth1_data: eth1data,
+      eth1_data: eth1_data,
       graffiti: graffiti,
       proposer_slashings: validator_changes.proposer_slashings,
       attester_slashings: validator_changes.attester_slashings,

beacon_chain/spec/state_transition_epoch.nim

@@ -936,20 +936,24 @@ func process_eth1_data_reset*(state: var ForkyBeaconState) =
   if next_epoch mod EPOCHS_PER_ETH1_VOTING_PERIOD == 0:
     state.eth1_data_votes = default(type state.eth1_data_votes)
 
+# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#effective-balances-updates
+template effective_balance_might_update*(
+    balance: Gwei, effective_balance: Gwei): bool =
+  const
+    HYSTERESIS_INCREMENT = EFFECTIVE_BALANCE_INCREMENT div HYSTERESIS_QUOTIENT
+    DOWNWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_DOWNWARD_MULTIPLIER
+    UPWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_UPWARD_MULTIPLIER
+  balance + DOWNWARD_THRESHOLD < effective_balance or
+    effective_balance + UPWARD_THRESHOLD < balance
+
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#effective-balances-updates
 func process_effective_balance_updates*(state: var ForkyBeaconState) =
   # Update effective balances with hysteresis
   for vidx in state.validators.vindices:
-    let balance = state.balances.item(vidx)
+    let
-    const
+      balance = state.balances.item(vidx)
-      HYSTERESIS_INCREMENT =
+      effective_balance = state.validators.item(vidx).effective_balance
-        EFFECTIVE_BALANCE_INCREMENT div HYSTERESIS_QUOTIENT
+    if effective_balance_might_update(balance, effective_balance):
-      DOWNWARD_THRESHOLD =
-        HYSTERESIS_INCREMENT * HYSTERESIS_DOWNWARD_MULTIPLIER
-      UPWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_UPWARD_MULTIPLIER
-    let effective_balance = state.validators.item(vidx).effective_balance
-    if balance + DOWNWARD_THRESHOLD < effective_balance or
-        effective_balance + UPWARD_THRESHOLD < balance:
       let new_effective_balance =
         min(
           balance - balance mod EFFECTIVE_BALANCE_INCREMENT,

beacon_chain/spec/validator.nim

@@ -8,10 +8,7 @@
 
 {.push raises: [].}
 
-import
+import ./helpers
-  ./datatypes/[phase0, altair, bellatrix],
-  ./helpers
-
 export helpers
 
 const
@@ -23,8 +20,7 @@ const
 
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#compute_shuffled_index
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#compute_committee
-# Port of https://github.com/protolambda/zrnt/blob/master/eth2/beacon/shuffle.go
+# Port of https://github.com/protolambda/zrnt/blob/v0.14.0/eth2/beacon/shuffle.go
-# Shuffles or unshuffles, depending on the `dir` (true for shuffling, false for unshuffling
 func shuffle_list*(input: var seq[ValidatorIndex], seed: Eth2Digest) =
   let list_size = input.lenu64
 
@@ -302,8 +298,9 @@ func get_beacon_committee_len*(
     get_beacon_committee_len(forkyState.data, slot, index, cache)
 
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#compute_shuffled_index
-func compute_shuffled_index*(
+template compute_shuffled_index_aux(
-    index: uint64, index_count: uint64, seed: Eth2Digest): uint64 =
+    index: uint64, index_count: uint64, seed: Eth2Digest, iter: untyped):
+    uint64 =
   ## Return the shuffled index corresponding to ``seed`` (and ``index_count``).
   doAssert index < index_count
 
@@ -315,7 +312,7 @@ func compute_shuffled_index*(
 
   # Swap or not (https://link.springer.com/content/pdf/10.1007%2F978-3-642-32009-5_1.pdf)
   # See the 'generalized domain' algorithm on page 3
-  for current_round in 0'u8 ..< SHUFFLE_ROUND_COUNT.uint8:
+  for current_round in iter:
     source_buffer[32] = current_round
 
     let
@@ -336,33 +333,59 @@ func compute_shuffled_index*(
 
   cur_idx_permuted
 
+func compute_shuffled_index*(
+    index: uint64, index_count: uint64, seed: Eth2Digest): uint64 =
+  ## Return the shuffled index corresponding to ``seed`` (and ``index_count``).
+  compute_shuffled_index_aux(index, index_count, seed) do:
+    0'u8 ..< SHUFFLE_ROUND_COUNT.uint8
+
+func compute_inverted_shuffled_index*(
+    index: uint64, index_count: uint64, seed: Eth2Digest): uint64 =
+  ## Return the inverse of the shuffled index corresponding to ``seed`` (and
+  ## ``index_count``).
+  compute_shuffled_index_aux(index, index_count, seed) do:
+    countdown(SHUFFLE_ROUND_COUNT.uint8 - 1, 0'u8, 1)
+
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#compute_proposer_index
-func compute_proposer_index(state: ForkyBeaconState,
+template compute_proposer_index(state: ForkyBeaconState,
-    indices: seq[ValidatorIndex], seed: Eth2Digest): Opt[ValidatorIndex] =
+    indices: openArray[ValidatorIndex], seed: Eth2Digest,
+    unshuffleTransform: untyped): Opt[ValidatorIndex] =
   ## Return from ``indices`` a random index sampled by effective balance.
   const MAX_RANDOM_BYTE = 255
 
   if len(indices) == 0:
-    return Opt.none(ValidatorIndex)
+    Opt.none(ValidatorIndex)
-
+  else:
-  let seq_len = indices.lenu64
+    let seq_len {.inject.} = indices.lenu64
 
     var
       i = 0'u64
       buffer: array[32+8, byte]
+      res: Opt[ValidatorIndex]
     buffer[0..31] = seed.data
     while true:
       buffer[32..39] = uint_to_bytes(i div 32)
       let
-      candidate_index =
+        shuffled_index {.inject.} =
-        indices[compute_shuffled_index(i mod seq_len, seq_len, seed)]
+          compute_shuffled_index(i mod seq_len, seq_len, seed)
+        candidate_index = indices[unshuffleTransform]
         random_byte = (eth2digest(buffer).data)[i mod 32]
         effective_balance = state.validators[candidate_index].effective_balance
       if effective_balance * MAX_RANDOM_BYTE >=
           MAX_EFFECTIVE_BALANCE * random_byte:
-      return Opt.some(candidate_index)
+        res = Opt.some(candidate_index)
+        break
       i += 1
 
+    doAssert res.isSome
+    res
+
+func compute_proposer_index(state: ForkyBeaconState,
+    indices: openArray[ValidatorIndex], seed: Eth2Digest):
+    Opt[ValidatorIndex] =
+  ## Return from ``indices`` a random index sampled by effective balance.
+  compute_proposer_index(state, indices, seed, shuffled_index)
+
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#get_beacon_proposer_index
 func get_beacon_proposer_index*(
     state: ForkyBeaconState, cache: var StateCache, slot: Slot):
@@ -378,19 +401,16 @@ func get_beacon_proposer_index*(
   cache.beacon_proposer_indices.withValue(slot, proposer) do:
     return proposer[]
   do:
-    # Return the beacon proposer index at the current slot.
+    ## Return the beacon proposer index at the current slot.
 
     var buffer: array[32 + 8, byte]
     buffer[0..31] = get_seed(state, epoch, DOMAIN_BEACON_PROPOSER).data
 
     # There's exactly one beacon proposer per slot - the same validator may
     # however propose several times in the same epoch (however unlikely)
-    let
+    let indices = get_active_validator_indices(state, epoch)
-      # active validator indices are kept in cache but sorting them takes
-      # quite a while
-      indices = get_active_validator_indices(state, epoch)
-
     var res: Opt[ValidatorIndex]
+
     for epoch_slot in epoch.slots():
       buffer[32..39] = uint_to_bytes(epoch_slot.asUInt64)
       let seed = eth2digest(buffer)
@@ -401,6 +421,28 @@ func get_beacon_proposer_index*(
 
     return res
 
+# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#get_beacon_proposer_index
+func get_beacon_proposer_indices*(
+    state: ForkyBeaconState, shuffled_indices: openArray[ValidatorIndex], epoch: Epoch):
+    seq[Opt[ValidatorIndex]] =
+  ## Return the beacon proposer indices at the current epoch, using shuffled
+  ## rather than sorted active validator indices.
+  var
+    buffer {.noinit.}: array[32 + 8, byte]
+    res: seq[Opt[ValidatorIndex]]
+
+  buffer[0..31] = get_seed(state, epoch, DOMAIN_BEACON_PROPOSER).data
+  let epoch_shuffle_seed = get_seed(state, epoch, DOMAIN_BEACON_ATTESTER)
+
+  for epoch_slot in epoch.slots():
+    buffer[32..39] = uint_to_bytes(epoch_slot.asUInt64)
+    res.add (
+      compute_proposer_index(state, shuffled_indices, eth2digest(buffer)) do:
+        compute_inverted_shuffled_index(
+          shuffled_index, seq_len, epoch_shuffle_seed))
+
+  res
+
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/beacon-chain.md#get_beacon_proposer_index
 func get_beacon_proposer_index*(state: ForkyBeaconState, cache: var StateCache):
     Opt[ValidatorIndex] =
@@ -413,7 +455,7 @@ func get_beacon_proposer_index*(state: ForkedHashedBeaconState,
   withState(state):
     get_beacon_proposer_index(forkyState.data, cache, slot)
 
-# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.1/specs/phase0/validator.md#aggregation-selection
+# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/validator.md#aggregation-selection
 func is_aggregator*(committee_len: uint64, slot_signature: ValidatorSig): bool =
   let modulo = max(1'u64, committee_len div TARGET_AGGREGATORS_PER_COMMITTEE)
   bytes_to_uint64(eth2digest(
@@ -471,7 +513,7 @@ func livenessFailsafeInEffect*(
 
   false
 
-# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.1/specs/phase0/p2p-interface.md#attestation-subnet-subscription
+# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/p2p-interface.md#attestation-subnet-subscription
 func compute_subscribed_subnet(node_id: UInt256, epoch: Epoch, index: uint64):
     SubnetId =
   # Ensure neither `truncate` loses information
@@ -495,7 +537,7 @@ func compute_subscribed_subnet(node_id: UInt256, epoch: Epoch, index: uint64):
     )
   SubnetId((permutated_prefix + index) mod ATTESTATION_SUBNET_COUNT)
 
-# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.1/specs/phase0/p2p-interface.md#attestation-subnet-subscription
+# https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.2/specs/phase0/p2p-interface.md#attestation-subnet-subscription
 iterator compute_subscribed_subnets*(node_id: UInt256, epoch: Epoch): SubnetId =
   for index in 0'u64 ..< SUBNETS_PER_NODE:
     yield compute_subscribed_subnet(node_id, epoch, index)

beacon_chain/validators/action_tracker.nim

@@ -10,8 +10,9 @@ import
   ../spec/forks
 
 from ../spec/validator import compute_subscribed_subnets
+from ../consensus_object_pools/block_pools_types import ShufflingRef
 from ../consensus_object_pools/spec_cache import
-  EpochRef, epoch, get_committee_assignments
+  epoch, get_committee_assignments
 
 export forks, tables, sets
 
@@ -214,19 +215,19 @@ func needsUpdate*(
 from std/sequtils import toSeq
 
 func updateActions*(
-    tracker: var ActionTracker, epochRef: EpochRef) =
+    tracker: var ActionTracker, shufflingRef: ShufflingRef,
-  # Updates the schedule for upcoming attestation and proposal work
+    beaconProposers: openArray[Opt[ValidatorIndex]]) =
-  let
+  let epoch = shufflingRef.epoch
-    epoch = epochRef.epoch
 
-  tracker.attesterDepRoot = epochRef.shufflingRef.attester_dependent_root
+  # Updates the schedule for upcoming attestation and proposal work
+  tracker.attesterDepRoot = shufflingRef.attester_dependent_root
   tracker.lastCalculatedEpoch = epoch
 
   let validatorIndices = toHashSet(toSeq(tracker.knownValidators.keys()))
 
   # Update proposals
   tracker.proposingSlots[epoch mod 2] = 0
-  for i, proposer in epochRef.beacon_proposers:
+  for i, proposer in beacon_proposers:
     if proposer.isSome and proposer.get() in validatorIndices:
       tracker.proposingSlots[epoch mod 2] =
         tracker.proposingSlots[epoch mod 2] or (1'u32 shl i)
@@ -237,8 +238,7 @@ func updateActions*(
   static: doAssert SLOTS_PER_EPOCH <= 32
 
   for (committeeIndex, subnet_id, slot) in
-      get_committee_assignments(epochRef.shufflingRef, validatorIndices):
+      get_committee_assignments(shufflingRef, validatorIndices):
-
     doAssert epoch(slot) == epoch
 
     # Each get_committee_assignments() call here is on the next epoch. At any

tests/test_blockchain_dag.nim

@@ -186,8 +186,18 @@ suite "Block pool processing" & preset():
       nextEpochSlot = nextEpoch.start_slot()
       parentBsi = dag.head.parent.atSlot(nextEpochSlot).toBlockSlotId().get()
       stateCheckpoint = dag.stateCheckpoint(parentBsi)
+      shufflingRef = dag.getShufflingRef(dag.head, nextEpoch, false).valueOr:
+        raiseAssert "false"
+      nextEpochProposers = withState(dag.headState):
+        get_beacon_proposer_indices(
+          forkyState.data, shufflingRef.shuffled_active_validator_indices,
+          nextEpoch)
 
     check:
+      # get_beacon_proposer_indices based on ShufflingRef matches EpochRef
+      nextEpochProposers == dag.getEpochRef(
+        dag.head, nextEpoch, true).get.beacon_proposers
+
       parentBsi.bid == dag.head.parent.bid
       parentBsi.slot == nextEpochSlot
       # Pre-heated caches
@@ -200,6 +210,7 @@ suite "Block pool processing" & preset():
       # this is required for the test to work - it's not a "public"
       # post-condition of getEpochRef
       getStateField(dag.epochRefState, slot) == nextEpochSlot
+
     assign(state[], dag.epochRefState)
 
     let

tests/test_honest_validator.nim

@@ -10,8 +10,7 @@
 import
   unittest2,
   ./testutil,
-  ../beacon_chain/spec/[network, validator],
+  ../beacon_chain/spec/[network, validator]
-  ../beacon_chain/spec/datatypes/[base, altair]
 
 from std/sequtils import toSeq
 
@@ -309,3 +308,17 @@ suite "Honest validator":
         @[16.SubnetId, 17.SubnetId]
       toSeq(compute_subscribed_subnets(default(UInt256), 400.Epoch)) ==
         @[16.SubnetId, 17.SubnetId]
+
+  test "Index shuffling and unshuffling invert":
+    const seed = Eth2Digest.fromHex(
+      "0xa0054f8b4dead1ac88bd2c50cf13eab88f86d020362708a97a13012a402c57d3")
+
+    for index_count in [1'u64, 4'u64, 52'u64, 2121'u64, 42616'u64]:
+      for index in 0'u64 ..< index_count:
+        check:
+          compute_shuffled_index(
+            compute_inverted_shuffled_index(
+              index, index_count, seed), index_count, seed) == index
+          compute_inverted_shuffled_index(
+            compute_shuffled_index(
+              index, index_count, seed), index_count, seed) == index