feat: implement is_better_update for light client

beacon-chain/rpc/eth/light-client/helpers.go

@@ -13,11 +13,16 @@ import (
  "github.com/prysmaticlabs/prysm/v5/config/params"
  "github.com/prysmaticlabs/prysm/v5/consensus-types/interfaces"
  v1 "github.com/prysmaticlabs/prysm/v5/proto/eth/v1"
+ ethpbv2 "github.com/prysmaticlabs/prysm/v5/proto/eth/v2"
  v2 "github.com/prysmaticlabs/prysm/v5/proto/eth/v2"
  "github.com/prysmaticlabs/prysm/v5/proto/migration"
  "github.com/prysmaticlabs/prysm/v5/time/slots"
 )
 
+const (
+ finalityBranchNumOfLeaves = 6
+)
+
 // createLightClientBootstrap - implements https://github.com/ethereum/consensus-specs/blob/3d235740e5f1e641d3b160c8688f26e7dc5a1894/specs/altair/light-client/full-node.md#create_light_client_bootstrap
 // def create_light_client_bootstrap(state: BeaconState) -> LightClientBootstrap:
 //
@@ -311,3 +316,74 @@ func newLightClientUpdateToJSON(input *v2.LightClientUpdate) *structs.LightClien
  SignatureSlot: strconv.FormatUint(uint64(input.SignatureSlot), 10),
  }
 }
+
+func isSyncCommitteeUpdate(update *ethpbv2.LightClientUpdate) bool {
+ nextSyncCommitteeBranch := make([][]byte, fieldparams.NextSyncCommitteeBranchDepth)
+ return update.NextSyncCommitteeBranch != nextSyncCommitteeBranch
+}
+
+func isFinalityUpdate(update *ethpbv2.LightClientUpdate) bool {
+ finalityBranch := make([][]byte, finalityBranchNumOfLeaves)
+ return update.FinalityBranch != finalityBranch
+}
+
+func isBetterUpdate(newUpdate, oldUpdate *ethpbv2.LightClientUpdate) bool {
+ maxActiveParticipants := newUpdate.SyncAggregate.SyncCommitteeBits.Len()
+ newNumActiveParticipants := uint64(0)
+ for i := uint64(0); i < maxActiveParticipants; i++ {
+ if newUpdate.SyncAggregate.SyncCommitteeBits.BitAt(i) {
+ newNumActiveParticipants += 1
+ }
+ }
+ oldNumActiveParticipants := uint64(0)
+ for i := uint64(0); i < oldUpdate.SyncAggregate.SyncCommitteeBits.Len(); i++ {
+ if oldUpdate.SyncAggregate.SyncCommitteeBits.BitAt(i) {
+ oldNumActiveParticipants += 1
+ }
+ }
+ newHasSupermajority := newNumActiveParticipants*3 >= maxActiveParticipants*2
+ oldHasSupermajority := oldNumActiveParticipants*3 >= maxActiveParticipants*2
+
+ if newHasSupermajority != oldHasSupermajority {
+ return newHasSupermajority
+ }
+ if !newHasSupermajority && newNumActiveParticipants != oldNumActiveParticipants {
+ return newNumActiveParticipants > oldNumActiveParticipants
+ }
+
+ // Compare presence of relevant sync committee
+ newHasRelevantSyncCommittee := isSyncCommitteeUpdate(newUpdate) && (slots.ToEpoch(newUpdate.AttestedHeader.Slot) == slots.ToEpoch(newUpdate.SignatureSlot))
+ oldHasRelevantSyncCommittee := isSyncCommitteeUpdate(oldUpdate) && (slots.ToEpoch(oldUpdate.AttestedHeader.Slot) == slots.ToEpoch(oldUpdate.SignatureSlot))
+
+ if newHasRelevantSyncCommittee != oldHasRelevantSyncCommittee {
+ return newHasRelevantSyncCommittee
+ }
+
+ // Compare indication of any finality
+ newHasFinality := isFinalityUpdate(newUpdate)
+ oldHasFinality := isFinalityUpdate(oldUpdate)
+ if newHasFinality != oldHasFinality {
+ return newHasFinality
+ }
+
+ // Compare sync committee finality
+ if newHasFinality {
+ newHasSyncCommitteeFinality := slots.ToEpoch(newUpdate.FinalizedHeader.Slot) == slots.ToEpoch(newUpdate.AttestedHeader.Slot)
+ oldHasSyncCommitteeFinality := slots.ToEpoch(oldUpdate.FinalizedHeader.Slot) == slots.ToEpoch(oldUpdate.AttestedHeader.Slot)
+
+ if newHasSyncCommitteeFinality != oldHasSyncCommitteeFinality {
+ return newHasSyncCommitteeFinality
+ }
+ }
+
+ // Tiebreaker 1: Sync committee participation beyond supermajority
+ if newNumActiveParticipants != oldNumActiveParticipants {
+ return newNumActiveParticipants > oldNumActiveParticipants
+ }
+
+ // Tiebreaker 2: Prefer older data (fewer changes to best)
+ if newUpdate.AttestedHeader.Slot != oldUpdate.AttestedHeader.Slot {
+ return newUpdate.AttestedHeader.Slot < oldUpdate.AttestedHeader.Slot
+ }
+ return newUpdate.SignatureSlot < oldUpdate.SignatureSlot
+}